Laminating Glass to Ply - Strength Question

HI Guys

Hopefully a simple one for you. I am making some walking board for the rear canopy - previously these were 18mm ply.. they sit on some 3mm stainless supports welded on the canopy tube (imagine a square formed by the tube, tabs welded on the inside of the square and the boards drop in on top of the tabs) The tabs are at around 300-400mm centres with the width of the panels around 650mm and length up to 1000mm

I have now put a 2mm fibreglass skin on the inside of the canopy to stop it from sagging and also so I can mount some lights etc. Because of this I cannot use the 18mm boards. I have a max of 12mm thickness.

I have 2 options,

1 go with 12mm ply, cut out recesses where the tabs are (or route a recess all around the edge about 3mm deep) so that the ply drops down, and glass the ply both sides. I know it will be strong enough to stand on, and thickness if recessed enough should be right (and overall probably around 14mm)

2, use 9mm marine ply and glass around 1.5mm skins on either side using woven and or CSM (advise taken) - This i'm not sure if it will be strong enough to take my not so lightweight bulk (110kg)

The end game is to get panels that are strong and lightweight and gel coated to match the new underside of the canopy... I'm planning on using standard poly resin and gel coat to finish.. sanded back and polished.

I look forward to any comments

Steve

 

Hello Steve - sorry, but I am a bit dense, and having trouble trying to understand what you are explaining above.
Can you post a photo (or a sketch) or two of the set up to help explain what you want to do?

Re glassing your marine plywood, I think that everybody is generally in agreement that you would be better off using epoxy and a stitched cloth rather than polyester with CSM and WR. It might be a little bit more expensive, but it would last a lot longer.

 

No not dense.. just not a good description... a picture may help.. the yellow highlighted area is the panel (really old photo) the canvas goes over the top and you stand on it to get the dinghy down.



understand that epoxy is stronger. I have the poly however.. and it gets stood on maybe 50 times a year.. so not high traffic.. I can certainly used stitched cloth though.. maybe a combo of csm and stitched

Apreciate your response

Steve

 

I don't quite understand why you didn't or can't build the thickness you want. There must be some constraint you are not sharing.

A deck of 12-18mm ply with skins each side would be best. But given the unknown constraints; a better option would be to relieve the panels for the tabbing and glass the bottom more than the top. The reliefs can be done so the entire panel gets glass and into the relief. Using an electric planer and sanding the step will work.

So, if you are limited to 14mm total, and something like 12mm relieved 2mm with 2mm glass (2 layers of 1708) on the bottom and one on the top should be fine.

Otherwise, 9mm ply with three layers on bottom and one on top. The panel spans are important and I am only guessing at them.

The other thing is confusing is why the piecemal approach? Glassing it all as a unit would be stronger. If it is for angles, I understand.

Also, epoxy is waterproof and will outlast. If the panels are bonded to the metal with screws; the screwholes need to be overbored and filled with thickened epoxy.

More glass on the bottom because the forces are all downward.

 

Not my best reply. Relieving a panel is an awful lot of work..

 

Thanks fall guy

Any reply is good, appreciate your time. Relieving it shouldn't be bad with a router table, so happy to do that.

My description isn't great. The thickness restriction comes from a 2mm pre made fibreglass panel that sits above these tread boards (highlighted yellow in above picture) and on top of the canopy frame but under the canvas. Think of it as a budget hard top, that still has canvas over it.. I don't have the skills or time to make a full fibreglass canopy so am improvising with this.. it allows me to mount lights etc too. The tread boards where 18mm when it was canvas on top so plenty strong with no glass on.

My issue is I can't get a feel for how strong 9mm glassed would be. 12mm I know holds me but bends a bit over 600mm (only supported at ends, actual case has more support.) Hence me looking for advice.

The 2 peices are because there are 2 'cavities' to fill in the canopy frame, ie the tube splits the standing area, meaning 2 boards have to be used..

It's gone midnight now but will try and get a drawing of the spans and support distance in the morning..

Again appreciate any help that is given..

Steve

 

I think (but I am not really sure, so please correct me if I am wrong), that I am beginning to understand. The white part is fabric (Sunbrella or PVC) streched over the frame to form a bimini. In the yellow area there are 18mm ply panels under the fabric screwed to the frame tubing via welded in tabs so you can walk on the bimini.
You have now inserted a 2mm fiberglass panel between frame tubing and fabric. The fabric is stretching enough to accommodate this additional 2mm thickness and whatever wiring will sit on top of it. But the frame tubing is only 12mm thick, so now you only have 12mm between the welded tab and the fiberglass panel sitting on top of the tube and can not mount the 18mm ply.
If my description is accurate, the solution is really simple. You cut the fiberglass panel to end at the transverse frame. You take the leftover piece and from it you cut two pieces the exact size of the plywood panels with cutouts to accomodate the stainless tabs. This pieces you glue/screw/velcro to the underside of the existing 18mm plywood.

 

Hi Rumars

Thanks for your response, yes your description is correct. The sunbrella doesnt shrink so has stretched slightly making it plenty well sized for the 'skin'

You are correct I could do as you suggested., however i wanted the upper skin to be continuous over the top, mainly for aesthetic reasons, as the walk on panels are not cut exactly to the size of the 'openings' (the openings are quite complex shape wise) the skin also continues to the upper side of the floor on the flybridge, so can be fixed to this and will prevent water ingress in the winter (we dont get much rain in the summer) down this 'gap'.. I could do as you suggest and make my life easier but there is no fun in that

I think the neatest way for me to acheive what I want is rebating a 12mm board all around the edge a couple of mm, at least I know a 12mm board will hold me without fibreglass.. fibrglass will just add to the strength...

Steve

 

Don't rebate the entire edge, just inlet the tabs into the ply. But, it's an academic question if the plywood does not conform to the actual camber of the bimini (maybe I'm imagining it, but I see a curve in your picture). You see, the fiberglass panel will conform, so there will be this variable gap between the curved fiberglass and the flat plywood. When you walk on it one of two things will happen, either the whole fiberglass panel will bend and twist, or it will crack and break underfoot. Neither case will contribute anything to the longevity of your sunbrella, and will make walking the area a hazard.
If you want it your way the ply panels will have to be contoured on the topside, matching the curvature.