Birch Bark canoe buoyancy

[culleyh]

Hi

I'm new this forum and I'm very interested in the construction and material of bark canoes.

Unfortunately I've never actually seen one being paddled, but I'm curious if anyone would like to comment on the buoyancy of these craft vs modern canoes (ie royalex, plastic, GRP etc). Footage I've seen of bark canoes in use give the impression of considerably greater buoyancy when compared to modern material based canoes.

If this is indeed the case, how does this affect tracking of the canoe when compared to modern canoes. Bark canoes almost give the appearance you would expect from an inflatable craft when on the water (which are often slated by canoeists who favour hard shell canoes made from royalex etc).

I look forward to reading your responses.

Howard

 

[Kathryn Klos]

welcome to the forums

The birch bark canoe would be my number one canoe-choice-- I've had the honor of paddling one of Ferdy Goode's craft, and it was like paddling a leaf-- very responsive, quiet, and totally wonderful.

I haven't paddled a canoe made from any of the "modern" materials... other than aluminum. For me, once I'd paddled a wood/canvas and a stripper canoe, the wooden canoe was all I wanted... except for that birch bark!

A large part of my preference has to do with aesthetics: wooden boats-- new or old-- are beautiful.

I made a video of our birch bark paddling experience: go to YouTube and log in 'kkloskklos birch bark launch and paddle". Note the feather decoration made from porcupine quills and the winter-bark etching. Many birch bark builders add these unique designs that make their canoes works of art, beyond the fact that they are enjoyable to use.

Kathy

 

[Todd Bradshaw]

Unfortunately, physics has little or no respect for classy craftsmanship. Buoyancy is all a matter of displacement and doesn't have anything directly to do with the material itself. It does, however, have a lot to do with the weight of that material. If we take a 200 lb. man, stick him in a 70 lb. canoe and put him in the water, the "dent" that the hull will make in the water has the same volume as 270 lbs. of water (water is in the low 60's, pound-wise, per cubic foot, depending on whether it's fresh or salt water). If we instead stick that same man in a 35 lb. canoe, built from some other material, the resulting "dent" only has the volume of 235 lbs. of water. Assuming that the two hulls have the same shape and size, this slightly smaller dent means that the canoe would be riding a bit higher in the water. The heavier we make the package (boat, paddler and gear) the more water it displaces (bigger dent) and the lower the hull rides in the water. The lighter the total package, (smaller dent = less water displaced) the higher the boat rides.

So for any given canoe size and shape, carrying the same load, any displacement variation is strictly a matter of the boat's weight. The lighter we can make it, the higher it will float, but a 50 lb. fiberglass canoe, or plastic canoe or aluminum canoe will float just as high as a 50 lb. birchbark canoe (you just won't look or feel as cool). The 50 lbs. will be the governing factor, not which particular material makes up that 50 lbs. It's quite possible for a birchbark canoe to float higher in the water than one made from something else, but only if the birchbark boat weighs less. Similarly, a canoe made from Kevlar or Carbon fiber (or anything else) will float the same way if it's the same weight. I don't know whether anyone has ever tried to build the same exact canoe from a variety of materials to compare finished weights, but that would be the determining factor in how high or low in the water they would float.

Maneuverability is also going to change with overall hull weight to some extent. A boat that has lighter ends (because the entire hull weighs less) will spin somewhat faster than one with heavier ends. This is mostly a matter of leverage, vs. distance. You have to apply more power to move something several feet away from you with a stick (paddle) if it's heavy than you would if it was lighter, and thus, you can probably move it easier/faster. Lighter boats will accelerate faster from a stopped position as well, since it simply takes less force to get them moving. These things are sometimes pretty obvious when paddling certain boats. In slalom kayaks, for example, you can feel a noticable difference in both acceleration and turning speed between a 32 lb. fiberglass boat and a 16 lb. carbon boat, both made from the same mold and identical in shape. Here again though, the issue at play is weight, not the material itself.

 

[Bob Holtzman]

Good answer, Todd.

 

[dumbquestionsguy]

That's really interesting - seems to be the opposite of what one would think. I suppose it makes sense, as it becomes more of a factor of displacement, and the buoyant force trying to "fill" that displacement.

Not to be provoking by any means, but by this definition, paddling 50lbs of styrofoam should be the same as paddling 50lbs of lead? Or is this flawed since lead doesn't float? (At least, not that I'm aware - though they do make concrete canoes, and ships are made of plate-steel so... I could be wrong). Is the weight comparison thing only applicable to "buoyant" materials? But where is this threshold, and isn't that related to the question earlier posed? I.e. what floats and what doesn't, and how do they compare?

Probably over or under thinking it. I do this often.

 

[pklonowski]

It's a matter of how much water the shape displaces, rather than its actual weight. A one pound aluminum ball will sink in water, because it will displace one pound of water, and nothing else enters the equation: Aluminum is more dense than water, so it will sink in water. But if that same one pound of aluminum is shaped into a hull, and placed on the water, it still displaces one pound of water, but the airspace inside the hull is less dense than water... when that airspace is included in the equation, the overall volume of the hull is less dense than the water, and thus the hull floats.

Physics is phun!

 

[culleyh]

Thank you all for the replies so far.

Its funny how the most obvious issue of displacement didn't actually occur to me! Can anyone comment on rough example weights for bark craft vs GRP or Royalex?

One would think that bark craft would have to be fairly light from the native perspective, to enable solo portage etc. Many cheaper modern materials appear to result in quite a heavy craft to be portaging solo.

Presumably if you were to compare the foam style sandwich materials used in modern canoes with say solid timber construction, the variation in material buoyancy must have some effect??

With an inflatable craft, does the air within the craft structure contribute to buoyancy, or does it simply form the shape of the hull??

Thanks again for your responses
Howard

 

[pklonowski]

Peter's correct... and I need more caffeine...

The ball of aluminum is more dense (mass/volume) than the water, thus it sinks. The hull shape has the airspce added, and is thus less dense than the water displaced, and it phloats...

 

[pklonowski]

The air within the hull structure that is below the water line contributes to the bouyancy equation. Air above the water line does not contribute to this.

You'll note that as you try to force a hull down into the water (inflatable or otherwise), it pushes back harder, as you push down harder. This is because you're adding airspace below the water line, which in effect makes the hull less dense overall.

Specific materials make a difference in the overall weight of a boat, but not in the bouyancy equation, as Todd describes above.

I'll leave the question about birchbark vs other materials to those more knowledgeable...

 

[Kim Ribic]

Last of the Mohicans

Morning
Would any one know what type of canoes were used in the movie,
Last of the Mohicans?
Who built them?
Are there plans available to build one?
Thanks for any info
Kim

 

[Greg Nolan]

There are at least 2 movie versions of The Last of the Mohicans

A trailer for the 1936 movie can be found at

http://www.youtube.com/watch?v=roVEH5QKEJA

The canoe shown at 0:26 of the trailer appears to be an ordinary then-modern (that is, wood/canvas) large canoe (maybe 20+ feet) to which were tacked on raised sheers and stems to imitate a canot du nord. The gunwales of the basic canoe which was modified were not covered over and are pretty clearly visible.


Glimpses of the canoes in the 1992 version of the movie may be seen at the point indicated in the following trailers/clips:

http://www.zuguide.com/#The-Last-of-the-Mohicans
1:02-1:06

http://www.youtube.com/watch?v=aHLoJfnFNv0
5:02-5:07


http://www.ovguide.com/movies_tv/the_last_of_the_mohicans.htm
1:32-35
The canoe shown seems to be a better reproduction of a canot du nord than was made for the 1936 film, but from what can be seen in these short scenes, I won't hazard even a guess as to how these reproductions were built.

 

[Tom Widney]

I seem to remember seeing an add for the sale of the 1992 canoes, they were in a warehouse somewhere in the UK. This was in the last year or so.