hole in hot molded hull

[albacore471]

I have a hole about 1/2 the size of a credit card right next to the bilge keel of my 1958 hot molded (mahogany) fairey marine albacore. I was stripping the paint off of the boat for a complete refinish when I found this small spot of dry rot which goes all the way through. Else so far seems very solid. After repair the boat will be painted and raced, I cannot find any good info on how to fix the hole. I of course do have access to the hole from inside and outside the boat.

[Tom Robb]

Never tried it myself, but I'm told you use a router and route out each layer of the layup stepwise (like a negative image of a step pyramid)largest on the outermost and smallest on the innermost layer. Then glue, probably w/ epoxy, patches (the same thickness as the veneer at each layer) that fit back in the hole one layer at a time. A form braced against the inside of the hole will likely be needed to keep the hull shape on the patches and if the hole is large enough, vacuum bagging may be useful.
Of course you'll have to make the crater big enough to loose ALL of the rot.
A more difficult, IMHO, way would be to taper the hole and matching patch as a section of a cone.

[Ian McColgin]

Doing just this with all the holes in Il Pipi, the Thistle.

It's actually easy, but tedious. You need a gizmo made for the laminate trade that is a router attachment for making inserts. It's in essence a pair of collars sized just enough to allow for the 'kerf' of the small cutting router bit you use with them.

Make a female template and use the smaller collar when cutting the inset, larger collar on same template when cutting the hole in the hull. Stunningly exact once you get the hang.

I temporarily screw some laths on the inside - plastic between them and the hull and hole - to give the first two layers something to land on.

Essentially, you're making a stepped dutchman. I go for one more layer in the new ply than the old had and figure while the little butt joints aren't as good as a perfect 12:1 bevel, it will be good enough and working in thin layers is the only way you're going to get the curves to match at all.

Depth control is all and it's especially hard to get the raw hole and the first step as there's not much strength there. Best cut the smallest (ends up inner) hole, put the laths in, and then make the step for the second layer. These are so thin and flexable they can be held down with push pins or staples through plastic for easy removal.

After the first two layers, perhaps it's best to go one layer at a time. Try to keep the epoxy off the top of each new layer so your between layer cleanup is easier.

You end up a toudge proud on each side - easy to fair down.

I make the inner and outer layers match the associated grain but the three (this is 1/4" hot mold original) inner layers I just plop at right angles back and forth. They don't match the inner grain anyway as the steps are between layers of the original.

Edited to add: If you've got much curve to the hull, get 'wiggle board' for template material. Best get the attachment and get used to fooling with it on your router a bit before buying anything else. I find depth control the trickiest part. On each layer going out, I've been adding a step of 7/16". That way I can cut the outline of the new step and move over to a 3/8" flat router bit to clear it all out.

G'luck.

[ 12-07-2004, 06:44 PM: Message edited by: Ian McColgin ]

[albacore471]

Wondering how the plunge router attachment for my dremel might work.

[Cuyahoga Chuck]

I believe there was a pictorial of a repair on a 3/4" thick cold molded hull in "Devlin on Boatbuilding". It was pretty much as was decribed above.
Charlie

[Cullen T.M. McGough]

Bah. Scarph it and don't be shy with the epoxy. An extra plate on the inside extending past the edge of the patch wouldn't hurt, either.

[Ian McColgin]

The Thistle is 5 layer 1/4" plywood with compound curves.

Previous repair attempts at straightforward scarfed in full depth plugs were failures. Even if you could get a thicker piece of wood to fit and planed it down inside and out, the structure would be all wrong.

Il Pipi's hull has inner and outer layers that are 1/32" and three central layers 1/16" each. My step scarf schedule goes with 5 1/1/6" layers but the outer two are fared after, so it works out about the same as the original. In practice, the flats - tred part of the steps where the lamination layers join - do not exactly register plug to hull. I consider this a good thing. I've been making each step 13/16" bigger than the one inside as I happen to have a 3/4" router bit that works well with the 1/16" cutting bit that fits with the template collars.

Albacore, note that in my e-mail I gave you the wrong numbers and these are right.

So it's 13:1 bevel. I'm not worried about the step structure as plywood has both cross grain and parallel to grain butt joints in it's layers anyway and the stuff seems to work fine.

On big developed surfaces, the veneers will not fold as a sheet into the compound curve. I have to strip cut narrow enough that they can lie flat enough and spile them to fit - an entertaining pain.

One great and cheap sourse of lamination material has been old door skins.

Again, a conventional dutchman cannot be worked into a compound curve. Building up cold plywood is the only thing I've hit on.

[Stiletto]

The plunge router technique should work, I dont know how powerful a dremel is compared to a regular router.

I have done some similar repairs on the bottom of a couple of dinghys with a router. I cut the outside patch and used a stanley knife to mark the shape around the outside of it that I wanted to cut,then marked out the smaller interior cut in the middle of this shape and cut right through with the router, and then starting away from the edges carefully freehanded up to the cut line with the router set at the appropriate depth. (practice this first as the router cuts well one way and runs off the other.)

Epoxy glue fastened the well epoxied outer piece, and then the smaller inner piece was glued to that. It was held in place while setting by a wooden strap covered in clingwrap screwed either side of the patch and removed later.
I have done this with a stepped shape piece of ply and also with two separate pieces.

This method may work in your case.

Good luck.

[RichardBlake]

Good information so far.. Did a couple of these many years ago - a couple of things which might help:
I don't know how many veneers in an Albacore - let's assume 7. I found I got better and easier results by stepping the veneer patches in from both sides - so the smallest patch was in veneer number 3, numbered from the inside of the boat. 7 is the biggest, followed by 6 and 1 about the same, 5 and 2 the same, then 4, and 3 the smallest.
Resist the temptation to rout out too much before you start, beyond tracking the rot - otherwise things can get too floppy.
From the inside of the boat, rout out for patches in veneers 2 and one, for your hole I should think the patch in 1 should be at least 6" by 6". The patch in 2 slightly smaller of course. Glue these two in. I found it best to use the absolute minimum of staples, and to hold the veneer in place with a curved backing piece covered in firm foam-rubber and clingwrap, well braced, plus some bracing support from outside the boat, around but not over the hole, so you can clean up the glue squeeze-out on the other side before it dries!
Then as you work on the rest of the repair from outside the boat you're pushing against two new veneers, from their convex side - stable working base.
I didn't have a router at the time, so I clamped a Stanley-knife blade between two steel flat bars, leaving the point exposed just to the depth of the veneer to be cut out. Make a lot of straight cuts, then remove the waste with a chisel and a small fine scraper. Sand and score the gluing surface.
One of the repairs I had to make was like yours - next to a glued and screwed bilge keel (just a light batten to protect the bilge from beaching damage, really)and the rot was coming from one of its fastenings. I'm afraid the only solution I found was to carefully cut away about 2 feet of the batten completely, then repair the ply as above, then scarph a new length of batten back on, 12 - 1 each end, stepped. I used epoxy throughout.
I suspect that a repair with the two largest patches in the outer veneers might even be stronger. It was certainly easier than the first one I did, where I tried to begin with the smallest pieces! Holding them in position was a ****! I suppose it would be more 'elegant' to have the smallest patch in veneer 4, right in the middle - but just two bigger patches on the inside of the hull were easy to glue up together in one go.
That was in a varnished hull. I confess to a little hubris when I saw it looking good three years later!
Oh yes - it also made a big difference when I lightly steamed (waving a kettle at them) the pre-cut patches, rolled them to a tighter curve than their place in the hull, and left them to set overnight.
Ian is right about compound curves, although I found that for the size of patch we're talking about the new veneer would settle well into place as long as I wetted it out very thoroughly with neat epoxy before applying the thickened stuff, and used the foam rubber pressure pad (about as firm as the stuff used for thin tumbling mats). I was using 2.5mil mahogany.
I expect I've burbled on too much about the obvious... I hope it helps a bit anyway, and as always that the real experts will shoot me down if I've said something dumb!

Richard