Hi Gardner,
It's bad but not a total conundrum. My calcs indicate rudder shaft at the lowest bearing
will need to be about 4" DIA solid aluminum. The shaft will taper by 30% to the upper
bearing. It also tapers significantly from lower bearing into the rudder itself, down to
about 1" DIA. That will allow larger bearings to disperse the loads into the beam and take
most the predicted load. If you broach badly or have excessive rudder angle obove 15
knots you could bend the shaft. Prefer that to carbon which may break off altogether.
You are right the Visionarry's are running with less rudder. Mine will be 1 meter
submerged by .5 meter chord or about 5.5 square feet each. The risk may be slow speed
manuevers. I am starting with an endplate on the bottom, which acts as a wingtip. That
changes the effective aspect ratio of the rudder by reducing the vortex shedding off the
end. Same as done on aircraft to improve efficiency. That should work well with keeping
draft low.
It has been suggested to add minikeels especially if you want to beach and dry out. That
will help leeway management. The ONLY consideration is maneuverability at low speed.
Working both rudders can help. THat small size you might want to add the end plates as
well. Also of issue might be rudder control in a fast following sea. Rudders too small and
broach is more likely. You won't be able to stop it if both rudders are losing effectiveness
and size is too small.
I will also add stops to keep the rudder from excessive turning. It will be one extra step
on shunts to release the stop but should only take 20-30 seconds. Have to keep me from
being stupid.
SEnt an email to Kevin Plank Re-e-power. He says they have sorted out a bunch of issues
with controllers and prop. He says he can nearly guarantee running two Epod 3000 @ 40 -
60 amps (48V) each to deliver 7-8 knots cruise speed. (5.7 KW per hour) . That's really
good as it makes genset smaller, batteries fewer, etc. Their early efforts needed controller
cooling and a reprop as they were over torquing the motor with excessive field current and
not achieving their desired RPM range (up to 2000rpm) They were only getting 600 rpm
out and burning up the controllers. THat's good news for those who want to try electric.
JT
--- In harryproa@yahoogrou
>
> Hi,
> This post prompted me to go back to "The nature of boats" and try to get
> some general idea of size, force, etc. From his rules of thumb, I am
> guessing the rudder size for a 50' harryproa should be about 6 sq ft (per
> rudder). The table for the rudder post for a rudder of that size, sailing at
> 15 kts is 3" solid bronze shaft. This is assuming the rudder is right below
> the hull. With a 3' seperation between the support and the rudder, I can't
> even imagine how strong that rudder post would need to be. Based on the
> difference between a rudder support on both top and bottom, vs just top
> support, it would seem that you would need at least the strength of a 6"
> diameter stainless steel solid shaft. And this doesn't even discuss the
> strength of the bearings or other structure by the beam. I would bet that
> this will pass more stress to the beam than supporting the windward hull
> does.
>
> So, let me see if I can take this discussion is a completely different and
> non-harryproa direction. What if we actually put the rudder under the lee
> hull? My 50' design draws about 1.5'. If I let the draft increase to 3.5'
> that gives me a 2' high rudder, but I can move it back at least 5' from the
> beams, so maybe I can get away with a 2' square rudder.
>
> To protect the rudder, put a mini-keel in front of it when using the rudder.
> Since the forward rudder isn't used, have a centerboard case for each
> rudder. Once you shunt, just pull the forward rudder up into the case to
> protect it in case of collision. Have a couple (or 3) asymetrical mini-keels
> to protect the rudder and provide some lift.
>
> The mini-keels should be just glued on. My Maine Cat 30 had its mini-keels
> glued on iwth 5200, so that in a severe impact, they would come off, instead
> of rippign out the bottom of the boat.
>
> Disadvantages: increased draft, more trouble raising and lowering rudders
> when shunting, possibly losing a rudder if you forget to raise it, potential
> leak if rudder is hit and breaks the centerboard case.
>
> Advantages: much, much less stress on the rudder. No worries abotu breaking
> if tiller is thrown over when sailing fast. Simpler rudder mechanism.
>
> I really like the idea of a 50' boat that only draws 18", but I could live
> with 3.5' of draft.
>
> More thoughts?
>
> - Gardner
> York, PA
>
> On Thu, Nov 6, 2008 at 10:50 AM, jjtctaylor <jtaylor412@
>
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