Boating and Sailing.pdf - Moja ladja

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Part 1: Getting Started in Boating
Boat Bytes
Boat props turn at only
about half the speed of the
engine rpms due to gears
where the propshaft meets
the drive shaft. The 2:1
gears cut the prop rotation speed
but increase the torque or twisting
force. More powerful
engines sometimes have lower
gear ratios of around 1.85:1,
and some racing engines are
direct 1:1 drive.
This is also true in relation to the number of blades. A
three-blade prop can be pitched a little higher than a
four-blade prop used on the same boat/motor combination,
because the four-blade has more gripping surface
to “load” the motor.
Propping is a science, and it’s best left to a dealer who
has plenty of props to play with and is willing to do the
job right for you. Particularly in performance boats, the
wrong prop can not only cut into speed, but also make
a given boat difficult or even downright dangerous to
drive, while the handling problems may go away completely
with the right prop.
Bet You Didn’t Know
How do you swap props? Props are held in place by a prop nut, which is sometimes
secured by a cotter pin. To remove the prop, you first straighten the bent end of the cotter
pin, and then remove it by inserting a flat screwdriver or punch into the rounded end
and tapping lightly until it slides free.
When you try to take off the prop nut, you’ll find that nut, prop, and prop shaft all turn
happily together—there’s no resistance to allow you to torque off the nut.
To secure things, place a pine 2 × 4 between one of the prop blades and the cavitation
plate of the motor. This holds things in place so you can get the nut off. The nut
secures a washer and a spacer—make sure to set them aside in the order they came
off so you don’t get confused later.
The prop should slide off easily once all the fasteners are removed. You may need to
lightly tap the prop with a rubber hammer to get started, especially if it hasn’t been
removed in a long time. The prop can’t be twisted off the shaft—it’s splined in place, so
you have to pull straight out.
Before you put on a new prop, rub a light coating of waterproof grease on the splines
of the prop shaft. This will help the prop to come off more easily next time.
Replace the spacer and washer, grease the nut, and reinstall it to torque specifications,
again using the 2 × 4 to hold the prop in place. And don’t forget to use a new cotter
pin, and only one made of stainless steel. Many a $400 prop has been lost due to use
of an old 25¢ pin.