Modified polycarbonate

For a long time now, I've had ambitions of building a Drascombe Lugger clone - what's been putting me off until now is the cost of marine-grade plywood and/or the cost of laminating a hull in glass and epoxy.

But earlier today I may have stumbled across a possible solution. I've been given - for free - 50-plus sheets of a modified polycarbonate called 'Compax', and I'm fairly confident that there's another 50 sheets available for the asking. As it's not a well-known product, I'm attaching a couple of graphics which contain just about all the data I've managed to source about this unusual material.

The sheets I have are 7ft x 3ft x 1.5mm and my provisional thinking is to laminate 3 or possibly 4 sheets together with a semi-flexible epoxy resin, to make up sufficient stiffness.

But - before embarking upon the use of such a novel material, I kinda thought it might be wise to canvas opinions about suitability and any possible pitfalls.

Thanks in advance for any opinions.

 

Epoxy won't work. You need to use adhesive that is formulated for the polycarbonate. Also, it will make a rather heavy boat. The density to stiffness is not good compared to wood or fiberglass. Considering the total cost of the boat, plywood may be cheaper than the adhesive you will need.

 

Polycarbonate is a ridiculously heavy material to build a hull shell out of. Compare it's density to most of the common materials employed and you'll see it's just not a good idea.

 

Specific Gravity 1.2 Would that be the problem, though ? GRP and alloy are somewhat denser, so I imagine it has other problems that render it unsuitable, or it would be in use for the purpose.

 

The density of Compax is 1230 kg/m3
The density of BS 1088 Marine Ply is >500 kg/m3.
i.e. Compax = 2.5 times as dense

BUT - wooden D/Lugger hulls normally have a 9mm plywood skin. The Russian clone designer suggests 4 to 5mm for grp or similar synthetic laminates. A 3 by 1.5mm laminate would be within that spec at exactly 50% of the equivalent plywood thickness.

The resulting weight would therefore be 1.25 times that of the equivalent marine plywood.

I won't bore you with the rest of the figures (unless you're really interested), but the end result would be an all-up dry hull weight of some 16% more than the equivalent plywood boat. That doesn't seem 'ridiculous' to me, especially as the D/Lugger is a heavy sea-boat design to begin with.


I forget to add:

in terms of stiffness - which I agree is a concern - the Russian clone, unlike the original plywood Drascombe Lugger, is of the same 'box-like' monocoque construction which was later adopted by those Luggers made from grp.
For example, there are full-length buoyancy chambers to either side of the open cockpit area. Quite clearly, such structures are essential in conjunction with the relatively thin skin to provide the required hull stiffness. However, the open cockpit itself does remain a 'weak spot' in terms of hull stiffness, and I'll need to give more thought to how this could be improved.

I spoke with the manufacturer's technical department this morning, and although lamination is not something they usually indulge in, they have confirmed that a 2-part adhesive, either polyurethane or epoxy, would be satisfactory. Indeed, as polycarbonate and epoxy hardeners are both made from bisphenol-A, it is anticipated that the epoxy adhesive I intend to use may well have a modest solvent effect upon the polycarbonate during the curing process, resulting in an extremely strong bond. Of course, test pieces will be required to confirm this.

 

You might be able to make a skin from plastic but what about all the secondary bonded structures. How will you fasten, glue , attach these components ?

 

Polycarbonate isn't a cheap material, but if it was up to the job it should have found an application in boat building other than windows and hatches. There has to be a snag(s) here somewhere.

 

Much will depend on the results of the test pieces I'll be making this afternoon. If the epoxy bonds as well as is suggested - then I'll use the Compax laminate exactly the same as if it were plywood - i.e. glass tape + epoxy joins, stainless-steel bolt-throughs with backing plates, and so on ...

If it doesn't - then I'll have to think around the problem somehow. But - I'll cross that bridge if it should materialise ...

 

Ah - but this isn't polycarbonate. Polycarbonate, as you know, is transparent ... and bullet-proof, and so on.

Palram have modified this stuff (see the sales blurb in post #1) to generate specific characteristics - hence all the dogma concerning bog-standard polycarbonate now goes out of the window.

This stuff is a unique product -which is both good and bad: it's good because it has very specific physical characteristics (well, 'good' if that's what you happen to be looking for ), but 'bad' when you come to the business of re-cycling. The sole reason I've been given these sheets gratis, is because the re-cycling centres won't touch it - 'cause it's not polycarbonate, nor is it PVC, nor polypropylene, nor ethylene terephthalate, and so on ...

It's a one-off material - and thus a pain in the ar$e for the firm who now has a whole stack of it in their yard, and who's management is committed to a rigid recycling policy. Several hundred sheets arrived from China as 'separator packaging' - seems that they don't want it either !!

So - it's either destined to be land-fill, a bl##dy great cloud of smoke, or a very substantially-built sea-boat - and one built for very little money.

 

Your boat is lapstrake ?

 

Yes.

So there'll be the equivalent of 9mm x 20mm stringers at the lapstrake join. Should help with stiffness.

 

The gains to bury the lap might by difficult

In general building with hard plastic doesnt sound like fun

How stable is plastic when wet ?

 

Hi Michael

well, I normally build structures in steel so I'm really enjoying the prospect of working for a change with something which doesn't rust.
If the sheets were thicker and didn't need to be laminated, then I'd weld it - but life is never perfect ...

The spec sheet talks about 0.16% water absorption after 24 hrs, so I don't think that'll be a problem. Hope not. It can always be painted of course - which is something I'm certainly considering for the decking, in order to obtain a lighter colour (sheets are dark brown) - maybe I'll paint the whole hull, who knows ?

Bit of an update - I've cut up a number of test strips using some strong scissors. Never again. My hands are now hurting badly. 1.5mm is certainly the limit for a manual cut - even then it's only half an inch at a time. 2.0mm would be out of the question. This stuff is *strong*.

Ok - I've now got some 2 and 3 laminate test strips epoxied-up and in front of a fan heater, as it's very cold today. Will post results of destructive testing in a day or two

 

One of the problems with polycarbonate is that, while it is "strong", it isn't very notch tolerant. That is, if you have a stress concentration (like where you put a screw through it) it can split out of that notch and just open right up. If it were covered with glass or some other cloth that would be better, but just using the material by itself is not a good idea.

There's a reason that materials like this aren't used in boats even though "on paper" they look acceptable.

 

Bit of a worry alright, stress cracking does seem to feature as a limitation of polycarbonate, at least the "unmodified" variety. I don't think it is worth the risk of finding out the hard way it wasn't suitable. The stuff has been round a long time now, yet it doesn't feature as a hull material, as you say there are reasons for that.