Boating and Sailing.pdf - Moja ladja

Chapter 3: Picking a Proper Prop
positive reverse. With nearly twice the blade area of a single prop unit, these systems do
everything except top end speed better.
The systems work via a prop shaft within a prop shaft, allowing the counter-rotation off a
single main engine shaft. They cost more than conventional drive systems, but for those
who can afford it, the advantages are well worth the price.
These Props Are Not Shiftless
Most props are shiftless, sitting there without ambition, mindlessly doing the job designated
by their designers. But there are a few mechanical wonders that actually do show
some unique capabilities to shift for themselves.
Shifting props use moveable blades that change position based on centrifugal force to
adjust their pitch while underway. Why would you want this? Because boats, unlike cars,
don’t have shifting transmissions. The prop that starts you off well in low gear is somewhat
of a compromise when it comes to top speed.
Particularly in heavy or underpowered boats, it sometimes takes such a low pitch to start
planing that the engine over-revs after the boat planes off. (Remember, the lower the
pitch, the faster the motor can turn it.) The shifting props solve the problem. They start
out in a lower pitch, say 10 inches or so, and
then as prop speed builds up after the boat
jumps up on top, springs and cams force the
blades to rotate slightly and become more
aggressive, up to a pitch of 17 or 19 inches.
When one of these props shifts, it feels like an
automatic transmission in your car—a quick
surge and you’re on your way. As you slow down,
powerful springs rotate the blades back to the
lower starting position so that you’re ready for the
hole shot again.
Sounds great, huh? But these props are not for everyone. For one thing, a shifting prop
has dozens of moving parts, while a conventional fixed prop has one. Guess which prop
gives you more maintenance problems, breaks more easily, and costs more?
Not only that, but no shifting prop currently on the market can quite match the top-end
performance of a well-designed single-pitch when used in performance rigs.
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Boat Bytes
Which boats are most
likely to benefit from shifting
props? Usually those with a
minimal power-to-weight
ratio; that is, a small
engine relative to the weight of
the boat. Heavy stern drives are
the best candidates.