10/01/2024 at 9:12 am #26357
I have embarked on repairing a Mk1, 12711. Some upgrades had been completed, the mast base for example, but the hull needs some attention, photos attached.
I believe I have cleared any rotten hull with the structure of the boat looking good(hog etc), to a first-time boat repairer anyway !
For the hull I plan to,
- Patch with 4mm-ply using scarf joints fixing with epoxy and screws
- Patch the bow
Based on the photos attached, anything I need to consider from those with more experience ?
The keel was rotten forward of the centre board casing. I have requested the Mk1 build plans from Julie, I assume I will be able to shape a replacement keel from these plans ?
I plan to use the boat for cruising.
Appreciate any feedback,
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10/01/2024 at 9:14 am #26362
10/01/2024 at 12:59 pm #26365
Don’t you just love it when you type a detailed response and then inadvertently lose the lot before submitting it!!!
I have seen your query, and have a response in mind; I will try to get back to you later today with a second attempt!
10/01/2024 at 8:06 pm #26366
Well done thus far!
One problem that you will very probably find is matching the thickness of the plywood. It is likely that your boat would have been built with plywood to an imperial thickness, somewhere around 1/4-inch (equivalent to 6.35 mm, but that is not of course a standard modern thickness); 3/16-inch (equivalent to 4.76 mm) seems perhaps a little too light – and certainly slightly less than the current class minimum, and I don’t remember whether 7/32-inch was ever offered. Your replacement will almost certainly have to be metric thickness, either 5 mm or 6 mm.
You suggest 4 mm, but that is definitely too thin, with 5 mm being the class minimum, and since you say that you intend to use the boat for cruising and make no mention of racing I would prioritise strength over weight-saving. So although I think on balance that 5 mm is probably the one to go for, I recommend that you at least consider 6 mm, but be guided by the thickness of the original.
Whichever thickness you choose will probably not be an exact match. For your use of the boat I recommend fairing in the change in thickness along the edges of the join; this can be done by a combination of planing and sanding what stands slightly proud, plus filling (use a 2-part epoxy filler) what is slightly recessed. The alternative – of choosing a thickness slightly more than the original and then thinning it to a precise match before fitting – is not worth the enormous effort involved, unless perhaps you have access to an industrial thicknessing machine; and even with such a machine, this approach might result in unsuitable cross-grain on the inside surface.
More to follow in a separate response.
10/01/2024 at 9:08 pm #26371
UPDATE re plywood thicknesses: you may actually be in luck.
On checking, I see that your sail number dates from 1986, so it seems likely that your boat may be just young enough to have been built with metric plywood. One website states that plywood has been made in metric sizes since 1978.
So that may be one problem solved.
You appear to be on a roll!
10/01/2024 at 8:23 pm #26367
I don’t know whether to be concerned, or to stand in awe and admiration, at your intention to use scarf joints.
A single scarf joint is reasonably easy, and with reasonable care a good job can be made. Of course the two gradients need to be a perfect match, but the precise location of the join is immaterial.
A pair of matching scarf joints is significantly more difficult, because (on a typically 12:1 gradient) a slight difference in the amount of wood you shave off the bevel shifts the joint laterally by 12 times that amount; and if you are trying to slot a graving piece into a gap in such a way that the base of it sits flush and the length is an exact match to the gap this requires some very delicate fitting. It is easier if both faces are accessible to the plane, so that you can use slightly thicker wood for the infill, and then plane both surfaces to get them flush; but all too often the “inside” surface is inaccessible because part of the structure of the boat is in the way. I have successfully done a few of these, on a variety of boats, when replacing damaged sections of rubbing strake, and I have been pleased and satisfied with the results; but I regard them as a demanding test of my skills, and about the limit of what I can successfully achieve.
Trying to do four matching scarf joints around the edges of a rectangle is a whole new challenge. The requirement is, of course, to get all four joints neat, and with the infill piece sitting at exactly the right height, and with no visible gap around any of the edges; and moreover to achieve this on a curved surface. I am sure that the professionals can achieve it, but when I tried it for two deck infills on my vintage boat I eventually gave up and settled instead for neat butt joints with a backing pad underneath.
If you are up for the challenge, and confident that you can make a success of it, I stand in awe; but if in doubt you might consider butt joints suitably backed on the inside.
More to follow.
10/01/2024 at 8:33 pm #26368
If you decide on butt joints it is worth considering whether you can use the existing framing to back up at least some of those joints, at least in part; and then consider taking the joint a little further outwards from the hole in order to use those structures. An obvious example is of course the hog, which is amply wide enough. At the other extreme, the (lateral) frames are almost certainly not wide enough, unless you bond in additional pieces as “sisters” to the faces of the frames in order to provide a greater thickness (fore-and-aft). Between these two extremes, the (longitudinal) stringer is narrower than I would wish for use as a backing piece, but it might make the basis of a backing pad if then further beefed up with judicious use (two or three layers) of woven glass cloth and epoxy over the stringer and extending onto the plywood for at least an inch (and I suggest two inches would be better) either side of the stringer.
Given the extent of the work involved, it is worth considering the question whether it might be both easier and more satisfactory to replace the full width of the panel from hog to chine piece, for the requisite length. It is difficult to judge from the photos, but it is worth seriously asking the question.
One further point to follow.
10/01/2024 at 8:42 pm #26369
It appears from the second set of photos that the damage extends right to the forefoot.
Although I have never had the misfortune to need to work on this area, I understand from my reading of the Bell Woodworking book and also seeing Searson Thompson’s film around 15 years ago that this one area is the most difficult part of the entire boat to fit, because the plywood is forced into a curvature that is just about on the very limit of what it will accept. According to Bell’s, their technique was to first secure the ply abaft this region, and then pour a kettle of boiling water over the ply where it is required to bend so severely, in order to soften it.
Link to the copy of the book on this site here, see page 9. This page also gives some potentially useful ideas on marking and cutting this area to shape, and also on the change of bevel at the chine in this area.
An adaptation of that technique might be a way forward for this apparently very difficult part of the job.
Hope this helps.
- This reply was modified 1 month, 2 weeks ago by Oliver Shaw. Reason: Inclusion of link to relevant page in the Bell Woodworking book
10/01/2024 at 8:46 pm #26370
As a final P.S.; on a matter of nomenclature, your boat is a Series 1, not a Mk 1. Mark 1 (and II, III, IV and IVa) were designators for successive models of early glassfibre boats, and I am sure you wouldn’t want her to be confused with her GRP sisters – especially as you appear to be sufficient of a craftsman and enthusiast to specifically enjoy the wood.
This is something that many owners are not aware of!
11/01/2024 at 3:48 pm #26374
Thanks for your very comprehensive response.
Starting to agree with your comments on a scarf joint, may try a routed scarf with 6mm as you have noted. Not keen on replacing a full panel, at this stage anyway ! Appreciate your comments on woven glass, sounds like a good idea.
For the forefoot, I am hoping to just patch the single ‘small’ plywood section that was rotten, I believe I have cleared back to solid wood. Layered epoxy with additives to ‘fill’ the remaining gaps.
Julie sent on the plans for the Series 1, but I do not see the measurements for the keel, any suggestions ?
One additional question if I may. I removed the bow bulkhead, it seemed to have been retrofitted and was in bad shape. On getting access to the bow interior I noticed most of the internal varnish has deteriorated badly and will need repainting. On a boat of this age is adding a few layers of epoxy of value in advance of a re-varnish of all of the box internals ?
From a cruising perspective I do not plan to replace the bulkhead.
13/01/2024 at 10:23 pm #26383
Will try to measure up for the keel sometime; but it is moderately unlikely to be convenient tomorrow, and if not done tomorrow I am then away for a week.
If no-one else comes up with dimensions feel free to remind me in about a week’s time.
13/01/2024 at 8:31 am #26379ChrisParticipant
When we did the bow repair on my series 2 we did the forefoot in two pieces of 3mm (or maybe 3mm and 2mm – it was a while ago!) laminated together to achieve the thickness and go round the curve. It needed a fair bit of sanding to fair in, but was much easier than trying too force a small piece of 5mm round the curve. This was butt-strapped as the chances of getting a decent scarf on a curve like that are just about zero!
My front end looked very similar to yours and we went a lot further to ensure all the rot was out. I think you need to do the same as if there is any rotten ply left you’ll be doing this again in 2-3 years time – you definitely don’t want that! The hardwood can be dried and recovered even if it looks awful. As long as its not powdery its fine. I removed the first 18-24 inches of keel, dried it all out and replaced the original.
I would reinstall the bulkhead even for cruising. If it was retrofitted it must be one of the last “bag” boats. Epoxy coating will be fine, and being solvent free you wont gas yourself in the process!
13/01/2024 at 9:12 am #26380
That is a good technique that Chris describes, always provided that you can avoid voids between the two layers of ply, and I am sure that he paid due attention to that point.
And staggered butt joints remind me that I have twice in my younger days repaired rot in Firefly dinghies by laminating up thin narrow hardwood strips in crossing diagonal layers, I think three layers. If the first layer is thin enough, and the strips narrow enough, it is reasonably easy to get them into a fair curve. Once they are bonded in and the glue cured they provide a reasonably stable surface for laying up the second (diagonally crossing) layer on top of them; and once that layer is cured you have a good firm surface to lay up the third and final layer. That method would seem to be adaptable for the forefoot on a GP14, and it is effectively laminating up plywood in situ.
I fully concur with reinstating the bulkhead “even for cruising”; particularly if taken to full height it achieves far more buoyancy than is normally achievable with bags, and in the event of capsize that additional buoyancy is highly desirable! The minimum buoyancy required for class rues is just that – a bare minimum; it will enable the boat to be righted, and bailed out by means of a large bucket, but it will not make life easy! Certainly if one wants the transom scuppers to do their job properly it is desirable to maximise the buoyancy in the boat, and particularly that in the bows.
Even with a full bulkhead you can still use the enclosed space for stowage; just fit nice large waterproof access hatches. Mine on A Capella are nearly a foot in diameter.
Again I concur with coating everything with epoxy before painting or varnishing. It might be worth considering Smith’s CPES, a wood-based epoxy originally designed for treating rotted wood but also increasingly used as a varnish primer. I have been using it for about the last 12 months, and am pleased with the results, although a longer period will be needed to fully evaluate how effective it is in the durability stakes.
Incidentally if the bow region is to be totally enclosed by a bulkhead it won’t normally be seen, so you might wish to consider using paint rather than varnish; it is probably more durable.
Hope this helps,
13/01/2024 at 1:39 pm #26381steve13003Participant
Hi having owned 4 Series1 wooden GPs I have on several occasions needed to make repairs to hulls, fortunately not due to rot just after accidents. When replacing hull ply wood I always cut back any damage to the nearest frame or stringer if possible, this gives a clean support for the new plywood and if the inside is still varnished the new piece is less obvious. If a frame or stringer is not easy to cut to, as if damage forward of the buoyancy tank bulkhead, if still in place, damage is cut out to a pre cut piece of ply and the inside of the prepared hole is backed with support pieces for the patch.
i have also replaced the bow section forward of frame 1 following the ply being damaged by a fire. After cutting the old ply out to the frame, keel and stringer, I managed to fit a new piece of ply to the correct curves! A bit of persuasion was needed in the form of two struts from the adjacent garage wall to hold the new piece in place until the epoxy set. A bit of careful filling and sanding resulted in a repair that can not be seen.
No need for scarf joints simple but joints with supports for the edges will always work with some filler. Always coat ply with an epoxy coating to ensure long life. My last Series 1 boat (13003) was epoxy coated inside and out when new and is still down to or below the minimum class weight.
13/01/2024 at 7:06 pm #26382ChrisParticipant
So I think we may have drilled holes in one of the ply layers to try to prevent voids on the bow. I think we did this with the outer layer. It never reared its head again in the 4 years I owned the boat and to my knowledge it’s still fine.
I agree with taking the ply back to a stringer. Yes, the hole is a lot bigger but you have something solid to fix the new ply to and should have removed any rotten wood. We definitely did this with mine, it would have been dat not to as it would have been within centimetres anyway! I think its easier to shape a larger piece too.
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