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I'm trying to understand propellers

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Jack Redington

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I'm trying to understand propellers

wrote:

Injam wrote:

It would seem to me that in order to go faster the propeller would have to
spin faster.

Prop speed is only part of the equation. How much work is being done
per rotation of the prop is another critical consideration.

Your prop will have two numbers on it. (#) X (#). The first number is
the diameter of the prop, expressed in inches. Rather obviously, a
1-inch prop cold spin a zillion rpm and not move a big boat very far or
very fast. The second number is the pitch of the prop, not entirely
accurately but closely enough for jazz described as the amount of
"curve" of the blades. Pitch measures how many inches forward the prop
would travel, (given a theoretical 100% efficiency), per revolution.

If your two boats weigh the same and are identical below the waterline,
the same prop turning in the same body of water (temp, salinity, etc)
should propel each boat the same distance per revolution.....but...if
there's a 20% difference in horsepower as you describe then at least
one of the boats is improperly rigged.

I am going to hurt myself here, for I am no prop expert. I would think
if all the things are equal as suggested by the poster. As check
suggested the one boat is not set up correctly.

But the boat with the additional HP would have a better hole-shot.

Capt Jack R..

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