This is a good picture. I have seen this quite often with my own first PVC skins.
My own experience at working with very large PVC skin panels is that bulges develop at the few remaining seams which show that the skin doesn't fit closely. These bulges were quite sharp since in the other places the skin would fit very smooth. Afterwards it was impossible to remove these bulges.
If you look closely, you will see the vertical folds in between the gunwales and stringers. This is exactly the problem zone, they tend to concentrate right before and after the cockpit. On the other hand, the keel area looks good... how about a single piece for both keel panels? Now, the keel needs a rub strip anyway, so there is also no real advantage in doing it in one piece. Doing it in two pieces yields much smoother stem and stern parts. You will find that narrow PVC panels are much easier to work into a pre defined shape.
My own conclusion is that a one-piece skin is perfect for testing the frame before doing the final skin. This one took less than three hours to make. Very motivating.
The Stamoid has unidirectional fibers which would only conform
in one direction, so it wouldn't take the shape of the hull
without getting all wrinkled at the stringers. In addition, and
the thing that bothered me the most, was the fact that the Stamoid was easy to tear when I
pulled on the stitching that I used to stretch the skin. This
happened in only one direction , parallel to the fibers.
I've never had a skin material
tear before, so it really caused me concern.
I'm not sure, but didn't
Patrick make a one piece PVC hull using regular PVC ?
I still feel that a one piece PVC hull is workable, as it has been done by others. The problem I had was trying to do it with Stamoid. The black PVC had multidirectional cloth , the same that I use on my sewn skins, so it had the ability to conform to the stringers. I didn't have enough of it for the entire boat so I purchased Stamoid thinking it was made the same way. It wasn't.
The Stamoid has unidirectional fibers which would only conform in one direction, so it wouldn't take the shape of the hull without getting all wrinkled at the stringers. In addition, and the thing that bothered me the most, was the fact that the Stamoid was easy to tear when I pulled on the stitching that I used to stretch the skin. This happened in only one direction , parallel to the fibers. I've never had a skin material tear before, so it really caused me concern.
I'm not sure, but didn't Patrick make a one piece PVC hull using regular PVC ?
About the Stamoid tearing easily when sewn; that is a good point. If I recall correctly, the thicker 0.6mm variant would not tear as easy as the 0.4mm one. Anyway, I had no other comparison and was satisfied with the 0.6mm stuff. Although I have never seen a skin tear on my boats, even on tar or stone beaches. And back then I simply took it as normal, but shame on me, I have used only a small amount of really thin Stamoid, mostly using the 0.6mm material. I stand corrected in having said the thinner material is better. Maybe the thicker material has a stronger cross weave respectively more cross weave?
Here is what I found today, 28OCT2002 when measuring the tearing resistance with a thin line that I would use for sewing the skin (25kg test Dacron). I punched the "sewing" holes an inch from the border and an inch apart:
Direction crosswise to roll length (in roll axis):
Direction following roll length, that is, the fibers,
if fabric were of unidirectional fibers:
To me the difference doesn't look so alarming, but yes the Stamoid actually is not at all multi/bi directional.
During my boat building runs I noted a rather large variation in quality even in a single roll. Also, the material sample I just have tested is 0.55mm thick instead of the supposed 0.6mm. The resistance to tearing with a strong line also varied a lot and was kept at bay in using a small distance between stitches. In the end I glued everything anyway. Also I used to use a thinner, weaker 10kg line that would rupture before the skin would tear and would not interfere with glueing.
I never could achieve anything with "lots" of heat shrinking on Stamoid. Only with narrow strips that were formed with the heat gun. Heat only re-aligned the weave within the PVC very lightly.
I also have heard of several people trying a one-piece skin, but my own results were frustrating (see above). See here for Patrick Poirier's results.
Contributors to this page: Thomas Yost (TDY), Patrick Poirier (PPR), Gerald Maroske (GUM) and Hendrik Maroske (HHM)