To some, choosing the right boat propeller can seem like a job
for a marine service center. Anything that has to do with the backend
of the boat can be a scary thing. Despite popular belief, choosing the
correct propeller for a particular boat and motor is actually a fairly
simple thing to do, once you understand what a propeller really is.
First think of propeller as a gear, whether it be on a car or even a
bicycle. The lower the gear, faster hole shot, more power, but
revolutions are much faster so at a point you can only turn that gear
so fast until you over rev the engine (or you legs). Now think of your
car stuck in only 5th or 6th gear, very slow out of the hole, little
torque, but eventually it will catch up with itself so you can cruise
to a good speed.
The same basic concept applies to propellers, except with a prop you
are committed to only one “gear". So what to do now? Find the prop size
that will give the best overall performance. Sound difficult? It's
really not.
Your engine has a recommended wide open throttle rpm range. Let take an
example of an18' boat with a 125 hp engine with a rpm range of
4500-5500. In most cases you purchase a boat with some sort of
propeller on it. Let's say this motor has a 21 pitch prop on it and at
wide open throttle under a normal boat load, the engine turns 4900
rpms. Well it's in the range so it's the correct prop, right? Well yes
and no. For the health and longevity of the engine it's okay, but for
overall performance, you want to run a propeller near the top of the
range. Well how do you change the size to get that?
Remember this rule of thumb: For every 2" of propeller pitch, rpm's
will change approximately 400 rpm's. As you drop in pitch, rpm's
increase, and as you go up in pitch, rpm's decrease. So if we want to
raise our rpm's we should choose a lower pitch prop. Following the
rule, going down to a 19 pitch should raise our rpm's to about 5300.
That would help the hole shot and also gain us some more speed.
As you follow the rule of thumb, remember that sometimes rules are
meant to be broken. This is not an iron clad rule, sometimes varying 2"
of pitch will only change rpm's by 200. Switching prop brands or
manufacturer, it can sometimes throw off the rule too.
At this point the most important thing to understand is that less pitch
means more rpm's, more pitch means less rpms. For your boat to perform
at its peak, you need to run at the upper end of your engine's
recommended rpm range. That's it. Once you're there and still want more
speed, handling, lift, etc, that's when choosing the right propeller
brand comes into play.
Here's a simple adjustment you can make to improvement performance
under varying boatloads: If you normally run with one other person on
board, use that boatload as your measurement when determining your wide
open throttle rpm's. But if you sometimes load the boat with 2-5 more
people or try pulling a water skier or tube, get another prop 2" of
pitch lower than your normal prop. It will compensate for added weight
and drag and make a huge improvement on performance in these
circumstances. But remember to keep an eye on you rpm's when running a
lower pitch for these circumstances.
Most props have the size marked somewhere on the prop, either on the
barrel, on the blades, or in the front or back of the propeller. Use
that pitch that you're currently running as your starting point. If
it's already running correctly then that pitch is your pitch.
The
decision between purchasing an aluminum or stainless steel propeller
can be difficult. However if you fully understand the differences
between them, your decision can be much easier.
The most common understanding of the difference is that stainless steel
propellers perform better than aluminum propellers. This is only half
true. Material (Stainless vs Aluminum) only accounts for approximately
10% of the actual performance of the propeller. The other 90% of
performance is in the blade design.
Well designed aluminum propellers will out perform an average stainless
steel propeller. So it's important to research a propeller's
performance before your purchase. It is also important to try different
pitches and brands to get your boat dialed in to the best possible
propeller.
The advantage of a stainless steel propeller over aluminum is
durability. Stainless props can withstand more of the damage caused by
small rocks, sand, or other loose objects in the water.
However, the disadvantage to a stainless propeller is that there is
“minimal give" to the blades, so if you hit an object hard enough,
there is a possibility of causing major damage to your lower unit is
increased greatly. With aluminum props, the blades will most likely
sacrifice themselves before any damage is caused to your lower unit.
If you run in deep or familiar waters, or in salt/brackish water, a
stainless steel propeller can be a good choice. But it is important to
always carry a spare propeller on board, and a spare aluminum is an
inexpensive choice.
Continuing
to use a damaged propeller can cause damage to the gearcase and
ultimately the power head of your motor. When a propeller has sustained
damage, it will not spin true, thus creating excessive vibrations that
will transfer all the way into your motor.
A
damaged propeller will also not perform as well as a new propeller
because the damaged parts of the blade can degrade hydrodynamic
efficiency.
Propeller
pitch is the theoretical forward movement of a propeller for one
revolution, assuming that there is no prop slip. For example, a 21
pitch propeller will theoretically move 21 inches for every revolution.
Propeller slip occurs with every propeller, but the amount of slip
varies depending on propeller design. More aggressively and efficiently
designed propellers will slip less.
When
selecting a propeller pitch for your boat, it is important that the
propeller runs at the upper end of your engines wide-open-throttle RPM
range. If you want your RPM's to increase, go down in pitch. To
decrease RPM's, go up in pitch. As a general guide, for every 2" of
pitch, RPM's will change approximately 400 RPM's.
For water sports or extra people on board, you should generally drop 2"
of pitch to help compensate for the added weight and drag on your boat.
It makes a noticeable difference in your boat's hole shot, fuel
efficiency, RPM's, and overall performance. You should ALWAYS carry a
spare propeller on board, and if you're into water sports or
occasionally load the boat with extra people, a spare prop with a
lesser pitch is a good idea. When thinking a propeller pitch, compare
it to a gear on car - lower gear, higher RPM's.
The same pitch from different manufacturers will run slightly different
RPM's due to a difference in blade design. Speed differences among the
same pitch from various manufacturers will vary even more. For example
a 17 pitch from one brand could run up to 7 mph faster than a 17 pitch
from another brand.
Pitch is the 2nd two digits that are listed in a propeller item
description (14-1/4 x 19, 19 is the pitch, 14-1/4 is the diameter)
Propeller
diameter is distance across the imaginary circle that a spinning
propeller makes. It can be easily determined by measuring the distance
from the center of the hub to the tip of one of the blades and
multiplying that number by 2.
Diameters between
different propeller manufacturers may vary slightly. For example for a
V6 outboard application, Turning Point uses a 14-1/4" diameter where a
different manufacturer may use a 14-1/2" diameter. This small variance
does not affect your performance as much as the pitch and overall
design. However, larger diameter propellers - 15" or 16" diameter - are
designed for larger boat applications - 23 ft +, and are not ideal for
15" - 22" foot run-a-bouts. These larger boats need more blade area to
push more water.
Propeller
rake is the degree that a propeller blade is angled in relation to the
hub. Props with higher rake typically have better speeds and greater
lift. The better performing propellers typically have between 20 - 30
degree rake angles.
Propeller
cupping is the curved lip at the trailing edge and/or tip of the
propeller. Cupping helps the propeller to get a better grip in the
water for better holding at higher trim and on turns. Cupping also
increases the efficiency of a propeller and can result in higher top
end speeds when properly designed.
Vent
holes are holes behind each blade designed to aid in the hole shot of a
propeller. When accelerating from neutral, these holes allow some
exhaust bubbles to flow through and flood the blades. The prop is then
spinning through more turbulent water, thus gaining RPM's and speed
more quickly. After approximately 1000 RPM's, the exhaust flows rapidly
enough that is will bypass the holes and flow through the hub,
eliminating any further slippage from the holes.
This performance feature can only be found in stainless steel
propellers. However Turning Point is the only one to offer this feature
in aluminum propellers as well.
CLICK HERE TO USE THE PROP WIZARD TO HELP FIND THE RIGHT BOAT PROPELLER!
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