Drive Ratio

[rayjay]

As the Bravo I on the 22 Classic project I acquired is a bit wonkey (a technical term meaning not quite flablunget) I have been trying to learn as much as I can about drives and drive types. Can someone explain how to choose what ratio drive to run? Is it better to run a 1.5 with a high pitch prop or a 1.3 with a lower pitch prop?

Also, where does the 22 hull seem to like its prop in relation to the bottom of the keel? Where a Bravo with a stock X dimension would put it, or higher by using a Bravo XR or an IMCO shortie? Somewhere in between? Or back and up by using a set back box?

Has anyone completed and is now running an Arneson on a 22? I know Chris has one for his 22, but has he tried it yet? Is anyone else running an Arneson on their 22?

rayjay

[GEOO]

Rayjay,

The gearing depends on your expected speed.
1.50:1 gear, 30" prop, 5200 rpm, 14% slip = 84 mph
1.36:1 gear, 30" prop, 5200 rpm, 14% slip = 94 mph

I think the small pitch prop with a 1.36 gear will run better then a large pitch with a 1:50 gear. As long as you have the power to spin it.
I found that larger pitch props tend to create more stern lift.

I have heard of a 20-22 running with a Arneson but it was a stock engine.
None of the 22's with built engine are completed yet.

My 18 should be ready to run in a week or two if you want to check it out. (I'm in Connecticut)

[MOP]

George the stern lift generated by the large pitch prop also relates to extra steering torque which costs speed. I have been chatting quite a bit with my prop guy, he says the least amount of reduction and a low pitch wheel will go faster and waste less HP which is lost in reduction. He wants me to swap to 1:36 on teh BH, I laughed told him having enough trouble finding wheels.

Phil

[rayjay]

My 18 should be ready to run in a week or two if you want to check it out. (I'm in Connecticut)

OH YEAH!! Would I ever like to check out your boat!! I've been wanting to do that for over two years now (probably along with everyone else on this Board!).

Okay, back to the subject... I remember reading somewhere that props were between 22 and 26 pitch were the most efficient, and that you should gear to run in that range. I'm only repeating what I read, not trying to state a fact. But what Phil was saying seems to bear that out. But I have been in boats that were runing really big pitch props, but with lots of horsepower, and seemed to be running pretty good. I guess cost is also a factor. Two big props are probably less cost than two new drives.

I'm just getting prepared if the Bravo I am getting is going to cast a lot to fix. (See cost comes in again) Rather than repair it I might as well get a new drive with a different ratio. Or possibly an Arneson... I'd just like to see how it works in a 22.

rayjay

[GEOO]

I have not tried a swap test on the same boat (gear ratio and prop) it would be interesting to see the exact difference.
I have read/heard of several people who have changed the gears in their Bravo's. Maybe you can find a used set to try?

[rayjay]

Trying different gears and props, low gear big pitch vs. big gear low pitch, on the same boat would be interesting. I'll have look into it.

Has anyone swapped gears and props on their boat? If so, what happened?

rayjay

[Ranman]

I've always heard that it's more efficient to spin a big prop slower than a small prop faster. I cannot, however, quantify that statement. Also, there are different forces at play in the small-prop-fast vs. big-prop-slow that may affect reliability but that's another topic.

IMO, the reason for different gear ratios is mostly for prop availability. Ultimatly, you want a gear ratio that will give you the best selection of props for the given horsepower of the boat.

Consider these scenarios:

My boat has a 300 PSHP 350 and a 1.65:1 Bravo drive. Because of the ratio I can turn a 25P Mirage+ to 5000+ RPM. This is not a bad ratio to horsepower combination as prop availibility in the 25P range is abundant. Now, say I change drive ratio to 1.5:1. Experience here tells me I'll need to drop to a 23P Mirage+ to get the motor spinning 5000RPM. Again, an acceptable ratio to HP factor as there's plenty of chioces in the 23P world. Lastly, say I switch drives again to a 1.36 ratio Bravo. What prop would work best now? Somewhere in the 21P to 22P area. Now the selection and availibilityof high performance props is shrinking. There's not as much to choose from.

Lets consider the opposite scenario: I decide to leave my 1.65 Bravo alone, but instead I swap in a 600+HP custom small block (dreaming, I know, but roll with me). What prop would I need to match up the ratio to HP factor? I'd have to swing a 30P, 32P or maybe larger. While 30's and 32's are out there, anything larger is going to be tough to find. If I were to change the drive down to a 1.36, the correct prop might fall in the 25P to 27P range.

Though in theory there might be slight efficiency differences in different ratio/prop combinations, I think that if you're looking for speed gains, there's not much here to be had in the ratio arena. The point of the drive ratio is intended to be used to match the HP of the boat to an available prop selection. Hope this helps someone out there.

[CDMA]

RayJay,

I still follow the board but posting time is slim. Regretably the Arneson boat has yet to be run and likely won't be for awhile. It has taken a backseat to school and career but she will get done sooner or later. I am very excited about it and think it has the potential to be a great package. At the same time it is a huge undertaking. I have restored boats, rerigged boats and I have never underestimated the size of a project like this one. Everything is different, custom and while fun, time consuming.

I have been doing a lot of theoretical work at school while I still have the chance and while the more I know the more I enjoy this stuff the one real nagging Naval Architecture question I have been chasing since I got to school was...will this 22 work and how well. Will the boat be able to generate enough bow lift without a big fin, will she be balanced and more important to me can I get her to run efficiently and take advantage of her potential advantages. That is the real question. I am sure the boat will run, it will turn it will work....just how well. A HP500 22 runs 80. If this boat with a 500 runs 82 is it worth it...no way. The Arneson is cool but if the difference is 2 mph there is no way she is worth the cost, lose of usability, and complexity. If she runs 90...then we are talking about a different story.

Go for a ride on Geoo's boat it is well worth it. But keep in mind Geoo's boat is not represtative of how it will work on a 22. If you look at Geoo's boat the real feat of that boat isn't the speed, its the control. Frankly, and Geoo will admit it, any 18 with a shorty with 750+ hp like he has should run those speeds...but keep in mind we never said controlably. In order to achieve that there is a large fin on the bottom which a) creates bow lift and b) adds huge amounts of directional stability allowing the boat to run well over 100 mph in complete control. The real trick would be to get a 22 with an Arneson to run with adequate bow lift without that fin. However this isn't as easy as it sounds. You get into changing CG's which is in many repects over rated. You move all the weight back and you optomize for high speed and all you have is a porpising pig bellow 80...what fun is that. Ok so go modify the bottom ( add rocker) same deal...fast speeds she rocks back and fly's...at low speed she porpises. Drag some tab to stop that...but then hook a tab going around a corner. Add controlable surfaces ( rocker plates) great...but don't forget you are adding wetted surface and in turn drag. Defeating the point of what you got with the Arneson. In addition I think something that is overlooked is with rocker plates they are flush with the bottom and from my experience there is something nice about having a slightly elevated surface to drop a little and keep the nose down in a seaway. Ok so how about rocker plates and K planes....now there is an idea... if you like $6000 of trim tabs on your transom, not to mention 4 trim pumps inside in additon to the one for the Arneson.

As you can see its a lot more complicated a problem then initally seems and I don't pretend to have the answers ...yet. I am not trying to scare you off by any means. The potential upside could be great...and thats why I will continue to head that route. I am young, dumb and stupid enough just to try it.

Chris

Ok...ban yourself from the board Chris...you must work...

I may be graduated by the time I get to a " my winter @ donzi post"

[CDMA]

Where did my avatar go??? :frown: :frown: :frown:

[gcarter]

Chris, reading your post reminds me of Gar Wood in his waning years trying valiantly to continue to lead the world in speed with what was in essance a mid '20's George Crouch design. In the mid thirties he had graduated to FOUR Packard 1M-2500 engines with a total of 6,200 HP but try as he would, could not better 125 MPH. Technology had passed him by! Just a few short years later, the Apel family and the Ventner yard were building boats with much higher capability with a fraction of the HP.
I think (but what do I know, opinions are like armpits, everyone has a couple of them) the goal of building a practical 90 MPH 22 is similar to Mr. Woods......
OK... everybody pile on, I can take it!

George

[CDMA]

G,

Funny you mention that because I absolutely agree, there are better high performance hulls out there then Donzi Classics. You can argue that Donzi's are better becuse of the history, the looks, the name, whatever you want...but to me its all about the people. That's what makes having, making faster, and enjoying a Donzi better then anything else. Even if they get passed from time to time...

Chris

Let the record reflect this is in refernce to Classics not say other models...ahemmm ZR, cause if you pass one of them you are pretty damn good.

[Rootsy]

hmmmm makes ya wonder how a pad would work on something like this chris... but then again.. i hated fluid dynamics so don't mind me... i am just purely theorizing in speculation... would she get up and run on a pad and given a slight angle generate bow lift yet run on the bottom and strakes at lower speeds? wonder how easy it wouild be to keep on a pad?

heck if i know... i can't bring myself to even go hacking thed bottom of my 16.. yet... :bonk:

JR

[Ranman]

Funny you mention that because I absolutely agree, there are better high performance hulls out there then Donzi Classics. You can argue that Donzi's are better becuse of the history, the looks, the name, whatever you want...but to me its all about the people. That's what makes having, making faster, and enjoying a Donzi better then anything else. Even if they get passed from time to time...

Well said Chris. I've always wanted a Donzi since I was a kid so I would own one regardless. I do have to say though, that being involved in this hobby with the others here who enjoy the same passion almost surpasses the enjoyment I derive from actually owning the boat.

[gcarter]

Chris & Randy, I completely agree with you.
I've wanted a Donzi for 30+ years and since I bought the Minx (even though it currently exists in a hull and numerous piles of boxes in the garage) my wife says it has made a different person out of me. I must say I haven't had so much fun in 20 or more years.

George

[gcarter]

Chris, we discussed Lindsey Lord once before, but in his book "Naval Archetecture of Planing Hulls" his passion was allways 40 to 50 knot patrol boats and cargo movers. Also he stated thatvee hulls were not particularly efficient, but that they can absorb large amounts of horse power and stay controllable. Also he was a big supporter of steps for going faster, with the caveat of keeping the vents clear.
The point being that a deepvee hull is a 50 knot hull, and if you want to go a lot faster, go to a step.

George

[CDMA]

Yes but one caveat to that, and I am not sure if it was Lindsey, Savitsky or someone else...but generally, some exceptions noted, stepped hulls do not really work on our size ( L/B ratio) boats until about 25-26. Purely a function of wetted chine and wetted keel length. 22 classic with steps...I don't think so (well I do have one step configuration I would like to try but I only give it a 40% chance of working). Steped 25-27 classic/ZR??? Now we are talking. I am not a subscriber to the idea that to be a classic it can't be stepped. Don would not approve of that mentality. He wanted everything he could get...so do I.

Chris


Why do one thesis when you can do two???

Phil: I will call you back
Rich: Likewise
Scott: I have your ramp
Cliff: I can't remember your number
Geoo: We need to go boating
Chris: I need to get back to work

[CDMA]

Added to that one of the only people to really get stepped bottom boats to work in the small boat region is a fellow named Harry Schoell (sp). That being said his step design, in my opinion, is more suited for mazimizing efficiency at about 50mph in a 22 foot boat as opposed to say 70+ which is what we are looking for. Sometime in the future I will try to go into it a little more.

Gcarter,

Step venting is without a doubt one of the most critical part of stepped hull design.

Chris

Back to the important question...where is my avatar??? :mad:

[gcarter]

Added to that one of the only people to really get stepped bottom boats to work in the small boat region is a fellow named Harry Schoell (sp). That being said his step design, in my opinion, is more suited for mazimizing efficiency at about 50mph in a 22 foot boat as opposed to say 70+ which is what we are looking for. Sometime in the future I will try to go into it a little more.

Gcarter,

Step venting is without a doubt one of the most critical part of stepped hull design.
Chris;
You , Harry and Dr. Lord all agree. I'm sure I do too, but I've never been on a stepped hull.

George

[gcarter]

OK Chris, I'm just ruminating...
So if you had a 22 ' hull that was stepped and was say 6' beam instead of 7', wouldn't that get around the L/B ratio problem ( I know things are never this simple) It would probably also have to be pretty light.
You could also always add a midship wing ala Fabio Buzzi...LOL!

George

[CDMA]

Yeah it gets around the L/B ratio but you wind up back where the banana boat was....too much deadrise for the beam resulting in transverse dynamic stability issues. To accomplish a 6 foot wide boat with the same dynamic stability you are going to have to go to less deadrise.

Make sense?

Chris

[gcarter]

Yeah, I understand.

It's not easy to do.

George

[CDMA]

Root, I have interest in pads but not really a traditional pad. To me a pad like a Fountain would be interesting. Say a full length pad about 8 inches wide but with say 11 degree deadrise on the pad. So a Vee pad for lack of a better description. But I want it all the way, more or less, to the bow. Well end of dynamic running surface.

Root you do the mechanical I will handle the hydro....


Chris

[mattyboy]

Chris,
1 is a thesis
2 is a thesi
and a classic with a step is a sci-fi novel

:tongue: ;)

Matty
still working on my thesis
classs of 83

[gcarter]

Chris,

and a classic with a step is a sci-fi novel

:tongue: ;)

Matty
still working on my thesis
classs of 83
Marry, thats why I added the Buzzi wing!

George