Optimum cloth weight?

I am going to build the stitch n glue dory here: http://cruisenews.net/construction.html This will be my first attempt at boat building. I plan to use ¼ Okume ply (which I can get for about 60 bucks a sheet at Yukon Lumber in Norfolk). My question concerns sheathing (excuse me, “ENCAPSULATING”) in glass/epoxy. It seems to me that the lightest cloth available- 2.5 oz- would be best because its only going to keep out the weather, not contribute strength. But in all the examples I see, people use 4 oz or 6 oz cloth. Am I missing something? Thanks, Shadroe Quiche.

 

The materials list says: "6oz glass cloth, about 15 yards"

 

That would be 200gsm if you are metrically inclined.

 

Thanks for the replys. I guess what I'm asking is why should I use such heavy cloth? I would like to keep the weight of the boat as light as possible- that's why I'm using wood in the first place. So, is there a reason not to use 2.5 oz cloth? Its only purpose to to seal the wood. Indeed, I'm tempted to use the super thin epoxy as paint without any cloth, but I like the idea of a little more abrasion resistance thin cloth would provide.

Too, the author of the article only glassed the bottom of his boat, and I'd like to glass the whole thing, inside and out. Another confounding factor is that he used ACX ply- not marine grade at all. I emailed him last week and he said the boat has held up well over the several years since he published the website.

Sorry if I'm being anal, but the journey is half the fun, right? Shadroe.

 

15 square yards at 6oz / yd = just about 5 lbs 9ozs. What are you going to save? 3lbs! Use lighter oars and row naked.

Pericles

 

Cloth weights of 4 ounce or lower, are considered a "finishing" cloth, which is primarily used when you intend to have a clear coating and a natural finish. The more weight in the fabric the more abrasion protection you'll have.

If you're interested in a light sheathing then there are better fabrics than traditional cloth. These are lighter, much stronger and much higher in abrasion protection. Have a look at Xynole, Dynel and Vectra fabrics.

The exterior sheathing does impart structure stiffness to the hull shell, so don't discount it out of hand. If the plans call for say 6 ounce S 'glass then you can substitute one of the high tech fabrics (mentioned) of about have that weight (3 ounce), but don't arbitrarily reduce the sheathing weight, just to save a couple of pounds, you may compromise things not considered or completely understood.

 

Pericles, no one wants to see me naked, though I'm tempted to moon my neighbors sometimes!

PAR, thanks for the advice. I see xynole isn't much more expensive than glass cloth- does it have a downside? Can I use it to make 'invisible scarphs' (butt splices?) like regular glass? Shadroe Quiche.

 

If you are painting the boat, there is no need to make "invisible scarphs", just sand off the join in fairing. Although I am not familiar with the other fabrics suggested, I see no reason to look past dynel for such an application.

 

Fiberglass is better for structural strength. If the 6-ounce is used as part of the scantlings, then you should NOT substitute anything for the fiberglass (e-glass) unless you're getting into using s-glass.

Xynole and Dynel have more abbrassion and puncture resistence. But they don't have structural strength. They also don't wet out clear. Not good for a natural wood finish but doesn't matter if you paint it.

You won't save any weight. In fact, you'll gain some because it take a lot more resin to wet out Xynole or Dynel than fiberglass.

Save the Xynole and Dynel for rub strips and the like. Otherwise, stick with what the designer specifies.

 

carl_shipwright, sorry if I was unclear about the scarph. I was asking if alternatives to glass are as strong as glass when used in a butt splice ( the type of scarph where the two ends of the plywood are joined by laying them flush against each other and applying a layer of epoxy and glass cloth over them. As opposed to an 8:1 scarph where each piece of plywood is tapered and layed face to face with the tapers overlapping).

JEM, thanks. What’s the difference between e-glass and s-glass? I’ve been looking for the answer to that question for a while. Shad.

 

There's different claims about the exact %, but I've always used s-glass being 20% stronger than e-glass.

Easy to understand info here: http://sweetcomposites.com/Fabric.html

 

Why is it that people who do not know how to design a boat, go out and buy a design and then want to change everything. You are forever reading about how someone or other built such and such a boat to the design, but just "changed a few things".

I am glad I am not a designer! It must be sooooo frustrating reading these sort of things.

 

Landlubber- yup, guilty as charged. I can’t leave well enough alone and I think I know a better way to do everything. However, this time I think I have the tacit approval of the designer. If you read the article, he says things like choice of wood, weather to glass the bottom only, the whole exterior, or the exterior and interior are all builder’s choice. He’s quite unspecific about lots of stuff.

And since this’ll be my first boat I’d like to learn as much as I can along the way. If all goes well I’d like to build a Simmon’s Sea Skiff later. There a just a few things that need changing with that design... Shad.

 

No worries mate.

 

The outermost fibers in a sandwich construction take most of the bending stress, on a typical glass/wood/glass lay up you would be losing a good amount of the strength if you go with lighter cloth. Even if the bending stress was not over loaded, the durability of the hull for landing and transporting it on trailer or car roof will suffer (known as "road rash").

Consider that fiberglass is often sandwiched with foam cores (though thicker), going with lighter fabric in a glass/foam/glass lay up would lose strength almost in direct proportion to the weight of glass used. Wood cores do contribute more to the strength than foam core construction (especially shear and stiffness), but you will still be lowering your overall strength.

I like to experiment too, but if this is your first project it is best to advise you to stick with the plans, and then experiment with the next one. But I am one to give this advice, I seldom "follow directions", for the same reason, I am always looking for better ways to do things. Though I have the background to do an engineering analysis if need be. But even so, many of my "modifications" turned out to be mistakes for reasons I discovered it was built that was not obvious before. These so far have been small craft and were not big investments in time or money, and I knew I was taking that risk. Usually I can rework them to an acceptable condition, though it would save time build it according to plan.

Put another way, if using the lighter fabric results in a mistake, will it cost you more than you are willing to throw away in both time, effort and material costs for the presumed benefit? And will it put you in jeopardy? In other words, do it with the understanding you are taking the risk, and it will mitigate any disappointment you might otherwise have had.

Good luck.