|
|
Close Help | ||||||||||||||
You have arrived at an old part of our website, to find the latest information please visit our new page about Building Carbon Masts. The new portal for the site can be found at www.uk-cherub.org please visit this for up to date news about the UK-Cherub class.
I never managed to get anyone to write this for me, so, as
I was acting as an amateur journalist, I thought a journalists
trick would do. I sat Simon Roberts and Dave Roe down with a pint
each, turned a tape recorder on, and asked stupid questions. Some
might also consider it characteristic that Simon ended up buying
the pints because I didn't have enough cash on me.
Introduction. Design.
Materials. Layup.
Finishing Off.
This document is compiled by an amateur, not a professional. It has been compiled in good faith, but almost certainly contains errors and inaccuracies. "Best practice" also changes frequently with changes in technology and materials. None of the procedures listed are guaranteed to work, and some or all of them may be hazardous. If you feel unable to take responsibility for your own actions and errors without resorting to the legal profession then you are advised not to read it, let alone build anything based on information here. In any case you are advised not to build a composite structure without someone experienced in the materials to contact for advice.
Mast building is probably the most challenging laminating job
the amateur boat builder is likely to take on. It means handling
a lot of material in a particularly tricky lay-up, and the consequences
of getting it wrong are serious. If you build part of your boat
with an unnecessarily strong lay-up then you've wasted a little
bit of material and added a few grams of unnecessary weight, and
if you make it too light then there'll be a small loss in stiffness
and possibly the need to reinforce it. The kind of lay-ups described
in other sections of these articles are very much on a "that
will be plenty strong enough and adequately light" basis.
Masts are more difficult. OK the possibility of making it badly
and it ending up breaking is there, but that.s not the major issue.
What's more serious is that the actual stiffness on the mast has
- of course - a huge effect on the performance, and the actual
difference between what we'd regard as a stiff mast and a bendy
one is not really very much. This means that's its reasonably
easy to end up with a telegraph pole or a fishing rod...
Don't take on a mast until you've done a good lot of laminating
and can consider yourself reasonable skilled. You're also well
advised to do a boom or a bowsprit first because the much smaller
size makes the project easier, there's less of a worry about it
ending up too stiff, and in any case its not such a big lump of
cash in the bin if you get it very badly wrong.
This article is also rather less of a "how-to" than
the others in the series, and rather more of a "how we did
it". The folk who are building masts within the UK fleet
are doing it with the benefit of some years experience and several
boats behind them. There's no substitute for "getting your
hands sticky". Its also well worth pointing out that there's
no consensus as to the best way of building masts yet, and there
are other methods that work just as well - maybe better.
Its possible to start with a theoretical list of stiffness
values and so on, and then go from there. This is what the companies
who build big one off yacht masts and so on must do. All any of
us have done is to start from a know alloy mast with published
data as a basepoint, and then say - well, a bit stiffer sideways,
much the same fore and aft, bendier at the top and so on.
Working out the values for mast taper is mostly informed guesswork
as the figures are not published, and although we have tried to
measure one, we didn't really get any results that we had too
much confidence in.
Designing the section isn't about the cross sectional shape itself,
so much as identifying the dimensions and fibre lay-up that will
give you the stiffness you want. The maths is roughly Mechanical
Engineering graduate student level, which may not daunt you, but
rather does me. If you have access to reliable information that
other people have worked out then all well and good. Basically
a carbon mast tends to consist of a substantial layer of unidirectional
fibres sandwiched between two layers of woven carbon which are
primarily there to keep the unidirectional carbon in column and
prevent buckling and peeling.
There are two ways of going about working out what this this lay-up
should be.
One is to start with an existing mandrel or mould and then work
out what lay-up is going to give you the stiffness you want.
The other alternative is to work out what lay-up you want to use,
and then build a mandrel or mould to give you a section that will
do the job you want. For instance Dave Roe's 1997 mast was based
on 4 layers of unidirectional fibre. The actual mathematics required
to design the lay-up is beyond the scope of an article like this
- suffice to say if you haven't got either the mathematical skills
or access to someone who has, or access to the technical information
on the materials you propose to use, then probably you shouldn't
be getting into mast building at the present state of the art
As an example one starts with the stiffness value of the particular
carbon that you intend to use for the main structure of the mast,
take off an allowance for the amount of resin that is likely to
be in the lay-up, add allowances for internal and external skins
(usually an order of magnitude smaller than the main unidirectional
fibres), add an allowance for the mast track and then see what
sort of figure you arrive at. This result may be unsatisfactory
for one reason or another, in which case you have to repeat the
exercise until you get something appropriate. A particular consideration
here - especially with dinghy masts - is that they get bashed
about a fair bit, what with beaches and trailers and roof racks
and so on. Its probably fair to say that anything with a wall
thickness of less than 1.5mm will be too prone too damage when
not in use, no matter how appropriate the structure is for sailing
with.
A couple of rules of thumb that can be of use are:-
Carbon rigs tend to be at least 20% stiffer than the equivalent
section in alloy. This means that you can usually go one section
size down when basing a carbon rig on an existing alloy one.
A 200g carbon cloth makes for about 0.2mm of section thickness
in a lay-up.
You must be using a low viscosity resin with a very long cure
time. It will take you hours to laminate up the tube. Resins from
the Ampreg range are the conventional choice - Ampreg 26 is good.
There are several reasons for using the Ampregs, which are, on
the face of it, very expensive, but basically it boils down to
"you get what you pay for" and in the case of the Ampregs
you get an easy to handle lower toxicity resin with lots of desirable
properties like low heat sensitivity, especially when cured. Don't
economise on resin. Especially, whatever you do, don't consider
using polyester resin!
There are all sorts of nasty small scale phenomena that can occur
in carbon masts which just aren't an issue in a relatively low
stressed hull construction that can cause all sorts of problems.
One of the worst is "microcracking" which occurs if
the resin takes up load before the carbon and cracks. Suddenly
the carbon is unsupported and... I'm sure you can guess what happens
next.
In the UK we tend to use fairly ordinary grades of carbon, which
seem quite adequate. For the woven cloth inside and out we use
a single layer of 200gsm carbon, which is the most economical
currently available. Two layers of 100gsm carbon, aligned in different
directions would be superior, but four or five times the cost.
Higher grades or carbon could certainly reduce the size and weight
- Dave Roe has calculated that he could make a 1.5 inch diameter
mast that would be around 70% of the weight of the current ones,
but that the material cost would be something like three times
greater...
In the past we've used a lot of paraffin wax as a mould release
agent that can be melted out. If you have appropriate facilities
and are using an aluminium mandrel then laminating and curing
at a relatively high temperature can help considerably as the
thermal expansion of aluminium exceeds that of carbon lay-ups.
Experience seems to indicate that ordinary mould release agents
just won't guarantee you to be able to get the mast off the mandrel.
This is an area that the commercial mast makers are looking at
a lot, and will need to be solved before true mass production
mast making can happen.
Lay-up consolidation is absolutely essential. The best bet
seems to be vacuum bagging. There are people who use tape successfully,
but there are a number of dangers in this, most especially that
of dragging the lay-up round the mast in a spiral which will probably
result in less stiffness than was planned. I don't propose to
go into a full treatment of vacuum bagging for the amateur here,
but there's space for a few pointers. Supposedly there are some
good books on the subject, but we've all learnt from talking to
people. There's a lot of people who use the techniques in one
industry or another these days, at least in our part of the world.
Peel ply is essential. Perforated release film is good, but hopefully
the peel ply will soak up the resin unless you've got far too
much in the lay-up anyway. Breather cloth is quite cheap, and
you may as well use the real thing, even though some people use
Chopped strand mat instead (about all its good for!). Even old
blankets from a car boot sale will do the job at a pinch though,
and are much better than nothing. You can use virtually any airtight
sheet plastic for bag film but proper bag film is very thin which
leads to smaller wrinkles, a better finish and less extra work.
You will need to smooth the bag down as the vacuum goes in to
minimise the wrinkles. Where the vacuum goes in is important -
in practice bags always leak, and so if you have a leak near the
inlet you may not get much vacuum at the other end. A tube with
holes in to distribute the vacuum is thus a very good idea to
equalise things out. SP recommend no more than 0.5 bar for the
vacuum but by the time you've spent 4 hours laminating it up so
the first layer is half set, you're working maybe in 16 degrees
instead of the specified 21 then you may need to go higher simply
because the viscosity is higher and the mix has half gone off
as well. In practice if all the bleed cloth is saturated in epoxy
you won't get much more resin out of the mix, and if you've put
4 layers of unidirectional on that's about right. The danger with
more tractable lay-ups is that you can actually suck so much resin
out of the cloth that the strength is badly compromised
Having said that the lower the resin ratio the better (well almost)
and carbon masts need to be as dry as you dare go. At a maximum
you should be looking at a lay-up that is one gram of resin for
every two grams of fibre. If you're not confident about achieving
that then maybe you want to tackle another boat before you do
a mast. Unidirectional cloth that has glass binder is useful for
helping you see how well wetted out the lay-up is. Its also worth
noting that the consolidation effect of the vacuum increases exponentially
with the decrease in pressure, and at the far end small improvements
can lead to big changes.
The technique we've used is to lay the mast up around a mandrel
- effectively a male mould - and vacuum bag the mast onto that.
Others have apparently made spars by making a female mould and
using a bag on the inside to consolidate the lay-up. I guess that
this would be more work on the mould, and more trouble to lay
up, but be considerably easier to remove mast from mould.
The first (inmost) layer of lay-up is local reinforcement where
fittings and so on will go. This is usually plain glass, and then
a layer of light kevlar. This has two roles. The first is to add
local strength where fittings and so on penetrate the mast, and
the second is that the glass insulates the electrochemically active
carbon from the fittings. Kevlar is chosen because its excellent
for resisting crack propagation and so on, but has a tendency
to go "furry at the edges", which glass doesn't suffer
from.
The next layer should be Carbon cloth, 45/45 degree aligned for
torsional strength. This is adequate for a dinghy mast, but something
bigger will need to be designed more carefully. Its worth noting
that, in contrast to a metal mast, fibre masts are not made out
of a homogenous material, and can and will have significantly
different properties according to the alignment of the fibres.
It is not especially easy getting the fibres properly aligned
and can be wasteful of cloth. None of this job is easy!
Once these layers have been cured and tidied up the main lay-up
of uni-directional fibre can be commenced. This needs to be done
as a single job, which will take several hours. Obviously it needs
to be done carefully, making sure that all the fibre is properly
wetted out (not always easy to tell with carbon,) and that there
are no voids and that the fibres are correctly distributed round
the spar.
Fair up this layer before you put the outer skin on. Here you've
got to be careful with the (virtually inevitable) wrinkles. If
you have longitudinal wrinkles of carbon sticking out then you
can sand them down without too much worry. After all what they
principally represent is fibres that have been squeezed out as
the laminate compresses down, so you can take them out without
compromising strength or lay-up thickness. Horizontal wrinkles
are trouble because if you sand them off you are breaking into
the carbon at a single point, making a spectacular weak point.
This is mainly a matter for very careful vacuum bagging.
The track - provided you aren't making a trackless mast for
a sleeve luff rig - should be made from half inch diameter pultruded
tube from whatever your handiest source is. Glass is cheaper,
heavier, and has less impact on the final bend characteristics.
Carbon is more expensive, lighter, and contributes more to stiffness,
especially fore and aft. Glue the track on with a reasonably strong
filled epoxy mix, and then fair up the gap between tube and mast
to your preferred shape with something a bit lighter.
In New Zealand they take a different approach. They make their
tracks from a rubber type caravan awning track which bends very
easily but does not seem to lose the bolt rope out of it. This
is glued onto the mast using a particularly flexible adhesive
called Sikaflex.
Take a good look at the end of the mast track where the sail feeds
in. Its very easy to let the luff rope get trapped across the
groove. This puts a very big load on the track and has bee known
to actually crack the track, after which the luff rope gets in
the slot more and more often, rigging is a pain, and the luff
rope can pull out of the groove whilst sailing.. Rainforce the
last inch of the track so that this doesn't happen. Similarly
be very careful about the end of the slot. If its too sharp a
corner you may risk tearing the sail, but a very even taper means
the luff rope getting caught for sure.
Lastly on goes an outside layer of 200gsm carbon, this time aligned 0/90 degrees to the mast, plus appropriate local reinforcement where needed. It goes round the mast track of course. Should you ever wish to modify the stiffness of the mast by adding (not always successful without great care) or removing fibre, you should sand off this entire layer and then replace it when you've made the changes. One has to be very careful about adding extra fibres later, because its very difficult to get really first class bonding between the original and added structure, which means that there's always the possibility that the load won't be distributed between the fibres that well. There is no especial constructional reason for having the 45/45 layer on the inside and the 0/90 layer on the outside other than it is easier to make a neat job of the 0/90 lay-up. As mentioned above the optimum would be a thinner layer of both each side of the uni-directionals.
Page IndexA lot of people commercially are working on reliable release
and easy lamination. It can be big trouble. Quite often its easier
to simply cut the mast down one side and take it off like that.
After you do that you'll need an outer layer of carbon again,
but its not as disastrous as you might think because your cut
is aligned with the majority of the fibres anyway. If you have
an alloy mandrel you can make it as cool as possible to shrink
it out from the lay-up, if you used wax then very hot water down
the centre of an alloy mandrel gets it out a treat, or alternatively
you can melt it out with a heat gun (don't get the lay-up too
hot!).
Cut out the slot in the mast track. I suppose in theory the slot
reduces the effect of the outer layer of fibres, but the track
itself contributes plenty of strength in that area, and it doesn't
seem to be a problem.
Fit out the mast out pretty much as normal - but be *very sure*
that the fittings have been placed where the reinforcement for
them was. Corrosion is a significant consideration - carbon is
electrolytically active and there's a considerable potential between
it and aluminium. Make sure there's no contact between aluminium
and carbon - stainless rivets are definitely preferred. This applies
to other fittings as well. Again glass can be used as an insulator,
but resin coat the alloy fittings too and generally do your best
to keep them isolated. In this context its worth noting that I've
heard reports that RS600 mast bottom sections - which have an
aluminium sleeve to shorten the mast to reduce sail area - have,
in some circumstances, actually failed because of the expansion
caused by corrosion of the aluminium. Be very careful with rivets
and avoid them if you can. In particular make sure that the holes
you drill for rivets are as tight as possible in order to avoid
the rivet expanding in the hole and causing a local distortion
in the structure. For similar reasons backing washers on rivets
are nothing but a good thing if you can possibly get them on.
Finally paint it. The main reason for painting the tube is to
provide UV protection. This most especially applies to a mast,
which unlike the boat tends to be out in all weathers uncovered
- at least if your boat lives at the Club all year round. For
this reason I painted my carbon mast white, as this should reduce
the amount the layup gets cooked on hot days. Having said that
all the mass production companies seem to think a clear lacquer
to stop the UV is more than enough, and if, like many Cherubs,
your boat lives in the garage, not the sailing club then its not
nearly such an issue, so paint it all the rainbow colours you
like!
Jim Champ, 1998, with grateful thanks to Dave Roe, Simon Roberts
& Tim Dean.
For further information on this page email the Webmaster
