hatch rib dementions

[whitby guy]

Hi all.
I've been quietly lurking whilst slowly building and have come upon a problem (or what might soon be a problem). I am cutting ribs for the hatch...hatch size will be 48" wide x 56" long...and have cut 5 ribs in 3/4" plywood, each about 2" wide. The problem appears that there is too much spring in the rib...they will move 1/2" when given light pressure, and before I get to the point of putting on skin (1/4" plywood) I thought I would ask...Are the ribs too thin? Will having more ribs on a closer spacing make the hatch rigid enough to eliminate the springback?
Thanks in advance for any advice.
Sandy

[toypusher]

Sandy,

I made mine with doubled up verticle rib (2 3/4" plywood ribs glued and screwed together) on each side and then filled in between with horizontal ribs. I skinned the outside with 1/8" laun plywood and inside with 1/8" birch plywood. Also used aluminum on the outside over the luan. No springback in mine and it fit the first time.

If you are going to stay with the verticle ribs, then you should put some horizontal braceing in between them, maybe a foot apart or so.

[mikeschn]

Kerry is right. Doubling up on the ribs, especially the two outside one will help a lot. So will adding the horizontal ribs.

Building a hatch is kinda like building a torsion box. It isn't strong until all the pieces are in place.

Mike...

[Weerider]

Sandy,

My plywood ribs are 1.75" wide. You just have to use enough of them to do the job. I use 1.5:X1.75" stiffening blocks also. The outside is skinned with two layers of 1/8"... much easier to bend and less likely to spring back than 1/4".

Howard

[whitby guy]

Thanks guys for the tips.
Went out to the garage (as soon as the wife went out) and doubled up the thickness of the ribs...seemed to take most of the movement away. I think I'll fab up using double outer ribs and a double in the centre with single widths inbetween (five rib total) and use the horizontal stiffeners as required.
Thanks for the photos as well...always nice to see that I am not blazing a new trail here.
Sandy

[jeepr]

I am at the same point in my build. I cut the ribs yesterday. Mine is 58" wide. I will be using 4 doubled ribs and two single ribs between them. I plan on pre-bending the plywood by putting a ratchet strap around it (bent in a circle) and letting it sit for a few days. I hope by doing that I will have less spring back.

Good luck..