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From: (no name) (no email)
Date: Tue Mar 04 2008 - 17:39:20 EST
Raining and cold in the Hudson Valley so another jeremiad. This time on
propellers.
Compared to most of the power transmission devices we are familiar with, a
propeller is comparatively inefficient, often losing more than half of the
available engine power to slip, turbulence, blade drag, friction, etc. Although a
well designed boat propulsion system can be as much as 60% efficient, most are
in the 40 to 50% range.
The maximum practical speed that a displacement hull can reach in knots is
the "hull" speed. This is generally defined as 1.34 times the square root of the
waterline length in feet. It takes very little power to move a displacement
boat at slow speeds but the required power increases as the cube of the speed
increase until the hull speed is reached. At that point the power requirements
increase even more sharply and few true displacement boats can carry enough
power to exceed hull speed by more than a few knots.
Contrary to the popular assumption, a propeller does not "screw" into the
water. A propeller moves a boat by accelerating a mass of water backward. In
accordance with Newton's law of action and reaction the boat moves forward. For
maximum efficiency, the water mass should be as large as possible and it should
move backward at 1.2 to 1.5 the boat's forward speed. Thus the recipe for
maximizing thrust and increasing efficiency is to use a large slow turning
propeller with sufficient pitch to achieve hull speed at the desired RPM. A smaller,
faster turning propeller will move an equivalent mass of water backward but
with lower efficiency.
As far as props themselves go, for displacement boats, it is the blade area
that counts. It makes little difference in power transmission if it is a three
or four blade prop. A four blader will have less vibration and can be slightly
smaller in diameter (say 5%) for the same performance under power but is a
bit less efficient. The pitch and diameter of the prop are interrelated. By and
large, the diameter should be as large as practical, allowing for a minimum
three inch clearance between the prop and all hull structures. It is the prop's
diameter that primarily determines efficiency. The pitch should be selected so
that the engine, if a diesel, is turning about 80 to 85% of maximum
continuous RPM at hull speed. This assumes adequate engine power at that RPM to reach
hull speed.
Dave Gerr's "Propeller Handbook" gives a variety of methods for calculation
of the best propeller for a given boat but requires some degree of mathematical
sophistication and complicated measurements for the best results. Skene's
"Elements of Design" gives a simpler and perhaps less precise technique but
adequate for most uses.
There is always a lot of discussion on T&T every time the subject of
controllable pitch props comes up. For me, the major advantage of the CPP is its
adaptability to various boating regimes. It offers another way of moderating boat
speed and thrust in addition to varying engine speed. Nothing comes free and
the downside is reduced efficiency and generally higher costs than a
conventional prop. For most trawlers, a CPP would be of only minor benefit. Trawlers tend
to travel at relatively constant speeds. The difference in weight between a
lightly loaded boat and one with filled tanks and larder is rarely more than
15%. Most of us would be better off with a fixed prop sized for maximum
efficiency at our typical cruising speed and tolerate the slightly lower efficiency at
higher and lower speeds.
The possible exception to this rule would be for owners of semi-displacement
trawlers with high power engines. The specifications for the "ideal" prop for
displacement speeds and those for the "ideal" high speed prop are so different
that a CPP might be an asset. In the past the loss in efficiency in one
regime or the other was largely ignored. However, in this era of high priced fuel
some owners might reconsider the decision.
I don't own a boat with a CPP now although I have owned one in the past. And
I have used boats equipped with CPPs frequently at my wife's family home in
Norway. The Scandinavian predilection for CPPs that designer Michael Kasten
cites has both a historical and practical reason. The fishing industry in Norway
and Sweden is a much larger portion of the boating market than in the US. Most
makers offered CPPs because of their practical advantages in handling load
variation. Engines were generally less powerful and squeezing the last ounce of
thrust out of the prop when dragging nets made sense. CPP equipment is
available at competitive prices. While Kasten is correct in saying that large
commercial ships in Scandinavia use CPPs, they usually don't do it for fuel savings.
The Chief Engineer of the Hurtigruten, Norway's coastal ferry system told me
that the main reason for the use of dual CPPs on the 500 to 600 ft. ferries was
maneuverability in harbors built for smaller vessels. With bow thrusters and
dual CPPs the big boats can squeeze into the smallest of harbors. He also said
that CPPs require more maintenance than conventional propellers.
In fact, I wish I owned a CPP now. I have an older Willard Horizon
motorsailer, powered by a venerable Perkiness 4-107 swinging an 18" x 14" prop. This
prop is ideal for cruising under power. But since I enjoy sailing in favorable
winds with the engine off, the water resistance of the fixed prop is equivalent
to dragging an anchor. It knocks 2 or 3 knots off the already marginal sailing
performance of the Willard. Feathering the CPC would help.
Larry Z
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