Understanding your Propeller by Caravelle Powerboats
Your Caravelle Powerboat has been equipped with a stock
propeller that we have determined is the best choice for most applications. If
you are considering a change or upgrade from your stock propeller, the
following information should be helpful. (To find your stock propeller see
online spec pages for your boat model.)
Identifying
your propeller
The size of a prop is described using two sets of numbers.
(e.g. 14 X 19) The first number designates the diameter, and the second number
designates the propeller pitch.
Propeller
Terminology
Diameter
Diameter is the width of the circle described by the rotating
blades.
Pitch
Pitch is the theoretical distance a prop moves forward in
one revolution. The following propeller has a pitch that will theoretically
result in 24” of forward travel in one revolution. However, in the real world,
the propeller experiences “slippage” so that its actual travel
per revolution is less than the stated pitch.
Prop pitch is frequently
misunderstood. Many people mistakenly believe that switching to a larger pitch
prop will make the boat go faster. If a prop with too large a pitch is used,
the motor simply doesn't have enough horsepower to turn the prop adequately and
the RPM level drops. Not only does the boat go slower, but the motor can load
up and the plugs become fouled. If a prop with too low a pitch is used the prop
can exceed the manufacturers recommended RPM level and damage the engine.
The ideal situation is to
select a prop with a pitch that allows the motor to reach the maximum RPM
suggested by the engine manufacturer without going over. If the prop selected
doesn't reach the recommended RPM level, the boat will sacrifice speed and
lift.
Here's a simple rule of
thumb to follow when experimenting with prop pitch. Remember that at wide-open
RPM increasing the prop pitch reduces RPM levels by roughly 200 rpm's per inch
of pitch. In other words, when switching from a 23- to a 25-pitch prop, the
maximum RPM level will drop approximately 400 rpm's. The reverse is true when
going down in pitch size.
Propeller Cup
Many
of today's propellers incorporate a cup at the trailing edge of the propeller
blade. This curved lip on the propeller allows it to get a better bite on the
water. This results in reduced ventilation, slipping, and allows for a better
hole shot in many cases. A cupped propeller also works very well where the
motor can be trimmed so that the propeller is near the surface of the water.
The cup will typically result in higher top end speed on one of these
applications.
Switching from an un-cupped to a cupped propeller will reduce your RPM. The
actual amount of RPM decrease is dependent on where, how much, and the quality
of the cupping. Typically, a cupped propeller of the same pitch and diameter
will reduce your RPM by approximately 200.
Ventilation
Air
from the water surface or exhaust gases from the exhaust outlet being drawn
into the prop blades causes ventilation. When this situation occurs,
boat speed is lost and engine RPM climbs rapidly. This occurs most often with
high transom mounting, over-trimming the engine, or sharp turns.
Cavitation
Cavitation
(often confused with ventilation), is a phenomena of water vaporizing or
"boiling" due to the extreme reduction of pressure on the back of the
propeller blade. Many propellers partially cavitate during normal operation,
but excessive cavitation can result in metal erosion or "cavitation
burn" to the prop's blade surface. There are numerous causes of cavitation
such as incorrect matching of prop style to application, incorrect pitch,
damage to the blade edges, etc.
Rake
Rake is the measurement of the angle of the tilt of the
blade's tip toward or away from the gearcase. The angle is measured on a line
extending from the center of the hub through the center of one blade.
Selecting the
right propeller
The best propeller size for your boat and engine combination is based on
the recommended operating range at wide open throttle (w.o.t.) for your engine,
which you will find in your engine operator's manual.
The
goal in prop selection is to determine what propeller style and size will
maximize performance for your boat, while allowing your engine to operate in
the recommended RPM range. The correct propeller will prevent the engine from
over-revving, yet allow it to reach the minimum RPM where maximum horsepower is
produced.
Run the boat/motor at w.o.t. under normal operating load to determine the
maximum RPM you are able to obtain. Adjust the motor trim angle for the optimum
performance. If during this test, you begin to exceed the maximum rated RPM of
the engine, reduce throttle setting to a position where maximum RPM is not
exceeded.
If your test results in your being able to over-rev the engine, you need to
increase the pitch of the propeller. Increasing the pitch increment by 1"
will result in approximately a 200 RPM drop. If your testing shows, however,
that you are only able to obtain a RPM somewhat lower than the maximum rating
given by your engine manufacturer, you would need to decrease pitch. Decreasing
pitch would increase your RPM. Once your wide open throttle RPM falls within
the recommended range of the engine manufacturer, you have a propeller that is
suited correctly for your boat with respect to RPM.
If
you use your boat for fishing, cruising and skiing, one prop probably won't do
all three things equally well. It is best in circumstances like this to have
two propellers; One to accommodate one set of circumstances and the other to
perform best under the different load. It is imperative, however, that the wide
open throttle RPM fall within the range specified by your engine manufacturer.
Aluminum or
Stainless?
Aluminum props are the
ideal choice for small- to medium-sized boats and motors. Available up to 23
pitch, aluminum props are also the logical choice for use in areas where prop
damage comes with the territory. The average aluminum prop costs about $150
compared to $400-$500 for stainless propellers.
Stainless steel props are
more expensive, but they offer several advantages. They deliver more
performance and are available in larger pitch sizes. One reason stainless props
provide more performance than similar pitch aluminum models is because the
steel blades are rigid and they don't flex under power. The blades on an
aluminum prop actually bend when the prop is in use, reducing the overall size
of the prop by approximately one pitch size. Stainless props also provide
increased bow lift, especially on heavy boats. The more bow lift a prop
provides, the faster and smoother the boat will ride at high speed and in rough
water.
Three-Blade,
Four-Blade, or Five-Blade?
Propellers are available in
three-blade, four-blade and five-blade models. Multiple blade props came on the
scene in response to high horsepower outboards that are typically mounted
higher on the transom than smaller motors. Because the prop runs closer to the
surface, the extra blades help to provide bite and stability at high speed
The advantages of props
with extra blades are they provide quicker take-offs, and allow the boat to be
kept on plane with fewer engine rpm's. However, they are slower at top-end
speeds. A quality three-blade prop is normally two or three miles per hour
faster at top end than the same pitch prop featuring multiple blades.
Maintenance
Check
your propeller often for nicks, rolled tips, or bent blades. Any distortion
from normal will cause a loss in performance and can create vibrations harmful
to the engine. A propeller with worn blades will allow the engine to accelerate
beyond the recommended operating range which can result in damage to the
engine.
Keep
the bottom of the boat clean. A fouled bottom, an accumulation of marine
growth, moss and barnacles in sea water, and the accumulation of dirt, slime,
lime, and other matter in fresh water, is the major cause of poor boat
performance. Cleaning the boat's bottom frequently during your boating season
will greatly improve boat performance.
Things
to Remember
1.
Engine RPM
at W.O.T. must be within the recommended operating range with the rated
horsepower RPM as the target number (see specification in your engine
operator's manual).
2.
Increasing
or decreasing prop pitch directly affects engine load throughout its RPM range.
3.
Trim angle
affects propeller performance.
4.
Atmospheric
pressure, elevation, temperature, and humidity all affect engine performance,
which directly affects propeller performance.
5.
Salt water
is more buoyant than fresh water; this may cause some hulls to run faster than
identical freshwater setups.
6.
Water
conditions can play a big part in boat performance and propeller efficiency.
7. The boat's
load and position of the load can significantly affect performance.
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