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From: (no name) (no email)
Date: Thu Oct 12 2006 - 21:02:33 EDT
There are two direct "costs" with operating active fins.
One is the drag that they add to the boat both when locked and operating.
Less of course when locked and more when operating and more even when they
are operating full on.
Two is the cost of the fuel to operate the pump that is driving them. Little
when they are locked but quite a few HP when the pump is working full on in
a bad seaway.
Trying to quantify the drag is quite difficult without tank tests. I am not
aware of published results for any in various sea states/speeds. Barring
this we can only try to guesstimate what it might be for various sea states
and speed. As you are probably aware sailboat racers will go to extremes to
fair their hulls to reduce drag. Adding two fins on either side in the water
flow must create a relatively large amount of drag. Turning them from side
to side to create lift increases that drag exponentially.
A boat that is instrumented with accurate speed & fuel burn equipment should
be able to see what the total effect of active fins "costs" in the way of
fuel burn and/or speed for any given sea state. These would be interesting
numbers to have if anyone has made such a table.
Other wise we are all stuck with "1/2 to 1 kt" or "maybe 1 GPH" or some
other non definitive number. I personally think the cost in fuel burn/speed
penalty is a lot more than the "estimates" we see from the manufactures when
you really need them in a contrary sea state.
The systems that were spec'd for Swan Song all had hydraulic pumps in the 15
to 20 HP range with double belts. The manufactures were skeptical, as I have
stated before, of the usefulness of these systems at 4-5 kts in rough seas
unless the fins were very big. As you know we didn't opt for active fins.
This much HP requires ~1 GPH and maybe more to deliver it by the time all
the losses are figured in. On a long cruise this 24 GPD adds up if you are
looking for range.
OTH, for a coastal cruiser this usually isn't an issue. But on an ocean
voyage it certainly would be as it affects range and the duration of your
passage.
Given a choice I'll take air conditioning and our no drag, no power
stabilization tank vs. active fins and being hot and sweaty with no AC :-)
YMMV
Dave & Nancy
Swan Song
I have been reading your posts and it sounds like your blinded by enthusiasm
for your flume tanks and it's causing to generate some pretty inacurate
information.
Brian Palmetto FL
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