Boating and Sailing.pdf - Moja ladja

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Part 1: Getting Started in Boating
Gay Blades—And Serious Ones
The most obvious difference between one prop and another is the number of blades. The
blades are the part that does the work, and the number, size, angle, and shape of the
blades determine how the prop performs. The more blades a prop has, the less vibration
there is. That’s why a complete disc would be vibration-free—but it wouldn’t move your
boat an inch!
Here’s a look at the characteristics of some popular designs:
◆ Two-blade props have the least resistance but the highest vibration. They are used
mostly for smaller outboards and electric trolling motors.
Boat Bytes
Blades on performance
props are usually “cupped,”
which means the tips
and the back edges are
slightly concave. This helps
the blade grip the water, particularly
on takeoff, at high trim and
in sharp turns.
Barrel Onward
◆ Three-blade props are the most popular design,
offering good performance with moderate vibration
at a reasonable cost.
◆ Four-blade props have very low vibration, good
takeoff, and good performance, at a higher cost.
They’re a bit slower at top end, too.
◆ Five-blade props have the lowest vibration,
excellent takeoff, and some reduction on top-end
speed, at a very high cost.
The barrel or hub is the central structure of the prop, the cylinder to which the blades
attach. Most props on outboards and stern drives feature a “through-hub” design. The
hub is large and hollow, and the exhaust from the motor passes through the center. It’s a
convenient spot to let the gases exit, and because they come out underwater, there’s a considerable
muffling effect.
Here’s a closer look at the parts of the propeller, which are shown in the following figure:
◆ Blade. Thin, curved sections that grip the water as the prop rotates.
◆ Hub. The central barrel to which the blades attach.
◆ Prop nut. The nut that holds the prop to the prop shaft.
◆ Prop shaft. The stainless-steel rod that couples the prop to the drive shaft and
gears.
◆ Diameter. The width, in inches, of a circle made by a blade tip in a complete
rotation.