View Full Version : Fiberglassing a wooden sailboat hull - recommendations

07-13-2001, 11:31 AM
I could use a bit of advice. My father and I are building a 20 foot sailboat. The hull is planked with three layers of 3mm Okume plywood for a total thickness of around 3/8". We are using a stringer / frame construction so there is a relatively substantial structure behind the hull of marine plywood and sitka spruce stringers. We've used WEST epoxy for all construction. I am searching for recommendations on what fiberglass fabric to cover the hull with. I know… I know… I’m a heritic and a looser for tainting our previously pure design with “frozen snot” but so be it.

We will be painting the hull white so a natural finish is not required. In general I'd tend towards being more durable as opposed to trying to be very light weight although I'd rather it's not so heavy as to be a 'pig'. I was wondering about using two layers of 7.5 oz cloth. Does this seem ok/too much/ not enough?

In addition, a cord struck at the widest part of the hull is 120" so if there were such a thing as 60" wide fabric in your recommended weight, I could do the entire boat with two strips (one on either side) and a seam right on the keel. If not I need to overlap / piece / butt the pieces of fabric and I've been unable to find any advice on how to do that. Do I just overlap the fabric? Should I run the strips along the length of the hull or the beam? How do I fair out the seam?

Any advice would really be appreciated.

Thanks a ton

07-13-2001, 12:22 PM
You might start by looking at the Abrasion Resistance thread under Resources for a discussion of this subject.
As to wieghts, did the designer have any advice?

07-13-2001, 01:00 PM
Tom, The boat is our design (see below). If seeing any of the specs for our design would help I could forward them. I’ll check that thread. Thanks for the advice. And I misspelled heretic. Darn.


07-13-2001, 02:00 PM
Since the designer is you, I guess you already asked him. And since I'm neither chemist nor engineer I wouldn't know how heavy the fabric ought to be. Others here are FAR more knowledgeable about this than I. Almost anything would probably work but I doubt, looking at your setup, that "good enough" would be good enough, rather that optimum is an important goal for you.
One thing I remember reading about glassing a cold-molded hull, probably in the Gougeon Bros. manual, was that the glass serves the/a function of regulating the thickness of the epoxy coating. Abrasion resistance may be better served by some other materials if that is your concern.

[This message has been edited by TomRobb (edited 07-13-2001).]

[This message has been edited by TomRobb (edited 07-13-2001).]

[This message has been edited by TomRobb (edited 07-13-2001).]

07-13-2001, 02:30 PM

Well I AM an engineer… and let me be the first to admit that given years of highly technical, professional and unrelated training I still have absolutely no idea what I’m doing in this area.. and would trade my ill-informed engineers opinion for that of an experienced boat builder any day (regardless of educational background).

I’m trying to figure out what adding fiberglass to the bottom of my hull is actually doing since there doesn’t seem to be consensus on this. From what I’ve heard, seen, read or deduced, the following are things I ought to consider:


1) Provides gross impact (dock and rock) protection by improving the “burst” strength of the hull. Improved tensile, compressive and flexural strength from plain wood laminate.
2) Improves structural integrity by adding to the flexural modulus of the laminate. Hull is generally stiffer as a composite laminate.
3) Improves the abrasion resistance in situations where the boat is likely to be dragged over rocks, sand, dirt, legs, etc.
4) Provides a means to fair out minor imperfections in the hulls.
5) Provides long term dimensional stability to the hull and prevents humidity, water absorbtion etc from distorting the shape of the hull.


1) Adds weight
2) Adds cost
3) Adds ugly
4) Adds scorn and ridicule from purists

I did find the following kind of useful article (which, not surprisingly enough recommends two 7.5oz layers for a 14 foot plywood john boat).


In my modest opinion, it would be very hard for me to analytically evaluate 1-5 and make a selection based on scientific data. I’d much rather base it on somebody’s 30 years of empirical experience.

Alan Peck
07-13-2001, 02:45 PM
Dave, Glen-L Marine has a really good video on fiberglasing boats. I think you would find it useful.

They have a website glen-l.com

Alan Peck
07-13-2001, 02:49 PM
Forgot to mention, Glen-L also has a good book on the subject. I think they have a package price if you buy both.

Tom Lathrop
07-13-2001, 02:52 PM
We assume that you have designed sufficient strength and stiffness into the hull with the structure. The function of added something to the surface coating is then to simply make the coating mor durable and puncture proof. Think of what the properties of an ideal sheath would be in that situation.

The paint, substrate and the sheath should be compatible in elasticity. A brittle paint or sheath on a soft substrate is bad. The sheath should pay its way by being resistant to puncture and abrasion. As Tom Robb said, look at the thread on abrasion for already voiced opinions on this subject. My choice is Xynole and epoxy for most instances like yours.

07-13-2001, 02:56 PM

Pardon my ignorace... what is Xynole?

George Roberts
07-13-2001, 07:01 PM
davef ---

I am also an engineer. I do a bit of work on composites.

You need .010" of epoxy to make the surface "waterproof." 6oz (.007") of glass will lay up to about .010" is you are careful.

How much glass you need depends on how you will be using the boat. If you never hit the bottom, you need none beyond that which provides waterproofing. If you hit a large rock at full speed, no amount will help. You put on what makes you feel good (safe).

I hope this was helpful.

[This message has been edited by George Roberts (edited 07-14-2001).]

Tom Lathrop
07-13-2001, 10:44 PM

Xynole is a woven polyester cloth with very fine filaments that soaks up epoxy very well and has very high resiatance to abrasion and peeling. It's quite a bit better than glass cloth in both properties. Xynole and the other synthetics are available from Defender Industries. There are a couple of other choices, like the kevlar paste mentioned on the other thread that look promising.

Scott Rosen
07-14-2001, 06:34 AM
I have no engineering background. But I've been sailing and hanging around wooden boats for over 40 years. In reading your post, I'm struck by the contrast between the large amount of "objective" analysis and the almost total lack of stated design objectives. If you could tell us what you're design goals are, the question would almost answer itself.

Three layers of 1/8 ply. Why did you choose that method of constuction? Consider instead cold-molding, which may get you a stonger, stiffer hull, practically impervious to water damage, and eliminate the need for a glass covering (although some cold-molded construction uses a glass layer BETWEEN the wood layers). Take your five points.

1. A cold-molded hull with an extra layer of wood will be stronger, stiffer and more impact resistant than a cold-molded hull with a layer of glass.

2. I can't comment on the engineering mumbo-jumbo, but see number 1 above.

3. Rocks will scratch any boat surface, wood or glass, and there's no paint known to man that can resist a beach full of sharp rocks. Once you get that deep gouge in the hull, wood will be easier to repair than glass. If, before you have a chance to do the repair, water gets between the glass and the wood, you're in for some big trouble.

4. Surfacing putty and sandpaper are the best and easist way to fair a hull. What are you going to use to fair the imperfections in the glass?

5. Wrong. This type of thinking is dangerous. Glass over wood has never had those effects. At best, glass over wood gives a good surface for paint. I've never seen a situation where a layer of glass improved the structural qualities of a well built wooden boat. In other words, if the structural integrity of your 20 foot sailboat depends on an outer layer of glass, then you've designed one very lousy boat.

Charlie J
07-14-2001, 08:10 AM
when I built my Cross 35, I glassed her with 1 layer of 10 oz above the water line and 2 layeres below. Done with WEST epoxy. She was launched in 82 and far as I know is still going strong. The layup on the main hull was four layers WR cedar and the outer huls were 3 layers 1/8 door skins, all cold molded.

I am currently in the procees of glass covering a Weekender for a customer, using 1 layer 6 oz and WEST epoxy.

I really don't think you'd want to fight a 120 inch wide strip of cloth. Would be much easier to use narrower stuff and do some light grinding at the overlaps - doesn't take a lot if you are careful with the squeegee at the edges. - Also - a Surform hand plane works really well to remove bumps and rough edges. The sooner you get to it after it first cures the easier it is to smooth. I try hard to do all the rough smoothing within a day of layup.

Good luck with the job

Dale Harvey
07-14-2001, 08:27 AM
After my last post I have mixed feelings about mentioning glass, but it is not the medium, rather it is its misapplication that causes trouble. In your case it would be a serious error to omit the covering. High abrasion resistant materials have one common defect. They do not sand or fair or feather. You have to do that with a buildup coat over them. Being new to the process, you will find glass much easier to deal with. If you put a layer of polyolfin or kevlar on wear areas first, putty within the timeframe of the epoxy without sanding, and then apply glass over it and the rest of the hull, you will find it easier. If you are going to have to sand a lot of the glass off to get the desired fairness, you will have to put on enough that the high spots will not be sanded thru. Ten oz. is a good place to start, and 12.5 wouldn't hurt a thing. The last cloth I delt with from Gougeon was junk. Same with West Marine. If it feels flimsey and loose, it is. Get Defender's catalog. www.DefenderUS.com (http://www.DefenderUS.com) or go with a local supplier who carries material on rolls. Never fold glass or especialy synthetic cloth. Keep it rolled on a tube.

07-14-2001, 08:39 AM
What George said.

John Fox, an engineer who specializes in boats, once told me (Compuserve Sailing Forum) that a layer of glass added the same stiffness as an equally thick layer of wood. Well, that is my understanding of what he said, anyway. There are variables.

Two layers of 10 oz cloth is beginning to get meaningful, stiffness wise.

Um, wouldn't glass on the inside resist punctures more than glass on the outside? Glass wood glass wood glass, even more. (Gougeon Bros. book, I think.)

The more opinions you get the more confusion obtains.

Best wishes.


07-14-2001, 08:08 PM
I think there are too many variables in our discussion. For example, using comercially available resin and glass fabric, tensile modulus (ie stiffness) could easily vary from 500,000 to 4,000,000. Thus talk of structural value of added glass cannot be very explicit.

I think you should cover your hull with fabric and resin. It doesn't matter too much which one you choose. The purpose should be to increase impact resistance. Okoume ply is very soft. Any of the fabrics mentiond will give at least 5 fold improvement in impact.

The resistance to moisture depends more on the resin you choose than the fabric. Higher molecular weight resins and hardeners tend to have better moisture resistance. However increasing the molecular weight of resins usually increases viscosity making them hard to use.

Given a particular resin, moisture resistance varies directly with thickness. My favorite compromise is to use a normal viscosity resin and hardener for laminating the fabric to the hull followed by using an epoxy product formulated for sealing out moisture to fill the weave. I usually fill the weave enough so that sanding will not get to the fabric. Any marine supply store can sell you epoxy products formulated to seal (look for the stuff used to seal osmotic blisters on fiberglass boats).

Beginners usually find small pieces of fabric easier to apply than large pieces.

When the laminated fabric is still in the green stage (consistancy of wax) you can trim any high spots with a Stanly surform.

07-14-2001, 08:15 PM
oops! By impact resistance, I mean localized impact damage to the surface of the okoume.

07-15-2001, 08:03 PM
Well everyone...

Thanks a ton for the spirited discussion. I've found this forum incredibly valuable and, if you're willing to endure occasional insult, scorn, and ridicule you can get some excellent advice. In case anyone cares, I've settled on two layers of 6oz glass/WEST on the outside.

Wish me luck and happy sailing to all of you!


Phil Young
07-15-2001, 10:15 PM
If it were me, I wouldn't agonize too much over whether its appropriate to cover a ply/epoxy boat with a layer of glass. I don't see a big issue there with loss of purity. And if it were me I'd glass the thing. Just seems like there'd be less chance of little scratches piercing the coating and letting water into the wood.