Boat structures 101

[Jeff79Sceptre]

I'll have to agree with Fas. When I bought the 23 last year I replaced the deck with 3/4 ply and epoxy. Then I replaced the fuel tank platform with 1/2 coosa. I made my deck in 1 piece then dropped it in. It was HEAVY. After putting the coosa in for the new tank it flexed. So I asked a few questions on here and my schedule wasn't correct. Cut it out, more 1708 and tabbed it back in. Super strong and light weight was the end result. Now I'm on my transom replacement. Coosa is what I'm going with because of my prior experience with the fuel tank platform. I'm upset with the deck in the boat and I may next winter hack it out and redo with either coosa or core cell. I don't know anything about the latter but I'll keep watching this post. Very informative!

[FLexpat]

Reading Jeff’s post started me thinking about something I ran into when I started on my 23 redo... The original hatches for the fuel tank and the forward fishbox in my 23 Sceptre are bowed. The cores don’t appear to be wet and they are not super flexible so it took me a bit to figure it out; the edges of the cores in the hatches had not been properly finished.

Like Dave mentioned, cored construction creates something kinda like an I beam. When it is supported on the ends and a weight is in the middle (like someone standing on a hatch), the top of the laminate is in compression and the bottom of the laminate is in tension. That is great if the lower laminate is rigidly tied to the upper one at the edges – near zero flex or bending. My factory hatches have a balsa core that just fits inside the shoebox-lid shaped hatch and a thin glass lower laminate that only touches the upper laminate along a 3/16” edge – or it did when it was made. As a result of the lower laminate being in tension whenever someone walked on it, it broke loose along the edges. The result is a pair of hatches where the upper and lower laminates are not tied together anymore – and took a set with a shallow dish shape over the last 40 years.

So what does this say about cores? The edges of the core and the structure have to be properly finished with the upper and lower laminates being tied together. It was done properly on the hatches for my stern wells; the core stops an inch or two from the inner edge. It is tapered to the underside of the top laminate (~30-45 deg angle) and the lower laminate covers the core AND is wrapped fully into the bottom of the upper laminate. That means the tension from the lower laminate gets spread across a 1-2” wide joint to the upper laminate which is in shear.

This cartoon shows the way the hatches are and the way they should be.


So that is what I need to do to my larger hatches. I also need to properly finish the edges of the core (Core-cell) when I replace the decks. If I just have a 90 degree angle on the edge of the core in the deck and don’t tie the upper and lower laminates together properly I will cause the Corecell to fail in shear on the edges. Then the deck starts falling apart and delaminating from the edges. And I will have to redo it.

There is a good bit of discussion with diagrams about this in Gerrs book and some in Bruce Pfunds article on core failures.

[Blue_Heron]

Thanks to everyone who has contributed so far. This is exactly the type of conversation I was hoping to stimulate. There's a lot of talent and skill on this board and it's good to see it come together.

I want to go back to a couple things Bushwhacker said and explore them in a little more depth. In regards to Gerr's scantling rules, Denny said:

Originally Posted by Bushwacker

...I've run a few calculations on his recommended Scantling Numbers and compared them to what SeaCraft used in the early 4-stringer 20' hulls. Although I believe Moesly arrived at his structure by trial and error, including some very creative use of thin laminates and brittle resin in prototypes to find the high stress locations, he ended up with laminate thicknesses that are very close to Gerr's recommendations!

Denny is right. In the early days of fiberglass boats, not much was known about how to design them structurally. There was a lot of trial and error. Carl Moesly knew from his background as a pilot that weight and balance matter. So he started out with lightweight structures, but he made a point to test his boats to find the weak spots and potential failure points before he put them into production. Carl was a pioneer on many fronts.

That brings me to another point Denny made:

Originally Posted by Bushwacker

... I'd be very careful about changing anything he designed!

My signature line is "Reinventing the wheel, one spoke at a time". This is an inside joke I have with myself to remind me that when I have a great new idea, there's about a 95% chance that either it's not a great idea, or it's not a new idea. But I can't help over-thinking stuff. I suspect there are others like me in this group.

So, any time you find yourself thinking you've got a better way to rebuild your boat than the way it was originally constructed, ask yourself why it wasn't done that way to begin with.

Moesly and Potter, just like all the other builders of production fiberglass boats, then and now, had an economic mandate to make them as economically as possible. They had some options (hull, stringer, and liner molds, for example) that aren't practical for us. But just because you can't pop a new deck and liner from a mold and install it in your boat doesn't mean you can't copy the same laminate and core thicknesses for the transom, stringers, deck, gunwale cap, etc. and rebuild your boat as good or better than original. Why substitute a different deck structure when the original lasted 40 years?

That being said... As Flexpat's hatch example demonstrates, there are a few things that could have been done better in these boats. Mainly, you'll find them in the details, not in the basic structures. So think about what you're doing, but try not to over-think it.

Dave

[FishStretcher]

It is 10F up here this morning. At least it is 10 ABOVE today.

I compiled a spreadsheet of commonly available resins and properties. It isn't as complete as I'd like, but it compares polyester, epoxy vinylester, and normal marine epoxies. All made for lamination or infusion. No oddball chemistries like BPA fumarate, or methacrylate modified systems.