cartopper scantlings
Johannes,
For someone who has such unconventional and eccentric opinions on thin skinned structures (steel, sandwich or otherwise) you are remarkably thick skinned!!!
When you finally do get your copy of “Boat Strength” by Dave Gerr, Go straight to the Sandwich Construction section (page 76) and read: “The Hammer Puncture Test”
In his other book ” The Nature of Boats” page 343, he give a good anecdote of the builder who proudly showed off the incredible “strength” of his all kevlar sandwich 90 ft boat by bashing it hard repeatedly with a hammer, and what the sceptical naval architect did to the same hull with one hit using the other side of the same hammer…..........
Cheers,
Rob
builder who proudly showed off the incredible “strength” of his all kevlar sandwich 90 ft boat by bashing it hard repeatedly with a hammer
There is a big difference between nylon and kevlar. Glass has a lot higher yield strength then rubber, but is much easier to break with a hammer. Same thing with nylon vs kevlar.
A thick layer of rip-stop nylon or similar lightweight elastic fiber in an elastic epoxy can absorb much more energy then kevlar before tearing. Kevlar is used in bulletproof wests because the bullet can not be allowed to flex the west 3 inch before being stopped, as that would crush the chest of the person wearing the west. Think of the foam as a deformation-zone, the elastic outer shell as wear resistance and the inner shell as the load carrying structure.
Cheers,
Johannes
What you are talking about is not a structural sandwich but a boat covered with padding and the inner skin needing to be of sufficient thickness strength and puncture resistance in itself to do the job…................Back to Square One???
Cheers
Rob
Things are not absolutly black or absolutly white. As in every thing else, there are shades of grey even in engineering.
A foam core sandwich can be absolutly symmetrical with a layer of foam in the middle and one layer of fiber-glass on each side of the foam, or it can be slightly asymmetrical with say 3 layers of 200 g/m2 fiberglass on the inside and 3 layers of flexible nylon weave on the outside. Even though the nylon has less tensile strength, it still has tensile strength, and that combined with an unusually thick layer of foam will create a very stiff, strong, bouyant, insulating and impact resistant shell.
Sometimes you have to design something to do more then one thing, and absolute strength might not be the only property that is important.
Yes I might have a thick skinn, but I seem to need that , since I often choose to try stuff instead of listening to whatever authority is being worshiped at the moment. I do belive that progress comes from testing things instead of just doing whatever everybody else does. I am very sorry if this offends anyone. I just enjoy actually testing ideas I have, and in the end that is the only important aspect in building proas or whatever hobby one care to have. Instead of being a thought police and telling people “it can’t be done” or “it should never be tested”, enjoy the fact somebode else wants to make and document the mistakes for you.
Cheers,
Johannes
Hi, Johannes.
I am very curious how your would bond ripstop nylon to a foam core. I have sometimes used it as a substitute for peel-ply in vacuum bagging. I guess you would have to treat it somehow as in my experience it peels off very easily on epoxy.
I think its a very good idea if there is a good way to get it to stick.
Trent
You can not use the normal nylon weave you buy at a store. You have to contact a mill where they produce nylon weave. It has to be sized for epoxy, and not treated to repell water. Sven Yrvind bougth polyester here in sweden and used it in the way I described in a previous post. The mill has moved its production to China so it is much harder to get hold of nowdays. I have learned about this method from Sven Yrvind. He has written several books, where he describes this method and the tests he did. I know Nils Malmgren (the Swedish epoxy maker) has created a more elastic epoxy for this kind of laminates. Polyester is stiffern than nylon, and easier to laminate with epoxy. Probably easier to get too.
Yrvind Marstrom
Small Craft Advisor Sven Yrvind
Cheers,
Johannes
You mentioned polyester, which reminds me of a funny story…
Frequently I use 100 percent polyester from the discount rack at the fabric store for peel-ply. Once, I got some that must have been mislabeled. I strongly suspect it was some kind of polyester/ cotton blend.
Maybe you can guess what happened; it took a few hours of grinding to remove that stuff and I had to re-laminate more glass to fix all the damage I did trying to get it off.
It was pretty tough and not very easy to grind off.
Ha, ha!
I can assure you it did not seem funny at the time!