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From: (no name) (no email)
Date: Wed Oct 11 2006 - 15:20:03 EDT
In a message dated 10/11/06 12:00:59 AM, Terry asks:
> Does anyone have any before and after data on what speed and fuel
> consumption they got a given RPM prior to installing Active Fins VS after
> installation what speed and fuel consumption they got at the same RPM?
There
> is a price to pay in economy as you are creating drag, I would like to know
> how much that price is before I make a decision on installing them.
>
> For those slower trawler folks that have them........Are they still
> effective when you are only making 6 knots speed through the water? I am
> not talking about 6 knots speed over ground. The Stabilizer only knows the
> speed at which they pass through the water. The lift is a function of the
> speed squared and if the speed is too low to create the needed lift then
you
> put larger fins on. This creates more drag and less economy.
>
All roll stabilizing systems except sails and roll tanks take some power from
the forward motion of the boat. The power is used to create a counter force
to the roll. With active fins, the amount of drag is a conjoint function of
fin
size, boat speed, and deflection. angle. So the answer to your question is
"it all depends." In light waves, where the rolling forces are slight, the fin
deflection can be relatively small and the drag is comparatively low. Nordhavn
estimated that it knocked about a quarter of a knot off the boat speed during
their circumnavigation. For a Nordhavn 40 moving at a speed of 7 knots, the
additional drag would amount to about 4 horsepower. Active stabilization in
extreme conditions can cost up to 10% in fuel consumption. But you need
stabilization only a fraction of the time and the additional fuel consumption
is worth
the comfort that stabilization brings.
As far as the minimum speed, Naiad suggests that 6 knots is at the lower
limit for active stabilization. The counterforce that a fin exerts at a given
deflection is a function of the square of the boat speed. A little bit faster
gives you a lot more force. At slow speeds, you need big fins and a lot of
deflection to stop a boat from rolling.
You can't fool Mother Nature.
Larry Z
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