Plywood thickness

Greetings

I'm just getting started on my first driftboat.  I'm working off of plans that I purchased from Spira International.  I chose them since he had designed a boat for the moderate speed, calm rivers of the midwest rivers that we have in Northern Michigan.

Any ways, I've been reading a lot on this site for the past couple days and loving it.  Among some of the things I've learned is the fact that where as my plans call for 1/2" thick (or optional 3/8" thick) plywood sides and bottom, almost everyone that I read here is using 1/4" ply for the sides!   Now I'm second-guessing my plans since if there's a chance that 1/4" will work and be durable, I would LOVE to cut that weight and improve my draft.   I'd love some input on the subject!

My boat is called the 'Canadian' designed by Dave Spira, Spirainternational.com.

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Permalink Reply by Lawrence E. Long on February 6, 2012 at 7:23pm

If its a conventional frame boat I would think 1/4" sides and 1/2' bottom would be adequate. !/2 " sides is real overkill. S&G 3/8" sides and 1/2"- 5/8" bottom sounds right.  Others that have built this same design will have better advice

Good Luck

Permalink Reply by Michael Rosso on February 7, 2012 at 5:24am

It's a conventional frame. No S & G.  

Thx

Permalink Reply by Riverman on February 7, 2012 at 5:06am

I have the same Spira plans...  I'm going with 1/4" sides. That 1/2" just sounds way too heavy.

Permalink Reply by Michael Rosso on February 7, 2012 at 5:22am

Excellent!

Thanks a ton.   1/4" is going to be ridiculously easy to work with compared to 3/8.  Should make the whole thing a LOT lighter too!  I wonder why Spira fails to even mention that you could even consider using 1/4"?   With the lack of snow and rain this winter, I'm worried about low river levels come spring.

Where are you at with your boat?  I've just constructed the strongback, but have to build two picture frames for my wife before I'm 'allowed' to really begin my boat... I mean 'our' boat.

Permalink Reply by Michael Rosso on February 7, 2012 at 5:58am

I appreciate the offer.  Thanks.   I'm going to stick with the Spira plans.  I always liked the fact that I can get the hull finished out first, then concentrate on the interior.  I'm looking at this project primarily as a learning experience so if I end up hating the Spira boat, I'll know better the second time around!

Permalink Reply by Jason Knight on February 8, 2012 at 4:30am
Yeah building sides with1/2" sounds like a pain... Dealing with a large 1/2" section for the bottom taught me to appreciate how heavy it is :)
Permalink Reply by Michael Rosso on February 8, 2012 at 5:58am

the boat I'm building is 14' LOA.  Would you guys say that it's worth purchasing 2 pieces @ 4' x 10'? and 2 pieces at 4' x 8' for the remainder?   Or is it easier to just join the pieces in the middle of the hull with 4 pieces of 4 x 8?   Is epoxy still the best adhesive to use for the construction?

I'm planning on painting the outside hull.

Permalink Reply by Eric Loudenslager on February 8, 2012 at 7:28am

The boat plans should specify the optimum number of plywood panels and how to cut out the sheathing for boat pieces.

The usual way to connect two plywood panels is to use a 1:8 to 1:12 scarf joint, glued with epoxy.  Do a google search and you will find several detailed explanations of scarf joints.

Most of the "standard" height drift boats are built using 4 x 8 panels scarfed together, 2 for a 15'9" panel, 2 plus some extra for a 17' or 18' panel.  Then both sides of the boat are cut from that single large panel.  See Roger Fletcher's book for more explanation.  If you are building a boat that calls for a panel larger than 15'9" then a 10' scarfed to an 8' would be a good way to go.    However all of my sources for marine plywood are 4 x 8, no 10's.  They may be available, but check on shipping costs if purchasing from a distance.  It may be cheaper to ship the smaller sheets and scarf.

Marine grade plywood is recommended.  The primary feature is no voids, which is essential when cutting, nailing, etc.  Plus, water can condense in the voids and rot the panel from the inside.

Permalink Reply by Michael Rosso on February 8, 2012 at 8:49am

Thanks, Eric.

I'm planning on using Hydrotech.  All my  plans say on the subject is to butt-joint the two pieces together using epoxy and a backer-strip of plywood with both pieces screwed to the backer and then later filling any gaps with epoxy thickened with 'wood flour' aka really fine sawdust.   I was going to ask how the heck you accomplish a scarf joint in 1/4" plywood since it's something like a 7 degree angle, but I just researched it to save you the trouble of answering.   Looks like I can easily make a jig for it.

thanks for your info!

Permalink Reply by Dave Barnes on February 9, 2012 at 7:48am

I used 5/8 on the bottom and 3/8 on the sides. a lot of the strength comes from the tension in the bend in the sides. I've hit some pretty good rocks and it seems to be fine. I used a frame-less design which actually has 3 small ribs in the center, supporting the side storage bins and the rowers bench. 

Epoxy - yes. scarf joint - power to you, I did butt joints and they are fine, You have screws all over the place in either case according to the instruction I saw and I just wasn't confident in my ability to get a strong joint in 3/8" without the backer piece.

Permalink Reply by Michael Rosso on February 9, 2012 at 8:42am

I've definitely considered that, Dave.  Our rivers here in Michigan our a whole different animal, however.  They run wide and slow for the most part and the term 'rapids' is used more as a punchline when drifting on 2" riffles over a sandbar. (at least the ones I'm planning on drifting down)   Thus my interest in reducing weight at the expense of durability. 

As for the scarf vs. butt joints, mostly I just like the idea of not having a backer boards to look at inside the hull and since I'm sticking with my original design (it has a total of 7 frames excluding the transom)  I'm not too worried about the durability.  (famous last words, right?)  :)

Permalink Reply by Guy Fredrickson on February 9, 2012 at 3:45pm

All-

I am waffling on these same questions myself.  I am building a 16-ft framed boat.  The options (as I see them) are:

1) 1/4 sides and 1/2 bottom

2) 3/8 sides and 1/2 bottom

3) 3/8 sides and 5/8 bottom

Having never built a boat, but having looked at plywoods, I am leaning toward Option 2) and away from Option 1).  Option 3) seems a little overkill.  I plan to fiberglass and paint the outside and I'm also conidering the additional UHMW sheet to cover the bottom.

But what is the proper thickness for the UHMW sheet?  Is it 1/4 or 3/8?

I ordered wood for my frames and that should arrive in about a week.  Once the frames are built I'll order the plywood.

Thanks

 

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