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schem
07-17-2009, 10:53 AM
I have an 89 18 foot classic with the king cobra OMC outdrive. Chevrolet ZZ4 crate engine with CNC ported heads. Estimated about 400HP/400lb.

The problem I have is with the drive all the way down the boat will porpoise badly. I have to put way to much trim tab to get the boat to stop.

Any idea how I can get the boat to stop porpoising with out putting in so much trim tab? I know that a full tank of fuel helps, but that is not the answer.

Thanks,

Donzi Vol
07-17-2009, 11:03 AM
Well, you're going to a plethora of thoughts here, as it's a common topic for discussion, so I might as well go first! The 18's are very known to do that. It's just characteristic of the hull. With the power package and weight that you have all I can say is to play around with the trim and tabs until you get it managable. Again, it's like no other boat when it comes to managing the porpoising. Lastly, and I don't mean to sound like a smarty pants here, try accelorating. If it's porpoising at 60mph, then stop accelorating...don't kill yourself, but if it's doing it at 20mph, then it should work its way out as you start going faster. Good luck and don't get frustrated...it's worth it!

All the best

dsparis
07-17-2009, 11:05 AM
I've got a 91 18c king cobra ho w/ nosecone , alum. heads, intake, and stainless marine exh. I don't have tabs and have no porpoising issues even with little fuel.
Is your drive going all the way to the down position ?

MOP
07-17-2009, 11:46 AM
Some much comes into play, schem having a nose cone and no tabs gives you an idea. Take the nose cone issue, some lift the stern and some help with bow lift. On my boat too much weight forward makes it porpoise to the point of being uncomfortable. For starters I would lighten the bow see what that does, then try to borrow a bow lifting prop don't worry about pitch until you see if it helps. I think that one or both will help, I have a hunch you are running a cupped prop with very little rake.

schem
07-17-2009, 12:29 PM
I would think that getting weight out of the would be the wrong direction being that is poroises less with a fuel tank. The faster I go the worse it gets. When it starts, I just put the tabs down until it stops but the other day I left them down when I put the boat on th elift and saw how far they were down and thought there has to be a better way. I have to believe it is killing my speed.

I don't know if the prop is original. It can with the boat. I have heard that the race boats change props to either get the nose up or stern up. I will have to look up the prop I have and see what the characteristics are. Is a cupped prop more of a nose lifting prop? What originaly came on an 18 classic?

BigGrizzly
07-17-2009, 12:36 PM
Remember too much negative (trim in)trim, causes it just as much as too much positive trim(trim out). the trim in gives handling issues and bow steer. the fact you can trim it to stop means there is need for seat time. Next there is NO trim Perfect for all speeds it is a changing boat attitude.

schem
07-17-2009, 12:49 PM
It seams that when I get the tabs down far enough to stop the porosing, the hull is stuck in the water.

BigGrizzly
07-17-2009, 12:53 PM
It takes a while to stop by that time you have gone too far and have to tab back up. Now are we talking about tabs or drive trim? There is a big difference.

schem
07-17-2009, 01:05 PM
I have my drive all the way down and add trim tabs as needed. When I get it to stop pporpoising, I start pulling them back up but it does not take much for the boat to start again.

BigGrizzly
07-17-2009, 01:27 PM
Leave the tabs up and only use the drive trim. Tabs create a very wet hull. This is a rocker hull and will porpoise at some point and will need correction. this is a Donzi a drivers boat, not a Bayliner. seat time and experimentation, it does not happen overnight. There is nothing wrong with the boat.

mattyboy
07-17-2009, 01:29 PM
porpoising on a deep vee is caused more by the prop than any other factor it is a cycle of bow lift then the loss of bow lift then the prop picks the bow up but can't hold it and the cycle starts over, trim , trim tabs , weight can all help but until you find the right prop you'll need some or all of them to cure it. after 8 years with my 16 i finally got it to where it would to it very slighlty at a very small MPH range like 23-26 mph out side of that range I needed no trim or tabs it was flat and stable all the prop or the right prop . you'll find with the right prop and trim setup that the range it wants to porpoise will be much smaller, some props I had it would do it on from just on plane to about 3/4 throttle like 20-40 mph.

schem
07-17-2009, 01:38 PM
I know all about driving a boat, I had a Sanger flat bottom V-drive drag boat before I bought this boat. (Wife and kids induced)
mattyboy, what direction in prop did you go to help with your porpoising? less pitch, more pitch?

mattyboy
07-17-2009, 01:50 PM
I know all about driving a boat, I had a Sanger flat bottom V-drive drag boat before I bought this boat. (Wife and kids induced)
mattyboy, what direction in prop did you go to help with your porpoising? less pitch, more pitch?


pitch has nothing to do with it, more blade shape and design. i would try going to the 2 extremes something that has alot of bow lift, which means it can hold the bow high all the time, or something with stern lift which will push the nose down. Big Grizz might be able to help in that dept . what prop are you running now???? it takes some time to find the right prop cause after you get by the porpoise things like rpms and top speed handling come into play as well. so a prop that doesn't porpoise all that much can be slow or be fast but a handful at WOT. at what speeds does your boat do it??

schem
07-17-2009, 01:56 PM
Would trimming the outdrive up be the same as a prop with bow lift or are the physics different. I will have to check to see what prop I currently have. Don't know off the top of my head.

BUIZILLA
07-17-2009, 02:06 PM
after 14 posts I have to ask....

what prop? :)

mattyboy
07-17-2009, 02:16 PM
Would trimming the outdrive up be the same as a prop with bow lift or are the physics different. I will have to check to see what prop I currently have. Don't know off the top of my head.


it can be but if you go too much with trim the prop looses bite and it can't hold the bow up and then the cycle starts over. you want to keep the nose either down or up not to much of either has that can be bad . it has been said here seat time and prop testing you'll find the right prop each boat and prop react different you'll need to find out for yourself. sorry I wish it was as easy as part number such and such ina 23 pitch but it is not

let me make it clear when i was dialed in it did it but is was a very small speed range and the prop had great WOT handling so i could live with dragging some tabs in the 23-26 mph range

schem
07-17-2009, 09:05 PM
It is a stainless steel Raker prop part number 394753. 141/2 X 20 It also has a stamped number on it C582

It was on theboat when I bought it in 2000.

BUIZILLA
07-17-2009, 09:18 PM
with 400 claimed hp you need a 25" at least.... the Raker WILL carry and bobble the bow, I suggest a 24 Turbo

Planetwarmer
07-17-2009, 09:35 PM
A 20p with 400 ponies?!? I bet it pings off of the rev limiter.

mattyboy
07-18-2009, 06:03 AM
do you know the drive ratio??? that will help also in sizing a prop

chappy
07-18-2009, 07:57 AM
The Raker will make it wobble a bit, sounds like the prop choice should make a difference.

Small request, I'd love to see a pic of the ZZ4/KC drive combo, that's a great motor with a tough as nails drive.:yes:

RedDog
07-18-2009, 08:19 AM
The 18 I had ran 400 - 420 HP (big block so heavy in the ass). The King Cobra ratio was 1.43. I am pretty sure it had a 22" Raker with some cup added to the blades.

73.4 mph @ 5300 RPM

MOP
07-18-2009, 10:55 AM
do you know the drive ratio??? that will help also in sizing a prop

Thought this chart may help him ID his! Though the 5.7 with a King states two ratios 1.41 & 1.5 He will need to chalk it and measure it.

Phil

schem
07-18-2009, 11:22 AM
I turn about 5,500 but I have the tabs down so far to get away from the porpoising at WOT that I might as well be draging an anchor. I have the boat up borth at the cabin and won't be there until next weekend. I will take a picture of how much tab I am putting into it.

As far as the HP. The ZZ4 comes with 355HP and 405lb. I had the heads cnc ported by weld tech. So I am somewhere between 355 and 400.

BUIZILLA
07-18-2009, 11:28 AM
those heads ported only pick up about 4hp per hole or about 30-35 tops...

the more I think about this, you need to try a 23 Solas, and start off with the tabs even with the bottom, and leave them alone, until you can RAISE them.... dragging is a sign of poor prop selection... Solas and Cobra's are happy partners...

Carl C
07-18-2009, 01:22 PM
I knew you were from Michigan as soon as I saw the "Up North" thing. Some of these boats just seem to have incurable porpoising; Ask Yeller. Maybe you will get lucky and a prop change will fix it. Maybe not.

blackhawk
07-18-2009, 01:37 PM
I have my drive all the way down and add trim tabs as needed. When I get it to stop pporpoising, I start pulling them back up but it does not take much for the boat to start again.

Very odd that the boat will porpoise with the drive all the way down. Next time instead of bringing the tabs back up trim the drive up a bit instead. Play around with different trim/tab combos instead of leaving your drive all the way down.

mrfixxall
07-18-2009, 03:31 PM
Believe it or not!!! i put a merc high 5 propeller on my buddys 18 with a cobra and all the porposing went away, it would chit and get too!

DC18
07-19-2009, 12:47 AM
porpoising on a deep vee is caused more by the prop than any other factor it is a cycle of bow lift then the loss of bow lift then the prop picks the bow up but can't hold it and the cycle starts over, trim , trim tabs , weight can all help but until you find the right prop you'll need some or all of them to cure it. after 8 years with my 16 i finally got it to where it would to it very slighlty at a very small MPH range like 23-26 mph out side of that range I needed no trim or tabs it was flat and stable all the prop or the right prop . you'll find with the right prop and trim setup that the range it wants to porpoise will be much smaller, some props I had it would do it on from just on plane to about 3/4 throttle like 20-40 mph.
mattyboy, I agree. My 18 came with a Mirage Plus, which I'm told is a bow lifting prop. I too had porposing problems.:eek: BTY I don't have tabs.:shocking: I tried several props. The ones that work best :yes: for me are the ones with low blade rake i.e. props that are designed more for thrust. With these props I don't have any porposing at all! I have also talked to Griz :worthy:on these matters.

BigGrizzly
07-19-2009, 08:44 AM
Don't take offense but driving that flat bottom is not even close to the Donzi, Been there and done that. For get everything you know with that flat bottom. I also had a Jersey Skiff, nothing is the same in any way. From what you said in the first post with trim and tabs shows this. Now driving theme is easy since I have had one since 1966 and we still have it. SO follow Matty and maybe Mr Fix Idea will work. As for one of My Solas props, that engine will not turn it.

BUIZILLA
07-19-2009, 09:07 AM
. As for one of My Solas props, that engine will not turn it. gotta disagree there Griz, I had a 23 Solas with a 310 carbed Cobra 1.65 drive combo and it turned 4900 in a 21 4Winns, handled awesome... it'll turn it just fine... :wink:

BigGrizzly
07-19-2009, 09:13 AM
Buiz, I said MY Solas, not the run of the mill internet version. The normal titan or Lexor will be fine, but they did not do well paired with the older Volvo drive.

BUIZILLA
07-19-2009, 09:17 AM
internet model????

his boat is a Cobra not a Volvo.... not sure what model Solas mine was, kinda looked like a Turbo 1... it was 98-99ish when I bought it but it damm sure did the job... it's still on the boat to this day after 11 years.. so what's the difference between yours and Solas over the counter??..

BigGrizzly
07-19-2009, 09:23 AM
Yes I know that prop you had talked to me about years ago was the Titan and is a really good prop. On our Honda applications it sometimes beat the Turbo, about 80%, but always beat the M+. in all categories. I am trying to get them to make ones for a Bravo classic combination, but it may take time in this economy.

BUIZILLA
07-19-2009, 09:26 AM
it would be a home run if Solas would do it... my good buddy I sold that boat to will NOT part with it.. :yes:

BigGrizzly
07-19-2009, 09:30 AM
Trust me Buiz, we are on the same page. As for my Solas I have never had a complaint except some want more bow lift. My answer to that is if that prop handles soo well WHY do you want more bow lift? Let the hull work and do its job.

RATATTACK
07-19-2009, 09:36 AM
SOUNDS LIKE YOU SHOULD BUY A SCARAB,HA HA:shark:

schem
07-21-2009, 09:37 PM
Griz,
no offence taken. I just wanted you to know I am used to a boat that is anything but a cruiser. My Donzi is alot less of an animal to drive then the flat bottom. I am on vacation next week and will try to use the drive only to stop the porpoising. Does any one have a prop I could try before I put out the money?

schem
07-21-2009, 10:00 PM
Does Solac make a prop for an OMC? I look at thier chart and had no luck.

MOP
07-21-2009, 10:05 PM
Just so you know any big gear case OMC outboard prop will also fit your drive, you will need to do some poking around. Take your time props get expensive fast, try to borrow!

Lenny
07-22-2009, 12:18 AM
As for my Solas I have never had a complaint except some want more bow lift. My answer to that is if that prop handles soo well WHY do you want more bow lift? Let the hull work and do its job.

OK Grizz, you come here, play in 20-30 knot winds in the Ocean, Freighters all day long and Ferry's up to 563' every hour (both into the waves and surfing) smaller ones evry ten minutes, and MASSIVE Cruisers.

Granted the prop will "stick" like glue, and it WILL handle like a Ferrari, but once you have stuffed the bow ( I have not yet in five years) by either going into or on top of (surfing) a few close HUGE ones you begin to appreciate the ability to immediately "lift" the nose or "hop" onto the next wave back without spending any time in between. :)

Come here, play in this $hit and tell me that the bow lift is NOT an issue with the Solas and the hull will look after you :rolleyes:

I will be the one with tanks and a wetsuit beside you and I DO KNOW how to stay alive in these waters.

schem
07-23-2009, 01:50 PM
griz,
Do you have any other recomendations other than the Solas? I can not seem to find one that fits the OMC.
The next step will be finding one that I can try before I buy one. Even if the prop helps the porpoising, I still don't want to have it over or under prop'd after spend a couple hundred on a new prop.

VetteLT193
07-23-2009, 02:00 PM
Did you get in the boat yet and try it? You must trim it out, all the way down will cause a horrible porpoise. I know from experience because the previous owner of my Minx reversed the trim switch. I kept trying to put the drive out, but was actually pulling it in. The porpoise is far worse when trimmed all the way in than I could ever get it by trimming out.

If you have a trim gauge, try to figure out about where the indicator reads when the drive is neutral. Start there, and go out more.

schem
07-23-2009, 02:07 PM
I am heading to cottage this weekend where I keep it. I will be there all next week so I will be putting many hours on it. I have in the past only run it with the drive all the way down and just messed with the trim tabs.
Thank you for the advise.

dsparis
07-23-2009, 02:34 PM
Schem, as described in post 3 my boat is simular to yours. I run a raker 14.5 x 22 with a few tweaks. Mine will porpoise a little at mid range speeds but is easily controlled with trim. When trimmed all the way down there is no way to make it porpoise. Good luck.

joseph m. hahnl
07-23-2009, 04:38 PM
Did you get in the boat yet and try it? You must trim it out, all the way down will cause a horrible porpoise. I know from experience because the previous owner of my Minx reversed the trim switch. I kept trying to put the drive out, but was actually pulling it in. The porpoise is far worse when trimmed all the way in than I could ever get it by trimming out.

If you have a trim gauge, try to figure out about where the indicator reads when the drive is neutral. Start there, and go out more.


???????????????:shocking::shocking::shocking::shoc king::shocking:


I have no such issues in my Minx:kingme:

Tucked in no porpoise:wink:


This is my experience: which may be different then other users and or operators of vessels that are similar but not exactly the same :wink: as well as vessels that are completely different but similar or the same but different or different but the same or even exactly the same:shocking:

The drive in = (Negative) should drive the hull through the water.
The tabs down should drive the hull through the water.
Tabs down and drive up = (positive) to cavitate and increase Rpm on the prop.
Not enough power to a (positive) dive will cause Porpoise
Shallow water will also cause porpoising no matter where the drive is


Schem .Some times your local dealer will allow you to test the prop, as long as you don't Ding it they will take it back:shades:

Also at the Gatherings,and or poker runs some times there are people who allow you to prop test.:crossfing:

This weekend is lake Winni the self proclaimed:kingme:"king of lakes" which may not be the actual king of lakes but is similar to the actual king of lakes which said lake may not be defined as the actual king, yet would bare the same resemblance as it would have water in it that does not contain salt making it fresh water therefore being a lake:shocking:

If you can make that, you might be able to hook up with some one that has what you need to try out.:crossfing:

blackhawk
07-23-2009, 05:13 PM
I am heading to cottage this weekend where I keep it. I will be there all next week so I will be putting many hours on it. I have in the past only run it with the drive all the way down and just messed with the trim tabs.
Thank you for the advise.

Where's your cottage?

Lots of advice here. I would go out and play with different trim and tab settings and see how the boat reacts. I'm sure with some seat time you'll get it.

schem
07-24-2009, 12:06 PM
My cottage is on Otsego Lake in Gaylord.

I have always been tolk that positive drive will create porpoising. Never heard that negitive would so I never tried to go positive to keep it from porpoising. I can hoop the boat with the drive all the way down and trim tabs all the way up to the point the boat feels like it is about to leave the water. Scares the hell out of the wife. Happens as I excelerate and gets worse as I pick up speed.

schem
07-24-2009, 12:10 PM
Joseph,

How shallow of water and why does depth play a part? I run in about 15-20 feet.

Lenny
07-24-2009, 01:04 PM
I can hoop the boat with the drive all the way down and trim tabs all the way up to the point the boat feels like it is about to leave the water. Scares the hell out of the wife. Happens as I excelerate and gets worse as I pick up speed.

Drive up, tabs up. That is the tabs are NOT in the water, and the drive is tilting up at an angle as in not as much in the water. Give her some throttle. See what 35-40 feels like...

I am confused as hell as to your down/up descriptions. Get that drive out of the water and lift that bow and get the tabs OUT of the water.

IMO

Carl C
07-24-2009, 01:12 PM
I always thought drive down to stop porpoising:confused:. If that doesn't work, drop the tabs:confused:. My 22 only porpoises a little at lower speeds. I can easily stop it by lowering the drive or just speeding up. I don't see how raising the drive can help. Then again I don't have a 16.

mattyboy
07-24-2009, 02:25 PM
I always thought drive down to stop porpoising:confused:. If that doesn't work, drop the tabs:confused:. My 22 only porpoises a little at lower speeds. I can easily stop it by lowering the drive or just speeding up. I don't see how raising the drive can help. Then again I don't have a 16.


the cause is bow lift then no bow lift then repeat. usually at a speed where the prop can't hold the bow up..

so drive down tabs down forces the nose down and keeps it there or from lenny school of thought ad more bow lift so the prop can carry and hold the bow up this goes hand in hand with more throttle where the drive down tabs down goes hand in hand with slower speeds or less throttle

Lenny
07-24-2009, 02:41 PM
Schem, I just think you need to get her up to speed, carry the hull and drive through the porpoising and get used to a new style of ride. :yes:

Porpoising is not a part of that, but certainly is there in some places but certainly NOT at mid/full throttle. It is more of a 25-35 mph cruising thing.

joseph m. hahnl
07-24-2009, 02:41 PM
Joseph,

How shallow of water and why does depth play a part? I run in about 15-20 feet.

That is plenty deep enough and would not be a factor if it is consistently that deep.

When the displacement of the boat is greater than the depth of the water, the water has no where to go and pushes the boat up and down like going through someone's wake.

It's like when a wave breaks on the shallows coming from the deep.

schem
07-24-2009, 02:55 PM
I have never run the boat at speed with the drive up from full down because of what I thought caused porpoising. Which was bow up. I will try several combinations over the next week while on vacation.

Lenny
07-24-2009, 04:56 PM
Schem, tell your wife it is supposed to do that and once you figure it out it will ride like this :D

http://www.bananaboatco.com/videos/Aronow-Cigarette.wmv

BigGrizzly
07-24-2009, 06:12 PM
There are a ton of options for props on your boat , A turbo1, TXP Mairage plus. don't believe everything on the internet chart. As for what Lenny said, I have been driving on the ocean in a Donzi since 1966. Stuffing the bow happens regardless how much bow lift you have under certain situations. Truswt me on this Lanier on a Saturday afternoon in the south lake takes its tome on 30 plus footers here.

schem
08-04-2009, 02:55 PM
Well just got back after a week of playing. Bad weather aside. The boat will porpoise at around 45 but after that I was able to raise the trim tabs all the way up and the farther I pulled the out drive up the faster the boat went. I stoped at about 5,500 RPM due to fear of blowing the engine. It was definitly singing. :shocking: With a hyd. roller, I don't dare push it harder. speedo was at 68 I will have to go back and run with my GPS. The boat was noticably faster. I never felt as though I got to the point the prop was starting to give up.

It may be time to put more prop in it to see if I can gain more speed with less RPM.

blackhawk
08-04-2009, 07:05 PM
Well just got back after a week of playing. Bad weather aside. The boat will porpoise at around 45 but after that I was able to raise the trim tabs all the way up and the farther I pulled the out drive up the faster the boat went. I stoped at about 5,500 RPM due to fear of blowing the engine. It was definitly singing. :shocking: With a hyd. roller, I don't dare push it harder. speedo was at 68 I will have to go back and run with my GPS. The boat was noticably faster. I never felt as though I got to the point the prop was starting to give up.

It may be time to put more prop in it to see if I can gain more speed with less RPM.

Like I said in my PM it just takes some seat time! This Michigan weather is brutal this summer. :(

Lenny
08-04-2009, 07:32 PM
Nice Schem :yes:

Much better...