Gardner,
All of us completely underestimate the rudder forces. No leeboards
or other restriction to maintain direction except for hull shape and
rudder make a big problem. The lenght of shaft, distance to bearings
and pivot points or knuckes really make HUGE loads. With the rudders
down and a broach into 10knots boat speed and load on the bearings
will reach 30000 pounds easily. Structure on Rudi's rudder were
beefed up but he still takes it easy. Turn the rudder so it acts as
a brake, 90 deg to travel, above 10 knots and break it may.
Making a pivot below waterline and we'll just twist the rudder shaft
till it either fails on torsion or the linkages to the wheel fail
which are even more weak than rudder and bearings.
Using the rudders for leeboards and directional control presents a
whole set of reliability problems that none of us have really sorted
out. Mathematically it looks bad, but we don't know how well any of
these HP proas will track in rougher seas from hull alone. Need to
solve our first robustness problems before venturing into alternative
solutions. I have contracted the same engineering group for Rudi's
Blind Date design to sort out the rudder issues using FEA and other
modeling methods. From that may be able to tell what happens at
speed in another config. Base design is not yet obvious, except for
modest conditions.
For now recommend to take Rudi's guidance and take it easy with the
rudders. Wait till the FEA models determine likelihood for success
pivoting below waterline or pivoting at all.
JT
--- In harryproa@yahoogrou
wrote:
>
> I once owned a one-tonne IOR racing boat from the 1970s, from a
designer
> named Carter. He was known for his swing keels. We had 7000 lbs of
lead that
> gave us a 7' draft when it was down, and a 4.5' draft when it was
up. The
> really neat thing about it was that it was a hydrofoil in both
positions.
> The reason I bring this up, is that it seems like the same could be
done
> with a rudder. If we took the Blind Date rudder, which is about 5'
by 1.5'
> and hinged it where the rudder post attaches inside the rudder,
about 17-20%
> back from the top and leading edge, it could be used as a deep
draft rudder
> normally, then flipped up (manually or by a grounding) to a 1.5'
draft with
> the same square footage steering surface.
>
> By having the hinge below the water line, we could support the
rudder post
> in 2 places.. the beam and a strut out from the hull.
>
> If we are worried about surface level impacts, the hull strut could
decouple
> and the shaft could be hinged again up higher, near the beam.
>
> It seems that this could be simple to construct, would not have to
be as
> strong (since it has support below the beam) and provides alot of
rudder
> surface at low speed in shallow water for shunting.
>
> Anyone think this idea might be practical?
>
> - Gardner
>
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