# Need help calculating what dia and pitch propeller to use?

Discussion in 'Props' started by Bob Rosa, Jun 28, 2023.

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### Bob RosaJunior Member

When I started this project I had no idea this would become so confusing. I had it in my mind that I would use a 12 inch prop just like most of my friends run on their CC or other boat of a similar size. So I purchased a Hall-Craft 9 Degree Strut off eBay which is suited for a 12 inch prop. Made some good progress following Mr. jehardiman excellent advice and one of my friends that is in the "Know" category stopped in. Well by the time he left I was very confused as to what size prop to use, so I'm wishing for some more excellent help out there. The hull water line is 25 foot long. The beam is 5 foot. approximate total weight should be around 2000 lbs loaded. Only draws about 3 inches of water. Plan is to use a modern GM LS marine engine of close to 400 hp. Hull has one step of aprox. 2 inches 10 foot from transom. The forward three feet surface of the stepped hull is in plane with the aft 3 to 4 foot of the hull at the transom. My friend seems to think this boat should easily do 80 MPH. I'll be happy with less.

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### jehardimanSenior Member

Ok, where to start...
Do you have a rpm vs torque chart for the engine....Speed is all about the thrust at speed. Thrust at speed is all about torque at rpm. Thrust is developed by accelerating flow through the prop. So the prop rpm*pitch needs to be significantly faster than the velocity of the boat. To use common terms, the thrust is proportional to the advance ratio J i.e. J= Va/nD where the pitch P is a function of the propeller diameter D. Let us say you have a 12" wheel with a 12" pitch....so P/D = 1.0. At 80 mph (69 knots) the boat is traveling 117.33 ft/sec. To just keep up with the velocity of advance (Va), the prop needs to be turning at 117.33*60= 7039.8 rpm; however this speed makes no thrust. In order to generate thrust, the prop needs to be turning more than 7039.8 rpm. How much more? Let us say you need 100 lbs of thrust at 69 knots. Thrust =1/2* rho*v^2 so 100 = 0.5* 1.9940* v^2 =>100/(0.5*1.9940) =v^2 => v= 10.01 so to generate 100 lbs of thrust, the prop need to be spinning an additional 10 rps for a total of 7639 rpm. However, as that prop spins at 127 rps, it absorbs torque from the engine. The engine needs to provide this torque...at this rpm. This is why determining what prop to use starts with the engine's rpm vs torque chart...

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### DogCavalrySoy Soylent Green: I can't believe it's not people

What he said. I'd recommend Dave Gerr's Propeller Handbook, but it's not meant for boats this fast.

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### Bob RosaJunior Member

The best answer I can find for this is 424 lb-ft(575nm) @ 4600 rpm. Hope this is what is needed?

Last edited: Jun 30, 2023
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### Bob RosaJunior Member

I have tried to follow the math but I'm afraid my mind has gotten too old and lost some of that ability. Too much water over the dam. At 77 just doing the checkbook has gotten challenging. So I read about finding a prop that hits a sweet spot in rpm and speed. Sure hope this calculation should get close to that. What am I doing building a boat? I guess it's my last hora. When I started this project it was to honor the friendship of a gent and his family that worked for Less Staudacher building Gold Cup racers in the 1950 and 60's. I didn't realize it was going to take me so long for my first boat.

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### DogCavalrySoy Soylent Green: I can't believe it's not people

My boat, Serenity took over three years. I'd have bet money it would take a year.

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### DogCavalrySoy Soylent Green: I can't believe it's not people

A member on here, @sandhammaren05 wrote a book specifically about props for boats like yours.

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### Bob RosaJunior Member

Thank you! Will check it out.

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### missinginactionSenior Member

Bob, you might have seen my post as I struggled to get a small cruiser to plane. The recommendation to get and read Dave Gerr's book is a good one. I understand what you mean about wrapping your head around the math. You have to understand how to use fractional exponents and solve fairly complex equations. There are a lot of variables involved in the selection and design of propellers.

My advice would be to pick up a copy of Gerr's book and start there. Don't, DON'T try to digest it all at once. Take it a little at a time and THINK about what you're reading. Let it sink in. Once you understand a part of Gerr's book, then move on to the next part. In time, I think that you'll come to an understanding. It's either that or take your chances by paying someone to do the work for you.

Good Luck,

MIA

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### Bob RosaJunior Member

Thank You for the reply. I'll work on finding a copy of that book. Interestingly, I spent Sunday with a group of boat builders and owners. So I popped the question as to what propeller they were using. I only got one answer. "I just tried a couple of different props until I got one I liked." Glen-L has some interesting charts on Prop diameter and pitch vs potential speed. Fortunately, I have plenty of time to do this research. Thank You! Bob

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