Engine Thrust Guide

[tboltmike]

Are there any guides or rules of thumb to estimate thrust ranges for motor horsepower?

It seems like thrust would be a necessary factor in designing transom strength but I have yet to see any. Boat builders must have some guide.

Thanks Mike

 

[Rancherlee]

Re: Engine Thrust Guide

Never really seen a HP to Trust ratio before. I have seen a 3hp electric outboard that claims 150# of thrust which would be 50# per HP but who knows, too many variables.

 

[hwsiii]

Re: Engine Thrust Guide

There is no direct relationship between thrust and horsepower. Irecently had a discussion with 4 marine architects and there numbers ranged from 30 pounds per horsepower to over a 100 pounds per horsepower. Torque is directly measureable but converting to thrust varies considerably, unless you are talking about a jet engine, and then you really have thrust.

H

 

[Frank Acampora]

Re: Engine Thrust Guide

Well, one horsepower is 550 foot pounds per second. I don't want to do the math but if you have a 100 horsepower engine driving your boat at 30 MPH, you can figure foot pounds and thus thrust. This will be for your application and that speed only. Note that for different hulls and props, thrust will change. Note that thrust will also change as you go from hole shot up onto plane.
It is not just horsepower, but how you use it that determines the thrust an engine is delivering to the transom.

I suspect that in an outboard hull, empirically, (that is, by experience of boatbuilders over many years) a thickness of an inch and a half of plywood properly reinforced and glassed will handle almost anything you can hang on it.

 

[tboltmike]

Re: Engine Thrust Guide

In determining ship horsepower, models are towed in a tank and the force to tow the model is scaled to the ship.

I think horsepower is proportional to thrust. Finer pitched props allow the the RPM thus horsepower to be developed at a slower speed.

Water has to be pushed opposite of the intended boat direction to generate thrust. It takes power to move water.

Respectfully, If thurst were not porportional to power, any size engine could propel any size vessel at any speed.

 

[Frank Acampora]

Re: Engine Thrust Guide

You are not factoring in all things. For example: when a deep vee hull is in the hole shot, it is displacement and the engine is developing a LOT of thrust for the horsepower it is developing. When the hull climbs out of the hole and goes onto plane, it experiences way less water resistance and engine horsepower is translated into speed. Sure, thrust is there but I'll bet dollars to donuts it is less than during the hole shot.

Remember: One of the reasons that a squirt boat is about 25 % slower than an equivalent prop drive boat is because the faster it goes, the faster it runs away from the jet column and thrust decreases. (simplistic explanation)

Of course, I will not deny that a low horsepower engine will develop less thrust than a high horsepower one--in that respect, yes, thrust is proportional to horsepower-- I don't think anyone would deny that. I just think it is a very elusive thing to measure--unless you took the trouble and expense to mount the engine on pressure transducers.

HEY: Major thought: Why not ask Mythbusters? That's the sort of thing they might be interested in answering.

 

[hwsiii]

Re: Engine Thrust Guide

Frank, you are exactly right in my opinion. Theoretically speaking a deep vee and a flat bottom boat could very easily be using different amounts of thrust at the same weight, horsepower and speed. And definitely I agree that during the rise to plane in most boats the thrust is significantly higher than after planing speed is attained.

H

 

[wca_tim]

Re: Engine Thrust Guide

Isn't thrust more directly related to torque... considering gear ratio of the drive (if applicable) and then tconsidering the prop as a "gear"??

 

[hwsiii]

Re: Engine Thrust Guide

Tim, I wish I had that answer. The formulas I use for homebuilt airplanes are directly related to horsepower such as:
(Thrust is equal to Horsepower times propeller efficency) times 375(as the constant for MPH) / divided by air speed.
((70 HP * .90 Prop efficency)*375 MPH constant)/35 MPH = 675 Thrust
I wish I knew a lot more about thrust than I do, I can calculate (at least theoretically) any moment in time for thrust, but I do not believe the answers are always right, just as Frank stated in his post on boats and the climb required to get on plane and the thrust required.
My personal belief is that it is a combination of Horsepower and Torque, and that both have to be taken into account if you really want a very close tolerance answer at all. Torque is the power on the front end when the RPM are low and Horsepower is the power on the rear end when the torque is diminishing.
Now, if you believe in Daniel Savitsky his formula is directly allpied to horsepower with NO relevance to torque whatso ever. I have had many discussions with very knowledgeable people in the aircraft and boating industry, and have never come away with an answer that made me feel I KNEW what I needed to know about thrust, and I could absolutely QUANTIFY it, at any moment in time. I personally believe it is ONLY theoretically quantifiable in boat and airplanes with aspirated engines.

But I am definitely interested in finding a real answer, if there really is one, as the formulas I have now do NOT answer the question reliably in ALL cases.

H

 

[tboltmike]

Re: Engine Thrust Guide

Gentlemen,

Thanks for all of these informative replys.

However, I was looking for something dealing with forces as might be expected on the boat's transom.

I would suspect that thrust forces must be known within some ball park in order to structrually design a transom.
Therefore, there must be tables boat mfg's use for the various horsepower ranges.

I was hoping someone might have seen or access to these or at least know of their existance.
Mike