Plywood Bottom???

Dennis of Douglas · Post by **Dennis of Douglas** » Fri Mar 27, 2009 10:44 am

Good day All.
We are progressing slowly on my 30' river canoe. We have finished placing the stations and are now cutting the 1-1/4" thk Ash stern. Dave, the fellow working with me discussed the possible use of solid core 8mm plywood for the bottom of a "stripper", which in our case is relatively flat with a small degree of tumblehome towards the bilge area for the center 2/3 of the boat.

We discussed the podssibility of shaping the plywood and coving the edges that meet the strips, and bead and cove any end to end joints of the plywood.

Sound somewhat intriguing. What do you folks think about such and idea in the strucutural sense ? I certainly realize that this is quite a departure from the "traditional" method of strips.

Regards, Dennis of douglas

AlanWS · Post by **AlanWS** » Fri Mar 27, 2009 11:01 am

One of the main features of a strip boat is that you can easily create hulls with compound curves -- those that curve in two directions at the same time. Plywood is used in stitch and glue hulls, which consist of several different simple curves -- those curved in one direction but straight in a perpendicular direction.

I would not be surprised if someone has combined the two approaches, but would not think that most designs of woodstrip canoes would be appropriate. I'm sure you could do it, but that you would probably need to reshape the bottom of the canoe a bit, which will make it behave differently in the water.

frugal · Post by **frugal** » Fri Mar 27, 2009 11:13 am

I would steer clear of this myself. Even with "relatively flat" and "a small degree of tumblehome" you would be bending the plywood in two dimensions. 8mm plywood is going to be pretty stiff and I can see a lot of stress at the ends where it is going to want to flatten out. Even ignoring the look of plywood I would be concerned that the stress would weaken the bottom. A couple of good bumps and you'll likely see problems.

On our canoe we put the strips in a herringbone pattern and my gut feel is that we added strength because the ends of the strips were beaded to fit into the cove of the strip on the other side. This ended up interlocking the strips.

Dennis of Douglas · Post by **Dennis of Douglas** » Mon Mar 30, 2009 3:44 pm

Good day, Thank you for your responses. You've made great points that confirmed my suspicions! I recall building two strippers back in 87 and we did indeed use the herringbone pattern on the center bottom. This seems to have almost disapeared in the current methods. Do more builders use the herringbone pattern, or do most use the straight line method?

Regards,
Dennis

Woodchuck · Post by **Woodchuck** » Tue Mar 31, 2009 2:39 pm

Well, IMHO, I believe that the strength of a cedar stip canoe or any other material for that mattter, is derived from the construction technique, not the strips. If you cut thru a cross section of the hull, you would see a top layer of glass, a filler and another layer of glassl If you took an I beam, you have a top web and a bottom web that are separated by a vertical rib, for lack of a better word. The top side or inside is in tension while the bottom or outside is in compression, just like a I beam. The filler or spacer which is the wood material is just that, a spacer and adds little if anything to the strength of the hull. It all comes from a good job of laying the fiberglass with uniform coats of epoxy. It's basic physics so gon't get mad at me, OK...

CYA, Joe

frugal · Post by **frugal** » Wed Apr 01, 2009 7:38 am

Your own argument can be used against you in this. Your I beam example is based on two webs and a rib that wre all formed essentially from a liquid state and there is no tension between these three parts.

If the I beam was formed from three distinct parts where the rib was curved or bent and had to be straightened before they could be joined, you would expect some degree of springback which makes the entire structure weaker than one formed with no internal tensions.

Plywood is made in a manner that resists curving especially if that is a compound curve. A strip on the other hand can be bent a lot easier. A heat gun or steaming can also be used to make these bends easier where this is not possible on plywood.

When I was stripping the canoe the only springback that I had to contend with was at the turn of the bilge where I used a heat gun to twist the strips. This reduced the springback considerably and reduced the tension between the strips. Now the filler or spacer as you call it does not have tension it works as you have described.

The way I see it is that as long as the internal tensions of the strips are not significant then the fibreglass and epoxy layers work as you describe.

Woodchuck · Post by **Woodchuck** » Wed Apr 01, 2009 8:50 am

Physics is physics... Initially, the I beam is stress free. When you apply a load, one side will be in compression and the other in tension. If you have built it with internal stresses, of course they will come into play but everything like a heat gun, steaming, etc. is an attempt to change the shape of a material without causing internal stresses. How you make a I beam does not change the laws of physics and I'm sure the forces required to bend a piece of 1'4" cedar are alot less than the strength of cured epoxy. I think you can go to the System 3 folks in Bay City, MI and in their archives, then have tecnical papers on how the whole system works. IMHO
CYA, Joe