I've got centerboard/case issues

[Joe Nelson CS#35]

I built my centerboard a couple of years ago. During the storage, it took on a bit of a crook. Not much...just enough to bind in the case. I am taking down one side to the wood and will re-glass. Note to self...anyone building a centerboard in advance of putting it in the case should take care of how you store it!!!

My dilema is as follows.

After turning over the boat (upside down) for the first time, I discovered that the glass that coats the inside of the centerboard case that is suppose to wrap around the hull bottom had some voids under it in the intersection of the case bottom edge and the hull bottom. By the time I got all the glass out that had air pockets under it...well, we dont have much or any viable glass. My question is...how much of that glass is structural and how much is for abraision resistance? I could grind down my centerboard some more to accomdate some fiberglass tape to secure the centerboard insides to the hull bottom. Or I could enlarge the opening of the case slot to accomodate a new glass layer.

I found out how much abrasion resistance epoxy/glass has! I took a belt sander and ground down to wood my centerboard on one side. It took me over an hour!!! That glass is tough stuff!!!

The good news is that I have the cockpit and all inside surfaces painted. Just need to finish the hull exterior and paint the hull and decks. I am glassing the whole hull exterior but not the decks. Woven glass gives a lot of longitudinal strength to a hull and reduces long term stresses of oil canning. My heavier decks and seat tops should make the boat rock solid!

[Tom Williams]

Pretty amazing Joe ... I wreslting a similar problem on my CS17 right now! The 1/4" plywood panels I had for my CB trunk was not very flat. I used it anyway hoping the glass on the inside an the support stringer would straighten it. In addition I made a spacer which I inserted into the trunk when I glassed it in (I highly recommend this step).

Bottom line: there's a little bit of bow in the aft end of the trunk. When I inserted my centerboard (all painted of course) it was binding at the aft end. I've added some taper now to the board and it now is working freely. If I could offer any suggestions to future builders it would be to make sure the trunk is straight and true before it goes in. Either find some good flat plywood stock or add some additional support stringers to make it straight.

From a structural standpoint, the tapering the centerboard is an ok thing to do. The bending stresses deminish as you go out to the tip.

Live and learn!

Tom

[Designer]

This is a tough problem. There is nothing worse than a sloppy centerboard trunk except probably one that is too tight. If there are voids under the glass tape inside the trunk it indicates that there was not enough radius around the slot. I might have to open up the trunk slot if ameteurs can't maintain the tight tolerances.

The glass tape is partly to keep water out of the wood and it contributes some to the strength. You should be okay if you if there are a few voids but if they cover a large area I would repair them.

[Aaron_G]

I second Toms advise for using a spacer. My CB trunk had a bit of a warp when I put it in too. I solved this problem (I think) by screwing the side of the trunk closest to the center line to that piece of wood on the center that runs the length of the boat (is this called the "keelson"?). i.e. I screwed from the bottom up through the slot. I then used spacers to keep the other side parallel to the screwed side. After the glass dried, i took the screws out. Seems to be straight and the CB fits inside.

On a semi related note: 70% through my filleting of trunk to boat, I ran out of my silica thickener. In a panic I decided to use the graphite powder (West Systems stuff, contains both graphite and silica) I bought for coating the centerboard. Seemed to work great, but I'm wondering what the adhesion qualities of graphite to figerglass are. I applied the fiberglass while the fillet was still tacky. Also, thanks to the graphite, I will have to do some sanding and painting because it looks like someone lost a fight with a bucket of tar on the inside of my boat.

[JeffM]

I originally thought to leave a bare glass edge hanging out the bottom of the trunk and then epoxy it around to the bottom. Easier said than done! I quickly gave that up, and simply used Graham's tape to glass the trunk/bottom seam after I had rounded the edge well and filled voids. I also trimmed the edges off the tape to avoid their tendency to stick up, and angled the strips over the curve both to increase strength (more fibers cross the seam), and make going around the curve easier. MY problem is a trunk that turned out (despite my best efforts) to be a little twisted... Fortunately, my board must have the same twist--it fits well and moves smoothly!

Jeff, CS20 Beatrice Ann

[Joe Nelson CS#35]

Jeff,

Sounds like exactly what I tried to do without success. I routed (rounded) the slot again last night and filled everything with wood flour/cabosil/epoxy mixture. Tonight I will sand and tape as you did. If the added thickness binds the board, I will shave it down.

I got a router table and put my cheap Craftsman router in it. I didnt want to take it on and off, so bought a new one. Got a Porter Cable plunge router...what a sweet tool!!! Didnt know what I was missing before.