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Date Posted: - Monday - 07/25/11 - 2:10pm
Author: Mark AEOLUS M31
Subject: Prps for the M31
In reply to: Destiny 's message, "three blade prop size" on - Tuesday - 07/19/11 - 5:38pm

I'm no expert as I have the old 2 blade, but Randall has said he use to have a 13x8 three blade and went to a 12x6 as the 1:1 velvet drive tranny had touble w/the 13x8. A quick search of past threads will give you some insight. Good luck

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Replies:

[> Three blade prop -- George Andrew (Rational-optimistic), - Sunday - 01/ 1/12 - 11:04pm

I tried a three blade 12.6"x7.87" on my heavy M31 (7 tonne loaded) but over pitched. Looks like a 13x6 could still need less pitch. Read that 1" pitch alters rpm by 150-200. With a 12% aperture tip clearance the max dia could be 13.75" and pitch even lower.
Does anyone have a spare prop to sell? - LH. 1.25 shaft.

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[> [> What's your objective? -- Paul M31 #106 - NJ, - Monday - 01/ 2/12 - 10:20am

If you have the Perkins 4-107 or 4-108, it will provide the necessary HP on it's power curve (at ideal air/fuel ratio) to push the boat at around 1500 to 1700 rpm.

If you were to insist that the engine runs at "design rpm" at hull speed, this would mean it would run at around 3000 rpm, but still produce the same HP, i.e. around half its rated HP, meaning its pumping the amount of air through it for 45 hp, but the fuel for 25HP, causing it to run too cool and glaze the cylinder walls.

An "overpropped" engine is unable to reach the design rpm (airflow) for the amount of fuel introduced and will produce black smoke, causing sooting of exhaust valves, manifold, waterlift, etc.

After a lot of calculating and experimenting, I have decided that my 13x8 three blade is matched perfectly to my boat and engine. At 1700 rpm the engine puts out a little under 30hp, which is about what is required for the boat to do maximum displacement speed, digging a very respectable hole.

Sorry for the rant, but I am always suspicious of mechanics claiming that ANY engine should run at max "cruising" rpm when running hull speed.

For argument sake, put a 100hp diesel in your boat - would you expect it to run at cruising rpm when pushing your boat at hull speed?

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