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The
higher the pitch your engine can turn near top horsepower and RPM, the
faster your boat can go. This
is also accurate as far as it goes. The greater the pitch, the greater
the distance your boat will advance each revolution. Since top engine
RPM is constant, increasing pitch means more speed. Then, why aren't all
propellers as small in diameter as possible, with gigantic pitches? The
answer is simply that when the pitch gets too large, the angle of attack
of the propeller blades to the onrushing water becomes too steep and
they stall. This is exactly the same as an airplane wing's stalling in
too steep a climb. There are optimum pitches and pitch ratios for each
boat. Within these limits it is worthwhile, on high-speed craft, to use
the smallest diameter and the greatest pitch possible.
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Too
little pitch can ruin an engine. This
is quite true if the pitch and diameter combined are so low that it
allows the engine to race at speeds far over its designed top-rated RPM.
Never allow your engine to operate at more than 103 to 105 percent of
top-rated RPM. If your engine exceeds that figure, a propeller with
increased pitch Qr diameter is indicated.
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Every
two-inch increase in pitch will decrease engine speed by 50-100 RPM, and
viceversa. This
is a good rough guide for low to moderate speed boats, draggers and tug
boats. Like all rules of thumb, though, it is no more than a rough
guide.
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A
square wheel (a propeller with exactly the same diameter and pitch) is
the most efficient. This
is not true. There is nothing wrong with a square wheel; on the other
hand, there is nothing special about it, either.
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The
same propeller can't deliver both high speed and maximum power. This
is true. A propeller sized for high speed has a small diameter and
maximum pitch. A propeller sized for power or thrust has a large
diameter. For some boats you can compromise on an in-between propeller,
but for either real speed or real thrust there is little common ground.
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Base
Source: Propeller handbook (Dave Gerr)
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