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Subject: Subject: TWL: New boat Propeller Effciency
From: Jim Slocomb (jslocomb@XXX.XXX)
Date: Sun Aug 04 2002 - 12:00:44 EDT
Message: 1
From: "Fernando Rodrigues" <rodriguesipm@XXX.XXX>
To: trawler-world-list@XXX.XXX
Date: Sat, 03 Aug 2002 11:18:53 +0000
Subject: TWL: New boat Propeller Effciency
Fernando,
My numbers come in at 210 horsepower for 9.5 knots..... same as you. However
I didn't come up with reasonable efficiency till the reduction ratio was
between 3.5 and 4 to one swinging a 38 to 40 inch wheel. I would never use
an oversquare propellor on a displacement boat. I understand the key to
efficiency being heavily weighted toward large diameters and small pitches
turning slowly. I found Dave Gerr's book on the subject to be excellent.
Good luck
Jim Slocomb
40' Skookum
Friday Harbor, WA
Gents:
I am about to start the construction of a steel trawler. The boat will
displace 106,000 Lbs full load with a LWL of 50.42'. The designer has
specified an engine rated 330HP @XXX.XXX
However, I already have a brand new engine with 316HP @XXX.XXX
I have calculated the hull speed and I came up with 9.5 K max at 210 HP.
When I tried to figure out the maximum propeller efficiency, I tried to
matched it with the engine and suitable gear to improve economy at this
speed, my calculations always point to a 38" propeller with 2.2 to 2.4 :1
gear.
Am I wrong? Am I doing some mistake? It looks like no matter what engine, a
32" propeller is totally inefficient.
In a trawler do you see any concerns in having a propeller with the pitch
1.3 times bigger than the diameter?
Should I leave 6" above and below the propeller between hull and keel for
water flow?
Can you guys comment on this?
Your experience is very much appreciated
Thank you
Fernando
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