The Hatch - how many ribs/foot

[Mark Gunter]

Hello everyone,

I’m finally at the hatch stage of my build (it’s an exciting time) and I’ve been reading up in old threads re: hatches. I read one post that suggested that if you have enough ribs you needn’t worry about spring back. And another interesting post where a fellow pre-bent his skin to avoid spring back.

My hatch door will be rather large: Approximately 85” x 53”. Approximately 34” inches of the height will be vertical, the remaining 19 inches at the top of the door is on a 40” radius.



I plan to frame the hatch door with 3/4” plywood ribs and 1x2 hardwood. 1/8” skin inside and 1/4” skin outside. I plan to double the ribs at each end (sides).

My question is, how many ribs to the foot should I use? 16” centers? 24” centers? Any advice is greatly appreciated.

[Sparksalot]

I used ribs of 3/4” plywood for the hatch. They’re about 12” on center, and I staggered the cross pieces. The depth of the ribs is about 2” and the skin is 1/8” plywood, so there’s no possible way of any spring back. I purposely did not use an inner skin, to allow it to flex with the cabin structure.

[tony.latham]

I do strong gusseted ribs on the ends, and that's it. Which... I don't think you can do at this point.



You build the hatch twice, in a sense, once on the front and once in the rear. And of course, you don't need extra ribs up front. But that may be a simplification.





Tony

[Mark Gunter]

Very nice, sparks, every foot huh? And here I was wondering if 16” would be overkill. I like it, but I’ll probably skin the interior on mine as I wouldn’t want much flex on a door this large.

Tony, yes I’d say that’s a big oversimplification since the front is more of a wall and the hatch is an independent, moving panel - a door, lol. And of course it’s way too late to use your methods, though I applaud them. I love your method, especially the seal. I wish I had bought your book or done way more research before building, but time was a factor and I just jumped in the deep end. No regrets, I’ve learned and am learning a lot.

Any other advice on construction that will work in my situation, anyone?

[Mark Gunter]

Based on sparksalot’s post I’m thinking I need to do at least a rib every 16” if not more

[Mark Gunter]

And since less than half the height is actually radiused, maybe my worries are misplaced?