Re: MDO plywood for transom
MDO (Medium Density Oriented Strand) board is now made with water resistant glues and could be used except it is still not rated for spans over 18". Forget that black diamond on your tape measure. Glassing it won't add enough strength to it for a transom with a large motor (300-400lb). I prefer to use Super Ply or Marine ply. The diff between them is Super Ply (Roseburg Lumber) is made with water proof resin glue and Marine Ply is made with water resistant glue. Both are rated as having few or no voids.
Um... I suppose it depends on which marine ply you use, but they're almost all made with phenolic glue (a form of epoxy, really) which is non water soluble and therefore water proof.
Side note: The "span" calculations for housing work or carpentry don't really apply to boats, because the transom wood is mostly used as the core of a composite panel.. the strength comes from compression resistance and shear resistance, not from the stiffness of the wood. The wood strength numbers would matter only if it's an uncovered transom like in an aluminum boat.
on to your next point...
If you're looking for bullet proof, lay in a piece of bi or multi-directional weave carbon fiber, soaked in West Systems 105/205, between two pieces of 3/4" super ply and then glass that.
I'm sure you've used this with success, but I need to point out a couple things. First, I'm guessing that placing the cloth between the layers of plywood comes from the old practice of gluing ply layers with CSM and poly resin. For this, the CSM in the middle is needed for adhesion (because of the way poly works).
However, if you're using epoxy like the west stuff, you don't need anything but the epoxy mixed with a little fumed silica (cabosil) or another thickener like west sells to glue the panels together. It'll do a fine job, and be much stronger than ply glued with poly resin and CSM.
In fact, placing a piece of cloth (as opposed to mat(CSM) in the middle of a sandwich like this gets you nothing at all, regardless of what resin you use. It's non intuitive, I know. Here's why this is the case:
1) Fabric doesn't help adhesion any, only CSM does, and it's only needed with poly
2) The strength of a composite panel like a transom comes mostly from the fiberglass. The wood is there to keep the two fiberglass skins apart and parallel, like the top and bottom of an I beam. The wood is the center of the 'I'. If you put fabric in the middle of the wood it's like building an I beam half as tall... you don't get even half the strength of the full height.
Here's a good reference to how the panels work:
http://www.sandwichpanelmachinery.com/knowhow.html
All this is not to say that the wood doesn't add stiffness... plywood itself is a composite panel, and it does make your transom stiffer than one made from EG balsa core.
Now if you want a super-strong transom, you can get one with a good quality plywood core. No voids is important(voids are holes in the center of the I-beam). If you want lower weight and better water resistance, use foam core or hex cell, but the plywood is actually stronger with regard to compression and shear than most foams.
Cover the plywood on the outermost surfaces with carbon fiber in 2-3 layers (depending on the weight of the cloth) and use a vacuum infusion process to make sure you get the ideal amount of resin in it. You'll have a transom that's 5-6 times stronger than an equally thick transom made with fiberglass.
This is OVERKILL though. A transom made with 2x plywood layers laminated with epoxy, tabbed with 3 layers of biax tape and covered with 2-3 layers of biax cloth is far stronger than any factory made boat transom and will last longer than you own the boat assuming it stays dry. The rest of the hull will fail structurally before it does too.
If you want to make an unusually thin transom for some reason you might go with carbon fiber on the outside, but if you're doing that you'll know the calculations to determine the strength of the eventual composite, and you probably wouldn't be asking questions here
Erik
