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post #11 of 16 Old 10-03-2018
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Re: Boat thrust calculator?

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

https://www.vicprop.com/displacement_size.php
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post #12 of 16 Old 10-04-2018
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Re: Boat thrust calculator?

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Originally Posted by Captain Canuck View Post
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.

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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.
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post #13 of 16 Old 10-05-2018
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Re: Boat thrust calculator?

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.
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post #14 of 16 Old 10-05-2018
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Re: Boat thrust calculator?

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.

S/V First Tracks
1985 Cal 33-2
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post #15 of 16 Old 10-10-2018 Thread Starter
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Re: Boat thrust calculator?

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.
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post #16 of 16 Old 01-28-2020
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Re: Boat thrust calculator?

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.
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