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
>
>
> IMG_7404.JPG
> IMG_7405.JPG
> 002.JPG
> 003.JPG
> 013.JPG
> 014.JPG
>
> These pictures were sent with Picasa, from Google.
> Try it out here: http://picasa.google.com/
>
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