Boating and Sailing.pdf - Moja ladja

Chapter 3: Picking a Proper Prop
If there were no slip, a prop would move forward a distance exactly equal to its pitch in
one revolution. Pitches for V6 outboard props typically range from 15 to 25 inches.
Smaller motors will require lower pitches, down to as little as four inches for a 2-horse.
Putting It Together
The diameter is always listed first in prop designation. A 14 × 17–inch prop would have a
14-inch diameter and a 17-inch pitch. A prop of these dimensions is very common on
runabouts in the 17- or 18-foot range, powered with a 120- to 150-horse outboard. The
pitch, and sometimes the diameter, is stamped on the prop’s barrel.
The higher the pitch, the faster the boat can travel for a given rpm if power is unlimited.
But power is always limited, so the pitch must be matched to the weight and power of the
boat/motor combination.
The right combo when the boat has five gallons of fuel and one passenger might be completely
overwhelmed when there are 500 pounds of gasoline (gas weighs around 6 pounds
per gallon), 800 pounds of people, and 500 pounds of ice, food, and gear added. It’s a
commonly made mistake when setting up boats. Instead, you want to select a prop that
allows the motor to run in its peak horsepower range at full throttle, usually somewhere
around 5,000 to 5,800 rpms for large and mid-size outboards, 4,500 to 5,000 for stern
drives and inboards, with its typical load.
An inch of pitch increases or decreases rpms about 150 to 250 rpms. But props are offered
by most manufacturers in only 2-inch increments, which vary rpms 300 to 500 rpms.
So, if your rig turns up 5,000 rpms with a
19-inch-pitch prop and the motor is rated for
5,500, you might drop down to a 17-inch to
get it running at the max.
The lower pitch would also give better takeoffs
or “hole shots”—the hole being the hole left in
the water when a planing boat takes off. The stern
squats and there’s a big hole pushed down for a
moment before the hull squirts forward like a cake
of soap to race along on top.
When One Goes Up, the Other Goes Down
Boat Bytes
Larger-diameter props drop
rpms and increase holding
power and lift, but the
diameters are limited by
the space between the
prop shaft and the anti-ventilation
plate on outboards and stern
drives, so there’s not much variety
offered.
For a given boat/motor package, the prop diameter and pitch are inversely related: The
larger the diameter, the lower the pitch; the smaller the diameter, the higher the pitch
needed for good performance.
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