Prop theory?

[oldsub86]

So, can a boat be properly set up with a propeller for whatever engine?

As a theoretical example, what if one wanted to install a lower horsepower diesel engine into a boat that previously had a V8 gas motor?
If we go from 200 HP to about 75 HP is it possible to change a propeller sufficiently to make the boat remain usable?
I expect speed to decline but would it be doable?
A diesel normally makes pretty good torque compared to horsepower so I am guessing that would help.

Anyone have any experience with something like this?

Randy

 

[BaileysBoat]

Re: Prop theory?

No problem on a displacement hull, planing hull would be another story. Would not be able to get off the step and plane, so you would not get maximum fuel economy.

 

[Lyle29464]

Re: Prop theory?

Lets say your gas engine would plane the boat at 3000 rpm and that was 150LBS of torque.
You would need a Diesel that would put out 150 lbs below WOT. I am guessing at the numbers But you would not be as far off as you think. Look up some real numbers.

 

[QC]

Re: Prop theory?

A diesel normally makes pretty good torque compared to horsepower so I am guessing that would help.

Do not make this mistake. 75 hp is 75 hp. Doesn't matter if it is a diesel or a turbine. Shaft horsepower is what matters. To answer your original question, with gearing and propeller pitch you can usually find a combo that will allow you to put that horsepower to the water.

Where peak torque might help is getting her to plane, but I doubt that 75 horsepower will hold her on plane regardless of whether or not you could get her there. This is a very confusing topic for most. Do not try and simplify it by believing that there is some magic way to separate torque and horsepower. In general you should always remember that torque makes horsepower when you add in RPM. The belief that torque alone gets you something is commonly held, but it really doesn't. That 75 hp diesel will not make 300 lb/ft. torque yet you can exert 300 lb/ft. with your hands and a decent length breaker bar or long handled torque wrench. Think you could plane your boat with your hands?

 

[rallyart]

Re: Prop theory?

Good example QC. In a boat you are displacing water over time, or lifting a hull and displacing water over time. As soon as you add time into the equation you must deal with horsepower and not torque. For any boat to move at any particular speed it simply takes some X specific horsepower. If you get that at a low RPM with 1000 ft. lb. torque or at a high RPM with 50 ft. lb. torque it does not matter. You just need the right horsepower.
If you had a 200 hp V8 I'm guessing it is a planing hull boat. See if you can get it to plane when you short four spark plugs.

 

[Frank Acampora]

Re: Prop theory?

The boat in the avatar is 15 feet long. I have powered it with a three cylinder 90 HP engine of 72 cubic inch displacement. I have also powered it with a four cylinder 90 HP engine of 99 cubic inch displacement.

90 horsepower is 90 horsepower! Both engines top out at the same speed--45 MPH. HOWEVER: The added torque of the fourth cylinder and 25 cubic inches gets me there faster. Thus at below maximum RPM, the four cylinder engine is making more horsepower at a given RPM than the three cylinder.

The greater torque of the diesel may help it to swing a larger prop than an equivalent gas engine and accelerate better, but with the significant reduction of horsepower, top speed will suffer greatly. Depending upon hull, if it only takes 75 horsepower to break over into a plane and if planing speed is low enough, the boat will still perform, although not well. If it can not plane, it will wallow like a garbage scow.

Any boat is usable as long as it floats. Usable depends upon your definition. Do you want to ski with the 75? Not usable. Do you want to putt-putt to the fishing hole? usable. Look at the pontoons with 9.9 engines. Way below max, but usable.

 

[Texasmark]

Re: Prop theory?

The boat in the avatar is 15 feet long. I have powered it with a three cylinder 90 HP engine of 72 cubic inch displacement. I have also powered it with a four cylinder 90 HP engine of 99 cubic inch displacement.

90 horsepower is 90 horsepower! Both engines top out at the same speed--45 MPH. HOWEVER: The added torque of the fourth cylinder and 25 cubic inches gets me there faster. Thus at below maximum RPM, the four cylinder engine is making more horsepower at a given RPM than the three cylinder.

The greater torque of the diesel may help it to swing a larger prop than an equivalent gas engine and accelerate better, but with the significant reduction of horsepower, top speed will suffer greatly. Depending upon hull, if it only takes 75 horsepower to break over into a plane and if planing speed is low enough, the boat will still perform, although not well. If it can not plane, it will wallow like a garbage scow.

Any boat is usable as long as it floats. Usable depends upon your definition. Do you want to ski with the 75? Not usable. Do you want to putt-putt to the fishing hole? usable. Look at the pontoons with 9.9 engines. Way below max, but usable.

Hey Frank, not hijacking the thread, but didn't the 4 cyl Chrysler come out in 105 and 120 hp....that IS a 4 banger in your Avatar isn't it? I had a '71 triple at 85 hp, not painted as pretty as yours.
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I like you guys explanations and I agree with all accounts....QC I really like the torque wrench explanation..had to laugh at that one, but true. That pretty much separates the two (torque and hp).

Mark

 

[Frank Acampora]

Re: Prop theory?

Yep, that is a 4 cylinder in my avatar. It was originally rated at 105 at the powerhead, but in the early '80s when ratings were changed to prop horsepower, Chrysler de-rated it to 90 HP with NO changes. They simply called it what it was really delivering to the water.

I compared it to the three cylinder 90 which I also have. Both swing the same prop at the same RPM. However, you can actually feel (and hear) the difference in pick-up between the engines. The three cylinder will pick up briskly and smoothly transition to plane while the four will "bounce" the boat up onto an almost immediate plane. It pops a skier out much quicker too.

'Aint no replacement for displacement.

 

[QC]

Re: Prop theory?

I like you guys explanations and I agree with all accounts....QC I really like the torque wrench explanation..had to laugh at that one, but true. That pretty much separates the two (torque and hp).

Thought you would've seen that Mark. I use it once in a while, seems to help clear up the confusion. BTW, you are making horsepower if you actually turn the bolt. You moved it within some time frame and that's horsepower . . . Maybe it's something like 300 * 2 RPM/5252 = .1142 hp

'Aint no replacement for displacement.

Well there are two. Think Verado.

Hijack complete

 

[Frank Acampora]

Re: Prop theory?

NA-Na-Na-- A normally aspirated engine can not fill its complete cylinder volume. A supercharger simply puts more fuel/air into the cylinder EFFECTIVELY creating more displacement. Semantics!

 

[QC]

Re: Prop theory?

Semantics!

Yup

 

[Texasmark]

Re: Prop theory?

NA-Na-Na-- A normally aspirated engine can not fill its complete cylinder volume. A supercharger simply puts more fuel/air into the cylinder EFFECTIVELY creating more displacement. Semantics!

Yes and more stress on the associated engine parts that may not have been designed to accommodate the additional explosive "boost"...and then there is the turbine.........on diesel engines anyway....yucko, I'll take nat aspired.

So I guess we "sinned"...thread is hijacked, but I think it contained enough info to satisfy the inquisitiveness (ha) of Mr. Oldsub86. If not we will hear about it and can continue with what we are supposed to be talking about......but Frank, I have for a long time wanted to compliment you on your beautiful paint job.....and artistic deviation from the norm.

And QC I use the term "potential" hp when talking about torque. Like you said taint hp till it moves.

Fun and games,

Mark