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Tue, April 20 2021, 07:09Login ] [ Contact Forum Admin ] [ Main index ] [ Post a new message ] [ Search | Check update time | Archives: [1]2345678910 ]

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Date Posted: - Monday - 01/ 2/12 - 10:20am
Author: Paul M31 #106 - NJ
Subject: What's your objective?
In reply to: George Andrew 's message, "Three blade prop" on - Sunday - 01/ 1/12 - 11:04pm

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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