Fibreglassing dilemma

A couple of years ago I built a IOM out of 4oz.fibreglass, epoxy and balsa core, I thought I had over built it but it was fairly light. I just tried building another hull using the same fibreglass and epoxy just without the balsa. Now I have taken it off the male mold and just letting it cure. It feels real flimsy, very easy to oil can it. So my dilemma is this,
-wait for it to cure more (if that is possible)
-put it back on the mold and add another layer of glass
-scrap it chalking it up to a learning experience and start again

If you only did a single layer of 4 oz. cloth - you probably are on the light side - and a second layer might help. Hard to tell how “soft” soft is without actually seeing/feeling. You can always lay in a second layer on the inside of the hull to add some stiffness. No need to get it up into the bow as a bumber will go there.

Also - It will gain some strength as you …

  1. let it cure (at least a week or so)
  2. add gunwales
  3. add deck and transom and keel trunk with cross bracing.

The biggest improvement in overall stiffness will be when the deck is added since that will complete the “circular” (roughly) shaped cross section. That however won’t adress the “oil can” issue.

Remember that you just need to keep the water out and support the mast, keel and rudder - no requirements to support a large boy or small man. :lol:

Any builders in a club near you to look, touch and advise?

Thank you Dick for responce,

I did use two layers of glass, I guess I will wait a week or so to see if it hardens more. Would adding heat to it help? Thee is no club around my area, but this project of using a male mold to pull off hulls is a start to give to people at a low cost to get a club going.

AB -

Heat will improve the “start” of curing (we used hari dryer when making emergency hull repairs on our big boats), but it can also soften cured epoxy. (heat the screws on wooden golf club heads so you can remove the contact surface for repair/refurbish).

Try a black plastic garbage bag in the sun, but be sure boat hull is sitting level so it doesn’t twist. Best if you can place back on the plug to provide support. Leave in sun about an hour - two hours and check from time to time. If it gets too hot, open bag slightly to adjust temps. May want to try on a scrap piece of wood covering with a 4 inch square of cloth - just to be safe. You can use fingernail or even side of knife blade to test how hard epoxy is getting.

I think I’d put it down to experience and read a book on sandwich structures.

Given a reasonable bala core, you should be able to make the glass substantially lighter than your last efforts. Almost 30 years ago we were building 28 foot offshore racing boats with a layup of 2 x 1,5 oz CSM + 1/2 " end grain balsa core + 2 x 1.5 oz CSM. Some ‘cruising’ versions had an additional 1.5 oz CSM in the outer sin, but this actually reduced the failure load of the sandwich in certain circumstances. Over a total span of 28 feet, the structure was divided into three panels by 2 bulkheads. Extensive stiffening was applied in the way of the keel root/mast step.

As fas as I am aware, all the 20-odd boats built are still in existence and none has experienced substantial structural failure.

It sounds like your original effort was Bismark plus. Improve by reducing the amount of glass drastically and possibly increasing the core thickness.

Best of luck,


Composite structures will gain much stiffness by incorporating a core, whether it be balsa, foam or other lightweight synthetic material. This is because the core acts as a “shear web” to allow separation between the compressive & tensile components of the skin material. This is directly analogous to a steel I-beam where the vertical “web” connecting the top & bottom flanges will create a much stiffer beam than if the flanges were to be simply welded to each other without the web. Come to think of it, ordinary corrugated cardboard gains it’s strength based on the same principle.

I agree with you about the balsa core, my triple crown that I made is very stiff. I just wanted to try using a male mold I made from MDF. After reading about other people just using glass and epoxy I thought it would work. So how do other model boat builders do this,
in all the more advanced posts and web pages I read about them, none say anything about using balsa core?

AB -

I understand the question you are asking - how do you stiffen up a 2 ply (4 oz. cloth) glass only hull you have already laid up and found a bit “soft”?

While both Angus and Bill are correct in that a composite core of foam, wood, glass mat, honeycomb, etc. will all add to the strength and stiffness you found already in your Triple Crown - the relationship Angus posted with 1/2 inch balsa end-grain while fine for a 28 foot long boat is a bit of overkill for a boat that is 39 inches in length. Likewise, Bill is on target with his reasons for “why” a core imparts stiffness and strength. Unfortunately to add balsa (or foam) “after” the glass is laid up and removed from the plug isn’t practical.

To answer what I believed your question to be - is that “if” the additional cure time and crossbeams and/or deck do not increase stiffness, the only option you have is to add another layer of cloth inside. Whether it is glass or carbon is immaterial, it’s the compounded layers that will stiffen the boat. My earlier suggestion is to wait a few days and see if the hull stiffens (which it will - but by how much I cannot say) as the epoxy resin continues to cure - and that by heating, you “may” speed up the cure time - being cautious to not overheat.

If reconsidering their posts, I don’t believe either is suggesting you try to go back and add internal strips or glue in small bits of end grain balsa at this point in time. Yes it can be done, but you will be talking to yourself as they lead you away for “treatment”. I suppose you could glue balsa strips to the exterior of your current glass hull and then cover that with a layer of glass - but then the boat is over it’s designed displacement - and it puts back in what you were trying to remove.

By the way - I have reconsidered my thoughts about removing foam after glassing since I am finding the foam and a single light layer of cloth may be less weight than multiple layers of resin and cloth without the foam. (see “Beam Construction” post in Multihulls section)

Dick - of course you can’t add the balsa afterwards. Open top sandwiches don’t work as structures!:devil3:

If you want to continue with what you’ve got, you have to decide whether you think it’s going to work out as is. If so, give it time and warmth.

If not, the best way to save it is to look back to offshore racers of tyhe 1970s to 1980s. Many of these wetre still straight single skin. Fit 1 or 2 fore-and-aft stringers, say 4 mm thick (upstand fronm the hull) and, say 6 mm wide at the top. They should taper outward slightly towards the hull - like a block of chocolate. They should be made of either very light weight balsa or depron foam, in either case coarsely abraded on the outside. Stick to the hull with any kindof glue you like tha will hold it for the time being, then paint with epoxide. Finally run a little carbon embedded in carbon down the top of the stringer. This is a perfectly good localised sandwich sructure. Ty epoxide on the sides beefs up the core material enough for it to constitute an effective ‘web’ without any further reinforcement.

Make bulkheads as ring frames (as in some model aircraft) using the sme technique. and, say 2mm depron sheet. Here you can have a carbon strip flange to the bukhead on the bench, before fitting it to the boat -much easier. Make sure that the hole in the ring-frame is well rounded without any sharp angles. Monocoque structures do not lke stress-raisers. Also rember that a ring frame is often actually stronger than a straight bulkhead.

Hope this helps

So, how do the other builders build there IOMs with just two layers of glass?
Also if I were to add on more layer of glass I was going to do it on the inside of what is already built, do I have to sand the existing or rub down with accetone or other suggestions?

I thought my first hull felt flimsy but after a weeks cure and deck and such were added there is no problem. They will still oil-can if you push a little. I used a female mold, 2 lays of 4 oz. cloth and epoxy. As long as it doesn’t oil-can in the water you’re fine. If you make it too strong it will be too heavy.

I will wait Don for a few more weeks before I add anymore glass, but if I was going to add some would I sand between coats, or acetone. The oil canning is more like a soft tupperware container not an oil can, almost rubbery.

If it has been more than a day you should sand between lays. Can you hold the boat without having it oil-can? What brand of epoxy did you use? Are you absolutely positive that you mixed it correctly? How stiff is the bottom of the boat where there is a good compound curve? I just went out and pushed on my boat and it does feel Tupperware-ish and it’s two years old. The deck make a real big difference.
Since you used a male mold you are going to have to do some exterior sanding, why not wait until the boat is mostly done and then decide whether you need an extra lay.

I used cold cure epoxy made buy Industrial Formulators, it works when its cold is why I used it, my garage is not heated. I checked at the compound curve and it is fairly stiff, just the sides and flat areas are tupperware like. I guess i was thinking that it would be more rigid. I think I mixed it well and with the correct ratio 2:1 maybe to much hardener ar the resinis a little yellower that I thought it should be. I will wait till I finish it to add another layer.

thank you

My Disco IOM oil cans when picked up by the hull. It is a little on the light side and requires a 30 gram corrector weight. I only carry it by the foil and supporting the hull.


Andrew - if you are off a bit - it might explain why there is a slower cure time. Eventually (most) epoxies will cure if the hardener and resin mix is off a bit (not a lot though). At least you don’t have chewing gum, so it sounds like you will be OK - and once you add some kind of deck, it will add strength to the deck beam/width dimension.

A few guys have had a “soft” cure and a coat of correctly mixed epoxy applied over the soft coat eventually hardened up to an acceptable cure. Normally (again, a very broad view) too little hardener is the cause for soft cure - not too much. I’m not familiar with the epoxy you are using, so an email to the manufacturer/supplier is always a help if it doesn’t end up to your liking - but remember, epoxy generally stays softer than polyester resin even after cure.

Of course, there is soft, and then there is “SOFT” :wink:

You lucked out. I use Cold Cure. You will be fine. Just finish the boat. It takes a week to cure. If you still can, put it back on the mold for another few days or as long as you can bear. If you leave it on a flat surface it will form a flat spot. Good epoxy, but slow. I think next time I will use 51 Cure. It can use a fast or slow hardener like West Systems. Unfortunately I just bought some Cold Cure($85 a liter and a half) so it will be a while til I try the 51 Cure.
Good luck, if you ever come to the Island,bring your boat.

You can Google Cold Cure. Its formulated to cure in wet, cold conditions.

Was in Vancouver and Victoria this summer (deuce days) and thought about trying to meet up with some other model sailors but only in town for a wedding then back to Calgary. I like the cold cure it wets out real good, I just checked it again and I think it is hardening a bit more, but were the keel will be I had added an extra layer of glass and it is a lot stiffer so is this does not work I will add another layer. I have the hull laying deck side down because I rolled the edges around the mold to give me a good area to bond the deck to. How heavy is your hulls (hull only) when you pull it of the mold or do you know what the weight should be?


It’s been a couple of years and I can’t remember the weight. I’m doing one today,female mold, vacuum bagged. I’ll let you know in a few days. I put an extra lay where the keel goes too.

6 1/2 oz or 185 grams. It could have been a hair lighter because the vacuum bagging didn’t go quite perfectly. That weight includes the primer that I spray into the mold before the glass.