Prop pitch plus diametrically equals what

titaniumneck

Petty Officer 1st Class
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Apr 11, 2016
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209
OK, I have been beating a dead horse.
New question. 15 ft sprint bass boat 750# plus fuel, gear, people, another 500# at least, 1200# to 1300#. 40 hp 2 stroke tohatsu. Dropped down from 12 pitch s/s prop no vent holes to 11 pitch aluminum with vent holes. Motor takes a minute to get on plane, but rpms are about right now and top end about 27 mph.
I am going to buy a new prop for it, probably a 10 pitch aluminum.
My question is what kind of difference does diameter make, I have looked at props from 9.5 in diameter to 11.25 inch diameter. Which diameter is best with pitch and boat style and weight. Looking for all around performance, just want to get the best out of my little but dependable 40 tohatsu.
 

jimmbo

Supreme Mariner
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May 24, 2004
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12,944
You wouldn't be the first or only person to flog a dying or dead horse, and you certainly won't be the last.
So what are the rpms with the 11 inch prop, and what is the WOT rpm range? You say it takes a minute to get on plane? That seems like a very long time.
It is very common within a prop line for the dia. to increase as the pitch decreases, maybe to carry a heavier load. Inversely as the pitch goes up the dia. usually decreases, maybe for less drag at the anticipated faster speed. 27 mph for a 40hp on a 15 ft boat is not too bad
If you think you should be going faster, perhaps an inspection of the hull to check for Hooks, and an engine check over.
 

flyingscott

Fleet Admiral
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Apr 8, 2014
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Try plugging one or more of the holes on the ventilated prop. You could just be ventilating to much on hoelshot
 

Sea Rider

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Sep 20, 2008
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Which Tohatsu is it, 2 strokes 2 cylinder 40C or 3 cylinder M 40/ D2 OB ? Those usually run 5000-6000 wot rpm range. Tohatsu have their own sweet OB/lower leg height, love to run with maximized props to get full HP out of them. It's important to know max wot rpm achieved as usually loaded independently from max MPH to dial best prop.

The only way to know wot rpm is with a tach, by ear not even if being a musician. If don't count with a tach buy a induction Hardline hour/tach meter model HR-8061-2. Install per instructions, can play with props all you want.
Happy Boating
 

titaniumneck

Petty Officer 1st Class
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Apr 11, 2016
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209
I have a tac, I will be putting back on. I'm not seriously considering it, but Ive seen where a guy cut a over pitched prop tips down, off by about 3/4 of an inch.
 

jimmbo

Supreme Mariner
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May 24, 2004
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I have a tac, I will be putting back on. I'm not seriously considering it, but Ive seen where a guy cut a over pitched prop tips down, off by about 3/4 of an inch.

Either he knew what he was doing, or he didn't have a clue what he was doing.
I have stated that as pitch goes up, dia. goes down, but...
You just can't mount the prop on a disk sander worktable and grind the outermost tips off. It would require a reshape of both the leading and trailing edges of the blade to be done right. A cupped prop adds additional challenges.
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
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There's a proper tech answer in the Non Repair Boat Discussions about ideal transom height for that 40 C, check

Happy Boating
 
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