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To: |
lindsey@xxxxxxxxxxx |
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Subject: |
Re: Rudder rip-off and other advice for builders |
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From: |
BDally6107@xxxxxxx |
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Date: |
Mon, 10 Sep 2007 11:56:37 EDT |
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Full-name: |
BDally6107 |
In a message dated 9/9/2007 10:03:19 P.M. Pacific Daylight Time,
lindsey@xxxxxxxxxxx writes:
The rudder had completely ripped off the boat. See the attached pictures.
I think it is fairly obvious what happened. A strong bending moment bent the
pin which lead to the upper gudgeon block pulling away from the bar. I think
this lead to the lower end of the pin bending and the whole works falling
off.
Dave,
I too suspect that the pintle worked it's way up and out of the lower
gudgeon. On the other hand, I believe your analysis is also correct regarding
the
need for the built-in cassette bushings to be close to the gudgeons to
minimize the free span (and bending moment) on the pintle. From a structural
standpoint, the bushings should be as far apart as possible--flush with the
top
and bottom of the cassette.
Oh, one final point; I used screw (4) to attach the gudgeon to the bar. I'm
glad I did. I very purposely wanted this to be a failure point vs. breaking
the bar. Now I question if the rudder box should be designed to pivot with
a shear pin. Comments?
001, 002, and 013 have a very high combined sailing time in a variety of
conditions. To my knowledge, this has never been a problem on any of these
"high usage boats". Remember, the centerboard is going to hit any obstacles
in
the water before the rudder. The one designed failure point that I do believe
in is the tiller. If you fall on it, it should break before the transom bar
and before you tear out the lower gudgeon.
I have a bit of advice to everyone building their first Swift (this is
directed at all builders): "Try to stick with the systems shown in the
manuals or
the revisions shown on the website (and errata) until you have some time in
the boat". I haven't talked much about all of the failures I've had in the
process of developing the current systems and methods used on the Swift. I
now see others trying systems and methods that I've tried and abandoned in the
past. My own recent attempt to develop a better spinnaker sheeting system
is a good example. After trying two "alternative" systems, I'm back to the
old one and can't tell you how much easier the boat is to sail (gybe) again.
If I'd installed one of these other systems in the beginning, I would never
have realized how truly bad they were and instead, simply thought that the
boat was difficult to gybe. I'm on design failure number 8 or 9 on trapeze
systems. There is very little carry-over from double handed boats to the
Swift.
This is a new world in which everything has to work flawlessly because
there is no crew to deal with problems. Exercise some skepticism when sailors
from other classes offer ideas.
On the other hand, it is my hope that everyone will try to improve the Swift
with new innovations. I have no illusions about my designs withstanding the
test on a hundred minds working on better ways. However, without the
standard baseline, you will have no point of reference from which to judge
your
ideas. Your latest "improvement" may be relative to a really bad system that
you had installed in the first place.
This advice is an attempt to save each builder time. For the most part,
changing systems after completion is not a problem (I am indeed an experienced
expert on changing failed systems / ideas).
I will be writing a PDF soon to show the process of installing new backing
plates on a completed boat (it is really very simple). This should minimize
fear that you may have regarding the inability to change things once the boat
is done.
Dave, with regard to your cassette problem, I'm sorry that the PDF on how to
make the thing was not completed until you had finished yours. If you
decide to build a new one, I have the new foam cassette and tiller blanks for
you.
Best regards,
Bram
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