Replace plywood bottom

GP14 Sailing Forums Forum Replace plywood bottom

This topic contains 16 replies, has 4 voices, and was last updated by  Oliver Shaw 51 minutes ago.

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  • #19031

    Taffymat
    Participant

    Hi,

    Has anybody here ever replaced the 2x centre plywood sheets on a GP14? I’m thinking of joining it into the original ply somewhere near the mast area, then working aft, approx 4ft.

    I’m wondering how difficult a task it might be to release and refit the boards at centre hog.

    Regards

    Matt

  • #19032

    steve13003
    Participant

    Hi,

    i have replaced several panels on various GPs.  Is your boat a Series 1 boat with buoyancy bags? Up to hull number about 13200 or a newer Series 2 boat with buoyancy under the floor?  First I would buy a set of the construction plans from the Association office, for which ever type of boat you have, this will let you see how the bottom panels and the keel are constructed.

    The keel piece is fitted after the bottom panels are fitted, so to remove then one would normally remove a section of the keel so that the ply can be removed and the new ply fitted to the hog ( the inner part of the keel).  The ply can be replaced between any of the frames I would normally identify the position of the frame and cut through the ply along the middle of the frame so that the existing ply panel and the new ply are supported on the frame, you could also try a half joint where you taper the old and new ply over a length of about 50 to 75mm over a frame.  All new ply and joints should be made using SP epoxy 106 resin and fillers and all new wood coated with epoxy to ensure no further rotting. After fitting the new bottom panels refit the keel .

    what you are planning is not an easy job but is not impossible providing you have a good work space under cover if possible.

    Good luck and let us know how you go.

  • #19033

    Taffymat
    Participant

    Steve, many thanks for that. I can’t find the construction plans on here anywhere? I’ll try and contact them directly to ask for a copy.

    Yes, you are indeed correct a mk1 “12960”. She’s in a rather sorry state atm.

    Removing the keel sounds like it would be a nightmare? I’m going to take another look later on in that case and see if I can come up with an alternative plan.

    I’ll post some pics later, but I did wonder if the plywood sat tight against the mahogany strip that runs the interior length of the boat? If that piece were indeed chamfered and shaped tightly along its length, then I could cut the plywood short of the keel, and epoxy straight to that instead?

    Regards

    Matt

  • #19035

    steve13003
    Participant

    Matt,

    i have dug out my copy of the plans and on Sheet 3 of 4 the keel to hog bottom ply detail shows the ply under the keel but there should be enough width in the hog, the inner part to clean off the bottom panel and fit it to the hog without removing the keel.  I would check the width of the hog along the part you want to replace to see how much of the hog you will have as the landing for your new ply.  You will also need to remove the bilge keels, these screw through into the frames.  Along the chine the bottom ply is screw glued to the inner chine stringer and the outer chine piece sits in a notch and is planed down to match the bottom ply.  You may need to replace part of the outer chine piece just depends on how cleanly you can remove the old ply, it may be best to cut into the inner chine stringer and refinish the outer chine piece to match the new ply.

    if you get stuck for buying a set of plans I do have a set of my own which I could trace the details for you, email me at [email protected] if that helps

    Steve

  • #19045

    Taffymat
    Participant

    Steve,
    <p style=”text-align: left;”>That’s really helpful thanks. I’ve also today been offered a grp hull only as a quick fix. Is that a feasible swap do you know? i.e. swap mast and all fittings over?</p>
    Cheers

    Matt

  • #19068

    Taffymat
    Participant

    Thank you so far to everybody for your help.

    I have just completed a membership form, as clearly this website, and you guys are going to be massively helpful to me moving forward! I really appreciate your help.

    Can I please run through my repair plan of MK1 (12960) wooden hull, and see if anybody has anything to improve or object to? It’s a bit longwinded sorry!!

    I intend to replace the floor section from frame 2(mast foot), through to frame 7 (8ft in length)

    1- Get boat under-cover, lay 2 car tyres on the floor alongside boat, and using a helper roll boat onto it’s side, then fully over into inverted position. Support the upturned hull in several places on blocks & stands.

    2- Cut old ply bottom out alongside keel strip (keel strip seems to be epoxied into place. I think a lot of damage will occur if I try to remove it?). This should leave approx 25mm of hog exposed in order to attach new ply base to.

    3- Mark position of frames underneath the hull. Remove any screws, use a router to remove the ply thickness and epoxy down to the frame below.

    4- Use the router with edge guide to machine away the ply base, up to the chine edge. Remove the plywood bottom. Strip varnish from sides of chine strip, hog, and frames.

    5- Repair any damaged frames with sapele scarfed & epoxied into place. Replace the stringer along the 8ft length each side with new “obeche” timber to same dimensions, scarfed & epoxied into place. Replace external rubbing strakes “bilge keels?” with new timber “ash or obeche?”

    6- Prepare an 8:1 scarf joint midway across the frames no2 & no7.

    7- Coat plywood in Epoxy internally & externally. Epoxy and stainless screw the new 6mm “Robbins elite” plywood hull bottom into place using scarfed & epoxied joints at frame no2 & no7, these scarfed joints are then screwed into centre of frames.

    8- Epoxy tape any joints onto frames, keel, hog, and the exterior of the chine.

    9- Paint it, re-assemble it and enjoy it?

     

    Any suggestions, or critique are gratefully received! 🙂

    Many thanks again

    Matt

     

     

    • This reply was modified 2 weeks, 3 days ago by  Taffymat.
    • #19083

      Oliver Shaw
      Moderator

      Second attempt,  now that I am back home and can use a desktop PC rather than only a tablet;   the latter is something of a nightmare!

      Delighted that you have decided to join the Association!

      I confess that this is one repair which I have never had occasion to do myself,   but I have seen photos and write-ups of several other projects of this nature,  and have done a fair amount of other boat repair myself.    So my observations are based on a general awareness rather than first-hand experience of this particular job.

       

      Your step 2;   I provisionally concur,  subject to checking the keel for water saturation (as Chris has suggested),  subject also to sufficient bonding width on the hog being exposed,  and further subject to the old ply between hog and keel being in sound condition.    This last condition must be regarded as fairly doubtful until you have actually removed the damaged ply and positively verified that what is left there trapped between keel and hog is sound.

      Note that if you terminate the new ply at the edge of the keel,  rather than underneath the keel,  the edge of the ply will be potentially exposed,  so it will be vital that this is well epoxied.

      If in any doubt about any of these conditions,  remove the keel;   except inside the centreboard slot the edge of the ply will then be protected by the keel.

       

      Your step 6:   I presume that the frame itself is either sound or will have already been repaired,  and the scarf joint you refer to is between old and new ply.    I would expect (without measuring up),  that the scarf will actually be significantly wider than the width of the frames.

      That being so,  locating the scarf joint (in the ply) exactly on the frames is irrelevant;   it could be anywhere convenient,   but actually making the scarf joint while the (old) ply is on the boat (and therefore curved,  in two planes) may be difficult.   There are alternatives.

      A butt joint is perfectly acceptable,  provided there is adequate width of backing piece.    The frame itself will not be wide enough to serve adequately as a backing piece for a butt joint,  but its width can be doubled by use of a sister piece epoxied to the frame,   or the butt joint may be made away from the frame using a ply backing piece.    In either case there will inevitably be a weight penalty,  but this should be very modest,  indeed probably trivial;   and if the boat is already down to minimum racing weight and carrying corrector weights you could compensate by reducing the corrector weights accordingly.    (I think you would then need to get the new hull weight checked,  and your certificate endorsed,  if you were to adjust the corrector weights.)

      A refinement,  after making such a butt joint,  would be to then rout out the top layer of ply for a short distance either side of the joint,  and insert an inlay piece,  so that the main part of the joint in the ply is not on the surface.

      Another alternative is a lap splice joint,  cutting away (by router or otherwise) multiple faces parallel to the surface rather than the single angled surface of a scarf joint.    Depending on your available tools,   this might perhaps be easier than a scarf,   because of the curvature of the panels.    https://en.wikipedia.org/wiki/Splice_joint#Half_lap_splice_joint.

      See pages 27 and 28 of “Home Boat Building Made Easy”,  by the Bell Woodworking Company,  for a more general discussion of repair techniques.   My comments above are an adaptation of that advice,   modified for the size of the job on your boat.   That book is long out of print,  but (with the permission of the late Searson Thompson)  I scanned it and uploaded  it to the GP14 Owners Online Community site some years ago.    https://groups.yahoo.com/neo/groups/GP14_Community/photos/albums/1717966512/lightbox/568223741?orderBy=ordinal&sortOrder=asc&photoFilter=ALL#zax/416512707

      Hope this helps,

       

      Oliver

  • #19070

    Chris
    Participant

    1) Use something hard for your bilge keels. Obeche is too soft.

    2) You’ll want to fillet your joints, glass tape can trap moisture which then cant escape. fillets are stronger.

    3) it might be worth drilling some holes into the keel to see what the core sample is like. If its dry and doesn’t want to come off leave well alone. if its wet then its worth encouraging it to come off. Wet and rotten are not the same thing, the front 3ft of keel on mine was absolutely saturated with water. I took it off, dried it, and replaced the same wood. I then plugged the holes with some mahogany. Its. better safe than sorry here as water can bleed down the ply and degrade the glue joints to the structural frames. Epoxy is better, but it still happens and you’d be surprised at how much cascamite is used in boats that you’d automatically assume are epoxy throughout.

  • #19072

    Taffymat
    Participant

    Thanks Chris, I’ll get ash laths made for the bilge keeps then?

    Does Obeche look similar to ash? The interior stringers look like ash to me? But they could well be obeche if it looks similar?

    Fillets only, great stuff. Less work for me!😁

    I’ll definitely check everywhere for moisture..

     

    How do I know if I have mast step conversion already?

     

    Cheers, Matt.

  • #19073

    Oliver Shaw
    Moderator

    Mast step conversion;   if you  have it,  the mast step will be a metal track,  with pins across it to determine the fore-and-aft location,  and the foot of the mast will be a tenon which fits into the track.   The track sits on a longitudinal member which is raised above the hog,  and which is tenoned into the centreboard case;    the purpose of this redesign is to spread the load over a greater length of the hog.    Although it won’t be obvious without actual measurement,  the mast is also 12 cm shorter,  to take account of the raised position of the step.

    If you don’t have the mast step conversion the step is a square mortise in a wooden block which sits directly on the hog,  and the foot of the mast is square,  rather than having a (rectangular) tenon).

     

    Oliver

  • #19074

    Chris
    Participant

    ash and obeche are not dissimilar in colour once aged a bit, both are a paleish yellow. Ash has a more obvious grain pattern. The stringers *might* I suppose be ash, its not especially heavy and a GP is a heavy boat but I’ve yet to see a 12xxx numbered hull with anything other than obeche stringers.

    Obeche is also significantly more prone to rot, so I’d say if they’re rotten they’re almost certainly obeche. We had a similar quandary with the stem on my boat which was very rotten and we couldn’t decide between the two – Alistair confirmed it was obeche.

  • #19092

    Taffymat
    Participant

    Oliver,

    Thank you yet again. Your suggestion of a simpler joint is greatly welcomed. I was concerned about the feasibility of a uniform scarf joint in situ.

    My initial thoughts of making the scarf joint midway on a frame, was in order to screw through the joint, and into the frame as a means of clamping it together and forming the correct shape.

    I could actually move slightly farther towards the stern, and make a lap joint with a backing piece. The hull has less shape here, so it would be a simpler task. A 75mm x 6mm ply backing strip would be ok, I think?

    Can I ask, is there somewhere I am able to find some guidelines as to what an acceptable minimum bonding surface width would be? Say X times material thickness?

    Thanks again

  • #19095

    Oliver Shaw
    Moderator

    I think the simplest joint,  and perfectly acceptable,  would be a butt joint with a ply backing piece.

    A good reference source,  indeed the standard reference book (so far as there is such a thing),  for all epoxy work is the Gougeon Bros. manual;   https://www.westsystem.com/the-gougeon-brothers-on-boat-construction/

     

    Oliver

  • #19141

    Taffymat
    Participant

    A little update for you guys…. I’ve stripped the top deck, and applied epoxy. Shes looking better already.

    I’m going to turn her upside down this week, and start on the bottom.

    Anybody know if it’s ok to use to paint stripper to remove the paint on the underside? The underside has already been deposited, and I’d rather not remove the epoxy that with the heat gun. The paint is failing with solvent pop. That’s why I want to remove it. I assume the stripper wont damage the epoxy?

    I’m trying to figure out how to add pics!

  • #19142

    Oliver Shaw
    Moderator

    Yes,  you can safely use paint stripper.

    Some are better than others.   My own favourite is one of the oldest,  and probably the most aggressive,  Nitromors Original.   Be aware that it is pretty ferocious stuff,  so use with all proper care.    Apply it to only a small initial area;   Ian Proctor recommended just one square foot if I remember correctly (in his 1950s book Racing Dinghy Maintenance),  allow it perhaps 5-10 minutes to soak in (Proctor recommends using this time to re-sharpen your scraper),  then paint a different small area,  and then scrape the first area.     Keep going in a sequence like this.

    Allow about 5 minutes to soak in,  perhaps 10 minutes at maximum;    certainly not the 20 minutes sometimes recommended;   the stripper will evaporate if left that long!

    Your scraper needs to be seriously sharp at all times.   If you use a conventional scraper you will need to re-sharpen it every few minutes,  and there is a technique to getting it sharp enough.    However that need has been overtaken by modern technology;    use instead a scraper with a modern tungsten carbide blade,  and then you won’t need to re-sharpen it after every few minutes’ use,  (and with that material for the blade you won’t be able to sharpen it anyway),   so you could now work a sequence of three or even four areas all on the go at the same time,  each at a different stage of the sequence  –  always provided you can keep track of where each patch is up to.

    Nitromors nowadays offer several different formulations,  designed for different substrates,  so that they can be sure not to damage more delicate substrates.   However the gentler ones are correspondingly less effective at removing paint.    Personally I always use the toughest of the lot,   their Original,   and work on the principle that if there is a substantial thickness of paint it won’t penetrate right through with the first application;     and once you get down to the level where it will penetrate right through it will have been fairly weakened by attacking the paint,  and it won’t remain in contact with the epoxy long enough to do it any serious damage.   However you are welcome to take a more cautious approach,  using a gentler paint stripper;   but the job will take significantly longer,  and will be significantly more work.

    -o0o-                    -o0o-                    -o0o-

    Adding pictures;   I think you have already found the mechanism,  but you need to re-size your photos first.  A box between the typing pane and the “Choose file” button states that the maximum file size allowed is 512 kB.    That is quite small if your photos are originally taken at high resolution intended for printing,  or for projection onto a large screen;   but it is in fact amply good enough for the size that will be displayed on this forum.   Use any photo-editing program of your choice to re-size your photos;   you probably have one (or more) already on your computer.

    As one tip for re-sizing;   you may find that your photo-editing program allows you to set the image size in pixels,  rather than the file size in bytes.    Try 1000 pixels for the longest size,   save to a different filename (so that you don’t spoil your high resolution original),  and then either try to upload that or  –  if you wish  –  right-click on the file icon first and select Properties to check the filesize.

    Hope this helps,

     

    Oliver

  • #19143

    Taffymat
    Participant

    Thanks Oliver..

    Hopefully the pics worked this time. Hopefully you approve of works so far.

    I know of a commercial paint company, i use through work, so I may give them a call about stripper tomorrow. Ill revert back to Nitromors tho if needed. Glad to hear it’ll not likely harm the epoxy. Seems a waste to strip it all off.

    I’ve also got one of the tungsten pull-scrapers. Fantastic bit of kit.

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  • #19146

    Oliver Shaw
    Moderator

    She is looking good.

    Well done.

     

    Oliver

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