Deck thickness - marine ply

[bradz]

Hi everyone
Well I am slowly getting through a rebuild which I alluded to nearly 6 months ago. Once done I'll load up all of the photos.

Im spending far too much time pondering over what thickness marine ply I should be using for the floor.

My boat is a 20 foot fibreglass runabout with 4 stringers. My main concern is that the distance between the 2 centre stingers (which form the fuel tank locker) is about 20 inches.

I am using polyester resin. On the underside I plan on using a layer of 1.5 ounce chopped strand. On the top a layer of 1.5 ounce chopped strand and a layer of 6 ounce cloth over that.

So, should I be using 1/2 inch or 3/4 marine ply?

I'm sure 3/4 would be fine but want to know if you think 1/2 inch with this amount of reinforcement would be sufficient.

 

[tpenfield]

You could use 1/2", but put 'cross members' over the tank and center stringer portions. If you don't mind the extra weight, then 3/4" may be sufficient alone. Plenty of threads where folks have put cross members between the center stringers (either 1x3 or 2x4 lumber), so maybe review those threads for details, etc.

Might also help to post pictures sooner rather than afterwards.

 

[Scott Danforth]

1/2" is more than fine

my boat has 1/2" of end grain balsa with fiberglass on both sides and you can dance on it. distance between the two center stringers is about 24"

the strength comes from the two layers of fiberglass

 

[bradz]

The cross members you are referring too...do you mean simply reinforcing the fuel compartment lid or do you mean adding another "bulkhead"?
The idea of a few ribs running east-west across the middle section would add alot of stiffness.
The fuel compartment lid needs to be removable so those ribs might be useful.

 

[bradz]

PS. I've tried to add a photo and my connection times out. I'll try later from my computer as my smart phone seems temperamental.

 

[JASinIL2006]

I used 1/2" on mine, too, with a layer of fiberglass on the bottom and a couple on top. I was a little worried it wouldn't be rigid enough, but it was quite solid, even before I foamed the hull under the deck. If you're really worried, maybe you could put an extra layer of glass on the bottom?

 

[bradz]

Here is a photo of the stringers. Please note that I've since installed a centre keel stringer (about 4 inches tall) and lateral ribs between the outer stringers about 24 inches apart. So, the floor above that section will be well supported, whereas the middle section has a reasonably wide span.

 

[ezmobee]

I used 3/4" on my aluminum boat restores. You've got double the # of stringers I did and are adding fiberglass. In your case 1/2" + glass should be plenty solid.

 

[Woodonglass]

I concur with the others. With your glass layup plan 1/2" will be perfect.

 

[bradz]

Cheers guys. Thanks for the guidance.