Plywood advice

[Simon B]

After the successful rollover of my 1955 15ft Cavalier and stripping of the old plywood bottom (as pictured) I have a few questions regarding the replacement material I should be using.


The keel and frames are in nice condition considering they are 57 years old!!

I hope I hold up that well in 23 years time!!

1.) The amazing packet of information sent to me by the Mariners Museum has an advert stating "full length marine-plywood panel construction" out of interest does this mean that the original bottom would have been comprised of full length plywood sheets bow to stern? Or did that simply mean it was comprised of plywood only? The bottom I just stripped off had a join on both sides on the 3rd frame from the bow but it was not the original bottom.


Original 1956 sales brochure

2.) The packet from Mariners Museum also advised that the original bottom was 5/16" Fir Plywood (as pictured). Now after going to Condon Lumber in White Plains NY today (what an amazing place) they only sell 5/16" Plywood in 4'x8' sheets and considering that I need 10' to get to the frame where the bottom was previously joined the 5/16" is not an option which leads to my question. As it looks like I will need to use 3/8" in 4'x10'or12' has anyone used 3/8" (9mm) material in place of the original 5/16" (8mm) and did it present a problem at the bow where it needs to curve? (Condon Lumber price list pictured with sizes in stock)


Original 1955 Spec sheet for 15' Cavalier


ML Condon Lumber List/Stock

3.) Should I use Mahogany Occume (MFC) plywood or Mahogany - Occume (Bruynzeel Holland) considering this boat is going to be used in mostly fresh water and will spend a maximum of 7 days straight in the water at the lake house.

Maybe I am way off in which case please put me straight!!

Simon

[mfine]

If it was a plywood bottom to begin with, I would at least give a cold molded bottom serious consideration. Instead of one thick sheet of plywood with a seam, you would use 2-4 layers of thinner plywood with joined with epoxy, such as West System, with non over lapping seams. This will give you a stronger longer lasting bottom that will effectively be one piece, seamless and curves are also much easier since each layer is thin.

[kleiner]

Scarf your 4x8 sheets to get a single panel 4'x whatever length you need. A scarf joint is fast, simple, and if done correctly, as strong as the rest of the panel. Use a minimum of 1:8 slope (e.g., a 2" long scarf joint in a 1/4" thick panel), but 1:12 is better. Wet both faying surfaces with unthickened epoxy resin and then glue together with thickened epoxy (wood flour from your sander works well). Don't over-clamp and starve the joint of epoxy.

This would be the fastest and quickest way. If your boat was originally planked with plywood, there is no need to cold mold it as plywood can be expected to take the shape of your bottom with no problems (i.e., there are no complex curves that would necessitate laminating thin layers of plywood strips into a curved panel).

- Kevin

[maritimeclassics]

Where the front plywood came off you should see a frame the has been doubled up with a small strip where the two pieces of plywood came together. Is there another frame about 7 to 8 feet from the stern that also has the frame double on it? The boats that I have done were 3 pieces of plywood on each side. From the stern it was about 8 feet then another piece in the middle to get you far enough forward to the small triangular shaped piece for the bow. This is the way I would do it, replace the bottom battens, clean off the frames, keel and chine landing, CPES everything and the plywood. Then add a 3 inch strip (glued and screwed) to the back side of one of the frames for you plywood joint at about 7 to 8 feet, 5200 and bronze boat nail the plywood down. I use boat nails for 2 reasons, 1 they hold very well and 2 if you hit one with a screw when installing the planks is no big deal the drill will easily go through the head or go off to the side.

[jim g]

West System makes a nice scarfing jig that bolts to your circular saw that cuts your 8 to 1 scarf and its not very expensive.

[jim g]

West System makes a nice scarfing jig that bolts to your circular saw that cuts your 8 to 1 scarf and its not very expensive.

[maritimeclassics]

I really don't see the need to scarf if you are putting another layer over the top. I do not know any builder who does this, most would offset the joints or the planking would be the strength with 5200 in between. To me scarfing is more if the plywood was the only layer that you had like a kayak or the drift boat I built. Just a thought.

[maritimeclassics]

Also a good time to tighten or replace all the bolts and screws in your frames, keel and chines

[mfine]

I really don't see the need to scarf if you are putting another layer over the top.

Is there another layer on top with the Cavalier division boats? The sides were not planked so I assumed there was not a plank layer on the bottom either. That said, 5/16" seems pretty thin for a single layer.

[maritimeclassics]

I am not sure on that I would think it is double layer construction. He would have to measure his keel and chine landings to see the thickness, I would double layer it for sure.

[mcisaac inc]

fyi,a 15'cavalier is a plywood boat with one layer of 3/8" inch ply as the bottom. They were scarfed at the factory mid ship, and some were then fiberglassed as a factory option.....................markmcisaacinc.com

[mfine]

I wonder where the 5/16" in the factory info came from. I would definitely feel much better with a 1/2" bottom, the 5/16 sounds way too thin.

[Simon B]

The 5/16" info came from the original boat specification sheet sent to me by Mariners Museum (I included a picture of the specification on my original post).
Also the bottom I just removed was 5/16".
You now have me nervous about putting back what was originally on it!!
I have just purchased a 3/8" mahogany sheet in 4x8 to see if I can get it to shape correctly to bow section if that proves easy I will take your advice and take another step up to 1/2"

Still working on filling holes with dowels and epoxy.
Thanks for all the advice on scarffing and joining - the bottom I just took off had the 1/2" backing supports on the doubled up frames with brass bolts down each side of the butt join. Still thinking about what I should do.

I think I am crazy but I am also enquiring about a barn find that I have come across!!

Simon

[mfine]

If it held up OK with 5/16" since 1955, it must have been strong enough. I would still probably laminate a couple of thin layers of epoxy coated plywood and then not worry about it again for 75 years. There are benefits to that method beyond just fitting curves, but it will be more expensive and more work.

[mcisaac inc]

sorry, i meant 3/8" from the factory...........my mistake

[maritimeclassics]

Well scarfing is what you may have to do if you are going to use only one layer as factory original which I am sure would work. If you go thicker you are going to have extra work with fairing and fitting to the keel and chine. If it was me doing it I would look at marine grade plywood to see if you could come up with the correct thickness using two layers and make up the 5/16 you need. Then you could offset the seams of the plywood from the 1st layer to layer 2. If you scarf make sure you land the scarf over a frame so it is not in the middle of 2 frames. I just like the idea or double layer bottoms.

[Simon B]

So after testing the 3/8" mahogany plywood I am going to go ahead and use it - it seems to cope with the bow transition area curve.
I have a question regarding the filler I should be using for the counter sunk screw holes in the hull?
The material is to thin to allow for bungs and the bottom I took off had a filler compound like bondo in the screw holes - I am seeing loads of different tyopes of filler compounds but can someone give me a product name they know will work in this application?
It will of course be sanded and painted over copper bronze.