engineering connection of transom to boat

[wilfungus]

I am looking for information about how much reinforcement is needed to attach the transom to the rest of the boat when the transom is for an outboard motor. Gerr's books give great information on hull strength and tell you how thick the transom is supposed to be - but there is a gap in the information when it comes to connecting the two. How much reinforcement is needed for the speed of the boat and the size of the motor? Any suggested reading or engineering resources?
Wil

[PAR]

It would be easier if you told us what you were trying to do, rather then recite the engineering calculations typically used in a specific structure. In other words, much depends on the construction type, the current application and of course HP and outboard weight considerations, etc. How much bottom loading have you anticipated?

[wilfungus]

Hi PAR,
I am in the process of designing an 18 foot boat that I intend to use for fishing on bigger waters in Minnesota. It is a pretty simple design: shallow v type bottom, fiberglass set in epoxy over plywood. The marriage of the transom to the hull has become a bit of a mystery. Obviouly it gets attached at the sides and bottom - but what is required for additional bracing to support the force applied by an outboard of 75 to 120 HP is a mystery. Full length stringers? Knees? Plywood "box" type brackets? How big? Where? I can't find anything other than what other people have done in their own designs - but no information for calculating what is really needed for a boat that is going to see tough use in big (by Minnesota standards) water.
Wil

[PAR]

Again, not knowing what type of build you're employing, makes much more then guesses and general suggestions the only possibility.

Assuming it's a taped seam plywood boat, with an abrasion resistant fabric sheathing, you generally want to incorporate two longitudinal hull stiffeners as far up into the eyes of the hull shell as you can. Typically these would run from transom to 25% aft of the bow, where the up curvature of the forward sections causes them to "disappear". These two bearers, can employ a transom knee and in conjunction with another set of knees at the sheer, usually tie things together well.

Plywood over frames usually have a similar arrangement, though typically have more longitudinal stringers, tying the frames together, providing additional planking fastener area and supporting the planking in-between the frame bays. These systems usually have transom perimeter framing, with is tied with similar knees and stringers generally don't make contact at the transom, but fall just short, often with weep holes worked in for drainage.

If your sheer clamp/shelf arrangement, chine logs and keel batten are sized properly, you only need a couple of knees on the two longitudinal bearers, maybe an athwart "cap" on the knees to offer additional resistance to transom flex in addition to the lodging knees are the sheer.

A taped seam build can involve the splash well into the structure, which can serve the knee function and tie the topside and bottom planking, plus the other longitudinal stiffeners.

Obviously, there are many different ways to approach the set off issues surrounding this area. These are a few of them.

[wilfungus]

Thanks PAR - The boat I am building utilizes plywood with taped seams. A transom thickness of 2" and 1/2" bottom consisting of 2 layers of 1/4" ply with 10 oz glass. Gerr's calculations, based on his scantling number system, say that I should use stringers that are about 2-1/2" square. These would be screwed in place and secured with epoxy grout fillets and tape. At one point I had conceived using a combination of knees and splash well framing similar to what you have suggested and by your advise that seems to have been the correct path. Thanks for your advice.