--- In harryproa@..., "Rob Denney"
<proa@i...> wrote:
>
>
> Testing with one engine on, both on and pivot trials should
> deterine where things are at. Maybe Rob & Mark know
already
> and they are secretely withholding the information, eh ?
>
> No secret, they do not work at the beam spacings we are
using. As to the point of balance, the boat sits pretty square to
the wind, but the rudders are so influential that you can make it
drift 30 degrees either side pretty easily.

Sir Rob,

Did some major math on on the current Visionarry in production.
My conclusions will pretty well match your experience as you
have described it.

The shape of the hull(s) above water is pretty much a square as
the sillouette is very similar at most angles to wind. My calc
estimates is at about 300 square feet. Mutiplied by wind force
and a standard factor of 0.75 (for rounded edges) results in 225
pounds of force at 16 knots wind and 350 pounds of force at 20
knots.

That's a lot of force and similar to experiences of other multihulls
with bows blown off course as low speeds.

But you have two BIG oars in the water. I calculate the lift of a
rudder with 1 foot chord and 6 foot span (depth) using foilsim II.
At 5 knots each rudder at max lift (no stall) generate 500 pound
of force on the hull. So given that speed there is no reason why
the boat would not crab up to just short of the max input rudder
angle. The rudders should overcome a drift in winds up to 32
knots if you can move it along at 5 knots forward. Very
impressive.

At 3 knots forward which is getting closer to docking speeds the
rudders still generate 300 pounds of lift (each) which can handle
a 26 knot blow.

At 2 knots the Visionarry rudders are only effectively producing
130 pounds of lift, only able to hold it's own against a 16 knot
wind. Only 50 pounds of lift at 1 knot. A slow approach in a
breeze will require some seamanship and experience with
handling both rudders. Crabbing won't help in these situations.

A thruster could substitute for the function of the forward rudder
in slow docking manuevers in winds up to about 20 knots.
Certainly not essential, but if marinas are tight and forward
progess ala station keeping needed then perhaps an alternative.

At least these are some numbers to kick around and see how
well they match real world experience. I agree the spacing of the
OB's won't generate sufficient torque.

Regards,

JT
>
> regards,
>
> rob