Hello All, have a question about FG cloth, and a couple of others.

Yeah new guy with a cloth question. Been perusing the forums for a few months as a lurker, assimilating as much as posssible. I'm in the process of gutting the interior to completely refab, remod, rebuild everything, right down to wiring, and nuts and bolts.

The boat in question is 28 ft cabin cruiser, I/O, and has 4 stringers, 2 offset from the keel, and 2 more outboard from there, spaced approx 2 feet apart. The run from 4 ft or so from the bow keel to the transom. The innermost 2, which the engine rests on, are segmented, and step down in size due to a partial bulkhead, that seals the engine space from the cabin. This bulkhead, due to water leakage at the cabin door, has some rot at the joint between the forward and rear segments. Also the stringer ends are rotted a bit, and the fuel tank, aluminum, rotted through last year, all at the same area, due too water leakage. As well, structurally, the hull is a bit flexxy in the bottom panel under the tank due to unsupported span and laminate thickness. There are some minor spider cracks in the gunnel where the cockpit and cabin meet, that would indicate, it is moving around at least a little.

The plan is to replace the stringers, bulkhead, transom and add some thickness to the weak area and address some other issues as well as add more bulkheads, and remodel.

I know the keel is a natural strong point, but where the deadrise flattens out, due to the style of planning hull, it's a bit oilcanny. Hasn't failed or shown any signs of working or stress, but,if I press on, it I can flex it, I'm thinking of adding a center stringer, from about 1/3 back from the bow, to just a couple feet shy of the transom. This will be epoxy glassed in. Also I will make the split stringers, a single beam and notch the bulkhead around them. Advise or criticism?

All things equal, cost, weight (20oz), eglass, epoxy resin, which would you use, woven roving, biax, or triax? Looking to use one cloth, I can get Triax at 3 bucks a yd

 

You should post a picture.

Heavy cloth like 20 oz is difficult to use.

12oz biax 45 45 is friendly stuff to use, 2 or three layers for tabbing in structural members is typical.

This preformed stiffener is good for panels.

http://boatbuildercentral.com/products.php?cat=60#.UXo7_EqClIk

 

Skua,
I spec out a lot of repairs using alternating plies of 1808 bidirectional and 1708 double bias. Epoxy resin is usually the best because of its superior adhesion and the fact that no water gets through it--epoxy is 100% solid plastic when cured, save any air bubbles that might be entrapped.

The bidirectional is 18 oz/sq.yd 0°/90° glass with 0.75 oz/sq.ft. mat sewn to the back. The double biax is 17 oz/sq.yd. ±45° glass with 0.75 oz/sq.ft. mat sewn to the back. I usually start with the 1708 and use equal numbers of layers of each, ending with an 1808. These fabrics are easy to wet out and build up sufficient thickness relatively quickly. The 1708 is easily drapable over and into corners, and ALL fibers cross over the joints which makes them particularly strong. The 1808 gives strength and stiffness along the stringer and transversely across the stringer for overall strength and stiffness. Using multiple layers like this also lets you taper out the overlaps very easily for a gentle taper and good look. I typically prefer to put the longest or widest pieces down first, followed by the shorter ones, but the reverse is also acceptable.

I hope that helps.

Eric

 

Thank you! This is what I was looking for. One more question. Why is considered bad form to mount the deck to the stringers? It would seem to me that properly located, and installed, that it would form an almost boxbeam, in the lower hull and prove a very strong structure.

 

You are right, a deck fixed to the stringers would make the bottom structure very strong and stiff. There may be other reasons for not doing that, such as:
1. The stringers are low down in the boat, usually, and a deck down that low may not be desirable. Is this a walking deck, one that you stand on?
2. Adding a deck directly on top of the stringers necessarily impedes access to the spaces underneath. There is a lot of stuff happening in the bilge area, with equipment, wiring, plumbing, and particularly bilge plumbing. Sealing off the bilge may not be a good idea if you need ready access to closed off spaces.

Of course, if you have a deck that is some feet higher than the stringers, and you want to support the deck with columns, then you land the columns on the stringers for the best support. This also works in reverse, the columns give support to the stringers.

Eric

 

Yes it is the walking deck. The Stringers are 3 inches taller than the gas tank, which is a belly tank. All the other equipmant is located below that level as well, but the deck is built on a seperate grid system of 2x4 2-3 inches above the stringers. I want to eliminate the grid, as is soft due to age anyway. I was going to place several stanchions on the stringers to keep the deck, same height, and to keep the deck from sagging. Also subdivide the long stringer spans with 2 additonal partial bulkheads under the deck. Currently there is more than 10 fet between the only 2 there now. One of which also forms part of the cabin rear entrance and bulkhead, the other is storage undger the v berth. Free span in between with only cabinetry as support in between, which in typical fashion is held together with half a dozen screws.

 

From your description, it's probably fine to drop the deck onto the stringers. Structurally, I don't think it will hurt either the stringers or the deck.

Eric

 

Thank you kindly!

 

Eric, why do you include the sewn on mat layer when using epoxy resin?

-jim lee

 

Good question. The mat layered fabrics are very common and are used by a lot of repair yards. And not everyone uses epoxy resin; some use vinylester or polyester in their repairs. The mat helps with interlaminar adhesion particularly with polyester and vinylester, but you don't need it so much with epoxy. With epoxy, you could just as easily drop the mat layers and not have them. If a yard does not typically carry the no-mat fabrics, then they would have to make a special order for the repair at hand. That's not too big a deal, but I do try to spec out repairs with the materials that the repair yard typically uses, and as I said, the mat fabrics are very common.

The mat also makes the ply a little thicker without affecting drapability and wet out too much, and you get strength and stiffness with additional thickness as well as by just having the knitted fabric construction. So layers with mat on the back will build up thickness with fewer layers than will fabrics without the mat on the back.

Eric

 

So if I don't need thickness for any reason, and I will be using epoxy, I can skip the mat? I have a surplus yard that has rolls of just about anything, which is why I was looking at the triax first, as I figured 2-3 layers at most would gain me all the strength I would need.

 

Sure, you can drop the mat and probably be OK. For broad flat areas, the triax should be fine, too. For the areas over the stringers and doing the bulkhead tabbing, the triax would be difficult to work, and so the 1800 bidirectional and the 1700 double biax will be easier to work over the shapes and into the corners.

Eric

 

Alrighty, that will change my plan only slightly and for the better it sounds. Easier drape and formability is desirable for where I"ll be working. 1708, 1700 1808 and 1800 are also available to me in price comparable to the Triax.

Thank you!