Replacing Catamaran Rear Trampoline - GRP Advice Needed!

[ausnp84]

Hey guys,


I've recently bought a 30ft cat and want to replace the aft trampoline with a solid walkway / deck. I'd normally fit wood strips but as it's a smaller cat I want to keep it pretty lightweight, so was thinking 3-4 foam / glass / epoxy panels would do the job.


It's not a great pic of it, but you can see the existing trampoline drooping from the back of the boat. The proposed panels would fit on top of that red bar, and then I'd attach them to the transom as well.


My question is, how best to make the panels? I know surfboards are very strong and lightweight, and they're often foam with a couple of layers of glass and epoxy. I've also got 11kg of leftover West System epoxy and 300g biax cloth leftover from the bow repair I did (over ordered by about 10x what I actually needed), not to mention all the peel ply and colloidal silica, so might as well put it to good use!


Any help greatly appreciated.


n

[toolbar]

With dinghy, davids and solar there, what do you gain by a solid walkway? I would keep the netting.

Paul

[ausnp84]

Originally Posted by toolbar

With dinghy, davids and solar there, what do you gain by a solid walkway? I would keep the netting.

Paul

I'd like to put the dinghy vertically on the walkway when underway, and it'd also make a good place to put a BBQ & gas bottle, fishing rods, rest the SUP against the transom when at anchor, etc etc.


From a financial perspective, replacing the tramp is going to run about £1k whereas I have almost all the materials to build the hard walkway.


n

[Chotu]

My advice:. Don't make 3-4 panels. Make one panel, off the boat and fit that into place, taping it in. Doing the bottom sides of the tape joining the panel to the hulls/bridgedeck will be the worst part.

[ausnp84]

Originally Posted by Chotu

My advice:. Don't make 3-4 panels. Make one panel, off the boat and fit that into place, taping it in. Doing the bottom sides of the tape joining the panel to the hulls/bridgedeck will be the worst part.

Ta. My only thinking with 3-4 panels is that with some gaps between them, it’ll allow any water / waves to disperse quickly rather than swamping the stern.

Attaching shouldn’t be too hard while it’s out of the water. Lots of epoxy and biax!

N

[Sparx]

I see the logic of building 3 or 4 panels with a good gap in between. Looks easy to build and I ike having ample drainage. Here is how I would do the glass work...

I am thinking build a cross beam that runs the width of the transom that will support the forward edge of the panels. Decide if this will be removable or become glued into place permanently on the transom.

Then build panels that will bridge the space between beam and your red bar.

Both the new beam and the planks can be built of foam with epoxy and bias using similar methods.

Layout a single foam panel that is 1 cm longer and 1 cm wider than the combined width of all the panels.

On a flat work surface wet out a layer of biax on top if the foam. After it this has started to cure add a second layer of biax, well wet out and pressed into the first curing layer. Cover tightly with peel ply and let cure to hard.

Flip the panel over and do the same for the back side.

When fully cured run the panel through the table saw to the finished dimensions of the individual panels. Coat the exposed edges with well thicked epoxy.

Remove peel ply. Paint with primer and a good two part paint. Hint: if you go easy during sanding the peel ply might leave a reasonable non skid surface on the panels.

[GRIT]

A single panel with drainage holes, or slots, as you wish, will be easier to build than multiple panels.

I don't think I'd bother with the slots, as the percentage of open area with slots, or holes, or gaps between panels, is so small that the downward (or upward) pressure of a wave will only be diminished by a few kg's. I'd build of one piece, and angle it downward slightly for drainage, so you don't get pooling water.

And, yes, build it like a surfboard. Foam core, glass on both sides. 2 to 3 layers of 300gsm on either side should be plenty strong for such a narrow piece. I'd use 3/4 foam if you have access to it. A good quality foam (corecell) and 1 layer of 300 gsm would do the job, but would dent easily. The extra glass is more for durability than stiffness. A little uni going fore and aft wouldn't hurt either.

Having said all that. You'd be safer with the net.

Cheers.
Paul.

[ausnp84]

Originally Posted by Sparx

I am thinking build a cross beam that runs the width of the transom that will support the forward edge of the panels. Decide if this will be removable or become glued into place permanently on the transom.

Both the new beam and the planks can be built of foam with epoxy and bias using similar methods.

Layout a single foam panel that is 1 cm longer and 1 cm wider than the combined width of all the panels.

I like your thinking - Ross from Life on the Hulls has been using composite angle a lot and I think it'd work for the front beam.


What foam would you use? Polystyrene?


n

[ausnp84]

Originally Posted by GRIT

A single panel with drainage holes, or slots, as you wish, will be easier to build than multiple panels.

I don't think I'd bother with the slots, as the percentage of open area with slots, or holes, or gaps between panels, is so small that the downward pressure of a wave will only be diminished by a few kg's. I'd build of one piece, and angle it downward slightly for drainage, so you don't get pooling water.

And, yes, build it like a surfboard. Foam core, glass on both sides. 2 to 3 layers of 300gsm on either side should be plenty strong for such a narrow piece.

Cheers.
Paul.

Thanks Paul - angling the panel(s) downwards makes a lot of sense as well.


What foam would you use?



n

[Chotu]

Originally Posted by ausnp84

Ta. My only thinking with 3-4 panels is that with some gaps between them, it’ll allow any water / waves to disperse quickly rather than swamping the stern.

Attaching shouldn’t be too hard while it’s out of the water. Lots of epoxy and biax!

N

Which you'll be showering in when you do the install. Lol

You'll see what I meant about installing/taping in the underside being the hardest part of the job.

My point was also that the rest will be easy.

[Sparx]

Originally Posted by ausnp84

I like your thinking - Ross from Life on the Hulls has been using composite angle a lot and I think it'd work for the front beam.


What foam would you use? Polystyrene?


n

A composite angle is great. Something like G20. Or build your own using a simple form and a bunch of layers of glass.

For foam I like marine boat building foams. 1 cm foam will work is you keep the panels short. Thicker foam adds stiffness very quickly with little additional weight, so 1.5 cm foam is better.

https://www.jamestowndistributors.com/mobileportal/show_product.do?pid=1588

https://www.generalplastics.com/products/tr-marine

[ausnp84]

Originally Posted by Sparx

A composite angle is great. Something like G20. Or build your own using a simple form and a bunch of layers of glass.

For foam I like marine boat building foams. 1 cm foam will work is you keep the panels short. Thicker foam adds stiffness very quickly with little additional weight, so 1.5 cm foam is better.

https://www.jamestowndistributors.co...ct.do?pid=1588

https://www.generalplastics.com/products/tr-marine

Thanks mate! Will have a look for a supplier on this side of the pond. 1.5m foam with two layers of 300g biax each side will take a person / 2 people's weight? As a guess the total measurements are about 70cm x 400cm.


n


EDIT:
What's the view on Airex / Armacore foam? I can get that pretty easy over here:


https://www.ecfibreglasssupplies.co....re-100kgm-15mm


https://www.ecfibreglasssupplies.co....ural-foam-10mm

[Sparx]

Originally Posted by ausnp84

Thanks mate! Will have a look for a supplier on this side of the pond. 1.5m foam with two layers of 300g biax each side will take a person / 2 people's weight? As a guess the total measurements are about 70cm x 400cm.


n


EDIT:
What's the view on Airex / Armacore foam? I can get that pretty easy over here:


https://www.ecfibreglasssupplies.co....re-100kgm-15mm


https://www.ecfibreglasssupplies.co....ural-foam-10mm

I

I think either of the listing you provided will be fine. I think the 10 mm foam is enough, but 15 mm will be even stiffer. For the small increment in cost for your small project I might go for the 15 mm foam.

I am always amazed at how stiff these panels get with 2 layers of biaxial cloth on each side. The foam actually provides the minority of the stiffness, the majority of the strength comes from the biax/epoxy. The foam provides a separation space to keep those biax layers further apart, making a stronger beam. This is why thicker foam makes a stiffer beam.

Test the panels before painting. If you don't think they are stiff enough there are a couple of easy things to do that will add more stiffness. I expect you will be delighted.

[Cheechako]

Long strips would work. Like a home deck but more space between to save weight. (I think I see a center support right?)
You could even use Cedar decking, it's rot resistant, cheap, not slippery and very lightweight. A lot less work than making up a foam glass piece that will be heavier.. and wherever you screw it down allows water inside the foam.
Another option is thin wall Aluminum rectangular tubing or even channel. Like 1/16" wall would work. Again strips like a home deck.

[Slainte1]

If you are in the UK I have a couple of sheets of Airex left over from my trimaran build.