Strip canoe with no glass, only varnish inside?

[Glenn MacGrady]

Many paddlers are looking for lighter weight canoes or building methods/materials to cut down weight. Not being a builder or restorer myself, I've wondered about reducing weight on a strip build canoe by not glassing the interior. Just varnish the interior like wood/canvas canoes have always been made. This would eliminate the weight of the glass or other interior fabric(s) plus the weight of the interior resin.

I don't think I've ever seen anyone do this, so there must be some flaw with the idea. What is it?

 

[Cruiser]

The strength using the regular inner and outer layers of glass comes not so much from the glass layers or wood core ... but all three working in tandem, forming a composite structure.
The inner glass layer handles compressive loads (forces from outside) and the outer layer handles the tension loading, while the core holds the 2 layers in place.
Remove a layer, remove the capacity for that loading ... all 3 are needed to make it work.

My opinion on saving weight, is that there aren't really any "miracle" ideas for using the existing materials for a composite build, just smart building choices to meet your needs (or wants).

 

[Gamma1214]

...but all three working in tandem, forming a composite structure.

That's my understanding as well. I vaguely recall someone, years ago (McCrae seems reasonable but he's never [to my knowledge] built a canoe) did some strength testing and determined that strip thickness was pretty much irrelevant.

It was the 3 layers working together and the fact that the glass layers were separated that gave the hull its strength. I'd think you'd be better off replacing the wood with styrofoam insulation panels than to eliminate the inner or outer glass layer.

 

[Alan Gage]

I'd guess the only way to build a usable strip canoe without fiberglassing the inside would be to use ribs like a wood canvas canoe, most likely more than negating the weight savings of skipping the fiberglass.

Those of us who have attempted to build lightweight strip canoes are well aware how flimsy the hull is when removed the from strongback before the interior is fiberglassed. Once that inside layer is done there is a world of difference in strength and rigidity.

Alan

 

[Jim Dodd]

Henry Rushton , built his canoes in that manner.
He must have had access to some pretty good wood.
His workers were very skilled no doubt !

Comparing those hulls to todays canoes ? I'll stick with todays.

 

[yknpdlr]

Several years ago (1997), my early Adirondack 90 mile canoe race team commissioned a well known local woodstip canoe maker to construct for us a competitive voyageur canoe. He did it by overlapping the unfinished butt ends of two 16' C2 canoes together. I saw it before it was completely glassed inside, and worse than any unfinished C2, it was quite floppy. The secret of making it even stiffer was to stress the way the six seats and thwarts were fastened from one side to the other, alternating the stresses to prevent it from corkscrew warping. We raced it for several years and did very well on the 90.

 

[stripperguy]

Any stressed skin composite MUST have skin on both sides...
The core need only not fail in shear, and of course adhere well to the skins.
I've mentioned this many, many times before---the strength of a hull, assuming geomtries and materials are the same, is a function of the core thickness cubed.
In other words, double the core thickness and the strength will increase by a factor of 8. All calculations of stresses and deflections are related to the moment of inertia of the hull section. And that moment of inertia is dominated by the section thickness cubed.

Rather than eliminate a required component of a stressed skin composite, look elsewhere to save weight/increase strength.
Where can you find the optimum weight reduction?
By substituting the wood core with another core material that's 1/4 of the density.
Something like, uhmmm, oh yeah, Divinycell. At 5 lbs/ft^3 compared to 20-22 lbs/ft^3 for cedar, you can see the savings.
Or, use 3/8" thick Divinycell and still save half the weight of 3/16" thick cedar while increasing hull strength by a factor of 8.

 

[ppine]

The added strength makes up for the slight additional weight is right.
I built a stitch and glue kayak from Pygmy. It was amazingly strong for its weight.
The inside glass also protects the boat from getting wet.

 

[Glenn MacGrady]

did some strength testing and determined that strip thickness was pretty much irrelevant.

I'd guess the only way to build a usable strip canoe without fiberglassing the inside would be to use ribs

Henry Rushton , built his canoes in that manner.

I take it that a hull made of today's typical cedar strip thickness and typical outer layer composite thickness is too floppy without an inner layer of composite.

Yet boats forever were made just of wood planks using the "carvel method", though perhaps too heavy for canoes; and canoes were made made by Rushton and others using the "lapstrake method". Neither the carvel nor the lapstrake methods used outer or inner skins of anything. Nor did they necessarily have ribs.

Available digitally HERE is an 1884 book entitled, "Canoe and Boat Building - A Complete Manual for Amateurs - Containing Plain and Comprehensive Directions for the Construction of Canoes, Rowing and Sailing Boats and Hunting Craft." On pages 27-30 the lapstreak and carvel methods of building, along with "rib and batten" and "ribband carvel" variations, are briefly summarized before later chapters go into all the construction details of a canoe and related gear.

(Tangent: On the WCHA forums, I have used the last paragraph of page 114 of this book, along with two 1876 letters to the editor of Forest and Stream Magazine, to argue that at least one modern rib/plank/canvas canoe existed in Yarmouth, Nova Scotia, prior to 1876.)

Nomad Boatbuilding claims glued plywood lapstrake boats "are as light as any strip-built or kevlar canoe, are faster and easier to build and have a classic look that can’t be beat."

Lapstrake Canoes

Visit the post for more.

www.nomadboatbuilding.com

Rushton's Sairy Gamp weighs 10.5 pounds.

 

[Boatman53]

Sairy Gamp is built lapstrake and has ribs. That combination is very strong.
Jim

 

[Gamma1214]

LOL Thanks for quoting the post where I had the facts 180 degrees wrong. (thickness of the core IS relevant to strength as stripperguy points out above)

I'm unsure of the other building methods but, when I built my stripper, I remember removing it from the forms and thinking if was really flimsy. Even without the gunwales, thwart & seat (mine is structural rather than hung), the inside layer made an incredible difference in rigidity.

 

[Alan Gage]

I'm not familiar with the process of lapstrake building but, if I'm not mistaken, the planks are considerably thicker and/or denser than the 3/16" and 1/4" cedar typically used today in strip canoes. The edges of the planks are also overlapped whereas strip built canoes are edge jointed and often with imperfect joints.

The thin cedar strips allow for more complex shapes, lighter weight, and easy DIY building methods. The fiberglass skins add strength and impact resistance that rivals thicker (and denser) materials and eliminates the need for perfect joints since the glue holding the strips only needs to hold them until the fiberglass is on, which effectually bonds everything into a solid piece.

I believe cedar strip canoes are better compared with full composite boats rather than wooden boats. Most composite canoes use a foam core to build thickness with thin layers of composite for rigidity. We're using cedar to do the same thing. Same concept of an I beam. The cedar strip method we all use today originated before these strong, lightweight, foams were available and at the time it was pretty revolutionary.

So while functionally I believe the cedar strip hull has been overtaken by modern composite hulls it's still a solid building method for those who like to do things themselves or want a somewhat comparable hull for less money. And some people just like the look.

Alan

 

[Glenn MacGrady]

The question posed in my title and OP about eliminating the inside glass on on stripper has been answered.

The 1884 book I linked says that the planks on a lapstrake canoe are generally 1/4" cedar or pine. Also, for the canoe historian, the book has extremely detailed illustrations and lists of materials and tools needed to build canoes, paddles, rudders, sails, wells, cockpits, aprons, centerboards (of iron!), tabernacles, plus gear such as tents, beds, stoves and lights.

If the 9' Sairy Gamp weighs 10.5 lbs., I now wonder how much a 15' lapstrake canoe would weigh if made like the Sairy Gamp with thin ribs, perhaps two thwarts, and a bottom mounted pedestal seat.



The Nomad site says their 12', 27" beam lapstrake canoe weighs 27 lbs. It has no ribs and one thwart behind the bottom seat.





Many here may recall the interesting thread from @Sabs in Copenhagen, Denmark, seeking help in caulking leaks in his unglassed carvel canoe.

Refinishing Unglassed Furesø wood canoe

Hi everyone, I hope someone can help. I just bought an unglassed wood canoe that has been out of the water for at least two years. It will need a new coat of spar varnish and a soak to re hydrate the wood and seal the gaps. But which comes first? Varnish or water? And how long do I soak it...

www.canoetripping.net

 

[Boatman53]

I just checked my Rushton book and the Sairy Gamp ribs were 7/16” x 5/32” and the planking was 5/32” thick. When this boat was made only the finest wood was used with nice tight, close grain the likes of which is hard to find these days.
That glued lap plank you show Glenn is most likely thin plywood.
Jim
Just took a close look and that canoe is made of plywood.

 

[sailsman63]

I know I'm late to the thread...

Seems like the key to some of the older glass-less builds is that the ribs provide transverse strength, which a modern strip core entirely lacks up until the installation of the glass.

Sairy Gamp, in particular, had elm ribs, so very compact bit of strength.

It's not clear to me if the bright finish was original, or if it was a lightly canvassed design.

EDIT: I'm feeling a bit foolish. It's a lapstrake, so of course no canvassed.