Friday, November 19, 2010

Centerboard Case

No doubt you have noticed that during the finishing process time was flying by and little was getting done on the boat, to combat that I started work concurrently on the centerboard and the case. The plans called for a solid mahogany case and centerboard but, I didn’t have that much mahogany kicking around and I didn’t feel like buying another mahogany plank—too cheap I guess. So we elected to build everything out of plywood, cheap exterior grade plywood.

Stern spacer and the two sides of the case
For the case I glued a sheet of 1/8” mahogany plywood to the 5/8” ply so that the exterior of the case will match the mahogany on the inside of the boat. Cutting out the case was just a matter of following the plans.  The stern end of our case is curved (following the shape of the centerboard) and was built up from two pieces of 5/8" ply epoxied together and cut using a router on a trammel.  It was a little tricky to match the bottom curvature of the boat but with a trial and error approach the shape came together.

Trial fitting the two sides
Atkins has an interesting way of dealing with the centerboard pin though. In most boats that I have seen a hole is drilled through the centerboard case and then that hole is sealed off on each side of the case by some elaborate means. Atkins instead has you cut a channel in each side of the case that secures the centerboard pin on each side. The centerboard can move up and down vertically along this channel but is constrained from moving the other two dimensions; fore and aft and starboard and port (seemed like a good time to start using some nautical terms). I have elected to run a piece of UHMW (ultra-high molecular weight) plastic that I purchased from Lee Valley Tools in the channel and to drill a hole in that to position the centerboard pin. I believe that if I run a piece of UHMW the full length of the channel then the vertical movement will also to stopped. We plan to find a way to make the top of the centerboard case removable so that we can access the centerboard easily if needed.

To make the case watertight we applied a few coats of epoxy to the interior surfaces prior to screwing the two halves together. A thin coat of Sika was applied to the joints and the whole case was screwed together with 2 ½ stainless steel screws.
Assembling the case note the Sika and the channel

The scary bit: cutting the slot in the bottom planking for the centerboard to pass through.  I must admit that this was a little intimidating.  A lot of hard work had gone into installing the bottom planking and some how it just didn’t seem right to start cutting a huge hole right down the middle of it.  I also have to admit that it still doesn’t make sense to me that you can cut a board in half that is only supported on each end and not have it sag significantly in the middle—but it didn’t.  So after carefully measuring (twice at least) and marking its position it was time to take the plunge.  First I drilled a hole at each end and then marked a line to each side of the hole.  Then with jigsaw in hand I carefully cut at least a 1/8” to the inside of the marked line (what a chicken I am) all along thinking that I will finish cut it from the bottom with a router using a pilot bearing bit.  Like most of our fears they are all between our ears and are never realized—everything went really smoothly.

All marked and in the midst of cutting

There's a hole in my boat
 Prior to installing the centerboard case we laid down a generous bead of Sikaflex and then carefully lowered the case into position (we pre-marked its location to facilitate this process). With the case in position a long clamp was dropped down through the case so that it made contact on both the exterior of the bottom planking and the top of the case. It was then lightly tightened before a second clamp was positioned at the opposite end and tightened. At this point we double checked to ensure that the case was square and then proceeded to apply a little more pressure to each of the clamps—not too much though as we did not want to squeeze out all of the Sika. We left the Sika to cure for a couple of days before screwing the case in from the bottom side through the planking. The case is approximately 48” long and on average our planks are 3”, so I expect that we will put over 32, 3” stainless steel screws into the case from the bottom—that ought to hold it!

All done
A picture that shows the centerpin channel from the top

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