|
|
|
|
|||||
|
||||||
Subject: TWL: Re: Propeller specification
LRZeitlin@XXX.XXX
Date: Sun Aug 04 2002 - 10:52:17 EDT
In a message dated 8/4/02 12:02:50 AM,
trawler-world-list-request@XXX.XXX writes:
<< 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.
>>
Fernando,
You are right. A 32" prop is inefficient. But you have overestimated the
power required to move your boat at hull speed. It is more like 150 hp rather
than 210 hp. I ran a couple of propeller calculation programs using the specs
you provided and the optimum prop at an 80% of continuous rpm rating for the
engine you have comes out to be 38" diameter and 30" pitch. The efficiency
will be about 45%. If you increase the prop diameter to 43" and reduce the
pitch to 26", your efficiency will be 51%. Both efficiencies are about par
for trawler power installations. You should have at least 6" clearance
between the prop and any portion of the hull structure.
Remember that propeller specification is still pretty much of a black art.
The above figures should put you within +- 5% of the right prop but you might
still have to tweak them a bit for maximum efficiency. If you do a lot of
cruising this might be important but otherwise all it will cost you is a few
extra gallons of fuel a year. Good lock on your ambitious project.
Larry Z
_______________________________________________
http://lists.samurai.com/mailman/listinfo/trawler-world-list
|