Deck fixing question

[AnthonyExmouth]

Just getting ready to screw the ply deck to the beams in the hope it'll get cooler soon and I can glass the deck in. Anyway, in the past I've always applied a good amount of polysulphide bedding between the deck and beams, is that still the preferred method or are the newer alternatives. I really don't want any squeaking.

 

[jehardiman]

What is the construction? Is the ply a sub-deck or the only decking? Is the glass going to cover the deck-to-hull join?
Polysulphide works well for classic construction where the structure moves. Other methods, like cold molding or lightweight monocoque where the deck is a significant part of the structure, could use other adhesives and methods.

 

[AnthonyExmouth]

What is the construction? Is the ply a sub-deck or the only decking? Is the glass going to cover the deck-to-hull join?
Polysulphide works well for classic construction where the structure moves. Other methods, like cold molding or lightweight monocoque where the deck is a significant part of the structure, could use other adhesives and methods.

18mm ply over douglas fir beams and glassed over for a sealed, self draining deck. it's a 25' grp hull workboat.

 

[jehardiman]

18mm ply over douglas fir beams and glassed over for a sealed, self draining deck. it's a 25' grp hull workboat.

Going to glass the deck to the hull? How are the beams attached? If one part of the beams and deck can move, then it all has to move. Likewise if everything is rigid.

 

[AnthonyExmouth]

Going to glass the deck to the hull? How are the beams attached? If one part of the beams and deck can move, then it all has to move. Likewise if everything is rigid.

Beams will be bolted to a beam shelf and deck glassed to the hull with scuppers for a self draining deck.

 

[fallguy]

I would apply the ply deck with a structural adhesive that does not flex and will stand up to water.

I’m not familiar with polyester adhesives. I would use epoxy putty made with about 2 parts cabosil to epoxy by volume adjusted to adequate stiffness to hold up a vee. Because resin suck is a real problem, I would precoat all bonding surfaces with resin prior to applying putty.

After overnite, I would descew and fill the holes with putty. I’d probably countersink the holes a bit first to clean up the wood. Then putty them all; avoid walking on the deck for at least 24 hours.

I apologize, I am not familiar with all potential bonding agents. Epoxy is a waterproof, permanent bond that cures fairly quickly for continued work, but there are probably a few cheaper options.

 

[AnthonyExmouth]

I would apply the ply deck with a structural adhesive that does not flex and will stand up to water.

I’m not familiar with polyester adhesives. I would use epoxy putty made with about 2 parts cabosil to epoxy by volume adjusted to adequate stiffness to hold up a vee. Because resin suck is a real problem, I would precoat all bonding surfaces with resin prior to applying putty.

After overnite, I would descew and fill the holes with putty. I’d probably countersink the holes a bit first to clean up the wood. Then putty them all; avoid walking on the deck for at least 24 hours.

I apologize, I am not familiar with all potential bonding agents. Epoxy is a waterproof, permanent bond that cures fairly quickly for continued work, but there are probably a few cheaper options.

Are you saying to remove the screws completely? I was going to put a single layer of csm on the underside of the ply.

 

[fallguy]

Are you saying to remove the screws completely? I was going to put a single layer of csm on the underside of the ply.

if you use a structural adhesive and the bonding area is sufficient, then yes; the screws are not doing much. They basically pin the work in the proper locations. But bedding in epoxy is a challenge because of the resin suck issue and you cannot slide the decking into place; it needs to be carefully laid over the putty. I recommend application of the epoxy putty with 1/8” or 3mm vee trowel to the stringers and cleats. Hopefully the sides are not floating connections and are resting on cleats as well.

 

[AnthonyExmouth]

if you use a structural adhesive and the bonding area is sufficient, then yes; the screws are not doing much. They basically pin the work in the proper locations. But bedding in epoxy is a challenge because of the resin suck issue and you cannot slide the decking into place; it needs to be carefully laid over the putty. I recommend application of the epoxy putty with 1/8” or 3mm vee trowel to the stringers and cleats. Hopefully the sides are not floating connections and are resting on cleats as well.

Plenty of surface area. No stringers, 160mmx80mm beams at 400 centres and beam of boat is 2500mm at widest so will be single sheet across the boat.

 

[fallguy]

Plenty of surface area. No stringers, 160mmx80mm beams at 400 centres and beam of boat is 2500mm at widest so will be single sheet across the boat.
View attachment 209729

If you precoat the wood and bed in epoxy putty; it becomes a uniform structure.

A lot of the guys here think epoxy is excessive; the only concern for me is the csm layer under the deck. It must be well bonded for the deck to bond to the beams. How is the deck attached at the sides then?

See if someone offers a polyester answer as well.

 

[AnthonyExmouth]

If you precoat the wood and bed in epoxy putty; it becomes a uniform structure.

A lot of the guys here think epoxy is excessive; the only concern for me is the csm layer under the deck. It must be well bonded for the deck to bond to the beams. How is the deck attached at the sides then?

See if someone offers a polyester answer as well.

Sides are just glassed in. 1x600 csm 2x 1708 and then a 1x 600 csm on top followed by flowcoat.

 

[fallguy]

Sides are just glassed in. 1x600 csm 2x 1708 and then a 1x 600 csm on top followed by flowcoat.

If using gelcoats, then you need to find a VE bonding putty for this work I’d say…

Maybe @jehardiman knows the best products.

 

[jehardiman]

If using gelcoats, then you need to find a VE bonding putty for this work I’d say…

Maybe @jehardiman knows the best products.

I'm not really a chemical bonding guy, that was handled by somebody else in the plastic shop. I'm more of the structural guy. But fallguy is right, you need to chemically (or physically) bond the deck to the hull if you are going to glass the whole deck. Anything else is just making a weak joint.

 

[AnthonyExmouth]

I'm not really a chemical bonding guy, that was handled by somebody else in the plastic shop. I'm more of the structural guy. But fallguy is right, you need to chemically (or physically) bond the deck to the hull if you are going to glass the whole deck. Anything else is just making a weak joint.

I was planning on stainless screws every 150mm and a bedding compound. I've never considered bonding the sheets as I'm always concerned about later work, maybe having to change the fuel tanks maybe hull repairs. Maybe it's time to be less cautious and just bond it with epoxy putty. What's the benefit from taking the screws out? Aren't they best left in?

 

[fallguy]

If you do a permanent bond; the screws don’t help and only provide an avenue for water ingress.

You cannot really have a deck in bedding compound and make it waterproof.

hull to deck joints need to be made waterproof as well

Consider a deck with a flexible or removable bond. It moves and if glassed to the sides of the hull; those glass seams flex.

You have to decide wet bilge or dry bilge. That simple.