View Full Version : Plywood scarfing

07-11-2001, 12:43 PM
Hello everyone,
Last night I found an amazing deal on 1/2" marine plywood at my local hardware store because they ordered the wrong wood by accident and are trying to get rid of it at 12 dollars a sheet (normally $60). The sheets are only sold in 4x8' and I am constructing a 15 foot v hull skiff. I was wondering if it would be practicle to scarf these or just lay them on my hull separately with someting for the joints. Also my plans call for the hull ply to be 3/8" thick and was wondering if the extra 1/8" will be an issue. It would be great if I can get away with using these dimensions because it will save me a few hundred dollars. Then I may be able to afford a mahoganey transom.

07-11-2001, 12:52 PM
I think I'd be tempted to but the whole lot. Even if you couldn't use it now. But what sort of "marine ply?" Is there a grade stamp, a brand, how many plys, voids visible at the edges, "boats"(dutchmen)?
Sure you can scarf it or use butt blocks. Your boat will be heavier but probably by less than all the stuff you may load it with. And if your plan calls for much twist it may fight you more.

Tom Dugan
07-11-2001, 01:11 PM
Yeah, pretty much what Tom R said. Around here, 1/2" (well, 9mmm to be exact) okoume plywood goes for $90, so I suspect the hardware store variety is something less - more likely fir. Inspect it and get some idea of what it really is (as regards glue, voids, and species, in that order) before you put your cash down.

As far as scarfing, I've seen it called for so often that I've sort of come to the conclusion that it's the hands-down favorite way of joining plywood for long planks. I'd say you definitely want to be scarfing.

As far as the hull being 33% thicker - its impact will depend on the boat, and how it's driven. That is, if it's a planing hull that you're going to hang a 90hp outboard off, I'd say there is little effect. OTOH, if it were a narrow-beamed racing sailboat with really tight twists to the planking, you'd need to be more careful.

Also realize that this opinion may be worth exactly what you paid for it! http://media5.hypernet.com/~dick/ubb/smile.gif


(Of course $12 for a sheet of any 1/2 ply is really good, so if you've got storage space, why not buy a couple o' ton?)

jeff pierce
07-11-2001, 02:32 PM
I'm with Tom: if the stuff is verifiably marine ply I would take as much as the've got at that price. Use what you want and save the rest for the future or resell the rest for a small markup and all of us other boatbuilders will line up at your door to take it off your hands. Hey, if the've got enough of it you could finance the rest of your boat project that way.

I used butt joints (with butt blocks) for my 16' hull because its easier and they came out great. I suppose scarf joints would be the more "professional" solution, but my boatbuilding skills are decidedly "unprofessional". Either way, I daresay most builders use standard 4 x 8 sheets regardless of the length of the hull.

The issue I would be concerned with using the thicker plywood is stiffness. Bending stiffness is proportional to thickness cubed (to the third power for those not mathmatically inclined) so your extra 1/8" thickness makes the panel more than twice as stiff (2.37x to be exact). (OK, so I'm a nerdy engineer, please don't hold it against me.) In actual practice, making the bends at the stem on my runabout with 3/8" ply was brutal. I never would have been able to do it with 1/2".

Incidently, I made my transom (actually all the hull framing as well) out of red maranti mahogany, and it really wasn't that expensive. I found a local source that sells 1 x 4 for 85 cents per linear foot and 1 x 10 for $3 per linear foot.

Phil Young
07-17-2001, 11:07 PM
Buy the lot if its still there, and build a bigger boat. The bigger the boat the more you'll save. Scarphing ply is easy because the glue lines tell you whether you've got your bevel even. But joins, even glued, won't quite take a fair curve, and will tend to crack at the join after time.

Ian McColgin
07-18-2001, 09:11 AM
Yeah, buy the pile, but go ahead with your first skiff. If you build on a good, well braced strong back, it'll bend on just fine.

I scarf with a 12:1 bevel. Make a bench that has a really firm 4' end at least 4' - 6' long, but the very back end of the plywood can be held up on horses. Place one sheet squared up nicely with the edge of the bench, and the remaining that you're gonna plane (total 2, 4 or 6 or whatever) staggered back 6" each.

A long bed low angle plane is best. Get it really sharp and just have at it. The remark about the glue lines is spot on. Just make one huge sloping surface and all will match.

8:1 is also acceptable but given that you're putting some bend in thickish stock, I'd do the extra work of 12:1.

Plywood is amazingly easy to plane. I've done 4 sheets that thickness in about 15 minutes and I'm not very fast. I'm also not good enough to use a power planer - too many ripples. Nothing like a good sharp plane, some nice tunes in the background, and a lit pipe while planing.


07-18-2001, 10:08 AM
What Ian said, but unless you need the width, I'd cut the sheets to 2x8 and scarf up 2x16. It's a little more managable that way.

07-18-2001, 11:44 AM
The thing that bothers me about all this is, we don't really know what is ment by "marine plywood." Half inch normally at $60 suggests fir. $90 or more would be more like okoume. If it is fir marine ply why wouldn't they throw it in the pile along with and sell it as A/B exterior fir at, what, $30? $12 a sheet! If it seems to good to be true is probably is, no?

Otherwise, buy the lot. Build more boats.


07-18-2001, 01:35 PM
Well as it turns out I went to the store that had this wood (Home depot believe it or not) the other night to check out the quality after speaking with a local shipwright. He said if it had too many footballs and if there were any holes in the end grain then I shouldn't bother with it. It turns out the grade of the wood is A-B and it did have several footballs and a few holes so I bought 2 sheets for my decking only.
As it turns out I am building my boat at my landlords house,(it was a way for him to keep me as a tenant) so I have no where to store a pile of wood like this for my future boat which is definitely going to be bigger. As it is my landlord can't wait for me to get done making a mess of his yard but I told him what he was getting into when he let me start construction. I just keep telling him how nice it will be when I take him fishing every weekend.

Bob Cleek
07-18-2001, 08:56 PM
ROTFLMAO! HOME DEPOT? You gotta be kidding! They're advertisments about their "eco-sensitive" "top quality" lumber... LOL I've wasted my time picking through Home Despot's lumber stock once too many times to ever bother again... and that wasn't for boat wood either! What can you say about an outfit that crows that their top of the line stuff "has no bark on it!" LOL

Anywho, it just occurred to me as I was playing with my calculator... that $90 a sheet half inch ply costs about $2.25 a BF. You ought to have no trouble finding decent real wood planking stock for not much more than that. (I'm talking cedar, not teak or Honduras.) And... you don't gotta scarf it, either!