Q.
What prop should I use on a 30' boat with 185 h.p.
engine?
A.
Much more information on the boat, engine, gears,
type of service is required to make a reliable prop selection.
Past performance information is helpful too unless
yours is a stock outfit with known performance characteristics.
Your
marine dealer or a major propeller manufacturer can furnish
a propeller analysis form which their engineers can study
to advise you.
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Q.
Can one propeller give top speed plus lots of power?
A.
Generally no.
To gain high speeds involves sizes or pitch rations
unsuited to load carrying or working conditions.
Decide what is most important and select the prop
accordingly, or change props with boating conditions.
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Q.
You suggested a 13" diameter x 14" pitch for my boat.
Wouldn't a smaller diameter with more pitch do the
same?
A.
Pitch cannot be substituted for diameter.
Diameter is directly related to horsepower, 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.
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Q.
Will a prop change correct a disturbingly large wake?
A.
Probably not, unless the change causes a non-planing
boat to plane.
Wake is directly proportional to hull displacement.
Fast boats in planing position throw less wake than
slow displacement boats even though the planing boat goes
faster than the slow, wave-throwing hull.
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Q.
In building a boat: a) Where do I set the engine?
b) Where do I locate the prop?
c) What size rudder shall I use?
d) What shaft angle is best?
e) What size shaft do I need?
f) What shaft materials are best?
A.
a) Engine location is closely tied up with trim
calculation so it's impossible to answer without complete
scientific characteristics of the boat.
Engine weight and center of gravity must be calculated
in connection with all the other weights and centers.
If it's impossible to obtain these characteristics,
use a loose rule that the best location for engine weight
is 55% of the load waterline length of the boat abaft the
fore end of that waterline. b)
Aft, just forward or the transom. c)
For boats of 20 to 30 foot length, rudder blade should be
about 5% of the figure found by multiplying the waterline
length of the boat by the extreme draft.
Boats of 40 to 50 feet need rudders of about 4%.
Larger boats need and area of about 3%.
Blade should extend downward, even with lower prop
tip and aft far enough to get area from the above formula.
d)
Generally, as low as possible, never more than 15o,
allowing proper clearance between blade tips and boat bottom.
A modestly steep shaft angle does not have a noticeable
effect on performance.
Normally the difference between, say, 8o and
14o is negligible. e)
This is related to horsepower, rpm, and type of shaft material
used. f)
Monel, stainless steel and bronze, in this order.
All resist corrosion.
Monel is strongest.
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Q.
I run on mile-high western lakes.
Should I adjust prop size accordingly?
A.
Yes, gasoline engines lose power when operated
above 3000' altitudes.
A smaller pitch prop will compensate some by bringing
rpm to correct operating point.
Boats operated at high and low altitudes need a prop
for each condition.
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Q.
I have a right hand prop but I need a left hand.
Can it be changed?
A.
No.
Propeller hand cannot be changed.
Turning it around won't change the hand.
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Q.
How much can you change prop pitch?
A.
Depends on individual propeller.
Average standard bronze props can be changed about
2" up or down.
Remember that the metal near hubs cannot be changed.
Excessive stress and strain can distort or kink blades,
causing poor prop efficiency.
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Q.
What are true pitch, variable pitch and controllable
pitch propellers?
A.
A true pitch propeller measures constant pitch
at any radius … a section of a true helical surface.
A propeller whose pitch varies radially, that is,
is not constant but increases from hub to the tip, is a
variable pitch propeller.
Variable pitch propellers are used on most high speed
craft and reduce cavitation which might result at such speeds.
Many larger boats use variable pitch propellers to
compensate for wake or varying inflow velocities which result
from the depth of the immersed hull.
With a controllable pitch propeller, the pitch angle
of the blades can be mechanically adjusted when the boat
is underway.
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Q.
Shall I get my propeller dynamically balanced?
A.
Most premium grade propellers are dynamically
balanced at manufacture.
All other propellers can be dynamically balanced
but only on special order and at additional price.
Such a balance is inherent proof of manufacturing
perfection insofar as blade thickness, width, spacing, ect.
are not concerned.
This inherent accuracy is not destroyed by blade
damage, making it unnecessary to dynamic balance during
repair.
A good static balance at the repair station will
suffice as the inherent accuracy of the propeller is still
there.
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Q.
What is electrolysis, its cause, correction?
A.
Electrolysis is the flow of electricity caused
by the difference in potential of two dissimilar metals
immersed in electrolyte.
It acts as a plating process and one metal is eaten
up and deposited on the other.
Prevent harm to underwater parts by attaching sacrificial
anodes (usually zinc).
Replace from time to time as they deteriorate.
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Q.
Do I use the same size wheel if I convert
to a cupped edge propeller?
A.
No.
Diameter remains the same, but additional load placed
on the engine by the "cupped edge" requires that
pitch be reduced 1", or 2" in propellers above
14" diameter.
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Q.
What benefits does a cupped-edge wheel offer?
A.
Cupped-edge props for fast boats and high rpm
reduce slip, giving as much as 3 mph more speed on the average,
and eliminate cavitation.
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Q.
How close to the bottom of the boat can the blade
tips run and what is the required aperture clearance?
A.
The clearance between the propeller tips and
the hull should be at least 1/6 to 1/7 the propeller diameter.
Clearance is equally important along the forward
edges of the blade where they swing in line with the deadwood
and should not be less than 1/12 of the diameter.
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Q.
Shall I use 2-, 3-, 4-, or 5-blade propellers?
A.
Excessive horsepower, direct drive and high advanced
speeds indicate that propellers of high pitch-diameter ratio
be employed.
Generally 2-blade propellers are used.
At somewhat lower advanced speeds and shaft speeds,
3-blade propellers with pitch-diameter ratios of approximately
1.0 are required for best propeller efficiency.
It is in this category that most pleasure boas lie.
At even lower advanced speeds and shaft speeds, 4-blade
propellers with pitch-diameter ratios of .8 or less are
required for optimum propeller efficiency.
On large cruisers and commercial applications such
as trawlers, shrimpers, tugs and work boats, 4 blades are
generally used.
If excessive vibration is a problem, 5-blade propellers
are used.
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Q.
What are normal slip percentages for various craft?
A.
With propeller correctly selected for the operating
conditions, the slip percentages would be as follows: racing
hulls 10 to 15 percent, planing runabouts 15 to 25, planing
cruisers 25 to 35, displacement cruisers 30 to 40, sailing
auxiliaries 35 to 40 and work boats 40 to 80.
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Q.
The salesman said my boat would go "X" mph.
I go only 2/3 as fast, can that be right?
A.
"Claimed" boat speeds are often overestimated.
Were his speed trials and yours accurate?
Were test conditions similar?
It's hard to answer with so many factors in the picture.
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Q.
Shall I use heat to install or remove my prop?
A.
Heat should never be used to install a propeller,
and, therefore, should seldom be required for removal.
If a wheel puller and hammering with soft hammer
on propeller and /or shaft end does not work, gentle warming
with a blow torch may help.
Do not use a welding torch as the quick, harsh heat
will change the grain structure of the bronze, setting up
internal stresses resulting in hub splitting.
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Q.
Can I put an inboard engine in my outboard hull?
A.
The big question here is whether you will throw
the hull out of trim with a heavy inboard mounted mid-ships
when the hull was designed to carry an outboard in back
of the transom.
Normally such conversions are never successful unless
the hull as designed to carry the engine weight aft, or
you have a V drive or a transom drive with the engine well
aft.
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Q.
My outfit doesn't have the speed I expect, though
I've tried several types of prop.
Why?
A.
Some boats do not have the power to "get
over the hump" or attain planing speed.
Other boats that achieve planing speeds may not have
the power required to reach speeds desired.
In such cases repowering is the only answer.
Power requirements are proportional to the speed
cubed.
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Q.
Will a reduction gear give me more power?
A.
Reduction gears in themselves do not provide
power … they absorb power.
They provide the means of using a greater propeller
diameter, which may be more efficient.
They are commonly used with high rpm engines in relatively
large hulls, requiring greater blade area than can be used
driving direct.
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Q.
To change from a standard 3-blade, what diameter
or pitch modifications must be made for a 4-blade propeller
( to retain comparable engine operation relative to rpm)?
A.
For the 4-blade Dyna-Quad, usually associated
with pleasure boats, maintain the diameter and reduce pitch
1". For
the 4-blade Work-Horse style, usually associated with commercial
craft, reduce diameter 5% to 6% to the nearest even-inch
diameter. For
the narrower 4-blade Trawler style, maintain same diameter
and pitch.
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Q.
My motor is rated at 3200 rpm.
Should I use a wheel small enough to turn this rpm
at full throttle, though I want to cruise at 2800?
A.
A propeller is most efficient at the maximum
rpm your engine will turn it; the farther back from maximum
your rpm is set by throttle, the greater the loss in efficiency.
It is well to run somewhat less, say 200 rpm, than
maximum.
In this instance, prop should be of a size that will
turn 3000 at full throttle and be the best size for cruising
at 2800 rpm.
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Q.
What is the propeller "slip"?
A.
Slip refers to apparent slip and is a non-dimensional
figure expressed in percentage.
It is the difference between theoretical mph and
actual mph divided by theoretical mph.
Theoretical mph is calculated by multiplying propeller
pitch and propeller rpm and dividing by 1065.
As an example, a boat that goes 20 mph measured speed,
is driven by a 12" pitch propeller turning 2600 rpm.
Theoretical mph is 12 x 2600 divided by 1065 equals
9.6 which divided by 29.6 equals 32.5%.
Slip is not to be confused with propeller efficiency.
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Q.
Why do hulls of the same length, beam, weight and
power vary so in performance?
A.
Here the effect of an engineered design is realized.
The difference between a boat designed by a qualified
naval architect and one built by backyard techniques may
be many miles per hour.
Boat design is both a science and a skill which ahas
evolved over many years.
The designer must be cognizant of the effect of weight,
weight distribution, deadrise and other hull characteristics
which can affect overall performance.
Otherwise, a poor performing boat results.
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Q.
Can't I get more than "X" miles per gallon
of gas?
A.
Don't think of miles per gallon … think of gallons
per hour.
Fuel consumption is related to the overall efficiency.
Engines all use almost exactly one gallon of gas
per hour for every 10 h.p. developed.
For example, if you burn 15 ghp, your engine puts
out 150 h.p.
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Q.
A prop larger than I can swing is indicated for my
boat, engine, and gear.
Should I use as large diameter as I can and add pitch
to hold motor rpm down?
A.
Change the gear, increasing shaft rpm, reducing
prop diameter requirement, or, change shaft angle or prop
aperture to accommodate correct size.
If impossible, use a wide 4-blade prop for diameters
18" and up.
Under 18", a 3-blade, larger in diameter than
required, can be clipped to maximum usable diameter.
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Q.
Won't a step-up gear increase my speed?
A.
With an extremely fast boat (step bottom hydro
or drag boat), depending on your engine, a step-up gear
may let you use a prop size better suited to your needs.
Gears for "class" boats are usually worked
out by trial and error, so stick to what has been done successfully
for similar boats.
Step-up gears are the opposite of reduction gears,
but we still figure h.p. and rpm at the prop.
With
step-up gears, shaft rpm of 10,000 is not unusual
in racing craft.
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Q.
How can I tell if I need a left or right hand prop?
A.
Stand in back of boat.
If shaft turns clockwise, a right hand prop is needed.
Most marine motors take right hand props.
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Q.
What causes squatting?
A.
Many things.
A slipping or cavitating prop causes a vacuum and
digs a hole which the stern settles into.
Correction of prop size of ten corrects this.
Poor hull design , or a good hull put out of trim
by misplaced engine or load is a leading factor.
Heavy keel, unfaired strut, scoops, or automatic
bailing fixtures__ or anything that disrupts
smooth, free flow of water to prop, can cause cavitation
and subsequent squatting.
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Q.
On a sailboat under sail, should prop be rotating
or locked?
A.
There is less drag when prop rotates.
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Q.
With twin screw, is opposite rotation required?
A.
Theoretically it is best.
It definitely aids maneuvering, by advancing or retarding
one engine as required.
For steering, non-opposite rotating motors cause
no steering problems.
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Q.
With twin screw, should rotation be inboard or outboard?
A.
Rotation is normally outboard looking at the
top of prop from stern of boat.
Right-hand prop rotates to starboard, left-hand to
port.
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Q.
What do the stampings on my prop mean?
A.Most
props are stamped on forward end with diameter, pitch and
shaft size.
Diameter is always first, pitch second.
Size is likely repeated on the side of the hub with
the manufacturer's name, type prop, hand, and date of manufacture. Back
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