Some folks like those Colemans
Posted by: pblanc on Dec-20-12 9:35 AM (EST)
Coleman made 17 foot long canoes out of Ram-X. Because of the inherent flexibility of non-foam cored poly those boats required extensive aluminum structural supports for bracing including a keelson, multiple aluminum thwarts, aluminum seat frames, and upright aluminum braces extending from the bottom of each thwart and seat frame to the keelson.
With all that aluminum and a boat of that length, the poly had to be necessarily pretty thin to keep the weight down to a non-obscene level. As a result, the sides and bottoms of those boats flex quite a bit.
I understand that linear polyethylene varies quite a bit in density. Some has a specific gravity less than 1 and some greater than 1. I haven't seen boat makers publish maximum and minimum thickness values for their solid poly boats. All of them tend to claim their material is "the best" without publishing any real scientific information.
Molded polyethylene boats are generally not of uniform thickness. The mold is constructed such that the finished boat is thicker on the bottom and at the stems than the deck, for example. Furthermore, the rotomolding process does not always distribute the molten poly as uniformly as intended so thin spots and voids can result. I know that some makers used to do quality control by measuring the thickness of the poly in various locations using a hand-held ultrasound probe. I don't know if they still do that.
Some folks think that the blow molding process for constructing solid poly boats results in a more uniform and stronger product. I know that the general consensus was that Prijon whitewater kayaks (which were blow molded) were significantly tougher than similar rotomolded contemporary models made by other manufacturers back in the late 1980s and early 1990s, but they also seemed a bit heavier so maybe it was just a matter of using more material.
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