Layup flexibility - how many layers?

[erikgreen]

This is a question most likely for experts or those who have done scratch building of solid glass (non cored) for boat work.

I'm going to mold some new side panels for my boat on a table, laying on a waxed mold surface. I'm doing this as an alternative to my original idea, which was cutting and gluing a plywood core to the boat then building it up by glassing around it, then fairing and painting (or gelcoat, but more likely paint).

What I'm trying to decide is whether to mold the panels flat and then bend them to the hull shape, or instead to go all the way and take the mold curve from the hull shape, hold it with appropriate backing structure, and then mold the whole panel with core there.

The mold method will save me a lot of fairing and trouble regardless, but I'm not sure which way to go because I'm not sure how easily I'll be able to bend the panels to the hull shape.

What I'd be doing is:

1) Mold panels flat on a waxed surface. Layup would be gelcoat or duratec primer, followed by a layer of mat, followed by two layers of DB1200, all in poly resin.
2) Mount the panels on the boat by epoxy gluing them at the right height, then trim the top to form the sheer.
3) Attach a thin plywood core with cabosil glue/epoxy, then glass over the outside joint with biax tape and cover the inside core with 2x layers of DB1200.
4. Attach the top edge, sheer clamp, and the rest of the structure.

This will work if the panels I make flex enough to form to the hull contour. I'm not sure how well that will work. Anyone with more experience have an idea?

My alternative would be to form thin MDF to the hull shape then hold that curve by attaching backing structure (boards). Then I could move the mold to a table and glass the whole panel with a core before attaching it to the boat.

For reference this is a standard deep-V Sea Ray hull, 22 feet long, and as mentioned I'm attaching to the top edge of the old lower hull (which is now minus the top cap).


Erik

 

[ondarvr]

Re: Layup flexibility - how many layers?

Woven or stitched fabrics will bend easily and be somewhat floppy in a thin laminate if its a flat panel, CSM will make it stiffer, partly because of the random fibers, but more because of the higher resin content. You can only get a single direction curve out of it though, no compound bends, so if that works it will bend fairly easily.

Go find an old glass sliding door and use that as a mold surface for flat stock. Just prep the glass very well though, gel coat and/or resin will bond very well to a poorly waxed glass surface.

 

[jonesg]

Re: Layup flexibility - how many layers?

If you look at the classic seacraft project by Strick you see he made curved moldings by just laying a couple of layers of 10 oz cloth on the outside of the hull, waxed and all that. When it was popped off it retained the curve.
I think some mat between layers would stiffen it.

But I'll tell ya how flexible glass cloth layups are, I used to make telescope tubes by rolling up glass sheets into 6 inch dia tubes. Using a couple layers of 6 oz cloth its still flexible. I made them stiff after forming with ply rings.

When I did the curved roof for my T top I just used whiteboard showerwall, put a 2x4 under one side to induce whatever curve you need.

 

[erikgreen]

Re: Layup flexibility - how many layers?

Good info guys, thanks.

I did some experimenting last night and I had another thought this morning...I'm wondering if maybe I'm approaching this from the wrong direction. Maybe it would make sense to take a page from Oops' book and use some arbourite or FRP sheets as an on-hull mold... basically attach them as a form to the outside of the existing hull, form to the curve I want with supporting wood, then do a layup to the needed thickness from the inside, right on top of the existing hull edge.

Of course there would be not much strength in the joint because it's end-on with a small surface area, so I'd overlap some during the initial layup, then reinforce on the inside with a few layers of epoxy resin and biax. Plus a few stiffening structural members down to deck level once the panel is in place.

If I spray gelcoat into the "mold" I can even have it color matched to the existing hull if I want... although I think I'll be painting it instead.

After the layup hardens I can trim it to the sheer height and glass as planned... or I can even build the mold so that it forms the sheer and a nice smooth edge for attaching the horizontal section later... hmmm.

I did a test last night laying up 2 layers of biax and 2 of mat with poly resin (440 premium from US Composites if anyone cares) and it is fairly flexible in one dimension at a time (as noted by ondarvr). I'm leaning away from that because it would be hard to form the lines I want.

Actually, thinking about it, if I used a mold in place strategy, I could cut the mold to form a "demo" sheer line before I glass.. that way I'd see exactly what it looked like beforehand, and I could adjust curves and height as needed... this is sounding better and better..

Erik

 

[erikgreen]

Re: Layup flexibility - how many layers?

More thinking... I examined a couple layup schedule possibilities for this.

Whether I can do this or not depends on 1) Whether I can make the molds fit, hold in place, and basically work and 2) Whether I can afford the cloth/mat/poly resin.

Poly is lots cheaper than epoxy, which helps, but not so much if I go to a solid glass layup, for example:

mat:mat:roving:mat:roving:mat:roving:mat

This would be about a 1/3" thick layup, which would work ok. But it would take more than twice as much (poly) resin as a cored epoxy layup I think, mostly because epoxy doesn't need/use mat, which sucks a lot of resin.

So I looked at using what cloth I have instead of buying roving, and using some 1.5oz mat and a 1/4" plywood core. By the way, I'd normally avoid coring any part of a hull due to potential rot issues, but this is the part least likely to cause a catastrophic problem and most likely to be noticed if it rots. If anyone reading this thinks it's a problem I could use a polypropylene honeycomb core instead.

Anyway, with a core the layup looks like this:

mat:mat:dB1200:mat:core:mat:db1200:mat

And that would let me get away with much less resin for a thicker result, plus I wouldn't need to build so much thickness so I could use my already bought DB1200 fabric. I'd just have to buy $100 or so of mat, and potentially core material.

So unless there's a problem with that which someone sees, I'll probably plan on a cored layup.

The seller says the 1.5 oz mat needs 32 ounces of resin per yard to saturate. So thats 40 yards (allowing for waste) times 32 oz which is 1280 oz of resin. I'd also be using 2 cloth layers plus some cabosil/resin mix for glue at the core.. I know the cloth needs less resin to saturate than the mat, but I'll assume the cabosil glue balances that out. So I'll figure on 14 yards of DB1200 x 32 oz too.. that's 448 more ounces, total of 1728. There are 128 oz. in a gallon, so that's 13 gallons of poly!

Seems like a lot, but I am building a largish piece of fiberglass. Hmmm.

I'm going to see if I can think of a cheaper way to do this yet... maybe I can't, but I have to see what I can come up with.

Hmm.. 3/4oz mat would take half as much resin and might be enough for the adhesion between layers of cloth and core I want...

Erik

 

[erikgreen]

Re: Layup flexibility - how many layers?

Or for that matter, I can probably relax about print-through to the outer layer... not sure if I'll gelcoat or not, but since I'm probably going to prime and paint, a high-build primer should take care of that.

So, if I switch to 3/4oz mat and cut the mat layers on the outside to 1 and skip the innermost one (going to fair there, too)... that gives me about 5 layers.. 7 yds per layer (50" cloth or mat) so 35 yards to saturate. 16 oz resin per yard plus a bit for cabosil glue is about 6 gallons... very doable. The mold will make sure the outside isn't lumpy because of the core, and the inside will be layered over with epoxy and DB1200 a couple times and then faired, so that'll look ok too.

The question is, would it be stiff/strong enough... I don't know.

Actually, the real question is, why am I talking to myself so much in an open forum? People will think I've gone a bit strange.

Erik

 

[jonesg]

Re: Layup flexibility - how many layers?

If cost is the issue, it is for me, I'd just use white showerboard from depot.
$14 for 4x8 sheet x 3/16 thick, completely waterproof and takes any resin.
Core the back surface to make it rigid with the induced curve you need.
I used urethane foam from depot again, $22 4x8 sheet. A single layer of mat over the foam and its rigid. Thats the cheap way.

 

[erikgreen]

Re: Layup flexibility - how many layers?

Well, price is really important, but I don't know that I'd go that far... yet.

After obsessing on this a few days, I've decided to go with the following:

1) Use 1/8" MDF, cut to height, to form a mold against the hull. I'll make a 3 foot by 12 foot section x4 and do the boat in quarters. I'd use FRP shower liner but it's much more expensive than MDF (and more floppy).

2) Once it's flush and curved as I want, and held in place to the hull with some screws (which I'll also use to clamp the final panel in place) I'll apply backing material to hold it in that form... not sure what yet, I'm thinking of either backing it with strips of wood or MDF glued with construction adhesive and turned on edge or with a skeleton backbone of wood. That's the part I need to work out yet, but basically it has to hold the mold in shape when I remove it from the hull and move it to a work table. I'll mark the level of the overlap with the current hull, and either trim the top to sheer height or mark that too.

3) On the table I'll laminate it including the core using poly, mat, and biax. I'll be using a coat of wax plus sprayed PVA as a mold release. Cost of materials should be similar to the cored layup above. If I'm feeling ambitious I'll measure the angle of the deck at points along the hull top and build a second part of the mold at the sheer mark so I can mold that corner and part of the gunwale too. This would get me a very clean edge and save labor later.

4) Once cured I'll remove the panel from the mold and test fit the panel to the hull. If all went well it should fit overlapping partially on the existing hull and curved to the lines of the hull.

5) I'll glue it to the hull with epoxy glue and overlap one or two layers of biax on the outside and inside to complete the joint.

6) Last step is fairing of the joint. The panels should be smooth and ready for paint because the mold will be smooth.

On the inside of the hull I'll also extend some structure up to support the new panel, like a rib end or fiberglass stiffener.

If I can't accomplish the mold formation for some reason or another problem develops I'll have to re-think this. But for now, full speed ahead.

Back to my other thread....

Erik

 

[ondarvr]

Re: Layup flexibility - how many layers?

I'm doing something very similar, but my hull has straighter lines so it will be a little easier.

 

[erikgreen]

Re: Layup flexibility - how many layers?

Oooo... post pics!

I'm just about ready to start mine this weekend... should be a lot of "fun".