Boating and Sailing.pdf - Moja ladja

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Part 1: Getting Started in Boating
Diameter and Prop Performance
Just as the diameter of your waist determines whether you are a fast or slow runner, the
diameter of a prop determines its performance. The measure of prop diameter is the
width, in inches, of the circle made by the blade tips.
In general, large-diameter props grip the water better and are particularly effective in
low-speed maneuvering and in pushing heavy loads. Smaller-diameter props can “wind
up” faster, and are often good choices for high-performance boats because they go
through the water with less resistance.
Typical diameters on V6 outboard props range from 13 to 151 /2 inches. Typical diameters
for 10-horse props are around 8 inches, while 20- to 40-horse motors carry props with
diameters of 10 to 11 inches.
Here’s the Pitch
Diameter is only one part of the prop equation. The other is pitch, which is the distance a
prop would travel through water in one revolution if there were no slippage.
Boater-ese
Pitch is the term for the
distance a propeller would move
forward in one revolution if there
were no slip. Slippage or slip is
the term comparing the actual
forward motion of a propeller
through the water to the forward
motion it would have if it were
turning in a solid, like a screw
moving through wood.
If there were no slip, a prop
would move forward a distance
equal to its pitch in one
revolution.
A prop is a water screw, remember: If it were screwing
through wood, a 21-inch-pitch prop would travel forward
21 inches in one revolution due to the angle of the
blades. In water, because of slippage, it might only go 19
inches due to the slip in the liquid medium. In this case
there would be 2 inches of slip compared to 21 inches of
pitch, or 9.5 percent slip. A slip of 4 to 15 percent is
common in recreational powerboats.
Do not confuse slip with efficiency! A prop with no slip
(after correcting for camber and wake) produces no thrust
and therefore has an efficiency of 0. At the other extreme,
a prop with 100 percent slip is not moving and does no
useful work, so it, too, has an efficiency of 0.
Pitch
Diameter