Monday, May 18, 2009

Centreboard and Case

I have grouped the activities related to the centreboard and case here so that I can add to this part as the build progresses. I had an interesting time with both the case and the board.
Having decided that the board itself was to be 50mm wide between the cheeks in the case, I proceded to build the case with a 9mm doubler on either inside faces. I put another pair of 9mm ply doublers on the outside of the centre case between frames 3 and 4 which was rebated into the bunk flat supports and the centre case logs on the bottom panel. This effectively give a 3x 9mm ply lay-up stiffened by the centre case logs and bunk flat supports to withstand the lateral loads of the centreboard.
I also installed a centre case bush arrangement with rubber o-rings as shown (the o-ring and sleeve in the pic are not fully seated yet) which virtually eliminates the possiblity of leaks and prevents the centrecase from "pinching" the centreboard which is sandwiched between the inner doublers. The foil is also bushed and seats against the case bushes on either side. I just realise the pic is on its side but if you turn your head you will understand, look funny and get a sore neck like me.

When I shaped the foil itself, I used one side of a NACA 2412 wing ordinates for the profile of the foil. I made a jig for my skilsaw which I used to cut grooves lengthways into the laminated stock. The problem is the jig sagged without me realising it and I ended up with slots that went deeper at the end of each cut...ayeeee!!!!! Step away from the saw, have a wine and go back another day when the head is clear. All is not lost thanks to the miracles of epoxy and a bit of thickener.
When it came to the lead for the board I was keen for a jelly tip so I set about making a mould for the lead. I started by using the offcut from the centreboard as a profile around which I wrapped a thin sheet of aluminium which I crimped closed. I took the mould to a local chap who casts fishing sinkers to see whether he would pour the lead thus saving me a lot of time and hassle. He agreed and I left him to it returning about an hour later to pick up the tip.

My reception when I returned was decidedly "cool". It transpired that the laminated offcut at the bottom of the mould "fizzed" and together with the burning epoxy blew a chasm into the bottom of the mould through which molten lead proceded to spurt all over the foundry! Luckily no harm was done and they all managed to see the funny side of the event. They even agreed to allow me another go using an improved mould design. The second mould was a huge success and the pics speak for themselves.

Oh, I nearly forgot...having bolted the lead tip to the board (I used two 8mm SS threaded rods screwed into the foil to a depth of about 150mm with liberal amounts of epoxy "lube" on the threads), applied two layers of glass to the cheeks and leading edge, I had faired the transition from timber to lead and applied two coats of epoxy to the board while it hung from a piece of wire. When I was taking it down I forgot I was dealing with 14Kg of lead plus the weight of the board and it slipped from the wire and smashed into the floor about two inches from my toe (I was barefoot at the time). I nearly wept with relief and then fear that I would have to start all over again. To my amazement the damage was negligable and I simply filed off the slightly mushroomed leading tip, reapplied some thickened epoxy and hey presto...we have a centreboard! Needless to say I am more than happy with the result.

I still need to do a final sand, dry fitting and paint. I can't do a real dry fit the centreboard to the case until later when I roll the boat or cut the obstructing cross members from the building frame - probably once the planks are in place. She seems to go in ok upside down from the top.

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