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From: (no name) (no email)
Date: Sun Feb 17 2008 - 23:24:57 EST
When we discussed the power required for a 50' trawler, we neglected to
discuss the efficiency of the propeller. That makes all the difference between
requiring a 100 hp engine and a 200 hp engine.
The weather has been miserable in the Northeast recently and I have a lot of
time on my hands. Just for fun I calculated prop specifications for my Willard
using a range of engine and speed variables. For the 13 years I've owned the
boat, I've kept accurate logs of trip speed and fuel consumption so I have
good baseline information.
The usual screw propeller is not a very efficient device, On most trawlers
the propeller efficiency ranges from 30% to 50%. Less than half the engine power
is used to move the boat forward. The rest is wasted in churning the water
and (very slightly) heating the ocean. Most formulas used to specify boat
propellers make an arbitrary generous assumption of 50% efficiency. In many cases
the actual efficiency is a good deal lower. Of course listees remember the 40'
Nordhavn circumnavigation where the boat almost ran out of fuel on the first
leg to Hawaii. Fuel consumption was estimated on the basis of a 50% efficient
prop. In actuality the prop efficiency was closer to 40% and the boat used
considerably more fuel than anticipated.
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.
I hope the weather gets better so you won't have to put up with any more of
this drivel.
Larry Z
**************
Ideas to please picky eaters. Watch video on AOL Living.
(http://living.aol.com/video/how-to-please-your-picky-eater/rachel-campos-duffy/
2050827?NCID=aolcmp00300000002598)
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