Mark,
This is a good topic for discussion.?
Let me start by saying that neither the original CB or rudder that I made with
foam core have failed to date (4 or 5 years now).? The rudder was an experiment
in which I used an even lighter layup than the pdf.? That having been said,
enough of these things have failed that it has become fairly clear that there
are issues.
I've been?concerned from the beginning that we?should use spartite gaskets to
reduce point loading to a minimum but even that has not eliminated some of the
failures.? The newer CB's were 1/8" thinner which makes them 19% weaker (all
other things being equal) than the originals like mine.
Marks solution should be a good one.? Using carbon ribs will make the blade
substantially stronger because it will keep the two?skins in column and an
equal distance apart.? I would reccomend carving a?1/8" V?groove in the foam on
each side of each rib at the surface.? You will then fill that grove with
epoxy/silica when you coat the foam blank?immediately prior to lay-up.? Make
certain that the carbon ribs?were layed up with peel ply or at least rough?up
and clean the surface prior to insertion and lay-up of the blade.? On the
bottom of the ribs (well short of the bottom of the blade) cut a deep V-goove
in each rib.? This will eliminate the hard spot and point load at the end.?
Since the rib is about 1" wide, I'd suggest a V about 2" up.?
I am very surprised that Mark's blade did not fail immediately once it
bubbled.? When that happened it lost well over half its' strength.
Keep the discussion going if you have?new or different ideas.
Best regards,
Bram?
-----Original Message-----
From: Mark White <mwhite4459@xxxxxxxxx>
To: Chris Bull <cbull@xxxxxxxxxxxxxxxxx>; swiftsolo@xxxxxxxxxxxx
<swiftsolo@xxxxxxxxxxxx>
Sent: Thu, 13 Nov 2008 12:57 pm
Subject: failing centerboard
Chris wanted to see some pictures of a failing centerboard, so I
thought an open discussion might be in order, (since I am going to
make another).
This centerboard was made as per the instructions from Bram and or
Greg, to the best of my ability. I started using it in the 06 regatta
in FL, so it has seen 3 seasons. Before going to FL in 08, I removed
some surface scratches by filling and sanding back to a smooth and
fair finish. The first blister appeared 2 days before the 08 regatta,
and every time I sail, it gets larger, and now is on both sides. It
still works, but has become increasingly difficult to lower and raise.
You can see 1 large bulge on the starboard side, and 2 on the port
side. The origional rudder broke under heavy air just after the 07
regatta. The pictures of the foam core are the new rudder made just
before the 08 regatta, with wood strips inserted to help handle the
loads it incurs, (which I believe are enormous)
I'm not an engineer, but it seems hybrid and carbon cannot handle
compression very well, or maybe it's just a limited number of cycles
it can handle. I don't know if altitude has anything to do with it,
but these boards were made at 4500' msl. The fact that they are black
probably has a lot to do with failure, although I tried my best to
keep them out of direct sunlight. If you add heat to epoxy resin it
will soften.
I will build another centerboard with a foam core, but it will have
stringers (4 or 5) to help with the loads the skins incure. Hopefuly
it will last indefinitly, or at least longer than the first.
If anyone has any ideas on making foam cored boards last longer,
please chime in.
Mark
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