Fiberglass cowling/hull above waterline construction

I tried posting this on the materials section but no responses, so here goes:

Ok, I have made a male mold that I am going to fiberglass(see attached picture). My question is what is the best way, or tips/tricks that you guys have used to reduce fairing once the part is glassed? I hate sanding.

I plan on glassing it with polyester resin, used for surfboards, because I have a lot of it and it is fairly cheap. I also have a lot of chopped-strand mat that will be used. Not sure the weight, but it is approximately 1/16" thick. After 2-3 layers of fiberglass I want to use gelcoat for the finish.

I have never made a fiberglass part from a mold before, but I have researched it and I am pretty sure I am going about this the correct way, I just want to minimize the finishing sanding.

Thanks

 

I saw your post over there, but kept my mouth shut because it appears you are using a technique I have never heard of before. I kept quiet because by opening my mouth I will prove my ignorance of what is almost certainly a valid technique. I'll use this as an opportunity to learn something, and maybe we can get enough discussion going to interest an expert.

What is that paper on your mold? Or is that the mold?

For the record, I have used molds to make hovercraft ducts for lift and thrust. Never a boat. We never used any type of paper. We used mold release wax to prevent bonding to the mold, generally 4+ layers of it. Then we glassed directly to the mold.

Here's what I know about making a good part:

1. Your mold needs to be as perfect as you can make it. Every flaw in your mold is something you must fix on EVERY part you pull off that mold.
2. The molds I used and helped make involved a stunning amount of time with up to 1000 grit sandpaper, polishing the mold. These were commercial molds for repeated use.
3. Make sure your glass is properly wetted, meaning not too wet and not too dry. Too wet, and your part is heavy. Too dry and you have something you need to fix later, if it can be fixed.
4. The mold must be made such that the part can come off when cured.
5. I've found that popping the part off when about halfway through the time to a hard cure is best. The part is hard enough to not deform under its own weight, and soft enough to flex a bit to get off the mold. In the case of RAKA, West System or System 3 epoxies, we are talking about 2 days to a week, depending on the epoxy and the conditions in the shop.
6. In some cases, the soft cure time is a great opportunity to fix the part. For example, a void or dry spot on the finished surface is best cured after the part is off the mold, but before the final hard cure. This makes a chemical bond rather than a mechanical one. Trimming the part so that excess glass is gone is best done here, with a razor knife.
7. In others, you want to wait for a hard cure. Sanding is best done when the part is fully cured.

 

Your mold does not look fair, which means that it will be difficult to get the lams fair either. You might consider seeing if your local library has a copy of Making Your Kevlar Canoe. Not normally a book that i recomend, but Kevlar aside, the guy is heading in your direction. His process is to strip plank the molds with blue styrofoam and then sand that plug fair, which is relatively easy since it is soft. He then glassses with Kevlar, that at least in the canoe world does not require so many layers that stuff goes out of whack. But that is a sorta a special case since such a light layup is pretty compromised for stuff other than lightweight canoes.

Your mat will make a heavy weak boat and is normally only used as the one and only material where the end result is the sort of hull that gets sold at gas stations, or into amuzement parks. However, for a sturdy hull, mat can be a useful part of the package adding bulk, but that requires a range of mat, roving, and veiling.

To get stuff fair off a male plug, in addition to the above, you might consider structural foam since it can be applied sanded, and then glassed, while maintaining pretty good fairness. The the whole thing is popped off the mould and glassed inside.

Another option is to use your mat to make a female mold. Then your whole gel coat, and mat process would allow you to make a smooth on the outside hull. Your current process is more suited to a smooth on the inside deal, like a sink, at least as smooth as your plug.

 

Thanks guys. Kroberts: I made the mould with masonite, hot glue & construction paper. Basically I set the stations for the shape of the cowling/deck(whatever the correct terminology is) along a strongback, then placed strips of masonite along the length of the stations, then covered it with thick construction paper. I plan filling in all the depressions and fairing it as well as covering the whole thing with wax, or as you said release wax before I glass it.

ThomD, this part is a non structural component for my small trimaran. Basically a cowling to deflect the water and streamline the area of the main hull as well as enclosing the main hull. Have you had bad results with chopped strand mat in the past? I have never used it, only 6 & 8 oz fibercloth cloth.

As far as the mold not being fair, I thought that the masking tape I used to tape the seams of the paper together would not effect the glassing, since the tape thickness is minimal compared to the glass thickness.

Before I did this I thought about using foam but someone pointed out that the blue or pink foam you get at home depot will melt when either epoxy or polyester resin is applied. What type of foam are you talking about, and where would I buy it?

Thanks

 

And release agent !

If that kind of "unfairnes" doesn't bother you why not tape the whole plug with brown packing tape and you dont need wax neither release agent.

Polyerster is trickier with certain foams, epoxy generally isn't . Test anyway allways before major decisions.

 

I use epoxy on polystyrene foam all the time. Polyester resin is what kills polystyrene foam. Polystyrene is a really crappy core material, but if you wanted to make a one-off mold it's probably fine. In that case, you could make your mold, sand it down to the last detail, put several layers of mold release wax on it and then glass it directly. When your project cures well enough, you could break the mold out of there if it won't slip out.

Another thing you could do is the same thing, only the first few layers of glass are actually part of the mold. So you use the foam as a plug/form for the mold, and then sand the mold until it's perfectly smooth. Then you could use that to make multiple copies of your boat. That is a LOT of work though.

 

Thanks guys. Damn, making a mold is a lot of work, especially if I am only going to use it one time. I gues I might as well do it right and use a foam core. kroberts, just curious you said that, "Polystyrene is a really crappy core material", what else would you reccomend as a core material?

 

Use PU (polyurethane) foam, it does´nt dissappear with Polyesther either. But Strandmat does´nt give any strenght to your laminate! It just makes it thicker. You´ll need some sort of glass fabric in your laminate for strength. Then cover the fabric layer with strandmat to get a more even surface and reduce sanding. But if you like it or not, boatbuilding IS sanding every other step.


Regards
Richard

 

Lots of people use polystyrene as a core on hovercraft. Universal Hovercraft (UH) has two basic construction techniques. They have foam with plywood decks, and they have stick frames with plywood. The foam they use is extruded polystyrene (EPS), just exactly as you would get from Home Depot or any other home improvement store.

The UH approach on this is to basically make a solid raft of foam and then put a deck on it. My UH-18sp is like that. The raft is shaped and the edge wood is glued on with epoxy, and then the top is formed and the plywood deck vacuum bagged on. So I have more experience with that than I do with almost any other technique.

EPS sucks as a core material for a huge number of reasons, most of which I knew before I ever bought plans.

1. The foam has very poor strength characteristics. It's easy to break off a hunk, therefore any skin you put on it will easily break off a chunk of foam too. You get the part off and look, and you have the skin plus some glue, plus a bit of foam maybe 1mm thick.
2. EPS melts when it comes into contact with gasoline, oil or almost any other petroleum product. A single drop of gas and a huge volume of foam turns into a gooey mess right before your eyes. Worse yet, if it gets under the skin it won't just have a huge void you can't see, but over the next month or so the cancer spreads, slower and slower. I know a guy who wound up throwing away his entire hull because of a crack and a gas spill.
3. EPS will actually absorb water when used as a core. You get a crack, the crack admits water. The bending and flexing of normal use works that water around and expands the area of delamination, and also forces the water into the foam itself. Worse yet, since you can't see the problem you don't know it's there, and you can't repair it either.
4. EPS is brittle compared to structural foams or honeycomb materials

There are more reasons than that, but those are plenty. There are warnings in the UH documentation too. They use this foam because it is adequate to the task as designed, and in order to enable inexperienced home builders to get a hovercraft running for low cost and low effort. In many cases, the concept of a hovercraft is totally alien to the builder, they may not even believe they work until they see it in their garage.

The USCG does not accept EPS as a structural material in any commercially offered boat. The manufacturer does not market it as structural, nor do they acknowledge it to be suitable as a core. It's really easy to build a hovercraft out of, and your mold would probably be a good idea that way, but don't make it part of your canoe.

Boats seem to be much more sensitive to materials than hovercraft are. I think the forces on the hulls are much higher.

 

Erik,

I went into all that just ranting about polystyrene. Sorry I never got down to what I would use.

I don't really have good information on that, I'm not an expert. I would look at something like Plascore or other non-foam structural cores. I would also look into the Lord method of strip planking, which makes a strip plank boat and uses that as the core for a composite sandwich. I am reading up on that right now, when I'm not at work.

Extruded or expanded polystyrene (expanded is the bead board like that used in disposable coffee cups) could be used safely in a mold. Expanded is hard to sand without the beads coming off, but if you intend to make a multiple-use mold then these foams could be used to make the foundation on which the mold is built. In this case, they are there purely to hold the shape of the glass until the glass hardens. The glass then becomes the mold, and the foam can either stay there or be taken out.

My warning was simply to not use EPS in a boat as a structural material. It is thermal insulation for a house, not a structural core.

 

kroberts, I guess I didn't mention that I would not using the foam as a structural core. The only reason I would use the foam is to make a smooth mold so that I can fiberglass over it to create the desired fiberglass canopy shape that I need. Once the foam is glassed on the top I will throw away the core because I need that space inside the hull.

Apex, thanks a lot. I still have a lot of 6oz glass cloth left, so maybe 1 layer of 6oc cloth, then 2 layers chopped mat, then a final layer of 6oc cloth. See attached pdf.

 

Do it the other way round, cloth inside! Read: cloth, cloth, mat, mat, filler, wax, release agent.
The mat helps to fill the "holes" in the fabric pattern and you have less to fill and sand.
And how much weight / stress has the part to stand? It may be better to add a third layup of fabric if it´s 6oz?!? Is it plain woven fabric, or something like biaxial, unidirectional?
Use PU foam anyway it´s much easier to fair. And polystyrene gets immediately destroyed by Styrol (polyesther). Styrene is good in your walls at home.
Regards
Richard