Minimally constructed deck - Strength of plywood

[Paul Moir]

I'm working on a scaled up version of a class C runabout and am having some trouble figuring out how to build the deck. In the interests of keeping the center of gravity low and total boat weight to a minimum, I would like to have the deck constructed light but still strong enough to walk on. The plans originally called for 1/4" douglas fir plywood over beams on 20 1/2" centers. That doesn't sound strong enough to support a person to me - am I wrong? <br />I was thinking I could either add longatudinal beams, make a sandwitched deck, double up the beams, or laminate some cedar onto the ply for a thicker (and prettier) deck. Does anyone have any ideas/experiences? Or should I simply emblazon "NO STEP" across the deck and leave it at that?<br />The deck has quite a camber (~5" over ~4' beam at the highest), so I should get some stiffness out of that.<br />Thanks for your expertise!

 

[Jamar]

Re: Minimally constructed deck - Strength of plywood

Hey paul i am no expert boat builder but am a carpenter by trade. I would think standing on 1/4 ply on supports 20" apart would be about like standing on a mattress and doubt if would very long. I would use a minimum of 3/8 thick ply, and supports probably 12" oc. 16" if you really want to stretch it.

 

[Jamar]

Re: Minimally constructed deck - Strength of plywood

Hey i just reread your response and may have misunderstood. My reply was for use as a floor or casting deck........if you are talking about the front top of your boat that never gets foot traffic then perhaps it will work.

 

[crab bait]

Re: Minimally constructed deck - Strength of plywood

sure,, it'll work... do the deed,, than add another longeron in-bettween the 20 i/2's... it'll add tremendous strenght without much weight.. <br /><br />that's an 'engineerin' dream'...

 

[jamessloan]

Re: Minimally constructed deck - Strength of plywood

Will you be glassing the deck? If so, consider that the cloth or mat thickness and taping some distance up the sides will increase strength and stiffness.<br /><br />The idea of running some additional longitudinal supports has great merit. You can situate them, if you like, as part of the overall stringer framework and stiffen up the whole boat.<br /><br />Just my .02

 

[Paul Moir]

Re: Minimally constructed deck - Strength of plywood

Thanks for all your replies. I guess the longitudinals are the way to go then. <br />Jamar - I was thinking of thicker ply since it's the simplest solution, but if 3/8" isn't going to work well then I won't do it.<br />Crab Bait - I had to search out 'longeron' My beams run shear to shear, so it would be more like a grid work with longerons. <br />James Sloan - I don't want to glass the deck if I don't have to, but it's a good idea. I'll stick it in the back of my head for an emergency solution. Your advice is worth more than .02 to me. <br />Thanks again all!

 

[12Footer]

Re: Minimally constructed deck - Strength of plywood

Have you concidered Divinacell by DIAB?<br />This stuff is extremely strong, extremely light, extremely durable, and, unfortunately, extremely expensive. But I'm looking into this myself,on "Project Cobra".