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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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