GP14 Sailing Forums Forum Centreboard case questions.

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    • #27051
      nicecuppatea
      Participant

      Hi. I have removed the centreboard case on 4924 in order to fix the leaking bottom join. The association YouTube “restore your beloved GP14” being very useful. There was the usual mix of grp tape and mastic has been applied over the years! Everything is still pretty solid with only a couple of minor soft spots. 1) I plan to make a new section of keel but I note that the slot is chamfered on each side by approx 15 degrees at a guess. Is this 60 years of wear or necessary. 2) the centreboard is 19mm thick and the slot is 25mm wide. Is this normal? Someone has previously glued some formica sheet to the pivot area to ease the slop. 3) the lower sides of the case have slots routed for the frames but the frames are cut flush to the case and the slots filled with hardwood. 4) when refixing, should I use brass or stainless screws?

      Thank you.

       

       

    • #27053
      Oliver Shaw
      Moderator

      Thickness first;   the permitted range of thickness of the board is 13 to 20 mm,  so at 19 mm you are close to the maximum allowed,  which is good.

      As regards the interior space inside the case,  you will need at least some slack in order to allow the board to move freely,  and to have a reasonable chance of avoiding it jamming.   A particular concern is the risk of mud or sand or small stones getting drawn up into the case and causing the board to jam.    I happen to have my very early vintage boat in the garage (no. 64,  dating from 1951),  and I have just measured the interior width of the case at the top (the only place where it is accessible),  and found it to be 26 mm,  just very slightly wider than your slot (25 mm).    Given that the class was originally designed and built to Imperial measurements,  rather than metric,  those seems to be to be a very sensible pair of dimensions;   a gap of 1 inch (25.4 mm) and a permitted board thickness in the range 1/2 inch to 13/16 inch.      So my first thought is that this slot width is very probably right.

      If you wished to check further you could check against the plans,  available from the office.    But personally I would leave the slot width unchanged.

      I am mildly surprised at the chamfering of the sides of the slot,  which I don’t think is normal.   But I winder whether a previous owner has had problems with the slot gasket being drawn inwards as the board is retracted;   this could be an ad hoc solution to that problem.   This discussion is very much open to other people to chip in,  but in the absence of any advice to the contrary I would suggest cutting the slot with no chamfer,  and fit new slot gaskets under tension;    cut them longer than the required length,  secure one end (which will become the far end from where you will next be working) by means of a screw (driven through that end of the relevant keelband),  pull the gasket taut,  and while holding that tension drive in a screw at the near end (again through the keelband).    Then fit the remaining screws,  and finally cut off the surplus gasket.

      I suspect that this will be sufficient,  and that you will not have a problem of the gasket being trapped inside the slot;    but if that does happen then at that point you can consider chamfering the sides of the slot.   However I doubt whether you will need to do so;  but others may think differently.

      Hope this helps,

       

      Oliver

    • #27054
      Oliver Shaw
      Moderator

      Your query 4:    certainly not brass screws, under any circumstances.    Over time they will dezincify, and become very weak semi-porous and apparently crystalline copper screws, which are then useless.

      (I think this problem may perhaps not have been known at the time the early boats were built;   it seems to have emerged in the late fifties and early sixties when brass screws,  which had until then been widely used,  started failing for this reason.    I was at university in the early sixties,  racing Fireflies,  and we were forever breaking masts when the chainplates pulled off the hull because the screws failed   …   …   Far more is known today about the behaviour of alloys in the marine environment.)

      The appropriate choices are either stainless steel or bronze.   Bronze screws are more expensive than stainless, but are very appropriate to the age of your boat;   however the screws securing the case to the hog are never going to be seen,  so there you may as well use stainless as being the cheaper alternative.    You may wish to indulge in the luxury of bronze for screws which will be visible,  e.g. those securing the case to the thwart,  etc.    Bronze fastenings (and much else) are available from specialist chandlers such as Classic Marine,  or Marinestore.co.uk,  or Davey & Co.   There may well be others as well,   but I have not found them yet!

       

      Your query 3:    I suspect that someone has previously done some work on the boat,  at some point since she was built.     There is no great need for either the floorboard bearers or the frames to be routed into the case,  but it is not wrong to do so;  historically some professional builders did rout them in,  and other professionals did not.   The floorboard bearers are not structural,  and serve merely to support the floorboards,  but the frames are structural,  and serve to fix the shape of the hull,  and provide lateral strength.

      Only one of the actual frames intersects the case;   frame 3,  which is the frame that also intersects the thwart.   Any lateral weakness arising as a result of splitting that frame (because the case cuts across it) would seem to be amply compensated by the very considerable strength of the two quadrilaterals formed on each side of the boat by the two parts of that frame and the case and the thwart,  together with the fact that the thwart is a single and very stout piece of timber bridging those two quadrilaterals.   I suspect,  but cannot confirm,  that the cut inner ends of that frame are also glued and screwed to the hog,  albeit that this will be either side of the slot,  which will therefore at least help to fix the distance between them,  further contributing to a rigid structure overall.

      By contrast,  joining the two cut ends into the sides of the case,  however it is done,  would seem to be a joint involving end grain at each surface of the joint if traditional woodworking joints are to be used;     so that is never going to be a strong joint.   A modern epoxy fillet joint  –  or alternatively a wooden fillet joint (with horizontal grain at 45 degrees,  to avoid exposure of any end grain)  –  would of course be strong,   but that is not what we are discussing,  and I am not personally aware of any boat which has needed this modification,  although I think it very likely that some owners will have done it.   However my own assessment is that ample strength would seem to be provided elsewhere.

      Frame 2 is immediately ahead of the case,  and frame 4 is some little way abaft it;  so each of those frames has its own integrity,  not compromised by the centreboard case.

      So I would not worry about this apparent alteration to the way the boat was built,  unless it troubles you cosmetically.

      Hope this helps,

       

      Oliver

    • #27066
      nicecuppatea
      Participant

      Hi Oliver. Thanks very much. Very helpful.

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