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richard

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Hi all,

My name is Richard Peeters and I am starting to build a (modified) WEEKENDER.

I am living in Belgium, about 15 miles east of Brussels and I am working as an air traffic controller instructor at Brussels national airport. This is my second attempt to build a boat.

I am deviating a bit from the plans for several reasons, some of them inspired by things I read in the different forums.

All plywood I use is lauan exterior (good stuff

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Hi Richard ... I think you're the first builder from Belgium we've had here! Welcome!

At first, I thought the keel didn't sound like it would be wide enough, but if I remember correctly 10mm is close to 1/2", so two 22mm layers would be about 2 1/4" thick. That sounds plenty thick.

The extra weight will add stability, and that might be important for you there, depending on sailing conditions. I'm considering adding some weight so I'll feel more comfortable in chop and small waves.

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Actually Frank' date=' 1 inch = 25.4 mm. So 44 mm = 1.73 inches[/quote']

Hmmm ... I think that is a bit narrow, then. Ply is strong, but I would feel more comfortable with more of it on the bottom. All the weight of the boat is on the keel when its on the trailer. You might consider that while building, Richard. (Others here may disagree, and we've had at least two Weekenders built with plywood keels, so maybe they will speak up with how thick they made theirs).

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I'll try with 2 layers - am pretty sure this is enough in plywood.

I can eventually add a thin layer on each side later.

I suppose the 3 layers is from the days there was a centerboard incorporated in the middle layer.

I've seen some pictures of trailers where the boat is not resting on the keel but on the bottom.

Richard

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My major concern is that you are using plywood for a keel, which in itself probably isn't a concern as long as you realize that it doesn't hold fasteners as well as edge grain on solid wood.

Secondly is you are planning on scaling up some of the dimensions of the boat by 8%. You are asking less keel to do more.

I would be scaling up the keel as well to add strength and ability to work well for you. Not making it thinner. There is a lot of stress applied when you try to pull the bottom down and attach it to the keel. If you fabricate the bottom as per the plans and attach the stringers, you are going to have a very solid and flex-resistant forward portion of the bottom. Drawing it down to the forward curve is asking a lot. Without all that much material to hold the fasteners, I think you are asking a lot of just two layers of ply.

There just isn't enough experience to judge how the plywood keels will hold up. They look to be a viable alternative. But the plywoods are for the most part soft woods. Dimensionally stable, but still weak in their ability to hold screws in end grain.

But having said all of that, it is still your project and your decision. I personally wouldn't be considering making one area weaker while asking the structure to do more. I would attempt to find a better balance in overall structural integrity.

But that is just my opinion.

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I think Lauan is not a softwood.

I intend to fillet the bottom to the keel with epoxy, which is to my experience very strong.

I will make the bottom with only 2 pieces cut from one 4 x 8 and one 5 x 10 plywood board (readily available here).

The keel is almost finished and it seems very strong. As I said, I can always add a thin layer both sides afterwards when needed.

Richard

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I would assume like yourself that the two layeres of plywood is strong enough in itself as a composite keel structure, but the main concern is that end-grain plywood (softwood or not is pretty irrelevant) gives very poor hold for fasteners. After all, you need to pull the hull bottom down to the keel curve with a good extra pressure to ensure that the glue between keel and bottom is actually biting properly, not leaving any voids and preventing that you ever get water in there.

With the plywood qualities I have had access to the fasteners would have had to be LONG!

You mentioned an aluminum strip at the keel bottom for protection. Gliding more smoothly on the trailer and a better combination in your case (I think), would be a bottom protection with a red oak or similar hardwood oiled 1 1/2" thick batten. If this one is properly glued on and through-bolted to pull the bottom down every 3-4 feet or so (recessed bolt heads on the under-side) you would achieve a solid connection between keel and hull bottom.

Just some thoughts! :roll:

Your idea about increasing the beam will probably pay off well. Building again I would have looked into something similar, especially from the widest point backwards.

Good luck :D

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richard, have you considered using three layers of the 22mm , that would be a great keel :) and shouldn't require you buying any more ply as I cut my three layers from two sheets of ply.

though I have made my keel from ply (standard thickness, three 18mm layers), only time will tell what the issues are I guess. My keel ply was made from okhume, which I think is a hardwood.. but not too sure. When I attached the hull to the keel I meant to angle the screws so that they cross the ply layers as someone suggested on this forum.. but I forgot and just drove them in straight. I had no significent liftoff.. my screw tests (subjective) indicated that screws did pull much more easily from ply end-grain that from real wood, but that they still took a bit of force.. I am pretty sure I could have hung my weight from a single 2 inch screw in the ply.. I put in a few more than was suggested in the areas of tightest curvature. and have recently expoxied a wooden fillet along the hull-keel join... and then covered with glass... I suspect this hull/keel join is now as strong as the proverbial ok.. but in reality the proof is in the pudding to throw in another metaphor.

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I suspect this hull/keel join is now as strong as the proverbial ...

... brick s#!thouse? That's the proverbial over here, wander what it is in NZ, or US?

The PC keel is only two layers (of 19mm=38mm=1 1/2"), and it has the extra width of this mod and more. Mind you it has a 4"x19mm capboard on top. Can't say how the use of plywood v hardwood would affect the strength though.

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has anyone given any thougt to using a hollow core keel?

What I am thinking about is ply on each side, and in the center, two 1 inch strips (one at the top and another at the bottom.

The ply could be glued and screwed to the strips and the strips and the hull would be glued and screwed to the top strip.

Any comments?

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Sounds tricky, but not impossible. You would have to scarf the plywood parts to get the length, as I don't think a butt jont would be adequate for the job. The hollow cavity inside the completed keel would be vulnerable to rot. The benefit of such a design would be light weight for the strength, this is not neccesarily a good thing for a keel. The hollow section could be filled with ballast I suppose?

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Yes I suppose it would go some way towards it, providing you were taking the boat out of the water and draining/airing the keel on a regular basis.

What would be the benefit of such a keel though? I would imagine that the price difference would be quite small compared to a solid one, especially with the epoxy encapsualtion. It would certainly be more difficult to construct. The principal advantage of this type of construction is it has a high rigidity for its weight. I'd have thought that durability would be higher on the list of priorities that rigidity - and the weight factor would , presumably, be countered by the addition of ballast?

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Simon,

I agree with you. I think it would be much easier to build a solid keel out of three 1x12s. Which is what I built mine out of.

I was just wondering if it would be better than two pieces of ply. I think it would be, primarily because it gives you a good solid surface to attach the hull to. I would be concerned about screwing into the edge of a ply keel.

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Yeah, I suppose we're talking about availability and cost constraints here.

Hmm, I suppose you could build the middle layer of a three layer plywood keel slightly smaller so that a track of solid wood could be built in between the two outer layers, screwed into these outer layers from the side. That would solve the plywood endgrain problem without resorting to a box structure. Alternatively a capboard (as on the Pocket Cruiser ) would add some strength, but would require through bolts to deal with the endgrain issue. The ply can be screwed to this.

My personal opinion is that if possible the keel should be layed up from solid wood - this can be done without 1 x 12 planks by staggering butt joints along the length. I think this is a better solution to the problem and should be done unless its completely impossible - in which case the box keel should certainly be considered as one possible solution.

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Richard,

according to the PC plans the keel is made from _two_ layers of 12x 1" (3/4" dressed) laminated together with a capboard on top, making a T shape. The lower deck is screwed to the top of the T. I made mine from smaller width pieces, my website (www below) gives a drawing of the layup and a not very good picture of the complete keel.

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