Where to splice sandwich wall skins?

[DougGirling]

I am probably making a mountain out of a molehill, but it's easier to tweak the design while it is still paper...

I'm using the Grumman 2 profile for our TD and will be building sandwich walls using 4x8 sheets for the skins. (I don't want to rely on the availability of larger sheets which will mysteriously vanish when I go to order them.) For aesthetic reasons, we're going for a woodie look on the sides, which allows me to hide and seal the exterior seams behind decorative woodwork and butt-join the seam over a framing piece. I'm not currently planning on glassing over the wood, though I will seal it in epoxy.

If I go for the stock 10' length with the bottom of the walls flush with the frame bottom, a 4x8 sheet nicely goes from the rear to just in front of the door hinge, which is a natural place for a stile in the trim, and puts the butt joint over a hardwood framing member. (sweet) A quarter sheet of ply finishes off the front of the exterior, using about 1-1/4 sheets per wall for the exterior. A 4x8 sheet more than deals with the interior skin for an interior wall in the sleeping compartment, going from nose to rear bulkhead. It also means that the inside and outside skin splices are nicely staggered.

OTOH, if I mount the wall on top of the floor and use the woodie trim to hide the floor and frame, I can expand the profile by about 8%, keeping the same proportions. This gives me a cabin that's a little shy of 11 feet long with more headroom inside. The interior sheet will still (just) seamlessly run from the nose to the rear bulkhead. The exterior will take about 1-1/3 sheets per side, given the extra length, but the exterior sheet running forward from the tail now falls about 3/4 of the way through the door. This would put the seam in a high-stress area and is not nearly as elegant, though the seam could still be hidden by decorative woodie-ness. Alternately, I could run the main exterior sheet aft from the nose, but I loose the staggered interior/exterior joints in the process and end up with both joints at/near the rear bulkhead. A 3rd approach would be to place the exterior joint near the middle (at the aft end of the door), at the cost of 2 sheets per side and a lot more scrap.

With the frame constructed of 2"x3" tubing, I don't think the wall is going to be subjected to much flexing, so the joints aren't going to be carrying much load. This should mean that there are few structural restrictions on where the exterior seam goes. With that in mind, what advice to you all have for where to put the exterior seam?

Thanks in advance,
Doug

[Woodbutcher]

I am just finishing a Woody design 9' long. I wish I had gone 10'. Mine did not follow any plan. It's a basic Benroy design. I put my walls on top of the floor. Then I seamed my plywood about midway up the sides, running length wise. The rear area because of the curve of the body allowed for a single piece vertically. So I have 3 pieces of 1/4" on the sides. I did layout the frame work in advance to be sure my plywood worked. I to wanted extra headroom. I was not as concerned with how far I could stretch my plywood. Because of the overall cost of this thing I didn't want to go cheap. Most all the cutoffs went into other areas of the build so really there was little waste doing it this way. I covered all the seams with the Woody framework. You may find some pics in my album that will show what I did. I will try to add a few current ones as well to show the near finished product. Good luck in your build. Good planning is important to a good build.

[Arne]

I moved my door back 4 inches so the front of the door (the hinge slot) is the vertical seam on a full sheet of plywood starting from the back end.

[Esteban]

This is a timely question. I'm going to build a 5x10 Grumman inspired teardrop too. I plan to splice the 1/4" plywood for the sides in the middle of the door area to minimize the finished spliced area. The splices will be backed by framing. That means I'll need some unspliced plywood for the outer door skin...maybe from scrap or from 1/4" plywood cutoffs from the bottom of the sandwiched insulated floor. I plan to fiberglass the plywood joints, and epoxy the rest of the side to get a smooth base for a final epoxy boat paint finish. I also plan to fiberglass the roof and round over the transition to the sides.

I'm toying with the idea of splicing about 4 inches of plywood at the top to have 52 inch high sides outside that cover the floor and frame, so the inside cabin area can be about 46 inches tall.

The welder starts building the trailer frame this coming Monday.

[Gage]

Has any one thought to splice the interior skin at the galley bulkhead. Then the exterior can be spliced anywhere if your going to do exterior trim framing for effect.

[jeepr]

I wanted mine a little taller than 4'. So my plywood is up seamed right at the front of the door and right behind the wheels. So the bracing for the front seam brace is used for the door and the rear one is part of the galley bulkhead.

[schaney]

Another option to consider is joining the sheets with a scarf joint. This is regularly used in boat building for making longer sheets. The joint will be stronger than the plywood, so it's a asthetic decision where it goes not structual. I scarfed to two sheets of 1/4" together for making this 13' kayak.



Here is a overview on scarfing plywood, http://www.duckworksmagazine.com/05/articles/scarfs/index.cfm

[Miriam C.]

I used 3/8" Baltic Birch (5'x5' sheets) I but joined the pieces by gluing and screwing a piece of luan to the inside. My insulation in that area is thinner and since I cut my framing to a real 1x2" it all fit well. My door lands at the splice so I thought it real important to back that up.