I have to admit to being unable to find specific reference to use above the waterline only and would expect anybody cladding the hull to use much the same laminate throughout.I've never,ever seen CSM of less than 225gsm,so I'm a bit puzzled by references to 50gsm.Mat over here was traditionally specified in oz/sq.ft which may have faded away but 2oz mat is 600gsm and I think some confusion may have crept in.I do know one or two aeromodeIlers who would perhaps be interested in 50gsm cloth and a link to such might help them.
General purpose marine laminating resin should be adequate for priming and subsequent laminating.
The reason you couldn't find it is that it was not there. I was having trouble wrapping my head around the use of OSB below the waterline, so I assumed this was a house on a barge type, not a boat. My mistake. Not too bright.
I am a dinosaur. I learned how to sheath a small boat before there was an internet. You could go years without seeing a strip canoe. There were only two or three at the Mystic Small Craft Workshop when Andy brought Sister Suzy and I brought a ADK guideboat. John Gardner was there, but the workshop wasn't named after him yet. Non-air inhibited polyester resin was not readily available to the general public, and epoxy was either mail order or you had to figure out what to use from the local industrial supplier. The few people who knew anything knew that fiberglass would peel easily from wood if you used polyester. As far as I know, no one tried air inhibited resin and no one tried soaking the wood before applying the cloth. 30 years of hearing about consistent failures leads you to the conclusion that there is no reason to look at polyester for sheathing wood, so I stopped looking. Somewhere in the last 20 years it appears that someone solved the problem. I remain skeptical, and if you know it works, you can call me set in my ways. Speaking of set in my ways, I would argue against the pontoons based on my prejudices and the difference in draft, so not a strong argument.
For a similar discussion
How much fiberglass over plywood? - Cruisers & Sailing Forums https://www.cruisersforum.com/forums/f55/how-much-fiberglass-over-plywood-24979.html
I have used 6 0z (142 gsm) plain~weave cloth to cover plywood on a pontoon hull. It adds very little strength. The tensile strength of 6 oz style 7255 cloth in the warp or weft direction is 300 pounds per linear inch of width in a single layer (pli) {(~135 kgf per cm) What is the SI unit used?}. In comparison Open mold
1700 and
1708 have tensile strengths of about 1260 and 1410 pli respectively. The difference can be attributed to the mat, so 8 oz mat appears to contribute 144 pli. The data sheets show tensile strength in psi (0.00689 MPa), so the pli figures above are multiplied by the nominal thickness of one layer. The
hexcel reinforcement handbook shows that the relationship between weight and strength is not always consistent, so the 300 lb figure only applies to style 7255 and similar weight '6 oz' fabrics can vary by as much as 50 to 100 pli.
As a dinosaur, I am familiar with woven cloth and since I deal with small craft, woven roving or heavy biaxial cloth is of no interest except for patching rust holes on a car. Rather than asking for misinformation from fellow internet experts, I decided to look up the properties of 1708 cloth. I found the usual conflicting information about binders in the mat layer and the weight of the mat layer. Sometimes one site gave conflicting answers for both properties on the same page.
Vectorply has datasheets. There appears to be no need for binder
because the mat is stitched. The mat weight is 8 oz per square yard, not 3/4 oz. The consensus on various forums seems to be that while the mat should be compatible with epoxy, it is more difficult to wet out with epoxy and it is generally easier to work wit 1708 with polyester and 1700 with epoxy.
2 oz CSM is available, in the States anyway, but I will not recommend it as anything more than a coating to prevent the plywood from checking. While counting on a thin layer of cloth on the bottom to resist anything harder than water is not recommended, 2 oz CSM is not something I would try on the hull. 6 oz cloth is the minimum as long as you don't hit anything with it. Once you start bumping things, 1708 or 1808 is barely adequate.
1808 is 0/90 and 1708 +45/-45 are essentially the same except for fiber orientation. The difference is that the long fibers are 0.11 oz heavier in 1708. Open mold
1708 or
1808 biaxial cloth with polyester resin have a tensile strength in the fiber direction of about 1300 to 1400 pli. When you look at the datasheet, the heading says open mold 45° and for 1808 it only says open mold. What they are not spelling out is that they tested both types in the fiber direction for one of the layers, not the long axis of the cloth. When tested at 45° to the fiber direction, o/90 or +-45 fiberglass cloth sheets such as G-10 have a tensile strength of about 1/4 of that in the fiber direction. The mat layer complicates any estimate because it adds a fair amount of strength in all directions and there are no direct comparisons available on the Vectorply site. The best I can do is look at quadraxial E-QXM-3508, which is similar to 1808 stacked on top of E-BX 1200. The difference in strength is only about 7%, so the loss in strength due to the off axis fibers is smaller than I would expect. I would still like to see the bias cloth tested on the long axis rather than at 45°.
Why choose 1808, 1800, 1708 or 1700? The differences are fiber orientation and the presence or absence of mat. The 45° should drape more easily over a corner, but that can be controlled by rotating the cloth, which can be inconvenient. The GSM layer is apparently more beneficial for interlaminar strength with polyester and does not affect handling. Epoxy does not wet out the GSM layer as easily according to several sources. The mat adds 77% to the thickness and 69% to the weight of a single open mold layer, possibly less for multiple layers, without affecting the strength per layer much. For a single layer of cloth, the added thickness and consequent reduction in tensile strength for a given overall thickness is not an issue.
Videos
How to fiberglass over plywood with 1708 and polyester. Note the importance of the radius on corners.
Applying fiberglass overhead with 1708 and polyester. Now here there is an advantage with poly.
