"jlrogers±³©" wrote in message
...
"Wilbur Hubbard" wrote in message
anews.com...
What is pitch?
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)
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What is 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.
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What is rake?
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.
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What is cupping?
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.
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What are vent holes?
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.
Thanks, but I guess I'm more of a sailor than motor head like most of the
people posting here. To a real sailor pitch is something that happens when
the bow and stern of a boat goes up and down at an angle. Propeller pitch,
while referring to the "set angle" of the blades can also mean the angle of
the centerline of the shaft from the horizontal. See defintion 8 below.
2pitch
vb 1 : to erect and fix firmly in place ~ a tent 2 : throw, fling 3
: to deliver a baseball to a batter 4 : to toss (as coins) toward a mark 5 :
to set at a particular level ~ the voice low 6 : to fall headlong 7 : to
have the front end (as of a ship) alternately plunge and rise 8 : to incline
downward : slope
Wilbur Hubbard