Tricky Fiberglassing Question

[kev_rm]

My boat has (had) a small shelf in the very forward part of the forepeak - the windlass battery box sits on it, and the plywood rotted. The plywood was glassed in to the hull from the factory. I got it out ok, but need a little help on properly replacing it.

My plan was to epoxy and glass 1x3 strips to the hull as two new "shelf tracks", and then screw a shelf on top of that. The challenge I have is I don't have a good way to hold those "tracks" against the hull while the epoxy is setting.

Any thoughts/hints would be appreciated.

Admittedly a newbie to fiberglass work, be gentle.

[billknny]

As a general rule, it is a bad idea to attach interior shelving and other cabinetry directly to the hull. It creates a hard spot which creates stress risers when the hull flexes--which it does. Probably more than you think.

Better is to glass the shelf to the hull, but held away from it by a 0.5" to 0.75" of foam. The type isn't important, it is just acting as a spacer. That gives everything the flexibility it needs. You can hold it in place with tape on the top, glass in the bottom, then when that sets, glass the top. Strong, yet without making a hard spot.

[s/v Jedi]

You need to use thickened epoxy so that it is gap filling. Then you can simply tape the rails to the hull until epoxy has cured. It does not require clamping pressure like a glue does.

Is you hull solid glass? If it is cored you can even use short screws that only go through the inside fiberglass skin.

Last method is superglue. Use epoxy along all the edges but not the center where you put thick/gel formula CA glue. Spray accelerator on the hull then hold the rail in place for a couple of seconds for the CA glue to cure.

I recommend you treat the new shelf with penetrating epoxy, with extra attention to the edges that are most vulnerable to moisture ingress.

[scherzoja]

When I have to affix something to a surface, I find some way to brace it or temporarily hold it into position. Sometimes I put a small spot of hot glue or wedge 1 x 1" material to jamb the part in place. I've temporarily hot-glued cardboard cleats to hold the part in position. In your case, you might be able to hot-glue a couple of cardboard chunks for the shelf to sit on while you use a bit of epoxy putty to holed the foam spacer and shelf in place. Once epoxy cures, remove the cardboard.

Don't use hot glue if you will be unable to scrape/sand all traces away and it doesn't take much to hold stuff in place. If you used just a little bit and it didn't smear, you can leave it alone and bury int in your layers of epoxy/glass

I've even temporarily glassed blocks of wood and used those to screw sticks/wedges to so they could hold the shelf or whatever in place.

Once the part is temporarily fixed in position, I use a small strips of chopped strand mat (CSM) in a few places, usually three spots with polyester resin with 2% or 4% catalyst so it goes off quickly.

Once cured, I remove the temporary bracing ( 4 1/2" angle grinder with 36 grit disk), run fillets of epoxy to from round curves and let cure over night. The next day, I grind away the polyester/CSM tabbing or remove the couple of spots of hot glue (the epoxy fillets usually are enough to continue holding the part, then continue with the epoxy layup.

No need for the 1 x 3 cleat is the shelf is not to be removable, just glass it in place with the recommended 1/2" or 3/4" foam spacers

If this is a wet environment, you might consider making a fiberglass panel instead of using plywood for the shelf. Unless the panel has core, it'll be pretty flexible, but you can stiffen it with formed "floor joists" on the underside (see attached diagram).

Lay up 3 layers of 1708 with polyester resin. On the underside, take strips of cardboard and from a triangular shape (base of triangle against the fiberglass panel and apex pointing down. This is the form for the stiffeners. Don't make it too pointy, you are going to lay CSM over this shape and adhere it to the underside of the panel. Don't worry about the CSM not adhering to or forming perfectly on the cardboard, you are just using the cardboard to form a shape to resist flexing (think I-Beam).

Depending on how big this shelf is, you might need to run 2 or 4 of these. Once resin (polyester) cures, you'll have stiff, lightweight, non-rotting panel that you an cut into the shape to fit the bow area. While designing this, plan for battery tied downs so you can secure the battery. Incorporate those tied downs into the lay up of this panel. Maybe thicken the edge area, cut a slot so you can run webbing through it, if you will use webbing to secure the battery.

One can also thicken some parts of the shelf and drill a hole. Use long bolts (vertical) and a wood (or DYI fiberglass) batten (horizontal) to clamp the battery down to the shelf with the long bolts.

Jedi's superglue idea is a good idea too.

[kev_rm]

scherzoja - hot glue cardboard is a great idea - I think I can do the same with contact cement as well. that will be easy to take off later too.

Jedi - hull is not cored. thought about screws into pre drilled holes but not confident in margin of error.

[s/v Jedi]

Originally Posted by kev_rm

scherzoja - hot glue cardboard is a great idea - I think I can do the same with contact cement as well. that will be easy to take off later too.

Jedi - hull is not cored. thought about screws into pre drilled holes but not confident in margin of error.

No screws in that case. I agree with the other poster that the rails are not required for a fixed shelf. So here is what I would do, using the newest technique popular on all the YouTube channels:

Use 1/4” G10 Fiberglas sheet for the shelf. I think you will find it is strong and stiff enough for your application. I made a floor with this for 4x an L16 battery which are huge.

For mounting, the new trick: Mark where the shelf attaches to the hull. Have two small boards for temporary support. Use 3M blue masking tape, put tape on the hull an inch under the marks where the shelf comes. Also put tape on the boards one inch from the edge so that tape meets tape when you holds them as temporary supports. Now use CA glue and accelerator to attach the boards, with just a couple of drops of CA glue on the blue tape. This will cure in seconds and is strong enough to put the shelf on. Use a thick epoxy like Total Boat Sixto in the caulking gun to put a bead on the hull to shelf joint. Form it to a fillet and let cure. Then remove supports and reinforce the underside with fiberglass tabbing. A single layer 1708 should be enough

[jimbunyard]

Originally Posted by kev_rm

My boat has (had) a small shelf in the very forward part of the forepeak - the windlass battery box sits on it, and the plywood rotted. The plywood was glassed in to the hull from the factory. I got it out ok, but need a little help on properly replacing it.

My plan was to epoxy and glass 1x3 strips to the hull as two new "shelf tracks", and then screw a shelf on top of that. The challenge I have is I don't have a good way to hold those "tracks" against the hull while the epoxy is setting.

Any thoughts/hints would be appreciated.

Admittedly a newbie to fiberglass work, be gentle.

If the shelf was glassed in by the factory, I assume you won't have to worry about built in 'stress risers', even if you wanted to glass the shelf in permanently...

But if you want to make the shelf removable for, say, easier access to forepeak deck hardware fastenings, build your shelf, sculpt your shelf runners to fit the hull, then assemble the shelf and runners outside the installation, insulating the joint between the runner, shelf and hullside of the shelf with a couple of layers of masking tape and use the shelf itself to hold the runners in place whilst the thickened epoxy sets. Once it sets you can separate the shelf and runners and reattach it semi-permanently, or if you've assembled it as you want it, just leave it until you need to remove it. This method will give you a perfectly flat mount for the shelf.

Grind the area where the runners meet the hull before gluing for permanent adhesion.

If you don't trust the epoxy-only runner attachment, after the thickened runner attachment epoxy sets, remove the shelf and glass a layer of 1708 or similar over the runner/hull interface.

It's much easier to drill your attachments for the batterybox with the shelf out.

If it were me, and I could afford it, I'd use 3/4" coosa board with an extra layer of glass top and bottom for the shelf, probably glassed permanently in place, for strength and rot resistance in this notoriously deterioration-prone area...

https://www.boatoutfitters.com/coosa-composites-board

[kev_rm]

Thanks everybody - this thread is one of the reasons I really like this forum - great insight on options and technique without too much judgement or politics

I ended up not doing the rails, I just cut the shelf (plywood) to the hull contour, sealed it with epoxy, let cure, then gorilla taped the shelf and put in a a couple of small temporary glass bits and epoxy, let it cure overnight then ran full "seams" of glass. Looking good now. Thanks again.

[JPA Cate]

Good on you, mate, it sounds like you did a good job.

Ann