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From: (no name) (no email)
Date: Mon Feb 18 2008 - 07:05:36 EST
Hi Larry -
Thanks for an informative and interesting post on propeller efficiency. Your
posts always get me thinking.
The differences propeller speed and diameter make are quite substantial,
and although a 3-blade is more efficient than a 4, there is a downside. The
blades on a four are so much less prone to bending when colliding with flotsam
that a four is usually fitted to trawlers at the expense of efficiency.
With today's higher-speed engines, maximizing prop efficiency becomes
difficult because the increased size of a lower ratio reduction gear is difficult
to fit, and results in an increased shaft angle, which is also less
efficient. Fitting ideal larger, slower turning propeller also has a limit because
the minimum boat speed (at idle) becomes excessive. Sufficient aperture space
and blade tip immersion are also likely to limit prop size to something less
than ideal.
One question - is your Willard's propeller a 3 or a 4 blade?
Regards,
John
"Seahorse"
> My Willard Horizon is 30' LOA and has a waterline of 27.5 ft. It displaces
16,000 lbs. Power is provided by a venerable Perkins 4-107 driving an 18x14
prop
through a 2.57 BW Velvet Drive reduction gear. Hull speed is almost exactly
7
knots. The power required to reach this speed is 22.4 hp, a figure confirmed
by fuel consumption over several long cruises.
At an engine rpm of 2000 with my current 18x14 prop, the efficiency is
approximately 42% and the static thrust is 665.5 lbs. Maximum efficiency
with my
current setup would be realized with a 23x14.8 prop. With the engine turning
at
the same rpm the efficiency would be 54% and the static thrust would be 793
lbs. The five inches of additional diameter would gain me a 28% boost in
efficiency and would be the equivalent of adding a number of extra horses to
my hard
working engine. Alternately it would reduce my fuel consumption by about 25%.
If I should reduce the prop diameter to 16", the efficiency would drop to
38%
and the static thrust would be 612 lbs. If I should choose to swing a 30"
prop, using a 3.5 reduction ratio, the efficiency would rise to 61%. With a
36"
prop the efficiency would be 67%. I doubt that such a prop would fit on my
boat
but the general idea is clear.
For displacement boats we can see that prop efficiency is very dependent on
diameter. The engine should be turning the largest prop that will fit the
hull.
The gear ratio and prop pitch should be adjusted to achieve hull speed.
Theoretically is is possible to get up to 75% efficiency with very large,
very slow
turning props.
So the range of powers suggested for that 50' ft. trawler, from 100 to 200
hp, are correct. It all depends on the efficiency of the prop.
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