18 classic hull design

[CDMA]

Some of this has been hashed out already but we need something to talk about and we all keep learning right… wink

18 bottom hull design and effects

I have been playing with my 18 now for three years with a 260 hp Volvo and then alpha. Followed by a 383 Bravo about 400 hp. I have been running my boat as well as many other 18’s and been going through in my head how each of them run.

The earlier boats ( barrelbacks), like mine, have sharp keels and full length running strakes. The later ones ( after about 69) all have rounded keels and short inner strakes.

The general characteristic I have heard is the earlier boat has a greater tendency to slam or sort of slap the water on reentry. I am not inclined to say pound.

After all this time I have come to a few other ideas. I think until about 64-65 mph the earlier boat, with the exception of the pre stipulated slamming, acts very similar to the later rounded keel boat. However above that speed she is a different animal. While testing an exact set up back to back has not been possible through some conversations with other 18 owners and running their boats I see the following ( all in reference to above 65 mph):

At high speeds the stern lift on the barrelback is significantly greater. In order to get the boat aired out much more positive trim on the drive is needed. I feel this is creating a boat that not only runs higher and looser but a boat that is not as inclined to carry the bow as well. You have to trim the drive more, create more drag and lift even more of the boat out of the water then in a later hull. This creates an entirely different riding boat.

After careful thought I have been going back and forth between which change in the hull design creates this difference. I have come to the conclusion that the sharp keel has nothing to do with it and it is the increased horizontal running surface aft created by the full length running strakes that creates this.

The effects of the sharp keel still elude me. From a theoretical point of view I don’t think, with the exception of a slight speed penalty due to increased wetted surface, there is any negative effects of the ride by the sharp keel. I have been thinking for going real fast the sharp keel will create a more directionally stable running boat albeit slower.

So I have been seriously considering removing the long strakes from the bottom of my boat and creating a sharp keel short stake boat. This is not a simple task as the main structure of the hull would have to be removed. The main laminate of the hul actually runs in the strakes as far as I can tell.

The idea of running a barrelback aside, don’t even start with me, I would like to hear peoples input on this. I know poodle, Kent Perroux and a few others might have some input.

Worst case it makes for good conversation…

Chris

[BillG]

Chris,
I have a '66 Barrel back and I can say I have aired it out on more than one occasion and I have never felt that it landed with anything but a smooth easy landing in any sort of seaway. As far as bow lift all you need is a wheel with a lot of rake in it and the bow comes up very nicely. As far as the full length strakes go, the more lift the less drag.
Bill G

[CDMA]

Bill,

Power? Drive? Are you above 70 mph? You need to be going at least 65 to notice this. I never did before I added more power. Also have you ever run in a later model 18?

Run all the props including the bow lifting ones. I can get the bow up for sure just takes more trim then the other ones. More trim= more drag.

More lift does not necessarily mean less drag. Less wetted surface drag for sure...but not necessarily less total drag. Appendage, discontinuity, and reactive force,and spray drag are by FAR more a factor on a planing boat.

Spray drag might be another answer here. Most don't realize that spray actually is a form of lost energy and is a huge part of drag on a high speed boat.

The "slamming" I refer to is hardly noticable. It is only noticable if you run the boats side by side same conditions. There is no argueing that either hull is an awesome rough water boat but there are differences.

Chris

<small>[ April 11, 2003, 01:02 PM: Message edited by: CDMA ]</small>

[MOP]

It seems to me BillG is on the right track, using more rake allows you to use less trim. I have always believed that the closer to perpendicular a lower unit runs to the surface the better it flows water which to me should make for the fastest given speed. My reasoning is the hydrodynamic shape is fully utilized and the prop blades are grabbing equally. I read about an Italian race boat years back that ran straight shafts perpendicular to the surface by stepping the aft hull section which made it considerably faster than down angle shafting used on its nearly identical sistership. My prop shop guy has talked of how on high powered diesel boat due the shaft down angle how the blades flex near the hub and finally snap off. He says due to the shaft angle the blades load and unload every revolution, that means the bite the blades have on the water does not stay consistent therefore losing efficiency and causing heavy strain. I have seen many props off big sport fish and motor yachts snapped off, some go right through the bottom. If the lower unit is running flat it will get a much better bite. How many of us have blown by a jerk that is blowing a big rooster tail and going no where, us old timers laugh like hell at those idiots.

[Rootsy]

Chris,

you know that i too have pondered this subject somewhat in regard to the 16. the 16 with it's extremely short strake and rounded keel runs excessively bow high, especially with high rake props and V8's (which move the CG considerably rearward). While you are shortening your strakes up to bring the bow up i'd like to be extending mine to bring the butt up some! i think by flattening the 16 out you can still achieve a happy medium and coax enough bow lift out of it to air it out sufficiently. As well as add stability by allowing the hull to run on a flat surface to an extent. i do agree with your theory of excessive trim because the full strake makes the boat run very flat compared to a hull without because the water just flows off of the baren hull bottom (where there is no strake). I also see the V bottom vs round keel as the boat being a bit looser with the rounded keel... it's that whole force vector thing... so where is your free body diagram!

[CDMA]

Rootsy,

I agreee. I think that is what I am getting at as well. I think the round keel will be faster but I wonder if at high speeds the sharp keel will help it track better and have less of a propensity to roll in waves...I think so.


Bill and MOP,

Guys I am trying to stay away from props. I have tried the same props the later 18's have run and the boat rides different. I am looking for the reasons why.

Chris

[boldts]

I'll just add some to support M.O.P. I had a 69 18' Classic with round bottom. Sorry Chris, I can't offer a comparison to your barrelback because the boat was no where near 65 MPH. Couple things that I really loved about the boat though was how smooth and level it was while in the air. The boat had none of that flotation foam in it and it didn't matter to much how big the swells on Erie were, she'd do 50 all day long. Always came down rear first, nice and gentle.

Back to MOP's story, while up on Erie, in 1 to 2' swells, I had a guy in a Four Winns Liberator pull up next to me. 20' I believe. This guy was bouncing all over the place. More air than water! You would think that with the less wetted surface, he should have dusted my little 50 MPH Classic. I blew his strakes off because my level flying, prop in the water was moving forward rather than straight up and down. It was a fun ride and always brought a smile to my face running that Classic in the ruff stuff!

[mattyboy]

I wish i could talk inteligently on this subject(or any other subject for that matter eek! )
But here's what I feel in my 16 at WOT bow lift in not excessive but the prop I'm running 14X21 Spinelli has way to much stern lift( running the volvo at nuetral trim middle whole). so the boat is very close to spinout( very similar I think to a step boat in a turn. As MOP mentioned having the drive more level is better but if the prop has a tendency to climb to the surface like mine it will take the drive and hence the stern with it. But Chris has mentioned slight positive trim will point the prop slightly down and in turn push down on the stern and lift the bow .I was not aware of the 18 having this problem but it makes sense the 16 does it in the high 50's and it sounds like the 18 does it in the high 60's.
And I'm sure the merc brings different tendancies to the table.Tabs are another thing to consider and can help or hinder.
Chris are these issues that can be driven thru or is it a constant readjustment.
Good post would like to learn more

Matt

[CDMA]

Matty,

With a hydromotive the boat is rock steady...like fall asleep steady. But she doesn't feel like she is aired out at all. This is confirmed to me by a 5mph speed loss over a mirage.

With a mirage I can carry the bow a lot better but it still takes a lot of trim, more so then in other 18's I have ridden in/driven ( Oak, Paul O, Poodle, etc) The issue is with the mirage if I trim her up enough to air out the boat she gets real loose. Driveable but real touchy.

This is the common denominator. I have tried way more then these props ( other mirages, turbos, weird stuff, mach, cleaver etc) I get enough bow lift to go fast and she is gets loose. More so then other later boats I have driven at that speed. For a long time I chased stuff like tabs, added external steering. Now I am at the point where I think it has to be hull design. Everything else has been changed. When I look at pictures of my boat this idea is confirmed because she seems to ride A LOT higher then other 18's at speed. Some at events have noticed this as well. I guess the other thing I would really like to find is another 70+ mph barrelback. I don't know of another one.


Chris

<small>[ April 11, 2003, 03:06 PM: Message edited by: CDMA ]</small>

[HyperDonzi]

CDMA:
Appendage, discontinuity, and reactive force,and spray drag are by FAR more a factor on a planing boat.
Intresting topic, could you explain what the above are? Im guessing I know what the spray drag is, but the others I have never heard of before...
Tommy

[PaulO]

Three things IMHO,
1. Chris, I think you are on the right track and it is primarily the strake length that adds additional stern lift at speed to the older 18s. More stern lift certainly equals less bow lift and probably a larger wetted section of hull that needs to be countered with more pos. trim.
2. MOP, I am not sure I understand how a downward angle of a propshaft could create any real difference between prop blade loading and unloading during normal rotation. Prop blades load and unload because of the differing density of water due to the slight change in pressure from the change in depth as they rotate through shallower water and then deeper, denser water. If anything, I would think a propeller shaft approaching verticle (more downward angle) would lessen that effect. Unless of course I am misunderstanding.
3. There is a boat that is available locally that incorprates just what MOP has described. It is a 21 foot performance boat built by Syler Marine and incorparates an inboard single engine that exits a false notched transom resulting in a propeller shaft angle parrallel to the water surface. One of these days I will restore that interesting boat unless one of you nuts would like first crack at it.
PaulO

[BUIZILLA]

Chris, I think your on to more than you realize...

I say this in a postive way...

Ji

[MOP]

Ok I'm back had to do SOME work today. I have seen a couple of posts that recommend cleavers for the 16 to pick the stern up a bit, but decent rake on the larger boats. I would like to try a cleaver one my self. For speed I run little or no trim, it was pointed out to me that the OMC drives sit a few more inches back off the transom then the small Merc that would also be on these 16's may act like a mini extension box. Any thoughts Chris? Another thought when you see the really wild out boards racing they also seem use very little trim just enough to get a little air.

?? PaulO that Syler Marine is that Wood?

A tilted prop grabs unevenly that is pretty well know by the guys that re-do them and is what causes them to break blades off at the hub.

Second Edit: take any prop and hold it up then tilt back you will see the difference in blade attack. I remember from OMC school the videos and talks about how much better it was to use the prop to lift one end or the other rather then trimming for speed on bass boats. But just last year Art and I were up at the St. Lawrence during the Bass Tourney and I did see alot of guys running real high trim. WE NEED A TEST TANK! Oh Chris do they have one at school????

<small>[ April 11, 2003, 04:00 PM: Message edited by: M.O.P. ]</small>

[KMLFAMILY]

CDMA:
When I look at pictures of my boat this idea is confirmed because she seems to ride A LOT higher then other 18's at speed. Some at events have noticed this as well. Yup. :) http//www.donzi.net/photos/klessard35.jpg


Chris [/QB]

[Ralph Savarese]

Hey if i can help ad to this discussion dont leave me out. I would say i am on or near 65MPH

[MOP]

KMLFAMILY I love it!!! CDMA in a Squirrely mood!

[Fish boy]

Chris,
I am sure this is a bonehead thought, but is the weight the same and in the same places as the boats you are comparing yours to? Are the fuel tanks in the same place and the same size? Engines weigh the same?

Is it possible that a difference in weight fore or aft is alowing/causing some of the stern lift or bow down?

Anyway, i remember you having a fat guy in the back of your boat when you were down here, and it seemed to run great. Let me know if you need me to hold the stern down fer ya. :D

PS here is a pic of you running with a less fat guy in the back- still pretty good angle if you ask me.

http://www.donzi.net/photos/jstaples73.jpg

<small>[ April 11, 2003, 04:26 PM: Message edited by: Fshboy ]</small>

[MOP]

Yeah I remmeber a post just a short time ago Chris said he lightened the boat, Guy must have finally gone home!

[BillG]

I think what M.O.P. is trying to say that the pitch of each blade is at a different pitch relative to the bottom of the boat if it's centerline in not in line with the boat bottom. This will cause varying amounts of load on every rotation. Cleaver props also have varying degrees of lift depending on how far behind or ahead of the prop centerline the trailing edge of the blade is. This how hydro guys balance their boats.

Bill G

[MOP]

You are right Bill, all modern props blade sweep back, as you tilt them the angle and bite changes. Trim a drive way out the upper area of rotation will grab more water than the lower area of of rotation. Try it take a wheel in your hands and tilt it you will see how the blades angle of attack changes, also the area presented to the water get smaller. I am sure there are better tech terms for this. Another thing trimming sucks the stern down as it lifts the bow. So what is the happy median!

[CDMA]

Interesting guys...

All those pictures I am running a mirage 25...plenty of bow lift...little short on control even with External sttering.

One thing with pictures is in this case they can tell a lie. In those sea states figureing out what angle the boat actually runs is futile.

I have seen 75 with the mirage on GPS but in anything other then glass she is all over the place. Very hard to drive.

Chris

[CDMA]

Or this...

http://www.donzi.net/photos/callardairborn1.jpg

[Rootsy]

now i had theories of water pressure on different surfaces going for more strake or less strake and rounded vs. sharp keel but after looking at those photos i think that is all blown out of the water... cause man you don't have much of a hull in the water at all! looks pretty much just like doug lovins 18 with the SS and his 23 mirage... cept his boat with the SS just hangs there like it is suspended in mid air... not a twitch of unsteady... it is really awesome.

now on the 16, and maybe you can transfer some to the 18 hull... with no strake in the water when trimmed (16 only) and a rounded keel the boat is squirrly especially with a deep drive providing all of that torque. the water has nothing to react against. the rounded keel provides more of a parallel surface area for lift due to the water pressure but does not create a "pad" type running surface because of the arch and the water just pushes sideways off of it... therefore it ends up running very bow high because of the CG and no strake to really lift the rear of the boat... so without much to balance on other than the prop and drive it is very difficult to keep the 16 stable and she wants to fall to the side and chinewalk... kinda like rolling a can back and forth on a table top... i feel that with extending the strakes rearward to some point (Not all of the way back.. maybe not even as far as current 18's) so that you can air it out yet have a flat surface to partially balance on, maybe 6 inches to a foot of strake, the hull would become more stable. i do have a few other ideas to help counteract and balance the hull... yet to be tried...

by running the cleaver it does not provide as much bow lift but really more neutral lift... the bow is not as hight when you trim and the whole boat picks up more evenly.. and you lose the torque reaction at the prop due to less blade area at the tips and the boat is much more stable.. because of it as well as having more of a running surface which is closer to parallel with the water instead of bigtime bow high which is like pushing a wedge through the water causeing all kinds of drage and spray...

least that is my analogy fwiw...

[Scott Pearson]

http://www.donzi.net/photos/spear18003.jpg


(NJ)Scott

[MOP]

Ok Damn it who has a 22" or 23" OMC cleaver for me to try????

<small>[ April 12, 2003, 07:40 AM: Message edited by: M.O.P. ]</small>

[BillG]

Scott,
Why are those people holding on to the rails? I thought they were there to hold the boat at the gas dock!
Bill G

[PaulO]

MOP,
Sorry for the late reply but I was traveling for work. The Syler is purportedly kevlar. It is an Omni model. Really cool boat. If you are interested, let me know. I would love to see someone restore this interesting piece.
PaulO

[GEOO]

Chris,

I don't have much time in a V bottom vs round bottom, But the V would create less lift then the rounded bottom and Chime walk would be more violent in a V bottom vs the rounded bottom.
However, the running strike would add more lift back into the V bottom. HUM!!!

So many things come into play; other items ,variables:
The higher the X dim. the less bow lift from the drive and more transom lift from the prop.
The higher the pitch on the prop the more transom lift.
The larger the prop blade area the more transom lift.
Add cup to the tip of the prop gain bow lift.
Add cup to the blade adds transom lift.
New lighter exhaust, more transom lift.
What size tank do you have?
The 25 gal tanks might be mounted farther foward then the 41 gal tank. The 41 gal tank might move the CG back?
Bring the boat over to CT, well play with some sand bags or water balast and take video's!!!
Measure your CG or do you have it??

Hey, do you think the X-18 would be slower do to the downforce of wind hitting the front curved deck? wink :rolleyes:

[Eugene Nahemow]

O.T.
Chris,
What drive are you running now?
What is maximum hp/RPM for Alpha?
What is maximum hp/RPM for Bravo?
Thanks.

[Eugene Nahemow]

Sorry guys, but I had to hit 100 posts! :D

[BigGrizzly]

I havn't do the hull change on the 18 but we did contiue the strakes on a round bottom 16 foot Donzi. It was a tad faster but chine walked and road like a brick. One thing I can say is in rough water a higher X dinmention handles worse than a deeper one. We did the stock bravo than switched to the Imco shortie(on the same boat) and found this to be true.

[CDMA]

Mudpuddle,

The effect of higher X dimensions on handling and stabilty also intrige me. My intuition say that the deeper drive whould handle better but from what I have observed in real life IMHO the heigher X dimesion boats seem to hanlde better and be more stable. The Alpha SS boats being the prime example. My only thoughts on this is maybe the shorter lever arm created from the prop produces less twist on the boat and allows her to fly straight. Roosty your input after your swap would be appreciated.

I don't think the higher CG of my 1" higher X dimension makes a big difference. Compared to Pearson's I bet the Stainless marine and the aluminum intake offset any height increase when compared to the Merc stock steel parts. Also in thinking back compared to your boat my engine sits considerably lower then yours in the boat. Noticeable to the eye.

Chris

[Scott Pearson]

Guys,
I raised my X dimension on my 18 1 1/2inches higher then stock. So there goes your theory.....


(NJ)Scott

[Rootsy]

my limited input says that the SS has made the 16 a different animal... but why exactly?

as far as the SS hanging off my 16 goes... being that is has an outboard lower gearcase it has a torque tab on the skeg. it also has side water pickups instead of low water on the nose.

from my hours of testing props on the GEN II i can tell you that the less bow lift you generate the slower it wants to go cause of more wetted surface. but at the same time the cleaver made the boat extremely manageable whereas even a turbo 3 blade was out of control. the cleaver seemed to lift the boat more evenly and it also generated less torque on the hull. the boat did not list under power anymore and it didn't want to get out of control at high speed unless you really trimmed it too far or got into a bad wave situation.

Now with the SS and still running a cleaver the hull lifts well from the water, maybe all of the way, i don't know since i havn't seen it run, and it just goes straight now without any squirrly feelng what so ever... the SS is 3 inches less in the water than the GEN II and i have a feeling this has some to do with the chinewalking issue but on the same token i believe that torque tab cast into the skeg also helps immensely.

i see the rounded bottom as more of a "pad" type surface too but i feel it makes the hull want to slip slide back and forth more than a true "V" does. i feel that the rounded bottom is more akin to a barrel in the water...

[boatnut]

Some trivia info on this subject: I was at Donzi in the early 70's and the techies there told me that the rounded bottom and shortened strakes were originally done to provide a hull that could accomodate a jet drive (rounded bottom for intake to bed in, and strake design to keep the intake more in the water). They had several customers that wanted jets which were the current fad at that time. I've seen one 18 with a jet in Calif. After doing this they tried a sterndrive in the prototype and found it handled better, exhibiting less hooking in high speed turns and displayed no negative characteristics (not sure what was said about top speed). They decided to switch production to all rounded at that time. I've owned both hull types (67 and 69) and noticed no significant differences but neither boat went over 60mph.

[CDMA]

Boat nut,

Interesting. I haven't heard the jet drive reason before but it makes sense. The reasoning I had heard was the high speed hooking you mention. I have pushed my boat pretty hard through turns and never hooked or felt like it wanted to...but I guess someone did.

I never noticed the difference below 65 maybe even 70. Before that the boats, in my opinion seemed very similar. Above 70 I think it is a whole new ball game.

Chris

[Craig]

Boatnut,

(Here's 2 more cents) One thing worth mentioning about the Donzi 18 in the 70's (or at least in exactly 1970), is that from what I've read and seen the average 18's weren't going fast enough for the stuff being discussed here (at least I don't think most were).

I have an old Powerboat Mag. Article about the 1970 18-2+3 hanging handily over my computer and it shows the 18 going 51 mph. Power options listed included: 165hp, 210hp, and 235hp (stern drives) and 300hp and 400hp V-drives. It doesn't say it, but I'm sure the 290hp stern drive should have been shown here also.

The 51 mph (@ 5,000 rpm) boat tested was a 235 hp 302 with a Volvo (Holman Moody) stern drive (1.61:1 gear ratio), and a 14x21 aluminum prop. Probably a large percentage of the 18's purchased came something like this one so probably ran around low to mid 50's (maybe upper 50's with the 290hp) without any further tweaking for speed.

It was interesting to me that you said they (Donzi) realized better handling with the rounder keel. That would seem like the only benefit to have been concerned with at those speeds, so I guess they made the change permanent at that point.

Seems consisitent with what everyone else has said too about handling vs speeds "below 60."

This was a great post! Very interesting.

[boatnut]

I believe that your conclusion that the majority of the 18's in the early 70's were running in the 50's speed wise is correct. However, some were cooking. On a visit to Donzi in about the same timeframe (may have even been the same visit where the jet drive discussion took place), I was given a ride on an 18 they were "propping" prior to shipment to a customer in Italy (if I remember correctly). This 18 had a SBC that I think was marined by Chris Craft (CC had a Z28 based package that was one of the hottest stock sterndrive packages at the time). Donzi had tweaked the engine slightly and most importantly had one of Volvo's hand built "speedmaster" lower units on the Volvo (270 I think) drive. On a water speedo we were showing mid-70s and they said they had seen more?? So they all weren't slow, especially if a guy in Europe wrote a large enough check. It was really cool how they had a couple of electric hoists on the lifting rings on the boat and it was a 30 second process from changing a prop or drive adjustment until another test run. Isn't it amazing that in the last 35 years or so no one has made a better 18' speedboat! Ed

[BigGrizzly]

I would almost agree with the jet drive theory except that the first were round bottoms than Don made the change and than it came back. A year or so ago Brownie gave the whole rendition of the bottom thing, and he was there. In my early years I had driven the sharp bottom owned by Mike Fennamore and it hooked on me slightly. It did have a Volvo drive on it. Neither my 16 or my 18 has ever hooked on me ever. I will say that to hook it you have to be real agressive comming out of the turn, not going into it,

[CDMA]

Alright guys before I rip this baby apart and go to something else...

I need to try a Bravo shorty. Who has got one I can try or know where I can get one to try. Will trailer...

Chris

The X must go up...I think

[CDMA]

Up Poodle Up...

But will try down...

Chris

[olredalert]

------Talking about average speed of as delivered DONZIs,I ordered two of those Chris Craft powered 18s in 1972 and inadvertantly ended up keeping one of them for a couple of years.With the standard 270 Volvo drive it was a 60 MPH boat,so the 70 figure with superior lower unit would be completely believable.Must have been quite a ride!Because I was ordering boats for resale I never ordered any of the exotic stuff back then.Wish I would of now!!!!!!!.......Bill S......ol red!

[CDMA]

Hmmmm boys... thoughts?

From Principles of High Speed Boat Design:

http://www.donzi.net/photos/planing.jpg


"Spray rails (strakes) on a planing hull are show in fig 10.11. The wetted surface at the two speeds is indicated. If the speed will not exceed 25 knots it may be wise to cut the outer rail at point D, since it will be useless further aft. However, if the hull may obtain speeds of 40 knots the wetted surface must be reduced. Had the outer rail been cut the water would have continued to clear the hull at the chine and the center of pressure would have been moved too far aft. This would have caused the bow to fall resulting in a much larger resistance and possible steering problems. If the rail is kept all the way to the stern the wetted beam ( between rails) becomes smaller, which means the wetted length is not so much reduced when the speed increases. The center of effort thus does not move that far backward and the hull maintains it’s trim better. Note that the beam used in the calculations above is now the distance between the rails. There is no accurate procedure developed for the extra rail lift, but their effect may be roughly included by measuring the deadrise angle from the keel to the outer edge of the active rail. This angle is smaller then that measured along the surface and so it yields higher lift."

Yes I know not easy reading....

[TORYSMINX]

Chris,

You continue to impress me....


-Tory

Always looking for that extra 1/2 mph.... :)

[Kent Perroux]

I am going to have to stop in more often here. Good thread.

Some thoughts from my experience:

Originally 1983 round bottom 18. Inside strakes stoped 24" from transome. Volvo 280 at standard x:
Needed tabs to plane quickly. Needed tabs to reduce porposing below 35. Very high angle turns. Good rough water capability. Top speed with best labbed props and nice 350: 63 mph.

Mod 1: Extended inside strakes to transom.
Noticed a lot of stern lift. Could not trim the bow up for additional speed. Rode hard. Top speed no difference. Turns had very little angle. The last turn at 55 boat skipped and hooked throwing driver (me) out. (Lifeline vest kept me from having several broken ribs)

Mod 2: Immediately after the medication kicked in from the chest bruises, I cut off 6" of the inside strakes.
Ride improved, gained slightly more angle in turns, could more easily lift the bow for more speed. Rough water running was improved. Landings harder than stock if you fly it. However, if you run level and try not to fly, you can maintain a more level attack because the back does not dip as much in the swells. Stuffing does become a greater concern. Top speed: 67

Mod 3: Removed Volvo 280. Replaced with Merc Bravo (w/nosecone). Installed 1-1/2" higher than stock. Much better all around performance. Tabs rarely used; not really required, just adds that extra 8" of boat length. Turning is excellent with moderate lean, and no blowout like the volvo. Testing props I did make some runs with a 23" race cleaver and was bouncing on the 6000 rev limiter. With too much slack in my steering I did not pay much attention to the speedo. I didn't have chine-walk, but was difficult to balance with steering slack and no power assist.

Went to an old 25 mirage was running a solid 72. 24" Hydromotive around 70. At these speeds, no chine-walk; solid feel.

Mod 4: Added External Steering ram(non-assist, full hydraulic). Nice! Absolutely no slack, but tight to turn. OK, but prefer easier steering to react better.

Mod 5: In the process of adding power assist.

[Kent Perroux]

Chris? what date was that "Principles of High Speed Boat Design" published?

Looks kinda dated. A lot changes when you go from 40 knots to 80 knots.

[Rootsy]

I know this is an old thread..but it relates directly to what i am currently doing and what i am going to be doing or not be doing as far as strake and X dim go...

So what i ask is to continue on with this discussion for a bit with anything anyone has tried (chris, scott, kent, etc)... conclusions, findings, opinions, what not... and just how "much" strake do we want to get rid of...

lot of theory out there but sometimes as we all know theory don't hold up worth poopy against actual "experience"...

the one OTHER thing that concerned me was chris' thoughts of the strakes being laid up as a part of the main laminate schedule... do we know this for sure? or were they laid up in the mold, filled, cored or what not and then the main laminate schedule applied?

i was all for taking the sawzall to the hull before i read that... now i'm gonna need some "expert" opinions on how to keep this puppy from propogating cracks in the bottow when i twist the living piss out of it...

I am still up in the air as far as X dim... if i got stock X (14 1/2) iwth an already lightened engine, i can conceivably get up 2" with an imco easily... but what if i do go 15 1/2???

anywho... enjoy this blast from the past.. .when we had real meat and potatos discussions around this joint ;)

JR

[Woodsy]

Jamie...

We know the BH drive runs pretty good on an 18... sooooo

I would go up 2"... you can always space it back down using drive spacers. They are alot cheaper than IMCO shorties.

Woodsy

[CDMA]

Don't forget the sharp keep Vs. round keel even with identical strakes is going to mess with your X dimension. We can't compare the X on a round keel to a sharp keel. I don't know enough to know what the difference should or would be to make it comparable but I do know it would be different.

Helpful ehhhh???

Chris

[mphatc]

James,

Your 18 is a barrelback with a sharp vee, as is CDMA's 18 .. .

Chris, if I recall following your boat across Winni 2 years ago your bow was always pointing towards the sky in mild chop, and you commented that your Bravo X was not right . . do you know where you installed it? Maybe it's in the earlier pages of this thread, but I didn't re read all of it. . sorry.

I raised my X dimension a bit with the Bravo on my Corsican , but that's a round bottom so my results don't count here. My boat is stable till I over trim :hyper:


You can always go high and lower like Woodsy said, just use the lowest intake manifold and all the space under the hatches . . atleast there is more under your hatches that a newer 18 . . .

Mario

[Rootsy]

chris, you've mellowed on us man... what happened to ya... don't tell me corporate america has assimilated you :eek!:

i fully believe from conversations with Chris, Cliff, and others that strake needs to be removed... current round bottoms are 18 inches shy of the transom... i was going to go somewhere between 6 and 12 inches.. targeting 12 but possibly less.

BUT if you read through all 4 pages you'll see the question posed... did Donzi lay up the main laminant schedule encompassing the profile of the strakes???? with that sharp corner that kind of seems difficult to do to me.. but possible i reckon... i have my fingers crossed that they were laid up first (after gel was put down) and then the main schedule was laid on top of them... but i don't want to be the guinea pig to find out either...

If i have it correctly, Mr Pearson is + 1 1/2, Cliff was like +3 with the volvo in my boat, and i have conflicting info for Chris, some places i read 2 1/2 but later threads say 1 inch...

for all intensive purposes i am going to start out with a hydromotive quad IV O/T 23P... prop from my 16... it is smaller in diameter than the quad IV and has more rake for lift... and runs a solid 9% on my 16 from 3000 on up...

BUT, taking into account the differenece in the sharp keel vs round... if the round keel boats are placed stock at 14 1/2 from the bottom of the radius... where do we put the sharp keel? what are some similar sharp keel full strake boats on the market and where do they put their drives... i've seen some i just can't think of the names at this time...

oh Lenny... get your butt back from disneyland or world or whichever one is in florida... we need ya here... :banghead:

[mphatc]

Rootsy,

extrapolate . . .


draw a horizontal line from each rear chine corner across your transom, and measure to the bottom of the Vee . . on my 69 Corsican that dimension is 12 1/4"
what I just noticed . .. top of my water line is ~ 2" higher, and my x is above that by just a bit ..

Mario

[penbroke]

the one OTHER thing that concerned me was chris' thoughts of the strakes being laid up as a part of the main laminate schedule... do we know this for sure? or were they laid up in the mold, filled, cored or what not and then the main laminate schedule applied? JR
FWIW: In MY boat I can clearly see the strakes from the inside. I would guess they were layed up with the hull and then filled (badly) on the inside for looks.

Frank

[Rootsy]

speaking of sharp keel vs round keel differences...

24 degrees of deadrise... 18 inch radius keel (brownies formula for 1 inch of radius per foot of boat) for the rounded bottom... vs the sharp keel 24 degree hull.. with a 1 inch radius to knock the sharp off, which my boat has...

[mphatc]

James,

Mercruiser wants 14 1/2" for a bravo

from a very simplistic view point . . .

from your drawing if the X is ~ at 14.5" above the radius . . . you would need to be ~16.6" up to be ~ equal. . . .as I don't believe the sharp vee will effect the flow to the prop as much as a round bottom with a raised X . . .and I don't see that the COG is altered that much sharp vs round as your engine height and location is really no different from a round bottom . . .

Does this make sense . .

Maybe it expalins why CDMA's bow point to the sky . ..?? I think Chris is lost to Corporate America . . .:frown:


Mario

[Formula Jr]

Nice discussion. However there is a big pink elephant in the room. And no has addressed that. How does Geoo's 119 plus mph boat fit into this discussion?

Up until about 70, his boat's hull is working in a convention manner, and our theories of the relationships of lifting strakes and keel design apply. However, beyond 70, it seems an entirely, and revolutionary, new set of concepts come into play. Something more akin to how a three point hydro with sponsons and a prop lifted stern or a Hydrofoil design works. To visualise this, lets eliminate the hull completely, except as a rigid platform for holding everything together.

Visualize how a three point, non-tail dragging, hydro works. The sponsons are the spaced contact points that control roll. The wider these contact points are spaced the more stable the boat will be against roll. The further back from these two contact points the prop is, the more stable the boat will be against bow to stern rocking. So hold that visualizarion in your mind: This triangle of contact points.

We want to talk about offshore Deep Vee designs for running in rougher water than hydros typically can run in. Yet that design only comes into play when contacting a wave and letting the Vee of the hull slice that wave.

Now take that triangle of contact points used in a three point hydro and move the whole concept in back of the transom of the boat using huge controllable surfaces much like conventional trim tabs but much more complex and able to introduce rocker or hook. And move the prop, much further back like an Arneson surface drive works, but add, as Geoo did, a hydrofoil wing to the prop shaft housing and lower skeg and then make that also mechanically adjustable in elevation. Geoo's boat does still use the last little bit of his keel at speed as a forth point of contact. However even that doesn't seem to be necessary. And the introduction of a small flat pad, or even a small reverse V tunnel could eliminate the hull entirely as a running surface.

We now return you to your regularly scheduled broadcast. :)

[Lenny]

Jamie, I am still here. Just trying to "suck up" some recent events and go forward.

In your reply (as per Brownies) in regards to 1" of radius to foot of boat,...


Was that from the "X" dimension as measured at the transom, given the drive, or from some other tangent in regards to distance down from the deck flange?

[Greg Guimond]

Old thread but a great deal of information..........great read!

[samjannarone]

Formula,


Cool ideas regarding the relationships between a V hull and a 3 point hydro. My brother and I built one when we were kids and it was amazing, once on plane, how much you could throttle back, how much horsepower (from our stock 20 hp Merc!!!) was left to just go fast. Felt like aero drag was more of a problem than hydrodynamic drag. It was the ultimate chine walker.
Nice thread, just read it for the first time also.

[Donzi Vol]

This is a GREAT thread for sure! Very helpful info for an 18 owner/user. I've found it interesting since I've been enjoying the blue and white '67 Barrel Back (see recent threads) that it doesn't do crazy things that those long strakes usually cause. I've been in a couple others that caught me off guard with a lean or a self-decided turn that got me on edge. I almost jumped out of one one time just by natural reaction (I wasn't used to Donzi's just yet!). This one, however doesn't do that. Granted I've only been to about 50 mph, but it's my experience that these BB's do some funky stuff below the 65 mph mark. The only time it does lean is when I hammer down to get on plane, but I'm convinced that's just because of the torque of the prop. It just feels like it's a power thing rather than a balance or weight issue. Anyway, all that to say it seems out of the ordinary, but in a good way I guess.

[Conquistador_del_mar]

I believe that your conclusion that the majority of the 18's in the early 70's were running in the 50's speed wise is correct. However, some were cooking. On a visit to Donzi in about the same timeframe (may have even been the same visit where the jet drive discussion took place), I was given a ride on an 18 they were "propping" prior to shipment to a customer in Italy (if I remember correctly). This 18 had a SBC that I think was marined by Chris Craft (CC had a Z28 based package that was one of the hottest stock sterndrive packages at the time). Donzi had tweaked the engine slightly and most importantly had one of Volvo's hand built "speedmaster" lower units on the Volvo (270 I think) drive. On a water speedo we were showing mid-70s and they said they had seen more?? So they all weren't slow, especially if a guy in Europe wrote a large enough check. It was really cool how they had a couple of electric hoists on the lifting rings on the boat and it was a 30 second process from changing a prop or drive adjustment until another test run. Isn't it amazing that in the last 35 years or so no one has made a better 18' speedboat! Ed

Just to corroborate, I am now restoring a 1971 18' 2+3 (my old boat that I first bought in 1973, but all original at the time) that originally came with an LT1 Chris Craft marinized "Corvette" engine complete with the Corvette flags on the valve covers. It was rated 350HP and was good for right at 60MPH with one of the old school props and the 270 drive. It drove very well until I added a speedmaster drive like he talked about in the post here. I never got quite the right prop for it before selling it. Bill

[maddad]

My boat is a 72' that was origlnally a HM 351C boat. Once I got tired of blowing up Clevelands, I went to SBC's. I have a 430hp+ small block and spin a 23" Solas around 55-5600 and have had a max speed of 74+. I use a hydromotive nose cone and pick my water up through the hull. This could of been done in 72' if they had performance props for Volvo legs. Once I have external steering, I'm hoping for a controlable mid to high 70's.

[maddad]

Let me add that I've always wanted a E lower, but have been told I'm making to much power to keep it in one piece.

[Conquistador_del_mar]

My boat is a 72' that was origlnally a HM 351C boat. Once I got tired of blowing up Clevelands, I went to SBC's. I have a 430hp+ small block and spin a 23" Solas around 55-5600 and have had a max speed of 74+. I use a hydromotive nose cone and pick my water up through the hull. This could of been done in 72' if they had performance props for Volvo legs. Once I have external steering, I'm hoping for a controlable mid to high 70's.

That is an excellent top end speed. Do you have a stroked SBC? That is a lot of power if it is a regular 350. Yep, if there had been the newer props and nose cones available back then............................


Let me add that I've always wanted a E lower, but have been told I'm making to much power to keep it in one piece.

My friend was blowing up the speedmasters, but he was throwing about 600HP to them. Kit bought every one he could find in the US back in around 1987. I really don't know their reliable capability, but the one on my 1971 has held up since I installed it in 1976 or so with 350HP or less. Bill

[BigGrizzly]

Bill the E drive was originally built to handle 300 hp at about 5100 rpms. Like many drives with the right driver and not prakie motor it will probably survive about 400 HP, maybe more. Remember it was designed as a race drive with an inspection plate. It was meant to be maintained. One good thing is that it starts to wine befor it comes apart which very noticeable. Now my personal opinion is that it is a really neat drive and way ahead of its time.

[maddad]

That is an excellent top end speed. Do you have a stroked SBC? That is a lot of power if it is a regular 350.
Thanks. It's a 0.060 over 400. The stroker comes next.

[Conquistador_del_mar]

Bill the E drive was originally built to handle 300 hp at about 5100 rpms. Like many drives with the right driver and not prakie motor it will probably survive about 400 HP, maybe more. Remember it was designed as a race drive with an inspection plate. It was meant to be maintained. One good thing is that it starts to wine befor it comes apart which very noticeable. Now my personal opinion is that it is a really neat drive and way ahead of its time.

I am going to continue to use the Volvo Penta speedmaster in my 1971 restoration with a 330HP SBC that I am prepping right now, but I will probably need help on getting the right prop for it. I will try to get a new prop seal installed before taking it out for the first run. Do you think the large shaft size is going to be a problem (picture included) in finding props? If it is going to be a problem, I could switch to my backup e drive that has a regular straight shaft with 15 splines for mercury props. Bill
http://www.donzi.net/forums/attachment.php?attachmentid=36393&stc=1&thumb=1&d=1212283236

[Conquistador_del_mar]

Thanks. It's a 0.060 over 400. The stroker comes next.

That should make around 430CID - right? I meant to tell you that I liked your boat pictures that you put on the site - pretty much original look. On Friday, I talked to a mechanic who likes to put together 400s with 350 cranks making 377CID. He said they make great accelerating engines for drag purposes, but I don't guess they would work too well in boats? Bill

[tommymonza]

Some good reading here.I was going to start a thread on OSO about extending the lower strakes on my boat but came across this and was wondering how you guys made out.

I will start by telling you i own a Baron .There i said it .and my older brother was the designer and builder of them.

Just to clear the record i am not in any way affilated with Badass Baron Boy .

The guy that goes by Baronmarine on here is a friend of mine that i sold my old Baron too and he put a Arneson with a 406 in it. He is a nice guy but he now has a big Cigarette addiction.

Only 14 were produced before my brother decided there is more money in building houses and he closed shop and stored the molds until he sold them to the people who started Powerplay/Powerquest in Holland Michigan.

He designed them after the style of the 21 GT Donzi but it is an all original 24 degree design It is a heavily built well constructed boat all hand built by my brother who is the most anal retentive perfectionist you could ever meet.With a center mount 52 gallon gas tank and a 260 merc package is the way all but 1 of them were built.The hardware on them was all cheap Attwood crap .The bottom is very similar to the 18 Donzi other than the keel area is radiused at the transom and the forward entry is not quite as deep as the 18 design.Just like the 21 GT

I spent a lot of time in the early 80s chasing around Donzis racing them Most were the 260 package with thruu hull and i was the same but thru hub . I am not bragging but I always had them by just a hair and i attribute this to the rounded keel area creating less drag.The guys saw my weakness though after a while and would lead me into a Quarter chop and this would get me rolling where i had to get out of it for a second to even things up and than back to the chase.It was always all in good fun and we would switch out boats and rides and talk all the time.I was only 15 at the time and a lot of the guys were older but were real nice to me and knew i knew what i was doing.

Just to get off Subject a minute and i know i am already off subject but i read a thread on here about somebody wanting to know if they should buy a 18 or a 22 Donzi.The response from one person summed it up.If you want to ride ON a Donzi and have a wild fun little ride go for the 18.If you want to ride IN a Donzi and and have a calm great riding boat that is going to need a big block to move go for the 22.That being said was the comments i always got from Donzi owners after they rode in my Barons was it is all the fun of a 18 with the secure feeling of sitting inside of a 22. The downside is the center of gravity is much higher and adds to the chine walk.The 22 is one of my all time favorite boats and i will own one someday along with a 18 because they are too cool looking.The Donzis have a much better look than the Baron any day not to mention the following and heritage they have.

So back on subject.


The Baron has quite a bit of rocker in the bottom and carries the bow and rides on the last 3 feet of keel with very little trim on this wide radius.The down fall is the chine walk you get .So my theory was extend the lower strakes so the hull has something to support from side to side instead of the hull being balanced until it falls off to one side and can't recover till the chine dips.
The current Baron i have has a 325 hp with CMI headers and some small Kplanes on it.I would like to go to an Arneson like Baronmarine did with my old boat but don't have the coin and I have a brand new Alpha Gen2 transom assembly and outdrive.I know i have got to get the drive up but being SS lowers are so hard to come by and i am a fiberglass guy i will have to redo my transom and raise it.So there i will lose a lot of bow lift also.

So my next brain fart is this.Going to a full length pad with possibly a shingle type step on it.The 21 Scarab 1 had these on them minus the steps and they always had me by more than a couple mph with the same 260 motor.I have ridden in them out on Lake Michigan and though they are a little firmer they have no tendency to chine walk at all even in some good quarter chop.

The pad starts gradaully at the bow where the keel meets the water at a deadrise that is close to the bottom and runs the length of the bottom till it flattens out into a pad that is maybe 8 inches wide.

I was thinking about going further and putting small steps in the pad to further loosen up the bottom.I don't like stepped boats and the way they handle but if you where only completly up on the steps at max speed it might be a good compromise for 5mph i don't know,Just asking what all of your experiences are with what you guys have played with.

I kind of hate to ruin the fun factor too much though by making the boat ride to flat.I was talking with Baronmarine about his Arneson setup and he said even though it runs flat like a raped ape in Lake Michigan it rides so flat and predictable in the mid 70s in flat water that it is almost boring.

Sorry for the long Rant. I Would like to hear back what you guys think and what experiences you have had with bottoms and drive heights on your 18's and what handling problems you encounterd after you went past the 260hp 60mph threshold and into the 70s and 80s.Also what power it took you guys to get there.

Thanks guys Look forward to hearing back from all of you.


I know this is the Donzi forum and all other boats will be burned and sunk for posting in here but I will post a pic of my old Baron just to show where the boat rides at 60 with very little trim.

[BUIZILLA]

awesome post.. :pimp:

[tommymonza]

Thanks Buizilla

One other thing .I know where 8 of the Barons are and was wondering if you guys could tell me if you know where any more of them might be.Most of them ended up on Lake St Clair because that was the dealer for them.They would be either red or blue with possibly 1 white ,1 orange, and 1 poop mustard yellow. Thanks guys.

If you guys are bored and want to hear more http://www.speedwake.com/upload/showthread.php?s=&threadid=57943&perpage=20&pagenumber=1

[BigGrizzly]

Tommy, Welcom, I like the baron. I am also familar with that Scarab hull, my neighbor has 24 with special graphics I forget who he got it from. The boat is really nice boat, runs about 61 with a 350 chevy in it.

[Greg Guimond]

TTT for an informative and super interesting archive post !!

[Edoardo18Classic]

Dear Bill

I need to register my Classic 18 with the Peruvian Marine and they request the boats Plans and Drawings. from you picture it looks like you have sone Classic 18 drawings.

If so kindly let me know.

Thanks and best regards
Edoardo


Chris,
I have a '66 Barrel back and I can say I have aired it out on more than one occasion and I have never felt that it landed with anything but a smooth easy landing in any sort of seaway. As far as bow lift all you need is a wheel with a lot of rake in it and the bow comes up very nicely. As far as the full length strakes go, the more lift the less drag.
Bill G

[mattyboy]

Dear Bill

I need to register my Classic 18 with the Peruvian Marine and they request the boats Plans and Drawings. from you picture it looks like you have sone Classic 18 drawings.

If so kindly let me know.

Thanks and best regards
Edoardo

the plans for the 18 designed by Walt Walters have been given to the Mystic Seaport Museum by Walt you can purchase a copy from them
not sure with the pandemic if they are in full operation mode but you can contact by email

https://www.mysticseaport.org/

[Morgan's Cloud]

Matty .. when I saw the request I remembered your input on this a while back and tried my best to find it but the search function here drives me nutz . I found everything but ! I was hoping you'd pick up on this.

[Pat McPherson]

Great read.
Wish there was more activity on this forum.

[mattyboy]

well I guess i should add this tid bit on the 18 here as the thread as come back up , I had a chance to spend some time with Walt Walters at lake george and also up at the maritime museum at Mystic CT. When they went to design the 18 from the 16 one would think they added the "2 feet" at either end but what Walt did was cut the 16 in half just fwd of amidship and added the length there, I guess they went with the sharp keel to make sure the longer 18 would cut thru rougher water.