Thursday, February 09, 2006

Weight vs Strength and Which Fiberglass Cloth to Use

Light but Strong - Building Cedar Strip Canoes for Wilderness Tripping

Since I have to order my fiberglass now, I am considering which weight or weights of glass cloth to use. The standard is 6 oz. However the article linked above from and recent posts on the kayak builder's forum have me considering the use of a lighter weight glass in an effort to save some weight. By using 4 ounce glass rather than 6, I am putting only 2/3 as much glass on in any single layer and am also only using about 2/3 as much epoxy (well, it should use significantly less epoxy anyway). Of course there is a penalty, and that comes in the form of strength. A layup using 4 ounce glass will not be as strong as that using 6 ounce. This begs the question, is the extra strength really necessary? Maybe not, according to the article above. The suggestion has been made in the article and by other builders, that these boats are often over-built, adding weight which is unnecessary for a boat's intended use.

The weight and strength considerations for this kayak are:

  • * It must be light enough for my wife to easily handle on land to and from the water, by herself, whether it is over a portage or from the car to the water.
  • * It must be durable enough to live through trips on the Canadian shield. This means it must be strong enough to bounce off of the occasional rock in moving water (no, not whitewater), and it must withstand the abrasion of carefully landing on a rocky shore.

Unfortunately these two factors are opposing in their build requirements. However, I think I can come to some compromise to satisfy both conditions.

After discussing this matter a bit with Martin, I am thinking of following his suggestion of using 6 once glass on the hull, and 4 ounce on the deck where there will be less abrasion and the extra impact resistance is not absolutely necessary (unless of course one finds themselves travelling upside down descending a rocky set of rapids, but then there will be other things to be concerned about rather than the integrity of the deck). Certain areas of the deck will still be re-enforced with additional fiberglass layers to strengthen the region around the cockpit. Since I probably want a second layer of fiberglass on the outside of the hull for abrasion resistance, perhaps doing the whole outside hull in 4 ounce, then adding a second layer of 6 ounce to the just the bottom "football" area would be a good compromise, or I could do it the other way around. I think I'll keep 6 ounce on the inside of the hull to aid in impact resistance.

Have some thoughts or insight on this? Leave me a comment.

By the way, I finshed stripping the deck on Wednesday night. I'll post some pictures soon. Those last strips that go over the peak of the coaming were buggers to get in the right spot because they have a large amount of twist in them and becuase the pieces are very short. But, they're in now. Next, I have to cut out the opening for the cockpit recess. I also pulled a bunch of the staples from the deck. I've just been yanking them straight out with pliers and while pulling the last staple in a big section of the rear deck, a whole section of the rear deck broke free when I yanked out the staple! Holy F*)&%! A section 3 strips wide by about 14" long behind the cockpit just lifted up, breaking at the glue lines. I used thick crazy glue (thin enough to get into the seams better than wood glue, I think) to put it back in place so hopefully now it stays there. My lesson for the night: staples do have some holding power on the thin cedar.

I'll post some pictures of the completed deck soon.