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-   -   Boat thrust calculator? (https://www.sailnet.com/forums/engines/326086-boat-thrust-calculator.html)

 Captain Canuck 10-02-2018 12:13 PM

Boat thrust calculator?

I'm trying to figure out how much thrust I need to push my boat at hull speed.

I have the specs, but I can't seem to find an online calculator that specifically gives me the trust required to push it through the water. Ideally, I would like that info charted on a graph.

 capta 10-02-2018 01:27 PM

Re: Boat thrust calculator?

There are so many variables when moving a hull through water that I would think it much easier to ask others with the same boat what they are powered with. Generally, a series of boats have the same engine because the designer has worked all that our before the boat was even built.

 Captain Canuck 10-02-2018 01:48 PM

Re: Boat thrust calculator?

I'm replacing the stock motor with a different one, hence my need for new calculations.

My current motor isn't capable of pushing my boat at hull speed. It might turn out that hull speed is impractical, in which case I'll dial back my expectations. But I won't know that until I can find something that can do the math for me. While I find hydrodynamics interesting, I don't find it interesting enough to get a college degree in it.

I'm sure someone, somewhere has already built an online calculator to do this math, or at least built an equation where I can plug numbers into it and get an answer. It doesn't even have to be exact, just close enough.

 overbored 10-02-2018 02:20 PM

Re: Boat thrust calculator?

is this the hunter 295 it should have a 18 HP 2gm20 in it and is way more power then you need for a 30 foot boat. if it is a 2gm20 then rebuilt it if it needs. you will be money ahead over a repower and ahead on resale also. if you do re power then a 29.5' boat 15 to 20 HP it is that simple. don't over think it.
Why does the stock motor not get to hull speed. different prop, engine is lacking, dirty bottom. what speed are you getting?

 PhilCarlson 10-02-2018 03:02 PM

Re: Boat thrust calculator?

Like overbored said, if it's the stock engine, 18 HP is enough to get you hull speed. Should be able to do that at 2800-3000 RPM unless other factors are slowing you down. Does the engine run properly? Is the hull reasonably clean? Are you reading speed through the water or over the ground (currents will invalidate your OTG speed). Do you know your prop pitch and diameter? Is the prop clean and in good condition? (I once had electrolysis eat two inches off my prop somewhere in a 6 month window).

Lots of things to consider before repowering is you best option.

 Barquito 10-02-2018 03:09 PM

Re: Boat thrust calculator?

I think the short answer to your question is, that there isn't an equation that can tell you what engine power and prop you need for your boat. Like others said, check with other same boat owners.

 denverd0n 10-02-2018 03:29 PM

Re: Boat thrust calculator?

There is a formula that will get you into the neighborhood. I can't remember where I found this, but I believe it was a boat design book that I have. Here it is, formatted for Excel:

=(A1/POWER(10.665/1.34,3))/0.95

Where A1=displacement in lbs, and "0.95" is a fudge factor assuming 5% loss between the crankshaft and the propeller. On some boats it could be much higher than that. Also, you'll note the "1.34" in there. That is a generally accepted factor for hull speed, but it is not precise for every hull shape. Some boats, the actual number might be closer to 1.33. Some boats the actual number might be closer to 1.35.

It is important to realize that this is just a very rough ballpark number, and it assumes a properly sized and pitched propeller. If you want to be absolutely certain of attaining hull speed, round up by several horsepower.

 Captain Canuck 10-02-2018 04:54 PM

Re: Boat thrust calculator?

Thanks denerd0n. Does this equation assume a 100% efficient prop, or is the prop inefficiency factored in?

 GeorgeB 10-02-2018 05:25 PM

Re: Boat thrust calculator?

DenverdOn, what is the "POWER" variable in your equation?

 hellosailor 10-02-2018 07:44 PM

Re: Boat thrust calculator?

See if you can find "The Propeller Book" by David Gerr. Properly sizing a propeller is based on hull displacement, thrust, all those things interrelated.

There was probably a reason the original engine was chosen. Remember, "hull speed" on flat water is easy. Now with a full crew and supplies on board, against a 20 knot wind in six foot waves, you're gonna need a whole lot more more. And, if your alternator is stealing two or three horsepower to charge the batteries and run things...you'll need that much more spare horsepower as well. In theory, the boat was designed with all of that in mind.

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