MOST FREQUENTLY ASKED QUESTIONS
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Propeller Installation, Determining
RPM, Propeller
Sizing, Propeller
Terminology
Q. What prop should I use with my boat and motor?
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A. First, determine how the boat will be used or what the
normal load will be. If this boat normally operates with
one specific passenger load, propeller size is relatively
easy. If it has multiple uses ranging from light to heavy
loads, the selection of one or two propellers may be necessary.
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Q. Why change propellers?
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A. The stock outboard with which most outboards are equipped
is a compromise. Since it has fixed diameter and pitch,
it is limited in its use and may not provide satisfactory
performance for every combination load that will be encountered.
One important fact to note is that the propeller moves the
boat through the water at a specific engine R.P.M., and
H.P. is directly related to that R.P.M. The engine cover
is marked with a certain H.P. rating, but in most instances
the full benefit of the possible H.P. is not realized. Along
with the H.P. rating, equal emphasis should be placed on
the R.P.M. at which the related H.P. is developed. This
is where the propeller comes into the picture. Outboard
engines are designed to run at peak R.P.M for full efficiency.
Excessive R.P.M. with its increased friction and wear is
harmful. It is equally harmfull to run the engine overloaded
to the point that it can not achieve its rated R.P.M. This
results in excessive carbon build up with subsequent problems
of poor fuel economy, pre-ignition, frequent spark plug
failure, scoring of the cylinder walls and even burned pistons.
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Q. How can I be sure my motor is operating within
the recommended rpm range?
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A. This can be checked with a tachometer. There are various
types commercially available.
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Q. What are diameter and pitch?
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A. If a propeller is specified as 10x12 in size, this indicates
it is a 10" diameter by 12" pitch. Propeller Dimensions
are always given in this order. Diameter is determined by
doubling the distance between the blade tip and the center
of the hub. Pitch refers to blade angle. In this example,
the 12" pitch indicates that with each prop revolution
the boat theoretically would advance 12". Due to slippage,
actual advance is somewhat less.
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Q. Why do outboards of the same power sometimes
take different props?
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A. This is due to differences in lower unit gear ratios.
Stock outboards are geared so that the propeller shaft turns
at a slower speed than the R.P.M at the powerhead. This
is usually expressed as a ratio such as 12:21 or 14:28,
referring to the number of teeth in the drive gears. In
the first example, the crankshaft gear has 12 and the propeller
shaft the gear has 21. This means the propeller shaft turns
only 57% as fast as the indicated R.P.M at the powe head.
The lower the gear ratio, the larger the propeller that
can be used. In other instances, engines of different makes
may develop their maximum horsepower at different levels.
Check your owner's manual to determine the correct R.P.M
for your engine.
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Q. What is the correct transom height for my outboard
engine?
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A. On average boats, it is best to mount the engine so the
cavitation plate is approximately 1" below the bottom
of the keel, or 1" below the bottom of boats without
keel. For racing boats, better speeds can be attained by
raising the engine to reduce lower unit drag and exhaust
back pressure. The best transom height can only be determined
by experimenting. The best performance will be obtained
by mounting the engine as high as possible, or to the point
just before propeller cavitates excessively.
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Q. Will a different prop correct bad torque action
(listing and hard steering).
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A.Usually not. Most likely it is the result of any of several
irregularities in the hull, the steering hook-up, or the
engine mounting. If an engine has a right rotating propeller,
steering wheel should be on the right or starboard side.
Modern outboards have built-in features in the lower units
to compensate for torque. Engine tilt should be such that
the prop is horizontal when underway. If it is up or down,
the propeller can have a definite pull to one side. See
that the engine is at the exact center of the transom and
is sitting level. Check boat bottom for warping., distortion,
which could cause difficulty.
