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Would you like to see FAQ's for: |
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Q.
What prop should I use with my boat and motor? A.
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 it has multiple uses ranging from light to heavy loads,
the selection of one or two propellers for best results may be
necessary. A.
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 it
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. Q.
How can I be sure my motor is operating within the
recommended rpm range? A.
This can only be checked with a tachometer. There are various kinds commercially available. Q.
What are diameter and pitch? A.
These are the two common propeller measurements.
If a propeller is specified as 10 x 12 size, this indicates it is 10" diameter
by 12" pitch. Dimensions
are always given in this order.
Diameter is determined by doubling the distance between
blade tip and 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. Q.
Why do outboard motors of the same power sometimes take
different prop sizes? 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 that the rpm 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 gear has 21.
This means the propeller shaft turns only 57% as fast as
the indicated rpm at the powerhead.
The lower gear ratio, the larger the propeller that can be used and vice
versa. In other instances, engines of different makes may
develop their horsepower at
different rpm levels.
Everything else being nearly equal, higher rpm engines
require smaller props to achieve greater rpm. Q.
Can a prop change help me in water skiing? A.
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 prop. Q.
Can I troll better with different sized wheel? A.
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 slow speed, they tend to
overload the engine. A
low pitched wheel relieves overloading, permitting the engine to
idle faster while moving the boat slowly. Q.
I have a twin outboard.
Can I get a propeller of opposite rotation and run one
motor in reverse? A.
No. This is a frequent
question, and we would caution anyone against attempting
to run any engine in reverse at high speed.
The power units are not made to withstand the reverse
thrust and this type of operation can only be result in lower unit
failure. Some lower units have been available with reversed
gearing so counter-rotating propellers can be used in a dual
installation. Q.
What is the correct transom height for my outboard engine? 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 backpressure.
Best transom height can only be determined by experimenting
… get the engine as high as possible, or to the point just
before propeller cavitates excessively. Q.
What is the best tilt-setting or shaft angle? A.
Proper tilt is extremely important, and is determined only through
experimenting. In any
boat the tilt setting can change as the load changes.
Tilt adjustment determines the planing angle and if
improperly set, the speed loss can be substantial or the boat may
not plane at all. Tilting
the engine in toward the transom pulls the bow up.
Vary the angle to find the point where the boat assumes the
best planing position. Q.
Will a different prop correct bad torque action (listing
and hard steering) 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.
Steering wheel must be properly located relative to
propeller rotation. If
and 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 adequate sized pulleys, properly
swiveled and with the right cable tension.
Check the boat bottom for warping, distortion, which could
cause difficulty. Q.
Will a bronze prop damage my motor?
A.
No. Bronze is an ideal
propeller material having numerous advantages over aluminum or
die-cast material. Bronze
is strong, ductile, repairable, corrosion resistant and in normal
service will last the life of the motor. Frequently you may hear that a bronze prop wears out
the shifting mechanism. Shift
mechanism wear and failure is not due to the type of propeller
used. Most frequent
lower unit failure is in the shifter dog which results form
improper shifting procedure by the boat operator.
Many attempt to ease an outboard into gear thinking that
this is easier on the mechanism.
The opposite is true.
Shifter lever should be snapped into gear with some force
… the only way the shifter dog will mesh completely with forward
or reverse gear. Q.
Is it advisable to have outboard props repaired? A.
Depends on the material. Those
made of bronze or sand-cast
aluminum are
repairable at about 1/3 to 1/2 the new propeller price.
Die-Cast propellers generally are not repairable.
The material is very brittle, breaks off easily in the
straightening process and will not weld satisfactorily.
(Original equipment propellers are usually made of
die-cast). It is
advisable to discard such propellers and replace with the more
durable sand-cast aluminum or bronze. Extensive service is offered by propeller
manufacturers at the factory and through authorized propeller
repair stations located across the country.
These stations are operated by factory-trained people
employing the same methods and equipment in use at the factory. Q.
What is the purpose of the rubber cushion hub in an
outboard prop? A.
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 should occur due to the jolt or shock
that is experienced in the process of shifting gears. Q.
The rubber hub in my propeller seems to be slipping.
Is this possible? A.
It is a definite possibility, but it does not occur too frequently.
Take a look at the propeller.
If the blades are visibly bent or distorted, you very
likely are experiencing cavitation__ and cavitation is
often mistaken for a slipping bushing. Have it checked by the prop manufacturer or a reliable prop
service station. The
bushing can be replaced if it needs it or the blades can be
restored to proper accuracy to eliminate the cavitation. Q.
Why do I shear so many pins without apparently hitting
anything? A.
The engine may be turning too fast in the shifting range.
The shear pin hole or slot may be sloppy or oversized.
The shear pin may be of improper strength for the amount of
power involved, or the cushion hub may be frozen tight. Q.
What about plastic propellers? A.
Nothing to date has been developed that has all the qualities of
propellers made of metals. A
good propeller must be durable, repairable and above all, it must
perform well. So far
the available plastic fail in these important requirements. Q.
My outboard seems to vibrate excessively, yet the propeller
hardly shows signs of use. Why? A.
This is not unusual. Propeller
blade damage very often is not discernible to the naked eye and
blades can become bent or distorted without showing signs of
impact or abrasion. Q.
Is an adjustable pitch propeller possible or satisfactory? A.
An adjustable pitch propeller permits more flexibility of operation than
the standard props supplied with the outboard. However, it is only efficient at one setting since the blade
is a true helical surface only at one particular pitch and angle.
Two or three well designed rigid props of pitches needed
for different loads will give better performance than and
adjustable propeller. |
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Outboard
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