Latest News: Forums Technical Centreboard thickness

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  • #21654
    David Stanford
    Participant

    Hi

    I am planning to make a new centreboard over the winter but am after some advice as to the best finished thickness to choose within the range 17-21mm. My current CB is 21mm (perhaps even a little more in places) and is a very close fit to the case. This would seem to be a good thing (it is a snug fit but still free to move). The problem that occurs with a tight fitting board is the classic jammed centreboard when left on beach. But with a tight fitting board this occurs with sand. And then you have a problem, because the board is so tight fitting that you cannot slide anything (such as stainless steel rule) down the case. When this happened to me a couple of years ago we had to hastily careen the boat and pick out the sand from the underside with the handle of a teaspoon.

    So I am thinking of leaving enough space to enable a stainless strip to be worked down from above. I would rather that it is a large enough gap to remove sand from the equation, risking large sizes but having a means to remove it.

    So my question is, can anyone recommend an optimum thickness of board, taking into account what I am trying to avoid?

    My slight concern is that making it thinner puts more strain on the bolt and the board in general. I realise there might be some variation in case width, so maybe I will just have to work it out – but if anyone has been there and done it, perhaps they could offer some wisdom?

    Btw, I’m happy with other aspects of making a CB, having made a laminated rudder previously, so no need for advice on that side of things.

    David

     

    #21655
    Anonymous
    Inactive

    Hello David.  I used the Danish pivot bolt solution, best found on the Wayfarer Institute of Technology website under the heading “The centreboard insert that keeps bolt tightening from jamming the board”.

    Briefly instead of a hole in the board only just big enough for the bolt, get a cylindrical rod of Delrin ( not nylon, which is hydroscopic and will swell in the water) about 25mm diameter and cut a piece that is a close fit inside the centreboard case. Try to get the faces true, I asked a mate to turn it on his lathe. Drill a hole through for the pivot bolt.

    When you make the centreboard, drill an oversize hole and put the board on a flat surface with parcel tape to prevent epoxy sticking to it, set the Delrin piece in the hole and drip in thickened epoxy around it. The Delrin won’t stick to the epoxy so once it us cured the board will pivot about it.

    Now with the board in the case you can tighten the bolt but it won’t bend/ distort the casing.  The bolt pinches the Delrin, not the board. You could make the centreboard a bit thinner (respecting class rules of course)  so that sand binding is not a problem and include some spacers to centralise the board.

    You can get Delrin from on-line Plastics suppliers, or if you PM me I can send you a piece as you have to buy a metre at a time so I have enough to send you an off cut.

    Mike

     

    #21656
    Anonymous
    Inactive

    Incidentally perhaps you could advise me, I am making a laminated rudder ATM.  Whet method are you using? I was planning to tongue and groove a set if 50mm wide pieces of spruce and mahogany before glass sheathing.

    I plan to make a new centreboard once I gave “practised” on the rudder.  I put the Danish pivot into the existing ply centreboard.

    Mike

    #21657
    David Stanford
    Participant

    Hi Mike

    Thank you very much – that is very useful – I will use that method.

    I made the rudder blade as follows – laminated strips of sapele (from 25mm board) around 75mm in width, each alternate strip flipped in order that any warping would be cancelled out by adjacent strips (seems to be stable after 2 years or so). I have a friend who is a joiner and I spent an evening at his workshop to cut the laminates, glue, and then plane down the finished laminated board to the thickness I wanted in his thicknesser.

    I believe that the use of this kind of machinery is almost essential. You get a perfectly perpendicular cut on each adjoining face with a highly accurate bench saw. I am not sure you could do it without.

    We used “biscuits” at intervals of about 200mm along each joining edge (requiring a biscuit cutter to get accurately aligned grooves). We then glued the joints with a fast set PU glue (D4 rated). This makes an extremely tough joint. A test peice snapped the wood in a different place to the joint without a biscuit. It is also a lot easier to use than epoxy – you don’t have to mix, but you do have to work fairly quickly. (The biscuits may be an overkill.)

    On the rudder blade the top half or so was sheathed with glass and epoxy, the whole with epoxy.

    If I was doing it again and for the centreboard I will probably use 50mm laminates and different woods (partly for aesthetics). I might not use biscuits (with narrow laminates that is a lot of cutting). However, I would also consider the density of the woods as I would like the centreboard to be close to the weight max.

    So thats what I did / intend to do for what it is worth. I would say the most critical thing is that the adjoining faces are perfectly machined, which probably depends on access to decent machinery.

    David

     

    #21662
    Anonymous
    Inactive

    Interesting, I had considered biscuiting but thought tongue and groove might be stronger.  Also interesting that you say the polyurethane glue was so strong, do you remember the tradename?  I plan to use epoxy as I am familiar with it but I am open to alternative ideas and advice.

    I am awaiting delivery of the timber from Robbins, I am buying it sawn then will rely on my (home made) router table to true the edges and cut the tongue and groove. I hope the timber is not twisted/bent and that my skills are up to the job.

    Mike

     

    #21663
    David Stanford
    Participant

    I am sure tongue and groove would be stronger if a good tight fit. It might also compensate for any slight imperfections on the adjoining edges, if there are any.

    I think the PU glue used on the rudder was Wurth. However I have subsequently used this one with similar results:

    http://www.wudcare.co.uk/fastgrab.htm

    The PU glues seem to work best on tight fitting surfaces. Epoxy may have better gap filling qualities.

    Hope that helps.

    David

    #21687
    Dave Barker
    Keymaster

    Regarding the original question, the only real solution to the problem of centreboards jamming is to avoid conditions in which this is likely to happen. However much or little space there is beside the board there is a particle size that will cause a jam, given the chance! Sand and/or stones will find their way up the case when the boat is bobbing about at the water’s edge, but much less so if it is either fully afloat or fully dried out.

    Opinions on the subject of slot gaskets seem to vary – we have them (they were already fitted) and haven’t had a problem – the boat has been rolled up and down various beaches, sandy, stony, muddy, without problems.

    #21688
    Dave Barker
    Keymaster

    On the subject of laminated foils, my preference would be for epoxy as an adhesive, mainly because I would sheath the foil with epoxy too, but also because some PU glues foam during curing, making it harder to achieve really tight glue joints. But clamping should prevent this.

    For anyone using hand planing to prepare the edges of laminations prior to gluing, just clamp adjacent pieces together in the vice and plane them at the same time. This gives you a wider edge (which is easier to plane) and means that the boards will automatically line up correctly even if you don’t plane at exactly 90 degrees.

    Biscuits can help with alignment during gluing-up. Just plan carefully where they’re placed so that you don’t have any nasty surprises when you trim the blank to final shape…

    I don’t think biscuits add significantly to the strength of the laminated foil, but remember the oft-quoted remark that the glue is stronger than the wood, and with glass sheathing the final result should be good for decades with a bit of care.

    #21829
    David Stanford
    Participant

    Thanks for responding to the original question. The point about a tight fitting board is that in the event of it jamming there is no means of freeing it from above and if singlehanded that could be a showstopper. You can avoid circumstances where it may jam, and indeed I will, but it seems sensible to have a means of solving it as circumstances are not always a choice, and a slightly freer fit seems to be the solution.

    I do have slot gaskets, which need replacing, and also the current board does not fully retract – just enough to prevent the slot gasket fully closing together. That is a factor which probably contributed to the jamming with sand, and which I intend fixing with a new board. But I still would like to know I have a solution if it occurs again.

    PU glue does foam where it is not under compression, but surely clamping is a given whatever the glue. It certainly does not prevent a tight joint – that is what it is intended for. Its advantage over epoxy is the fact it is much easier to use. There is no mixing, and any excess is easily cleaned off after curing.

    #21830
    Dave Barker
    Keymaster

    I doubt very much that a significantly jammed board is going to be freed from above. The kind of narrow implements that people tend to carry for such eventualities are typically too thin to work well as a pusher, and would you really expect to be able to pull a jammed stone all the way through the case?. There used to be regular adverts in Wayfarer News (and I now discover there still are, in the Classified section on this website) for stainless steel tools for this very purpose – stone pulling I mean – which are 1.0 mm thick, with a hooked end. The idea, presumably, is to beach the boat, turn it on its side and try to yank the offending stone out the way it went in. As you say, not easy on your own. (A trivial jam will free just by pulling up on the board’s loop and stamping on the trailing edge while afloat, as we’ve probably all discovered…)

    As for adhesive, I find epoxy simple enough to work with, and I like the way it wets out the timber surface. If properly clamped PU will be fine, but for anyone improvising at home without a good set of clamps PU will try to open those joints. Epoxy won’t do this, and it may save buying two products instead of one. That was my thinking 🙂

    #21839
    Swiebertje
    Participant

    Closure strips are not only good for racing, for cruisers they do a pretty good job keeping debris out of the CB slot while beaching. And not even a cruiser would sail with two fingers in the water, which has the same effect on boat speed as a CB slot without closure strips.

     

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