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- 1. What kind of prop is right for my boat?
First determine how the boat will be used, or what the normal load
will be. If this boat usually operates with one specific passenger load,
propeller size selection is relatively easy. If this boat has multiple
uses then two propellers may be necessary.
2. Can a propeller change help me in water skiing?Yes, in most cases. Original equipment propellers are pitched a
little on the high side. Not knowing the boat the engine will be used
with, the manufacturer pitches the prop a little high so the engine does
not exceed top rpm, If placed on a light boat. However on a heavier boat
or with water skiers, this propeller tends to overload the engine,
resulting in poor speed, poor acceleration and sluggish performance,
making it difficult to get a skier up. This is corrected with a lower
pitched propeller.
3. Why change propellers?The stock propeller with which most outboards are equipped is a
compromise. Since it has fixed diameter and pitch, it is really limited
in its use and it does not provide satisfactory performance for all the
combinations of hulls and loads that will be encountered once it is
installed. One important fact to note is that the propeller moves the
boat through the water at a specific engine rpm, and h.p. is directly
related to the developed rpm. The engine cover is marked with a certain
h.p. rating, but in most instances the full benefit of the possible h.p.
is never realized. Along with the h.p. rating, equal emphasis should be
placed on the rpm at which the rated h.p. is developed. This, of course,
is where the propeller comes into the picture. Outboard engines are
designed to be run at peak rpm for full efficiency. Excessive rpm, with
its increased friction and wear, is obviously harmful. It is equally
harmful to run the engine so overloaded that is cannot achieve its rated
rpm, since this results in excessive carbon build-up in the cylinder,
with subsequent problems of poor fuel economy, pre-ignition, frequent
spark plug failure, scoring of the cylinder walls and even burned
pistons.
4. How can I be sure my motor is operating within the
recommended rpm range?This can only be checked with a tachometer. There are various kinds
commercially available.
5. What are diameter and pitch?These are the two common propeller measurements. If a propeller is
specified as 10 x 12 size, this indicates it is a 10" diameter by 12"
pitch. Dimensions are always given in this order. Diameter is determined
by doubling the distance between the blade tip and the center of 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 slip loss, actual advance is somewhat less.
6. Can I troll better with a different sized wheel?Low pitched propellers are always best for trolling. The lower the
pitch, the better. Standard propellers with relatively high pitch troll
too fast and in throttling down to extremely low speed, they tend to
overload the engine. A low pitched wheel relieves overloading,
permitting the engine to idle faster while moving the boat slowly.
7. What is the correct transom height for my outboard motor?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 a keel. For racing boats, better speeds can be
attained by raising the engine to reduce lower unit-drag and exhaust
back pressure. Best transom height can only be determined by
experimenting... get the engine as high as possible, or to the point
just before the propeller cavitates excessively.
8. Will a different prop correct bad torque action (listing
and hard steering)?Usually not. Most likely it is the result of any of several
irregularities in the hull, the steering hook-up or the engine mounting.
Steering wheel must be properly located relative to propeller rotation.
If an engine has a right hand rotating propeller, steering wheel should
be on the right or starboard side. This side normally would tend to lift
as the result of torque action and the driver's weight offsets it.
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 engine is at exact center of the transom and is
setting level. Steering linkage should have enough adequately sized
pulleys.
9. Can one propeller give top speed plus lots of power?Generally, no. To gain high speeds involves sizes or pitch ratios
unsuited to load carrying or working conditions. Decide what is most
important and detect the prop accordingly, or change props with boating
conditions.
10. You suggest a 13" diameter x 14" pitch for my boat.
Wouldn't a smaller diameter with more pitch do the same?Pitch cannot be substituted for diameter. Diameter is directly
related to h.p., rpm and speed, which indicate your requirements. If the
operating conditions call for a 13" diameter, a 12" cannot be
substituted without adversely affecting performance by decreasing
efficiency.
11. What is the purpose of the rubber cushion hub in an
outboard prop?It is not intended to prevent blade damage as is sometimes assumed.
This device protects lower unit parts by cushioning the shock of
propeller impact. Its prime purpose, however, is to prevent excessive
breakage of shear or drive pins that otherwise would occur due to the
jolt or shock that is experienced in the process of shifting gears.
Written by Rich Rockwood
867 Raynor Road, Dixon, IL 61021
Telephone 815-766-2757
Fax 815-284-3022
4:00 - 6:00 PM CST
E-mail sales@marinepropshop.com
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