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.

 

[Matthias]

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.

Wow, Interesting! I had no idea that foam could be used in that way, so never considered it as an option. Unfortunately the closest supplier of Divinycell is 1.5hrs drive away and I would need to buy a full sheet for $130.

I’m gonna go check out my local hardware store and see what they offer in the way of rigid foam insulation boards. Checking online — looks like I can get half a sheet for $20. If it feels rigid I’ll buy it and do some experimenting.

 

[Huntindave McCann]

Wow, Interesting! I had no idea that foam could be used in that way, so never considered it as an option. Unfortunately the closest supplier of Divinycell is 1.5hrs drive away and I would need to buy a full sheet for $130.

I’m gonna go check out my local hardware store and see what they offer in the way of rigid foam insulation boards. Checking online — looks like I can get half a sheet for $20. If it feels rigid I’ll buy it and do some experimenting.

Not sure if "regular" foam board would have the same properties, it might. My understanding is the foam core used with resin and/or epoxy will absorb and bond with the resin.

 

[Michael McCord1]

Is the lip that the hatch sits on built to hold the load? I’d also want a way to secure the hatch in place so it doesn’t slip out from under me and send me in the drink headfirst

 

[Matthias]

Is the lip that the hatch sits on built to hold the load? I’d also want a way to secure the hatch in place so it doesn’t slip out from under me and send me in the drink headfirst

It will be secured but I haven’t worked it out yet. Somehow it will be hinged and latched.

The little lip on the deck is just epoxy glued and only intended to mitigate water running into the anchor well. I have a drain built into the well, but didn’t want excessive water entering.

The lip on the lid itself is epoxy glued and will have glass around its perimeter for added strength.

 

[Huntindave McCann]

Just an example of how a foam core is used. This example is a lightweight solo canoe I have. The photo is stolen from the manufactures web page.

The foam core is used as ribs for the sidewalls and across the entirety of the floor area. The inner layer of glass has no colorant. Don't remember just how thick the core material may be in this example. I'd have to get into the attic area of my garage to look at it and make a guess estimate.

Ultra-light hulls are stiff and incredibly light. They are ideal for speed and distance paddling and for all people or conditions that demand the lightest gear. In our Ultra-light construction a structural-foam core and ribs are laminated into the hull and sides. We also add extra fabric layers to strengthen specific targeted areas. Aluminum plates are strategically laminated into the hull. Seats, footbraces, and all hardware is then riveted into these plates. This gives a very secure connection and eliminates rivet heads on the exterior. Aramid, the fiber used for aerospace, body armor and bulletproof vests, is amazingly light and strong. We've used it for more than 30 years to construct extremely light and tough canoes. Wenonah Canoe is the worlds largest maker of canoes using Aramid.

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That said, using single or multiple ribs is certainly an option. The ribs themselves don't need a lot of rigidity. The strength comes form the vertical walls of resin/glass laid up and over each rib. Yes, vacuum bagging would be the ideal method for either a foam core or rib(s) style "core".