Composite panel design

True, but I've always considered the idea behind stitch and glue to be a simplified design that was easy to make, and not necessarily the most structurally efficient design. The idea being to thicken the panels a bit and use the goo and tape to connect relatively stiff panels together.

Once you get down lower than 6mm ply, you can't have very wide (more than 8 inches wide) unsupported flat panels between stringers on the planing surface because the plywood itself will simply crack in the middle of the panel under pounding loads that you can see in a light planing hull. In order to prevent that you need to start using more and more glass and the weight goes up accordingly.

If you were to make the boat above without stringers from 4mm ply you are going to need a at least two layers of 9 oz on each side to get it to be anywhere near stiff enough and you'll need a couple of layers of tape on the seams to keep it together. At that point its going to be heavier than if you put some stringers in the bottom and edges to keep the boat in shape, or used a bit thicker ply in the first place.

Most of my experience is with small racing boats built with stringers and 4-5 mm plywood. These boats don't get used much, but they get bounced around and eventually fall apart from the pounding they take. In my experience, when the ply gets thin you need stringers, at least in the corners to keep the joints tight. Without them the panels will start to "oil can" and the joints will crack and unzip due to the concentrated stress and the whole thing falls apart. Been there and done that.. More bulkheads will help, but it doesn't solve the fundamental problem of the panel itself too flimsy.

I've also seen a lot of problems where thin bottom skins meet the transom. Even with a 1 inch thick transom, the twisting and flexing of the skins causes failure of the joint and the hull develops leaks and rot problems primarily due to the change in stiffness that results where the stiff transom meets the thin skins.

Designing with very thin skins is a specialized case in that you have to be careful or you will get joint cracking for a variety of reasons and the joints all have to be carefully designed to spread the loads over large areas or the structure will start cracking.

When you start to optimize the structure for light weight you start to encounter problems that don't exist in more conventional construction and things tend to fall apart more frequently.

For that reason alone, the original design, with thicker cores and glass skins is probably the preferred method of building this hull. If you made it from thicker ply (like 3/8" with glass on the bottoms and sides and 1/4 decking) and did it as a stitch and glue I think it would be a very good result, but probably not as light as the cored glass original version.

If you want to make it out of very thin (4-5mm) plywood, you need to rethink the structure and build it with stringers much more like a small outboard racing boat. That would likely be lighter than a thicker stitch and glue, but if you don't have experience or studied the construction methods of small race boats, it is very likely to fall apart in short order for a variety of reasons. If stitch and glue methods worked and were more structurally efficient that stringers using thin plywood, there would be a lot of racing hydros made that way since that is a lot easier to do than cut a lot of stringers and glue and screw everything together.

JMHO, but having a good bit of experience with this type of construction and being a structural design engineer I think it qualifies for something.

 

Does anyone have a suggestion for the transom thickness. I will be running bracing down the tunnel walls for support. I was thinking of using Coosa but not sure about the thickness. If I had an equivalent thickness in ply I could work it out.

Thanks

 

I think initial taped seam designs did just go for good enough and simple structures, but with the advent of new software and of course racing, plus kit design, developing light, strong structures has taken on new meaning now. Eventually you do get to a point of the plywood being just too thin, but practical boat sizes seem to level this out a bit as does reasonable design processes.

Transom thicknesses have been pretty much standardized for a while. Portable engines (30 HP and smaller) can live with a 1" thick transom up to 20 HP, which is best done with 2 layers of 1/2" a 30 HP could have 1 1/4", though the industry standard is 1 1/2". A typical 1 1/2" transom is 2 layers of 3/4" plywood, but 3 layers of 1/2" is stronger, especially if each layer is canted at 12 degrees to the previous layer.

 

Most racing boats use a 1" transom across the entire hull, with additional thickness (an additional 1/2" to 3/4") in the center section between the cockpit walls. Your tunnel walls are very similar to the cockpit of race boat so that approach should work just fine for you.

I would look at runabouts and hydros as a guide in attaching the transom to the center section. What they commonly do is notch the transom about 6 inches and then create a notch in the cockpit side panels and have about 5 inches of plywood sticking out the back of the transom. They then glue a one or two inch block to that behind the transom. Also an extra piece of 6mm ply is put on the side of the cockpit as a doubler, so the piece extending through the transom is roughly a half an inch thick and extends forward for about 4 inches. Vertical uprights are put on the inside of the transom and these are screwed to both the transom and the side panels. Here is a picture of a classic transom mounting arrangement that would work fine for your type of situation and would be plenty strong for motors up to 60 hp or so. In your case the transom outside of the motor area is taller, hence the notch, but the idea is the same.

 

What is missing in some of the arguments is that a wood panel (Occume ply in this case) is always stiffer than glass of equal weight. Sheathing thin ply with glass will increase stiffness but not as much as an equal weight of added wood to the thickness. Woven glass is a poor choice for sheathing to increase stiffness and uni or biax is far better. Of course plywood is not always available in the increments to satisfy the particular case.

Other needs like rupture and abrasion resistance make a sheath desirable in some cases. S&G construction has gone way beyond the "simpler and easier" choice, if that was ever the case, which I dispute anyway.

The battle between stressed skin and birdcage (with thin skin) construction does not always favor either one. When localized loading is the issue, stressed skin usually wins and for uniform loading, the thin skin and interior cage is usually the best choice. For this boat with its narrow panels joined at an angle to their neighbors, I think the method employed in the photos is best. That is a thick skin with only enough interior structure to hold everything in shape.

 

Full foam or balsa core would be great, unfortunately those prefab panels cost a fortune. My idea now is to use sheathed 6mm ply outer skin with prefab panel internal bracing. Coosa transom & knees, duflex foam floor & bulkheads, Coosa stringers where needed. This way I can keep it to two or three prefab panels, the rest in sheathed ply. Yes I will need to design a more substantial internal structure than the photos. You got me thinking now Tom..