Sailing plywood and epoxy cabin roof - compound curves

What I really need to know is advice on where I go from here? mould is 95% done and the money is spent. Needs abit more work if gel coat if first layer
1. The 3 layers of plywood as above
2. first layer in plywood(as per constant camber method) and then foam core and fibreglass
3. fibreglass with foam core

Qs are
1. can I do a non-gloss finish as the first layer on the mould? (under the roof) what sort of finish? Fibreglass and then spray paint after the top is finished?
2. weight of each of the above options
3. For non-skid (traction) finish on the top, do I need to do a gelcoat layer? I was told I had to lay up gelcoat before the non-skid finish
4. release plastic over the mould?

 

Can you explain the deep grooves in the mould? That is gonna make things harder to do by a bunch.

The only reason to glass the mould would be to preserve it for repeat use or prep for vacuum work.

The mould surface needs to be prepped for the planned method. So if you are going to vac bag; then glass is not a bad idea, but you'd need reliefs for the glass overlaps. You could also do a couple of lifts of resin and skip the reliefs. I use 2 oz per yard for epoxy for weave fills. Usually two lifts is enough to seal things if they are well glued already. But the lifts will be tough with those deep grooves if those are in the surface.

No plastic on the mould. The mould surface needs to be finished to about 220-320 grit and possibly painted and then waxed and then sprayed with a release agent. A higher gloss finish like paint or buffed out epoxy can be waxes and then sprayed with release to avoid bonding the part to the mould.

I don't understand the need for gelcoat if you are painting other than it would provide a wear layer for those times the paint does wear through. The only real concern is print through which can be alleviated through the use of csm. The csm specs you have seem very thick for print through.

Did you oversize the mould to allow for vac?

 

I will help as best I can, but I am an amateur and not familiar with gelcoat, so be patient and give others a chance to respond.

 

I would tend to think using the mould as a former would be much wiser, by the way.

Let us know about those grooves please for better replies.

 

Qs are
1. can I do a non-gloss finish as the first layer on the mould? Yes with great difficulty and the matte finish is easily damaged.(under the roof) what sort of finish? Special release agents
Fibreglass and then spray paint after the top is finished? Most durable low gloss option
2. weight of each of the above options No weight adjustment for finish sheen other than planned weight of color coat material
3. For non-skid (traction) finish on the top, do I need to do a gelcoat layer? No. Gelcoat could be used to create non+skid texture. Many non- skid options, molded, paint, stickers.I was told I had to lay up gelcoat before the non-skid finish
4. release plastic over the mould? Only if you want to use alot of fairing compound.

 

Deep grooves are for two laminated beams

 

Okay, so who told you to make the beam integral or is it just to keep the shape on release?

If so, were you planning to tab the beam to the roof at the end? If so, then provide a relief in the core if possible to avoid fairing at the beams.

I see how things started to get spendy here. You are trying to build a production quality mould, sort of, for work that only needs to be done once.

And the two beams add a lot of drama, but might help keep the shape when parted.

Just use that mould as a jig. Glass the bottom after flip.

 

See attached rough drawings.
I agree with the production quality comment.the way costs were going, the cost of the boat was going to double. A master craftsman boatbuilder with 30 years experience (he told that me many times) advised me the way we went. the beams are integral to the structure; supporting the mainsheet traveller and gantry. BTW. I'm using 3mm wallled aluminium tubing for the gantry. half the price of stainless steel. I'm leaning towards your idea of the jig with the first layer being 4mm gaboon plywood.
I don't need to make a decision for another month. off on holidays

 

How are you going to keep the ply down?

And for the compound do you have to strip plank?

You will need to brace the top on the non-beam side when you take it off the jig or it will deform without glass on the bottom (a bit).

 

Spoke to the boat builder. Thinking 4mm layers in whole sheets running across the boat.only join in the middle which will be hidden by conduit for wiring underneath and along the sides of the sheets(3 of these). release tape under the joins between the sheets. hold down with battens across with small screws on the edges of plywood
6mm+ strip plank running fore and aft held down with battens with small screw/staples in the end. Could run these diagonally as I've seen elsewhere
12mm ply on the lip/flange with release tape in this area and fixed to the first plywood layer. routed after last sheet of plywood
any ideas welcomed

 

Are you still going to glass it? I would think it wise, but not the same glass.

And, of course, you will need to tab the beam to the ply. That tabbing will be a bit unsightly, but could be faired a bit.

 

Yes. Will glass it? What glass would you recommend?

 

In order to make a recommendation; the ply thickness must be known.

If you are going 4mm plus 12mm or 16mm total; some glass would help. I would tab to the beams with some 600g mat backed tapes. Like a 6" and a 4". Do the 6 first.

Then I would probably use 400g biax no mat both sides. It is major overkill, but will keep the ply stable and help strengthen things a bit. And it is thin enough that fairing overlaps won't be too horrible. You'll need to prewet the plywood to avoid dryjoint. It will be a lot of work, but you can walk the job if you prewet the ply and let it cure and rough sand it for some bite in a secondary bond.

I am no architect, nor a designer, so take my advice with a grain of salt.

 

If you get it built and it seems floppy at all; I might go up to a 600g glass on the bottom.

All my glass ideas are for paint only.

 

What does "tab to beam mean? what are 600g mat backed tapes?
Precoating plywood with thinned epoxy stop dry joint?
Prewetting the ply - isn't moisture trapped??
pardon my ignorance

I will pay boatbuilder for advice on my final solution

The steps I'm looking at as a DIY are as follows with some questions

My stepson has done lots of fibreglassing from moulds and doesn't have the experience of how to go about anything from the normal


Join the two moulds side by side
Do the two sides in stages to allow walking/crawling on as it is too wide to do at once.

1. insert beams
2. precoat 4mm marine plywood
3. lay 4mm plywood (or 3mm bendable aircraft ply if 4mm doesn't bend to mould) across - athwartship-
lay release tape in joins
4. fix plywood to beam - how?? tab
many 9-12mm thick 300mm wide strips of plywood clamped to edges
5. screw ends and edges of plywood with small screws which I can take out later
6. 400g bias
7 epoxy with balloons
8 remove screws
9 spread more epoxy with baloons over holes and where strips were
10. let cure and then fair?
11. epoxy layer
12 lay 300 wide strips fore and aft. screw
13 many 9-12mm thick 300mm wide strips of plywood clamped to edges
14. fix 12mm plywood edges (on the flanges) precoated with thinned coat of epoxy
15 400g mat??? with epoxy - what sort???
16 15mm foam core type of foam??? (for insulation as we live in the sub-tropics and want to sail in the tropics)
weight down foam core with strips of plywood clamped
17. epoxy (what sort?)
18. 4mm plywood
many 9-12mm thick 300mm wide strips of plywood clamped to edges
19. final epoxy with 400g bias ??
20 fairing primer
21. use peel ply??? or long board sanding (I've read the forum on the pros and cons of peel ply but don't know about such a large area)
22. spray gel coat over top
23 mark where strips of gel coat are going to remain and prepare/finish off
24. tape off rectangles for applying non-slip/traction painting
25. turn over roof and apply 400g/600g bias gsm and epoxy over bottom and beams and tie in the beams
26.apply gel coat to the edges and prepare/finish
27 spray paint bottom with two part epoxy non-gloss paint

I'm going away for a while. My head is hurting