>. . . In their catalog, Aircraft Spruce has three different
> synthetic fabrics for covering aircraft. The fabrics are: Poly-fiber
> fabric, the heaviest being 3.4 oz., "recommended for severe operating
> conditions and high wing loading aircraft where a tough, durable"
> ..... "great for agricultural, aerobatic and bush planes". I
> don't know how this translates to using it for kayak skin... but, it is
> 70" wide and is $6.95 per lin. Yd. . .
> . . .in the catalog is a non-certified 3.7oz Dacron fabric,
> 66" wide and $3.80 lin.Yd. It has a tensile strength of 130 lb/in. I
> have no idea how this compares to cotton duck.

Airplane fabrics don't usually have to deal with scratches -- whether from hitting rocks (not a lot of boulders a 1000 ft above the ground), or scraping on sand, while kayaks encounter these at practically every launching and landing. As a general rule, thicker fabrics absorb this type of abuse better than thinner fabrics. Dacron may be a great material, when compared to cotton, but to be a better comparison you would have to compare this dacron to a cotton fabric that weighed about the same (say somewhere in the 3.5 to 4 ounce range).

As a reference, denim is in the 10 to 12 ounce range, and a fabric that would be a bit less than half that weight would probably be suitable for a dress shirt. Fabrics this thin just don't fill me with a lot of confidence ! However, when you add a nice thick coating of synthetic rubber or vinyl these fabrics may be great reinforcements, and highly durable.

The rating of 130 pounds per square inch is not one that fills me with glee. Some of the more ample (10 to 15 ounce) nylon and cordura fabrics have ratings in the thousands of pounds per square inch, and I think the 20 ounce nylon I've seen is rated at well over 10 thousand pounds. How they measure the tensile strength is unknown to me. They may be using totally different methods for these fabrics, which would make comparison of these number irrelevant. (Heck, I'm still trying to figure out why it is so hard to break 5 pound test fishing line -- and if someone can tell me how people manage to catch 20 and 30 pound fish with this stuff I would appreciate it!)

> The above fabrics are heat shrinkable, so a nicely skinned boat should be
> easily attainable. Platt Monfort mentions these fabrics for his Airolite
> boats (though I think he generally uses coatings that are less exotic than
> the ones in the catalog).

Well, now, here is a design choice. If you are going for extreme light weight you not mind compromising on fabric life. If replacing a more fragile skin does not impose too much on your time or your wallet, then the convenience of an ultralight boat made with these materials may be greater than the additional weight of a boat made with heavier materials.

Once you have the frame, most of the boat building work is done. I'd suggest experimenting with the lighter weight materials. If the lightweight stuff is not durable you are going to need to replace it. At that time you can go to a heavier fabric. As long as the frame is intact, it is an afternoon job to strip off the old fabric and put on a new skin. Waiting for the coatings to dry takes a bit longer.

Good luck with your boat. If you haven't cut the frames yet, sometime this week I'll send along some newly-calculated graph points that may make cutting these simpler, and cheaper. Putz makes his about 30 inches high, I'm making mine much shorter to save on plywood.

Paul G. Jacobson