Plywood Thickness

Hello all...
I'm thinking of building a 13 foot Culler Butternut double-paddle canoe in glued-lapstrake using Okoume plywood. At 13 feet long, should I be using 4 mm plywood (I already have some), or would I be better off using 6 mm (I may need to refinance the house to buy it)?
I read somewhere that the rule of thumb is to use 1 mm thickness for every 4 feet of boat length. I'm not sure this is accurate, but if it is, then 4 mm is what I can use.
The boat will be used exclusively on lakes, ponds and slow rivers. No rough waters. I'm too old for that stuff now.
4 mm seems quite thin, but Okume is pretty tough stuff. 6 mm would certainly be stronger, but heavier.
So, what do you think? 4mm or 6mm? You guys probably know more than me about this.
Thank you for your time.

Ed

 

What is it, three-ply ?

 

Yes. 3 ply.

 

Have you bought a set of plans like these Ed, or are you designing it yourself?
BUTTERNUT 13' Canoe | Mystic Seaport Ships Plans https://store.mysticseaport.org/ships-plans/butternut-13-canoe.html

If you have plans, I guess they don't make any mention of scantlings?
I just googled a couple of threads on the Wooden Boat Forum about the Butternut, and they suggest that the info on the plans from Mystic is pretty basic.

In this thread they are suggesting using 3/16" cedar -
Need Help with Lines & Offsets, Culler's Butternut from "Boats, Oars, and Rowing" http://forum.woodenboat.com/showthread.php?64099-Need-Help-with-Lines-amp-Offsets-Culler-s-Butternut-from-quot-Boats-Oars-and-Rowing-quot

 

I'm using the plans from Mystic. And they do not have much detail at all, aside from the table of offsets and the 3 views. I also have the plans for Cullers 17 foot double-paddle canoe. Those plans give much more detail, but still not a lot. I read somewhere that the 2 plans were originally sold as a set, and the scantlings are the same between the two boats. Mystic broke them apart into 2 different plans.
I believe the original boat was planked with 1/4 inch cedar.
6 mm plywood would be more consistent with the 1/4 inch cedar, but if 4 mm plywood is adequate, I'll use what I have. But, I have no problem with buying 6 mm, if 4mm is inadequate. I just don't want to end up building a battle ship if I don't need to.
Thank you for your time and insights.

Ed

 

It depends on the framing. A stitch and glue with no framing would need a thicker skin than one with stringers space 8" apart. As an example, I raced and crusied extensively a 35' sailboat that had 12mm (just under 1/2") mahogany skin and no fiberglass.

 

4mm can work well with the right glass

I built a 16' Laker canoe 20 years ago with 0.1875" strips which got sanded mildly

I might step up the sheathing.

I would sheath 2 layers of 6 oz bottom inside and out.

what is the sheathing plan? Just step it up to what I said...

you will see waves ripple a flat bottom, but the glasswork copes well

 

4mm good quality ply.

No question, after all the info you've given.

Best of luck with your build.

 

Thank you all for your input. It's very much appreciated, and got me thinking.
One of me early boats was a CLC Sassafras 12 foot canoe. It is made from 4 mm Okoume, and the garboard is covered with glass and cloth on the outside, and the garboard plus the next strake up (I forget the proper name for it) is glassed on the outside. Since I'm not a fan at all of covering a hull (fully or partially) with fiberglass/cloth, I was not going to use it on the butternut. The strake laps will be epoxied, but no cloth.
With all this info given here, I'm leaning toward 6 mm (5 ply) with glued laps, and no ribs. If I were to use glass and cloth, I would go with 4mm.
I said in an earlier post that the rule of thumb I read was to use 1mm for every 4 feet of hull length. But what it didn't say was if that rule requires glass/cloth. So, barring any compelling reasons to use 4mm without glass/cloth, I'm going with 6mm.
If you have different ideas, then don't be shy. I still want to hear them.

Thank you.
Ed

 

I am not sure how the bottom is built, but another idea is to use one sheet of 6mm on the bottom and then where you lap to use the 4mm which laps to 8mm and would be quite strong.

That would be my approach no glass. But I would put some glass on it and my glass callout was probably very excessive for that little boat, btw.

 

Oh, if no glass then 6mm vs 4mm.

 

I built a simple 14' canoe, 50mm 24kg styrofoam flatt bottom glassed with 450gsm double bias on the outside and 600gsm inside. Sides 3.5mm 3 ply no glass just epoxy coated.
Perfectly happy with it.

 

Ed, how did the 4mm vs 6mm decision pan-out?