Carbon fibre DIY basics

[Michael Rogers]

Can anyone point me to a source of practical information about using carbon fibre? I'm talking idiot-proof basics here, and using it rather than making it. Cutting it: joining it (to itself, and to other materials) - so bolting, screwing, gluing (which glues?), any other techniques: things to avoid doing and absolute no-nos: compatibility with other materials such as wood, aluminium, s/s: does it corrode, if so why, when/where: painting and other finishes.

I googled without much success (I don't want to build a bike or a racing car). Any ideas or pointers please? Thanks!

[Julian Swindell]

What are you planning to do? I don't know that significant carbon fibre work can really be done on a DIY basis because a lot of it depends on autoclaving and such like to gain the benefits. But I think you can screw and glue things to it quite sucessfully using epoxy glues and maybe timber backing pieces. I don't think it corrodes at all but it will conduct electricity, which might lead to corrosion of unlike metal fixings on the same piece of carbon. I have screwed a couple of cleats and cheek blocks to a carbon fibre mast just using self tapping screws into pilot holes, and it has worked fine so far. Drilling the pilot holes was surprisingly difficult, it is much harder than GRP.

[Jeremy]

I have made a fair few parts from CF over the years, it is fairly easy to work with using DIY techniques, but you need to be reasonably comfortable with basic composite work.

There are a few basics, like only use epoxy or vinylester resins, never polyester and never put CF directly in contact with any metal that is less noble than carbon in the electrochemical series, for example, CF will corrode aluminium alloys pretty rapidly if you bolt it up to a bit directly.

It is also worth knowing a bit about the quirks of CF as a material.  It is markedly less strong in compression and shear than it is in tension.  It behaves differently under bearing stress (for example, bolted joints in shear) as it does not deform before failure, so, unlike metals, a bolted joint will fail with no warning when the bearing stress of the material is exceeded.

Bolted joints in CF sheet or tube will generally be dominated by bearing stress, rather than fastener strength.  This means that larger diameter fasteners may be needed, just to keep the bearing stress within limits.

The most effective way to join CF is by bonding, preferably with some additional reinforcement to transfer the loads into the material evenly.  It is perfectly possible to laminate and bond CF using ordinary epoxy resin and CF cloth or tape.  The only thing that makes it slightly harder to work with is the difficulty in seeing when the CF is fully wetted out (unlike glass cloth, CF doesn't dramatically change colour when wetted out).

If you want to make a bolted joint, then it helps prevent galvanic corrosion problems if you finish the area where the bolt will go with glass cloth and use sealant around the bolt.  The glass acts as an insulator between the CF and the bolt and the sealant prevents water getting into the joint to act as an electrolyte.

Finally, it is worth examining the structure you are building to make sure that CF is really the best material.  It is good stuff, with a high strength to weight ratio and fairly highly stiffness, but it is not really a wonder material and is not as well suited to some tasks as a less anisotropic material, like metal.  It is also ridiculously expensive!

Jeremy

[Michael Rogers]

Thanks, Guys, that's all helpful. I should have given the context, which is the possibility (no more than that at present) of using a CF mast in my boat, in which case the practical issues would be things like mast step, partners and securing mast fittings. I had already been warned that CF and aluminium don't get on (Jeremy almost has me scurrying back to the periodic table!). For some reason I had the obviously wrong impression that epoxy and CF didn't go well together, so I'm sorted on that one.

[Jeremy]

Sounds interesting.  The mast step should be fairly straightforward, as long as you just want to fit the mast into some form of socket.  If you want to be able to pivot the mast around a big bolt, then it might be an idea to bond in a sleeve where the bolt fits through, to increase the bearing area.

The other fittings only need some local reinforcement, probably best done with some glass cloth and epoxy for safety and to address the potential corrosion issue.  One rather nasty characteristic of CF is that it fails suddenly and breaks into vicious, spiky shards.  This problem is so serious that for aircraft crew compartments we have to use a glass or Kevlar inner skin to protect the crew from injury from the CF in a crash.  Although mast failure is a remote possibility, the consequences of being speared with CF shards perhaps needs to be considered.  I believe that windsurfer masts sometimes (maybe always?) have a glass or Kevlar layer to minimise this risk.

In my view, a CF mast is great for an unstayed rig or for spars, as unlike alloy, it is well suited to taking the bending loads, has the advantage of being able to designed to bend predictably and makes for a nice, light set up.  Its less well suited to a stayed rig, in my opinion, as its relatively poor compressive strength, coupled with the fact that the fixed rigging will add a fair bit of weight (and take most of the bending loads) means that the weight and stiffness advantages that CF gives are reduced somewhat.

Having made a wood mast and spars and owned a boat with an alloy mast many years ago, I would be very tempted to build an unstayed CF rig if it were me.  I'm pretty sure that a CF rig could be made at home (from pre-fabricated CF tube) with less time and effort than a wood one.

Jeremy

[Michael Rogers]

Jeremy, my interest is precisely what you describe - an unstayed rig (junk rig actually) and looking for lightness benefits. My new mast is aluminium. It is jointed about 15 ins above the deck (partners), with the joint angled so that it sits down and then doesn't rotate: there is a longish internal sleeve in the upper part which slides down into the lower part (the 'stump').

The upper part is 12 ft long plus the sleeve, and weighs about 20 lb. That's a bit, but not a lot, heavier than the previous mast + topmast (gunter gaff). However the weight issue is that, to slot the upper mast into the stump, I have to balance it vertically, lift it to nearly shoulder height, and extend my arms almost straight to drop the sleeve into the lower part - maintaining the 12 ft spar in vertical balance the whole time. It's not as easy as it sounds, and although I'm getting better at it, it feels like an accident waiting to happen, to the potential detriment of me, my boat, neighbouring craft and passers by! Also, I have to face the fact that I'm 70+ and not likely to get stronger as the years go by. Hence my interest in a carbon fibre mast, which would make the whole business relatively easy (am I right that we are talking here about a weight of less than 7-8 lb?).

I have made a removable internal wooden hinge between the two parts of the mast. The hinge itself works beautifully but, for (mechanical and ergonomic) reasons it would take too long to describe, the physical effort required to get the mast up is even greater than my vertical caber balancing act, which seems to be inescapable. If I was 8 ft tall a different approach would be a doddle.

Incidentally, the 'stump' is essential for trailer/sailing with junk rig, because nearly all the running lines (parrels) remain rigged round the stump when the sail is furled, ready for setting the sail next time. It works a treat. So I need a similar joint arrangement in any replacement mast.

The biggest practical obstacles to a DIY approach using carbon fibre are 1) getting CF tube of the right size, knowing that it's strong enough (my sail area is 68 sq ft - not exactly huge), and being able to afford it; 2) finding a second tube of exactly the right size to make a sleeve (I wouldn't have access to any machining facilities). I've approached Carbon Fibre Tubes Ltd (Hayling Island), but they don't deal with Joe Public (they do have two standard tube sizes which I think would fit the bill). Does anyone have any ideas on this? I suspect that Swallowboats may be customers of CFT, and I hope to pick Matt's brains on this during the rally at Teifi on June 19 - 20. I could try a 2nd hand windsurfing mast, but the sleeving problem remains.

Incidentally, aside from this mast-stepping issue, the switch to junk rig has been a complete success, and I'm chuffed to bits with it.

[Julian Swindell]

Hi Michael
What hull are you putting junk rig on? Is it a Swallowboat? My mast is a two part carbon fibre tube made, I am pretty sure, on Hayling Island. It slots together extremely well, but you have to line it up exactly. I wouldn't want to be doing it it over my head. I have only done it with the mast lowered and horizontal. Is there no way you can do without the stump? It is quite esay to pick up my whole mast, and I'm no strongman. It is about 7m long, say 23ft. If you could just pick up your mast and drop it in a hole, it would be very easy.

[Michael Rogers]

The new junk rig is on my Storm Petrel "Cadenza', Julian. The mast steps through the kingplank which is part of the foredeck. I need some way of minimising potential water ingress through the 'partners' hole because, with a water tight hatch cover in place, the whole space forward of the forward bulkhead is (or should be) buoyancy. The present arrangement, around the permanently stepped stump mast, is the best so far.

However, that's a relatively minor benefit of the stump. It's a bit difficult to explain why it makes such a difference to have the parrels (running rigging) remaining essentially in place when the sail is lowered. I can easily demonstrate what I mean to anyone who is coming to the Teifi Rally. There is a 'lot of string', but it's very manageable, and control of the sail can be very exact (although the whole rig is also very forgiving).

Unlike the previous sprit-boom rig, where the mast could (and did, to some extent) rotate, a junk mast needs to be fixed, so the step needs to make this possible, without too much grovelling through the hatch hole in the bulkhead. Any joint in the mast above deck must also be 'directional' so that masthead fittings and associated running rigging are properly aligned.

I don't see why an angled joint shouldn't be feasible in a carbon fibre mast, but am seeking advice from people like Jeremy who know what it's all about! Also, it occurs to me that the sleeve through the joint area could be solid wood without a big weight penalty, rather than tubing?

Jeremy mentioned CF's reputation for sudden collapse. Did anyone else see footage of one race in the final series of the America's Cup some years back? - both boats elegantly close hauled in no more than a force 3 - 4. Almost more quickly than the camera could catch, the New Zealand boat was reduced to a heaving heap of crumpled sails and tangled rigging as the CF mast suddenly shattered in two places with absolutely no warning. A vivid demonstration!

[Julian Swindell]

Hi Michael
I'm intrigued. How high does your stump have to be to hold your parel lines when the sail is stowed? Could you have a lower fixed CF tube which was bonded into the deck and to the keel, which formed the stump and allowed a single piece CF mast to be simply slid down inside it? There would be a slight step in at the top of the stump, but if that could be below the height of the boom when set it should not be a problem. I can't imagine a Storm petrel mast would be very heavy to lift into place.

[Jeremy]

CF Tubes on Hayling are a bit of an odd bunch to try and do business with, in my view.  I tried to buy a considerable quantity of small diameter CF tube from them some years ago and formed the view that they really weren't that interested in doing any business with me at all.

More recently some acquaintances have had excellent service from Hardys (see here: http://hardyadvancedcomposites.org/default.aspx) not only were they happy to sell small quantities directly but they also offered some good advice on using their products.  They started off as a fishing rod manufacturer (traditional fly fishing rods) developed a technique for making composite rods and then solved a major defence problem for us with a small missile casing and branched out into specialist forms of composite tube!

I can understand why you want to keep the "stump", it must be very handy to keep all those bits of string and beads wrapped around something, rather than have to thread the whole lot on every time you rig.  

You can certainly bond wood plugs inside CF tube, but often you will be able to get a section of smaller diameter CF tube that you can just bond inside as both local stiffening and a joiner.  I might be inclined to fit plugs in the tube at either end anyway, just to add a bit of mast buoyancy in the event of a capsize.

Your weight estimate sounds reasonable for the CF section, and should certainly ease the task of lifting it into place.

Jeremy

[Michael Rogers]

Thanks again, folks, for your interest and advice, much appreciated. Jeremy, I have contacted Hardy's and await a reply. I hope also to have a word with Matt this w/e. Incidentally, junk rig parrels don't involve any beads! I don't know whether the word parrel has been 'developed' (eg by Hasler et al) in a junk rig context. It seems to mean a running line (as opposed to lacing or whatever) which passes round the mast, once or several times, for sail control purposes.

[Carbon Fiber Technology]

There are many forums that can help explain about the carbon fiber DIY basics, you can type in Google carbon fiber tubes:forums. Hope this helps!