Fiberglass recommendations

[Pburk]

Hey all,

I have a separate thread going about a slow going core replacement, but wanted to see if I could get any specific recommendations about the fiberglass product I should use. Two layers of 1708? Something else? I have no experience with fiberglass and am covering a pretty large areaI (all the way from the berth to the stern), so something pretty simple and durable would be my goal. Would like to stick with the West Systems epoxy and any thoughts would be much appreciated.

Thanks!

Philip

[Salish_Explorer]

I would use either a medium weight epoxy -compatible roving or one of the double -bias roving/mat combo products (roving on one side, mat on the other), mat-side up to minimize print-through. I believe but an not sure that Biaxial mat like 1708 is probably going to be somewhere between mat and roving, strength-wise. Of course, using straight mat would be fast but not as strong.

I suspect you'll get a number of good answers here - many ways to skin the cat.

You might grab a copy of West System's fiberglass boat building manual. It's like five bucks and will have illustrated answers for most of this stuff that will be much more authoritative than what you'll get on a forum.

[Salish_Explorer]

Apologies: Upon educating myself further, I learned that 1708 is actually itself a newer kind of combo mat, and does typically have a mat backing stitched onto it, and is generally agreed to be a bit stronger but also potentially a bit harder to wet out.

I don't think my fiberglass knowledge is fine-grained enough to make a recommendation between the two, but it seems like you're generally on the right path.

Side note - it seems worth considering that the primary mode of failure in CDory construction that gets reported here is not failure in the field (failure of the laminate itself) but rather failure at lateral bulkheads, which make "hard spots" in the hull and commonly develop significant stress cracks over time. You may already have it dialed, but I think having a plan for really stout tabbing is as/more important as achieving a marginally stronger laminate in the field.

Just my (obviously only semi-educated) two cents.

[thataway]

Let's establish. a few facts. You have a cruiser model of the 16, and have removed almost all of the core. Core replacement is again Balsa. That is covered in your other thread. You are using West Systems Epoxy.

Your question is about the inner layer of the hull. Yes, 1708 is a proper material. It is a stitched bias about 17 oz roving bundles, at a 45* angle with a filler and mat on one side. It is epoxy compatible. There is also a similar cloth which has 90* orientation of the fibers (blocking on what the number is on that one.). There is also a 1700, which does not have the mat, plus there are 12xx and 22 or 24xx also. It used to be we always used layers of 24 oz roving with mat between each layer and that was difficult to work with in some cases, and in my experience not so good with epoxy.

Currently my Caracal is having part of its deck renewed--we use Coosa 20 for the core, and epoxy plugs any place there are going to be screws. We are using two layers of 1708 with 90* orientation difference, which gives a better strength. This new part is stronger than the original. Do the plugs before you finish the bottom layer. If you are going to tab the seats in, which is my recommendation, then no screws into the core anywhere, and the tabs are only one layer to hold the seat box in place. They can be easily cut, if you want to access the core again for some reason 30 years from now....

Two layers of 1708 would be perfect, mat side up as noted. You can orient the 1708 in different directions or use the 90* material for the second layer. Your boat will be stronger than new,

Although there are mats which are epoxy compatible--unless you can easily find those, just stick with the 1708.

I agree with Salish_Explorer that bulkheads are very important. But I am not aware of any issues with C Dorys and hard spots in the hull. I am always open to learning--so if you have any examples of this, please let me know.

When we were building 38' sailboats, we put trapazoid pieces of foam between the 3/4" marine plywood bulkheads and the hull, so that in no place did the bulkhead touch the inner solid glass (from 1" on the bottom to 3/8" near the hull to deck joint (which we fully glassed in while the hull was in the mold.).

We then laid up concentric (larger pieces first) of mat and 10 oz cloth concentrically smaller--usually 6 to 8 layers, and used epoxy for this critical area (especially where the chain plates for the mast were located.) For example the first piece might be 10" wide, the next 9", then 8, 7, 6 etc. This allowed the bulkhead to "float" and not pressure contact point. But as I noted above, I have not seen that as a C Dory failure.

What I have seen is that there was not adequate or even absent tabbing on both sides of the aft cabin bulkhead in the 19's (with Alaska bulkhead), 22's and 25's. It is especially a problem in the 25's. I know of several boats where this has had to be repaired in the last few months. As for the Tom Cats--(and I looked at a TC 24 which a Cbrat is considering purchasing on Friday), this does not seem to be an issue.

If I have not answered your questions, please send me an email and I can send illustrations easily.

[Salish_Explorer]

[Reelin2]

If it was my project, I’d use 1700 biax (no mat backing). My experience with 1708 and epoxy is that it takes a lot to wet it out. Also the mat backing doesn’t add much in strength but it does sucks up a lot of epoxy.

[thataway]

The feature of the mat is the smoother finish on the top. I have used 1708 extensively and found with the proper techniques, that the amount of epoxy used is not at all an issue. It is one heck of a lot better than the old 24 oz roving which the 1700 series (and similar) have replaced.

[thataway]

The feature of the mat is the smoother finish on the top. I have used 1708 extensively and found with the proper techniques, that the amount of epoxy used is not at all an issue. It is one heck of a lot better than the old 24 oz roving which the 1700 series (and similar) have replaced.

[thataway]

[Pburk]

Thanks all! It sounds like two layers of 1700 or 1708 will do the trick. I understand that I likely want the mat from the 1708 as my topmost layer, would it make sense to do a layer of 1700, then a layer of 1708 (mat up)? I'm pretty compelled by the idea of using less epoxy, although I guess I've come this far. This whole project has taken a lot more epoxy than I thought. I guess, I am still well under what it would have cost to have this done for me, but in hindsight I should have definitely just purchased the C size West System components.

Just so I am clear, when you are referencing "tabbing" does that mean running the fiberglass up the side of the hull a bit? I think I am going to have to do this, as I pretty much cut the entire floor out. It sounds like I should run the first layer up 9" or so and the second layer at about half that.

[thataway]

Should you run a layer of glass up the side of the hull, contiguous with the bottom? It would stiffen the hull up and not be a bad idea, but you do need to sand that area and be sure that the edge of the glass is smoothed in.

An example is the way we did the deck on my Caracal Cat. We left about 3" of old deck all around the sides of the hull. In this case we removed balsa core (even if good) out to the side of the hull. We put new core (Coosa 20) in with slightly thickened epoxy, but with a 1/4" gap between the horizontal edge of the Coosa 20 and the vertical hull side.

That is not tabbing: The below illustrates tabbing;

.

There are many ways to form the radius where the wood meets the hull, but it boils down to the hard 90* of the bulkhead at the hull, will cause print thru--a hard spot of the bulkhead against the side of the hull. If the laminate is sufficiently thick, I do not put the layor on the hull to begin with. I have used microballons and Cabosil or various foams which do not dissolve in the resin used. I have a tutorial on how to make a fillet in my album however this is hard board to hard board--no space as there should be as illustrated above. I don't taper the wood, and start with widest glass strip first. You do a single layer tab to hold furniture in place. Tabbing can be complex as illustrated, or just a single layer.

Unfortunately C Dory seems to use a single layer of cloth between the aft cabin bulkhead and the hull. That is not adequate and has led to rot in the core of that aft bulkhead, and water intrusion from the bilge (especially in boats with decked over cockpit floor. Why? Again price. Each step, and a bit more cost.

A few builders did it correctly. Way back when I was racing--there are a certain number of racers who represent a brand--hull, sails etc. They get discounts, often "factory" help, and then sell the boat after XX years with a good racing reputation. Those boats were often featured in ads for the various companies. I ordered a 35' boat from a major builder which was one of those project type situations--in that case I had Baxter and Cicero sails, other times I had Watt's or North Sails. I put the 35' boat thru its paces driving it hard for a couple of weeks, then called the engineer at Columbia/Coronado and suggested some "H" section be glassed into the hull to stiffen it up on the run aft, and to beef up the major bulkheads where the chainplate load was distributed.--also some more stiffening the boat for and aft, so that the hydraulic backstay tensioner would not distort the hull, plus put a larger aluminum extrusion in the way of where the mast was stepped on the forward edge of the keel to take that added load...After some discussion, the changes were effected at the local shipyard, rather than sending the boat back to the factory. The results were that I had a "stronger" boat than the average--and it made a difference. I had no hull distortion from wave action, and from higher than "usual" tension on the forestay and backstay. (For the benefit of those who have raced their sailboats.). The sails often went back to the loft a number of times to be slightly reinforced and recut to give a better shape as they stretched from use. Again--a faster boat.