Reinforcing side decks - ply or plastic?

[matg]

Hello,

I'm looking to reinforce the underneath of my side decks where the shroud u-bolts are fixed. It currently has ply glassed-in and water ingress has softened it.
I was looking to replace with 18mm marine ply, but then read on here about HDPE (high-density polyethylene).

>Quote from Dave 2452 'Have you considered HDPE http://www.directplastics.co.uk/hdpe-sheet easy to work as ply, sold cut to size and will never rot.'<

I wondered whether anyone has worked with it and whether it's suitable for the what I'm doing? I like the fact that it won't get softened by water, but is it as strong as ply, or even stronger? It comes in various thicknesses - I was looking at 15mm.

Any advice or other recommendations would much appreciated.

cheers,
Mat.

 

[David2452]

It really is very strong, I've used it for all sorts of stuff including backing for cleats, there is a product called Starboard that the septics use for all sorts of stuff including making deck boxes and the like, I'm sure its just HDPE with a bit of a textured finish.

 

[TradewindSailor]

The problem with using any material is whether it adheres to the remaining structure or is just a load spreader. If you use ply and epoxy it should become integral with the deck. In addition homogenous plastic is likely to deform and crack over time - Starboard certainly does under load.

 

[Tranona]

Ply is the easiest to fashion and bond to the existing GRP. Seal any holes with epoxy and use good sealant for the fastenings and you won't get leaks.

 

[lw395]

One of the best things to reinforce GRP with is more GRP.

 

[TradewindSailor]

I wouldn't disagree with that except it can be messy and it takes time to build up a thickness .... and it's the thickness that spreads the loads ..... i.e. increases stiffness

 

[William_H]

Chain plate type U bolts through a deck are really a design black spot. You could lay up more fibre glass under the deck and of course this will stiffen the area. But the thin dimension of the new fibreglass will not give any real stiffness against bending.
You could get a bit of carbon fibre which is stiffer but still it needs thickness to provide stiffness.
I have used some of the plastic sold as cutting boards for spacers against my swing rudder blade. Much better than plywood. However this kind of plastic is the type which is very hard to get resin to adhere to. Certainly rough the plastic up but I fear its apparently oily surface will not adhere to resin.

For a completely different approach to the problem. I would fit a saddle to the bottom of the U bolts a piece of rigging wire with thimble and swage down to the side of the hull. This would connect to a stainless steel plate with a tag to attach the wire to. The plate would be glassed to the inside of the hull with epoxy. Put lots of large holes in the plate and glass over the plate so glass top and bottom join. Getting tension on this wire will take some of the load of the stay and also support the deck. You could fit a rigging screw in the wire or you may be able to tighten the saddle up towards the deck using the threaded part of the U bolts. Then fit packing under the saddle if necessary.
Or you could have made up stainless steel box angle support from hull to the deck where the bolts are fitted. it could be made of plywood. Both need to be fibreglassed to the deck and hull.

good luck olewill

 

[cimo]

Or you could have made up stainless steel box angle support from hull to the deck where the bolts are fitted. Both need to be fibreglassed to the deck and hull.

good luck olewill

this would be my approach (whenever I get around to it).
got some hairline cracks in the gelcoat around the u-bolts, testament to some flexing here.

 

[TradewindSailor]

One point to remember when you are trying to attach fitting is the different stiffnesses of the materials. Loads will always travel along the stiffer material first. Suppose you stiffen a deck by placing a metal beam beneath it. Any loads in the area will be attracted to the beam as a pathway. Once in the beam, the loads have to get back out again. This causes stress concentrations and failures where they wouldn't have been had the two materials been better matched. This is why brackets are used, the ends of beams should be tapered, and holes such as windows should have well rounded corners.

 

[oldsaltoz]

The simple answer is add a stainless steel * mm or Aluminium 10 to 12 mm to support the 'U' Bolt.

The ensure the load is properly spread cover thee underside of the deck with cling wrap and load the top side of the plate with a mixture of Micro-fibres and resin, then offer it up and tension it enough to ensure you have excess coming out all round. Leave it to fully cure before re tensioning.

If the area is a bit narrow, just add a gusset to the lower side to stiffen it.

No glassing needed, and you can remove it in the future without damaging the hull/deck.

Good luck and fair winds.

 

[michael_w]

Look for some stuff called Garolite. It's a glass reinforced epoxy board. Superior to ply, and you can epoxy it to your boat's exisiting structure, unlike Starboard.

 

[matg]

Many thanks for all the replies.

Based on all the advice it's probably best to stick with marine ply. It has lasted nearly 30 years, so if I ensure it's epoxied and sealed well it should do fine.

cheers.

 

[alahol2]

Hello,

I'm looking to reinforce the underneath of my side decks where the shroud u-bolts are fixed. It currently has ply glassed-in and water ingress has softened it.

While you are at it I would replace, or very closely inspect, the U-bolts. I have had two U-bolts let go because of crevice corrosion inside the deck. One was a main shroud, luckily only one side snapped and I caught it before it could totally let go. I have since removed, inspected and re-bedded just about every through deck fitting on the boat.