Hello, Andreas-

: I am an engineer and worked in aircraft design for a number of years. You are
: mixing two very different problems.

Great! I have a BS in engineering, but decided to work in the operational end of things, so became a pilot. Flew F-18 military, B-767 currently. I'm not intentionally mixing problems, only trying to appreciate the numerous influences on the design of a good hull. As I wrote earlier, I'm fairly new to this kayak building stuff.

You might appreciate two stories about composites in aviation that have shaped my thinking about them:

1. F-18 was probably the first military aircraft to make really significant use of composites in structure. In late 1970's, the wings (which are only 4" thick at thickest and have to hold huge bending and twisting loads) were not working right. When rolling, a deflected aileron cause the wing to bend and twist so much that the aircraft actually rolled the other way! The engineers quickly added another few hundred pounds of fabric and epoxy, and got the wing to stiffen properly. Several things going on here, but there was no doubt that the engineers overestimated (significantly) the strength of the original layup. Different problem with the plane was the tails; you may be familiar with that from your background. There was (is?) a real learning curve with the composites in aviation.

2.I was doing an early morning walkaround on a 767 two years ago. I noticed a little high spot near the edge of a leading edge flap, which is composite. I figured the flap was just a little out of rig, no problem, I called a mechanic. He got a lift, and came back a few minutes later and told us we better have a few extra sets of underwear. I went back in the cabin and looked from above. A ten year old leading edge was completely delaminating. I was amazed, and I do not think that the folks who designed it expected for it to do that.

Am I saying that because of a few observed problems with composite layups that I am writing off the entire field, or the planes? Absolutley not! The F-18 was/is an incredible airplane, and a huge engineering and composite sucecess.

I do say that my experience with the stuff lends me to think that long term, the wood in my new kayak will probably outlast the glass. Therefore, I was simply wondering about the various interactions with materials and thicknesses and how they might affect kayak hull strength. And whether there was a possiblity of taking a few eggs out of the fiberglass basket and putting them in the wood basket.

: 1. Stiffness and strength: If you calculate the strength of the Cedar with
: fiberglass, you will see that the cedar has only a small influence on the
: strength.

No argument, at least partly due to the fact that cedar is a relatively low strength wood. My point was that by substituting a higher strength wood, perhaps it might be possible to shift some of the responsibilty to the wood and away from the glass.

All bending loads are carried in tension and compression by the
: outer skin (fiberglass).

Once you get close to the failure the bond to the
: cedar becomes important. If the fiberglass is able to seperate from the
: cedar core in compression, it will buckle and break, thus the laminate
: fails. If the bond is excellent you may see the layer in tension fail
: (depending on lay-up).

: 2. Impact strength is a completely different story and it is very difficult
: to predict. Your assumption is that your skin construction is superior. It
: will really depend on the bond between the core and fiberglass. If you
: achieve a better bond to the hardwood than to the cedar you may have a
: superior product. Set up a small experiment where you drop a hammer in a
: predictable fashion on you hardwood/fiberglass layup and then compare it
: to a cedar/ fiberglass layup. I expect that you see delamination between
: the fiberglass and wood before you see compression damage in either wood.
: The impact characteristics are also influenced by the overall stiffness of
: the sample (if you support the samples on the outside and drop the hammer
: in the middle, i.e. our kayak hits the rocks - "oh shit" story).
: To compare apples to apples you should use the same stiffness of the
: samples, i.e. the hardwood/fiberglass sample will be slightly thinner than
: the cedar/fiberglass sandwich. Let me know what you find.

: Andreas

Thanks for the suggestion. I'm out of the boat mode in the shop until next year, but I'll try to work something up.
Mike