Boat building: Stiffening a plywood hatch cover -- I don't want it to flex.

[Matthias]

At the moment I'm working on the lid for the anchor well.
It's 2 layers of 6mm (1/4-inch) Gaboon plywood and I want to be able to stand on it, even step down off a dock onto it without it flexing.
I'm not sure if a couple layers of glass inside and out will increase its rigidity enough and thought it probably needs some sort of webbing on its underside.



At work we just laser cut a bunch of acrylic plaques, and looking at the waste got me thinking! (insert eye roll)


What if I glass in the acrylic waste, double stacked, as webbing to the underside of the lid? I'd probably have to vacuum bag it.
Is this a valid idea or should I make some sort of webbing or stiffening batton from plywood instead?

Lid is about 19.5" long by 15.5" wide.

If it was your boat, how would you make it? What's the best way to get the lid super rigid?

 

[Steve Sanford]

Matthias~

This is one of those seat-of-the-pants engineering challenges we all face. Sometimes we just need to try something. My instinct would be to glue-and-glass 2 cleats that were scribed to the curve of your cover. I would use lengths of Mahogany or SYP - 3/4 thick by about 1.5 inches tall. All edges rounded.

Here is how I stiffened a much larger cover (for an Arthur Armstrong Blackjack). The crosspieces were PVC lumber (no inherent stiffness) and the longitudinal was clear SYP.



Having said that, your approach could certainly work - especially if you have ready access to vacuum bagging. Can you test on a piece of hatch stock you will not need? Your webbing would probably weigh less and rob less volume from the hold.

All the best!

SJS

 

[Matthias]

Matthias~

This is one of those seat-of-the-pants engineering challenges we all face. Sometimes we just need to try something. My instinct would be to glue-and-glass 2 cleats that were scribed to the curve of your cover. I would use lengths of Mahogany or SYP - 3/4 thick by about 1.5 inches tall. All edges rounded…

Thanks for the reply Steve, probably while you were typing I was sanding and came up with a simpler, more elegant solution similar to what you did.

Just went with one rib 5/8 ply about 1.25 high…
If I go down this path it’ll be simpler to construct — glue and fillet, then cover with 400gsm biaxial tape. Also sheath the top of the hatch with HD 800gsm woven cloth leftover from glassing the motor mount.

 

[Steve Sanford]

Matthias~

Looks good and strong to me. I'll send you your degree: Seat-of-the-Pants Engineer !

All the best,

SJS

 

[Huntindave McCann]

If it was your boat, how would you make it? What's the best way to get the lid super rigid?

Foam and Foam core material is the "professional" choice to add rigidity to a panel. Divinycell was the material used in my previous vessel. Several options to choose from if you wish to go this route. The major advantage to a core of foam, is for the most part, a foam core tends to spread the load out evenly across the panel. A single or even multiple ribs will carry the load along each rib. Each rib becomes a place where the stress concentrates.