Boat structures 101

[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