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.

 

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.

 

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?

 

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.

 

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.

 

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.

 

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

 

As I recall it does NOT assume 100% prop efficiency, as that is impossible. Rather it assumes a propeller of proper specifications. Another way to say that would be that it assumes you selected a prop that is perfectly sized for your boat, but then accepts that there is a small loss even with a perfectly sized prop.

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

POWER is a math function within Excel, raising the value to the power provided. Has nothing to do with engine "power" if that's what you were thinking.

 

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

 

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.

 

Here's a decent prop calculator that can help describe a suitable prop.

https://www.vicprop.com/displacement_size.php

 

Be careful with the concept of "hull speed".
Your boat often will go much faster than the calculated "hull speed" figure.
My own 11 ton ship has a waterline length of about 33 ft, and the calculated hull speed for that waterline is close to 7.7 kt.
In flat water (no surfing), in the right wind, I have seen her reach 10 kt and hold it for minutes on end.
The 35 hp motor will move her at 8.5 kt, flat out, in flat water, no wind. I don't hold that for long on the old engine though.
If we add more propulsive force to any hull shape and any displacement, it will always go a little bit faster. It has to to obey the action:reaction law of that fella Newton.
So treat hull speed with caution.... it is a somewhat nebulous concept.

 

Not sure why you think you need thrust. Note that the propeller thrust will equal the hull drag, and as others have said, this is very complex and is not something you can calculate by just plugging into a simple formula. And the BIG variable is the prop. Sailboat propeller efficiencies are pretty low, many in the vicinity of about 50%.

You are better off using one of the rules of thumb to estimate horsepower and then get help selecting a prop that matches your installation. I'm guessing a fixed 3-blade will be the most efficient, but will hurt your sailing performance.

If you really want a thrust estimate, the following from high school physics (well maybe freshman college statics) could be used:

Power = (Thrust x Speed)/33,000

Power would be the engine horsepower at that operating point times propeller efficiency and speed would be in feet/minute. The thrust will be in lbs.

 

Better propeller efficiency is my primary goal. Looks like "bigger and slower" are the key to better efficiency. I saw a guy selling electric outboards at the Annapolis show last weekend that were *considerably* more efficient than their gas counterparts. His max prop speed was 600 rpm. He was using a fairly big, relatively skinny prop with a high pitch, and it worked beautifully. It allowed for a tremendous amount of thrust from a far smaller package than I could have believed possible.

I need to know the thrust so I can reverse the calculation and work out the pitch and size of prop required to push my boat at cruising speed at 600 rpm for a given horsepower. This might lead to a prop diameter bigger than I can safely put on my boat, but that's OK. There are always compromises. I'm aiming for a best-effort rather than sacrifice-all-for-efficiency dynamic.

 

Captain Canuck did you ever get your answer? I'm also looking to re-design the propulsion system for a catamaran and it seems the answers aren't very straight forward. I'll be changing the motor and prop and I have an infinite window of options so I need to start the same as you, with thrust/torque required to push the boat at hull speed and work back from there to determine RPM requirements and HP at those RPM's. I'm even working in cad to design and machine my own prop based on those specs so I'm not even limited on prop design or specs.