View Full Version : Cost of Glassing
04-12-2006, 11:41 AM
I was wondering about the difference in cost of using bare high quality plywood and glassed medium grade. How much epoxy does it take to fill out a given area of a given weight of clothe? Any thoughts on glass versus polyester clothe?
PS - Why do I rate Senior Member status?
04-12-2006, 11:50 AM
I think those of us who survived the forum's "Regime change" are "senior"...or could it be over a certain # of posts? Wondering the same thing meself.
As for your question, I don't have a lot of specific knowledge, but suspect it is determined by the use of the boat. If you are talking about a large boat that lives in the water, choices might be quite different from the same materials used for a trailer-sailed small boat. And there is the longevity factor -- how much is an additional 5 or 10 years worth in 2006 $$?
On the Yahoo group for DuckWorks, there has been a lot of recent discussion on "cheap boats" -- daysailed / dry-stored ply boats with no epoxy and either just porch paint, or canvas / other fabric and porch paint. In many cases they seem to last a long time and work well for their purpose -- as long as they don't scrape thru the paint / material.
Let's see what the other folks say....
04-12-2006, 12:07 PM
50% cloth/50% epoxy if you are good.
40% cloth /60% epoxy if you are not.
04-12-2006, 12:38 PM
Sheathing with epoxy and cloth of some type has several functions. Depending on the boat and location, different functions may be more important.
1. Tensile strength. In this case, the fiberglass provides the tensile strength and the epoxy just holds it in place. This is important for impact resistance, and for holding together light strip-planked boats (which are really wood-core composites). George's point about epoxy-to-glass ratio applies here, particularly when weight is important. A special case is the seams of taped-seam boats, where the fillet is actually in compression sometimes, and the outer skin of cloth is mainly a tension member.
2. Abrasion resistance. This is important on decks, and on the bottoms of boats that are beached regularly or are used in white water. Other types of fabric (Dynel, Xynole, etc.) work better than fiberglass for abrasion resistance, although their tensile strength is lower. Again the fabric does most of the work, and the epoxy holds it in place.
3. Waterproofing. In this case, the epoxy is the functional element. The fiber reinforcement is necessary, however, to allow a thick enough layer of epoxy that doesn't crack and leak (think rebar in concrete).
4. Keeping fir plywood from checking. This is similar to waterproofing. The structural integrity of the sheathing layer, which needs to keep the plywood surface dry and immobile, is the most important thing.
04-12-2006, 12:40 PM
If you can avoid epoxy and glass by using good quality wood, then you will also save yourself extra labor and time.
George is on the right track with his estimate of assuming a mix of 40% glass and 60% epoxy by weight, maybe go 30/70 to be most conservative. Estimate the amount of area covered, convert to square yards, multiply by the weight of glass/square yard and then estimate how much epoxy (by weight) would be needed to fill the cloth. Estimate the costs from your favorite vendor.
Epoxy and glass aren't cheap, and they also add weight. If you can use the same amount of wood and reduce or eliminate the need for epoxy/glass by going with better quality wood (and you can get it), then I would go with the better quality wood, IMHO. One exception may be with fir marine plywood, which has a tendency to check and can be avoided only with a layer of glass.
04-12-2006, 12:50 PM
I think you're right, Thorne. You can change the Senior Member on your profile.
Andrew, can you please expand a bit on how you define 'medium' and 'high' grade plywood? Got a price comparison? What boat would this be for? The reason I ask is that a glass sheath on poorer wood is no substitute for good marine grade ply so I'm wondering why you are asking the question in the first place.
04-12-2006, 01:16 PM
This is quite off topic, but has anyone else noticed a haunting similarity between Thorne and Keith Wilson's avatars? Both lovely, by the way;)
04-12-2006, 03:37 PM
No comparison at all -- I'm facing the other way and my hat is MUCH nicer!
Baaaack to the topic, what size boat for what sort of use? Really don't think the question can be answered otherwise...
04-12-2006, 04:11 PM
Baaaack to the topic, what size boat for what sort of use? Really don't think the question can be answered otherwise...
Me either. If it's big, took years to build and lives in the water, the smart move is high quality ply and epoxy/glass. If it took a couple of weekends to throw together and lives in your garage, slap some house paint on it and have fun.
04-12-2006, 04:19 PM
I think the main difference is that Thorne's may actually be his own picture. I don't really have a hat anywhere near that elegant.
04-12-2006, 04:51 PM
LOL - I travel to the UK annually for English Civil War reenactments, and usually pick up a hat whilst I'm there. I wear it onto the plane to avoid having it crushed in the luggage, plus it gives the Customs folks something to talk about. And for some reason they *never* search your luggage when you are wearing a hat the size of an extra-large pizza.
This summer I flew back with a real beaut -- huge brown wool with an enormous black ostrich plume. We were transferring planes in Chicago, and as I walked off the plane a kindly local gent told me, 'Don't think they'll like that hat here in Illinois." I smiled at him and said, "That's OK, I'm flying through to San Francisco -- where they'll LOVE it."
As long as we are well and truely hijacking this thread, here's some pics of my dory with the new interior and exterior paint -- Rustoleum marine white and Kirby's Blind Green on the sheerstrake:
04-12-2006, 06:05 PM
Another thing about glass sheathing is that it makes fir plywood practical again. Fir plywood of good quality is much less costly than the imported stuff, but it has equal or better strength and decay resistance. The problem with it is that it's very hard to apply a good enough finish that it doesn't check badly. Glass sheathing solves the checking problem and adds strength and impact resistance to boot. For some uses, glass sheathing on fir ply is a good choice, both functionally and economically.
04-12-2006, 06:32 PM
I went through this same thought process when I started my 26' Calkins Bartender. I ended up choosing the lighter, less expensive okuome plywood and sheathing with glass/epoxy. In a lot of ways, I think it is a wash, in terms of expense, and also possibly in terms of durability. And glassing has been the least enjoyable aspect of the building process. Were I to start over, I think I would buy top quality mahogany plywood and paint it. Maybe.
One thing about glassing plywood that doesn't get much mention is that, unless you are really skilled with wetting out the glass, you will get a wavy surface that is less fair than the plywood you started with. If you are careful in your framing, plywood planking will lie very fair and pretty, and can be painted (if it is not Doug fir ply) without the application of fairing compound. But add a layer of glass, and if you want a really nice finish, be prepared to work for it. My own fairing process consumed probably 100+ hours on a 26' hull. Not to mention a couple gallons of epoxy and boxes of sandpaper. It looks good, but I'm not sure that the finished product is much superior to what I had before I glassed! I did correct some minor unfairness in my planking, but I don't think it was enough to be worthwhile. Kind of a circle of futility, and an expensive one at that.
I've heard that the use of peel-ply can help in this regard; I think I'll give it a shot next time I glass.
04-13-2006, 01:06 PM
JimD, the current project is a little dingy that will be left unsecured in public area so I wish to mimimize cost/maximize utility. I wanted some idea on the cost of glassing to get an idea of whether it was worth the cost.
Andrew, in that case you may wish to build a boat no one would want to steal. Seriously, since glassing the dink might cost you maybe an additional $150 or more and it will be subject to public abuse and even vandalism and theft I wouldn't glass it or use good wood. I'd build with exterior construction grade plywood, fasten with ringnails and glue such as a good construction adhesive or Titebond or the like, and paint it.
04-14-2006, 09:48 AM
Jim, I've almost finished building a Bolger nymph with cheap 1/4" luan ply I picked up at big blue. I chose it over 1/4" fir because I've had checking problems unless glassed. Hence the question. The luan I used is the real crappy stuff with two paper thin outer plys and one thick inner ply. Of course the next trip to big blue their luan had three equal plys. I'll probablly go back and pick some up for future quick'n'dirty projects. If this boat disappears or falls apart I'll use the "good" cheap ply for the replacement.
Wow, I just made four aces in Posts count.
04-14-2006, 10:05 AM
I built a multichine plywood dinghy over stringers in 1988. It has lived on the beach for 7 years, always been outside except for two years. I taped the seams and coated the outside with polyester resin ( gasp!). Some of the tape has come off, the inside is a bit checked, ( just repainted interior for the first time) Annual coats of cheap white paint do fine. If your going to drag it up the beach, as I do, I recommend a chafing strip of glass. Otherwise, the plywood is ok.
04-14-2006, 10:21 AM
I think it really comes down to whether your boat is going to be a 'quick and dirty' project that is only expected to last a short time, and will result in no significant tears if it crumbles a few years down the road.... or if you intend to labor over it for a long period, obsessed with fit, finish, longevity, etc.
Personally, I enjoy the epoxy-glass process... it's not difficult, and the result is amazingly strong, abrasion resistant, and durable. It's not cheap, for sure, but I never had a problem with any surface that was covered by glass/epoxy.
The cost does vary, but you can save some money by choosing the lowest cost/sq yd glass (usually 10 oz.), and shopping around for epoxies. I've always used West System products, which are expensive, but I see that System Three, MAS, and a few others are somewhat cheaper, and I suspect they'd be just as good.... some even claim to be 'blush-free', although it's no big deal to wash off the amine blush before sanding.
I wouldn't even attempt it, however, without one essential tool: a 5" or 6" random orbit sander that uses perforated discs that attach with a Velcro system... and that has an exhaust port you can hook up to your shop vac. With one of these tools, sanding to a fair surface is quick and clean... without it, sanding epoxy is a misery! I have a couple of Bosch 5" versions that have always worked well for me, and are a bit lighter than the Porter Cable equivalents... but there are many other brands to choose from. Epoxy glass sanded this way (using 80 grit discs) provides a perfect surface for primer and paint... many say you can even skip the primer.
Two ounce fiberglass cloth is the lightest I've ever used and would be enough to stop the checking on fir plywood and would require the least amount of epoxy to wet out and fill properly. I just hope you'll be leaving the boat unattended in a nice neibourghood.
04-14-2006, 12:17 PM
Yeah, I'd agree that you should keep the costs down if it will be left anywhere near the public and vehicles -- I've had one dink stolen and had to be very wary to avoid losing the others.
Why not try fabric and paint? Use it just like epoxy and glass fabric, but use plain old cotton muslin or whatever is cheap at the discount fabric store. There was a long discussion of this method on the duckworks Yahoo forum, and many of the "cheap dinghy" builders swear by it.
I'd also say to install two LARGE drainplugs in the bottom of the dinghy and remove 'em -- that might discourage kids taking it out into the water. As for keeping punks from tossing it in the back of a van or truck - a cable lock can help somewhat...
04-17-2006, 11:54 AM
The location has a lot of small dinks, kayaks, etc that don't seem to have problems and I will use some sort of token nuisance lock. I almost hope it will "go missing" so I can take another (better) crack at it.
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