That bulge looks like an offgassing bulge to me. That would be caused by
chemical breakdown of the foam itself which can happen under heat and
especially if there is an incomplete cure of the epoxy holding the stringers
in, or on the interior layer of the layup.
But a bubble that big would not be caused by stress load failures. I've
broken rudders, skegs and daggerboards before. They don't fail gradually
when loaded, they snap. Bubbles are gradual delams.
So as much as I hate to point the finger at the builder, that's where I
think the problem lies.
An easy way to do the I-Beam CF stringers is to run the core on the table
saw and then glue the pieces back together under vertical pressure after
wrapping a strip of CF on each side of the "I-Beam". To get alignment you
can either put it back into the outer shell, or you can prefit 2 pegs
longitudinally and let those align the merge.
_____
From: Mark White [mailto:mwhite4459@xxxxxxxxx]
Sent: Thursday, November 13, 2008 4:37 PM
To: bdally6107@xxxxxxx
Cc: cbull@xxxxxxxxxxxxxxxxx; swiftsolo@xxxxxxxxxxxx
Subject: Re: failing centerboard
It was Greg Ryan's idea for carbon stringers, not mine. I was going with
wood origionaly, and thinking about it, my say with wood. The carbon
stringers will be more work than the wooden counterparts.
I don't think point loading is a factor here, the bulge is 4" or so lower
than where it exits the bottom block.
Another possibility why Bram's have lasted longer, may be because he handles
the boat better putting less stress on everything, unlike myself.
Mark
On Thu, Nov 13, 2008 at 4:42 PM, <bdally6107@xxxxxxx> wrote:
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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