light and strong seat base

Nick_H

Nick_H

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www.ybw-boatsforsale.com
I want to make a couple of bench seats, about a metre x 0.5 metre each, only supported at the ends, and capable of taking the weight of a couple of people sitting on them. They will be in and out of position regularly, so I want them to be really light and easy to handle.

I think in marine ply I would need at least 3/4", probably 1", which would be fairly heavy once foam and vinyl was added, so i'm looking for something lighter. I could use thinner ply with a stiffening batten or two, but then they're less easy to store

How strong is GRP foam sandwich with lateral loads, and how thick would it need to be. Would it work out much lighter once enough GRP was added each side to give it the required strength. The application doesn't warrant anything too exotic such as kevlar/carbon fibre.
 
Even 24mm ply will bend over a 1m unsupported length if you put a couple of 100kg humans on it. You're going to need to a brace it underneath. As you suggest though, 24mm ply is v heavy. I'd use starboard or kingboard (or whatever it is called, can't remember right now sorry) and then brace it back and front with aluminimum angle, and glass it in.
 
This stuff:

http://www.totalplastics.com/products/255

Like this:

Capture_zpsad51af55.jpg


and then glassed in.
 
gjgm said:
havent checked the sizes, but if you are going to cover/foam it anyway, can you dismantle something like this for the seat base? I didnt understand if this was to be a permanent, fixed seat, or a drop in/removable one?
http://www.ebay.co.uk/bhp/step-up-work-platform
Click to expand...

Nice lateral thinking, though I think i'll go with Jimmy's idea. It will be removeable, stored in the cabin and used when we anchor up. I want it light and with no sharp corners so it can be virtually thrown in the cabin, rather than having to go down and place it in carefully.
 
Tricky.
19mm ply will be heavy-ish and bend a bit so will need battens. 25mm ply is very heavy. The plastics boards can be stiff if you get the solid type but that is very heavy. The lighter version with the plastic sort that has been expanded a bit (a bit foam like) has remarkably little bending stiffness. I have both types on my boat

5-20-5 ply-closed cell foam-ply sandwich would be nice but it is a crazy price. You could just fabricate the thing in 5-20-5 sandwich but where the filling of the sandwich is 4 off 10x20x1000 hardwood battens evenly spaced, with the ends closed up by battens too. And to state the obvious, glue the battens to the ply over their full area with Titebond or similar, and assembly it flat to make sure total contact between ply and battens (brass pins maybe)

But getting back to RTFQ, GRP-airex-GRP is very stiff and could be a nice job, if a bit messier than the above to make. The foam is quite rigid (has compression stiffness) and of course the glued bond is perfect if you do it right. You can make a mould out of a sheet of cheapy Contiboard, with battens to form the edges. Of course you'll have only one of the faces (plus all 4 edges) gelcoated but that should be fine it you are upholstering over it. When you come to upholster it, you'll need a pneumatic stapler if you want to thwack s/s or monel staples through the GRP. Alternatively, if you want the cushion to be loose, you'll have your work cut out creating a GRP/foam board with two gelcoated sides while keeping it lightweight
 
Nick_H said:
Yes, I like the hardwood frame with ply bonded both sides idea as well, then maybe glassed all over. thanks.
Click to expand...
Glassing it adds nothing but weight surely? The critical glue bond so far as stiffness is concerned is on the inside, twixt battens and plywood, and glassing it adds nothing on that score other than to the outer battens. Better to paint it?
 
I was thinking of a single layer of glass mat, more for waterproofing in case the the foam absorbs some sea/rainwater. It's for use in an open cockpit.
 
Yup, that's what I was wondering - how will you bond the edges of the honeycomb to the plywood? That bond is absolutely critical to getting stiffness in the finished item. You'd frankly be much better using 20x10 wood strips because making a bond using wood glue (ege titebond) is easy and reliable.
 
Use a good epoxy (and plenty of it), paint the flat surface lay the honeycomb on top, add a weight and let it cure.
Turn over and repeat the process

TBH I have no point of reference doing this with plywood
 
although I do like the honeycomb idea, works wonders if done properly, I'm with JFM on that.
no matter how much epoxy or whatever glue you like apply it's got to hold to a under a mill "endgrain" of the honeycomb "frame"...
how about ordering and getting the right size of honeycomb object from one of them companies and you should be able to specify what type of skin you want.
THEN on the properly assembled thing, you can lay a wood veneer and smoothed batten for the corners and be done with it.

cheers

V.
 
Nick whatever option you decide to go for it would be great if you took a few construction photos along the way and posted. I am planning to do something very similar for this season.
 
Trust me, glue along the edge of the honeycomb works fine :) Use enough and it creeps up the side wall a bit too ;)

Aircraft and high performance cars have been using it for years, albeit with Aluminium or CFRP skins

Personally I'd use an aluminium skin http://www.metals4u.co.uk/aluminium...showvat=true&gclid=CNjE7Of2hbwCFQsCwwodJ34AIw
and egde it with anodised channel to make it look nice http://www.aluminiumwarehouse.co.uk/Aluminium-Silver_Anodized_Channel/c120_133/index.html

But each to their own :)

PS all the links above are examples. I'm in no way recommending these sites or saying that these are the best prices
 
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