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Subject: TWL: Re: Re: Trawler-world-list digest, Vol 5 #496 - 37 msgs
From: Bob Walters (bobandching@XXX.XXX)
Date: Fri Nov 01 2002 - 07:27:54 EST
> Larry Z posted:
>You need another calculator. (SNIP)
> You can't fool Father Neptune.
>
> Larry Z
> _______________________________________________
I would argue that it is you who are trying to fool Father Neptune.
Keith's formula does not give accurate hp predictions. In fact the
predictions it gives are routinely less than 70% of the true hp required in
the real world.
For example, I took a look at the data from the recent circumnavigation by a
Nordhavn 40. It was widly publicized and, as far as I know, the guys tried
their best to accurately log the data they were collecting.
Even then, if you read the PR stuff you'll get out of the paragraph where
they make mention of a run in favorable conditions (read strong favorable
current) and then quote a really nice sounding fuel burn without compairing
it with their true average. It's really hard to resist the urge to
fudge.<g>
However, picking through some of the daily logged data, I came up with some
figures that I believe represent some real world conditions, i.e. nice
weather but no unusual currents.
Using Nordhavn's data for a repersentitive 24 hour run at 6.29 knots Keith's
formula would predict a fuel burn of 1.435 gph while the actual burn was
2.08 gph. The formula I normally use predicts a burn of 2.045, which is
within a couple percent of what the Nordhavn actually burned.
You are correct when you say you can't fool Father Neptune.
I refer you back to my uncle Bill who's '52 Ford was so well built that he
claimed he actually had to remove oil between changes.<g>
As I've said before, there are plenty of folks who take their peak GPS SOG
reading for the day (let's say 8 knots) and claim that they're "cruising at
8 knots". Chances are they're cruising at less than 7.
Then they take their engine operating hours from the hour meter and divide
it by their latest fuel purchase and come up with a GPH figure.
The next step is simple. Divide 8 by the latest GPH calculation and head
straight to the bar to brag about getting fantastic mileage.
I find it interesting that often when someone posts fuel burn figures based
on a carefully calibrated fuel flow meter using real average speeds, etc.
etc, the figures are significantly lower than what the salesmen and some
enthusiastic owners claim.
How can the difference be explained? Are some guys dragging a bucket
around?
I stand by my statement that no KK 42 can average 7.2 knots using 38 hp.
Tossing around high efficiency fuel burn numbers is plenty of fun, but it
ain't necessarily the real world.
Sorry.
Bob Walters
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