You have arrived at an old part of our website, to find the latest information please visit our new page about building your own Ply/Glass Cherub. 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.
Introduction. Design. Preparation. Recommended materials. Construction. Cost & Time.
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 ply/fibre boat without someone experienced in the materials
to contact for advice.
These days foam sandwich is the popular option for the home
builder of Cherubs. It is probably the easier material to work
in and gives a low maintenance boat. On the other hand wood is
still a nice material to work with and does produce a really pretty
boat if you get it right! A wooden boat built in the hi-tech manner
described here, which is some way removed from traditional boatbuilding,
will also be a little more proof against knocks and dents than
a foam boat. This is intended as a selection of hints about Cherub
building in particular, not a complete "how to". There
are many books available in shops and from libraries that will
give you most of what you need. Other good sources of information
are the suppliers of boat building materials. Traditionally Cherubs
were built out of Aerolite 306 or Aerodux. These still work fine
for all wood boats, but the techniques are very different to those
described here, and weight is a major problem with this sort of
construction. The best wood boats before fibre/reinforced plywood
became used were probably the cold moulded ones, either fully
or just the area below the chines.
Home building is usually a rewarding process. A mixture of blind
determination and some skill in handling tools is important, although
enough determination and patience can make up for a shortage of
skill! Probably the most discouraging factor is the air of mystery
that surrounds boatbuilding. Just remember that skilled traditional
boat building is an arcane and complex art that has almost nothing
in common with building a modern Cherub!
I suppose it has to be conceded that the Cherub, being a very
light boat with considerable performance and very high stresses,
demands careful consideration in the building process. It means
among other things careful assessment of materials, quality of
jointing, use of suitable adhesives, and above all finding out
what other people have done successfully - and unsuccessfully
in the past. There is no substitute for talking to other people
who have built new boats or owned second-hand boats. The class
association will always be happy to put you into contact with
people who have had considerable experience of wooden construction.
Fortunately Cherub sailors tend to be a friendly and open bunch,
and are always ready to help with genuine enquiries.
There are a great many designs of Cherub. Most recent
designs were designed for foam sandwich, so you may be a bit
on your own working out details of bulkheads etc. However the
designer will always be happy to make suggestions, and most have
enough experience of wooden boats to be able to give you some
extra information in exchange for their design royalty. In selecting
a design it is best to talk to others in the class. At any given
time there is usually a dominant design that is a safe bet, and
they can give you advice on what suits your crew weight or sailing
area.
Having selected your design and bought your drawings you will
have, at best, a large selection of drawings, a table of offsets,
a selection of detail sketches, and a list of suggested materials.
At worst you'll just get a series of scale drawings and sections.
If you don't have a table of offsets you will have to cross check
regularly as temperature and humidity changes can distort drawings.
The aim is to get as fair a hull shape as possible and one as
near to the designer's intentions as possible. You will need to
end up with a set of formers no more than 18" apart (12"
near the bow), set on a rigid foundation if you are to produce
a fair and symmetrical hull. There are two ways of doing this.
Firstly you can trust to luck, make the formers to the shapes
given and, once it is all set up, fill in the dips and pare down
the bumps that will almost inevitably appear when sighting from
various directions. This sometimes works well, especially when
you are on your 5th or 6th boat and have had some practice! It
can also be a bit risky if some of your measurements - notably
rise of floor - are near the limits before you start paring!
The second method is to 'loft' the lines, which is basically drawing
them out full size (emulsion painted sheets of hardboard are good),
having drawn an accurate grid of station and water lines on which
to plot the offsets. If you don't have a table of offsets you
can scale them from the drawings, although this is tedious and
it is easy to make mistakes. The great advantage of lofting is
that by referring back and forth between elevations and plan you
iron out bumps and hollows and can easily check that measurements
are within the limits allowed. Another benefit is that many of
the construction details can be plotted and the details taken
off by paper patterns. For information about lofting refer to
'Complete Amateur Boat building' by Michael Verney (pub. John
Murray) and 'Boat Building' by Howard Chapelle (pub. Allen &
Unwin). Don't refer to them too much for constructional details
though!
Hull floor
4mm ply. 60g kevlar or 105g glass coating on the inside, 105g
glass on the outside skin.
Topsides, decks, bulkheads, daggercase sides etc.
3mm ply
all the ply should have a layer of 105 g glass or 60g kevlar on
the inside.
Use lightweight Gaboon ply (hardwood veneered looks very nice
but can be a bit heavy). Nowadays by far the best source is Israeli
made WBP gaboon, which is light, strong and very cheap.
Stringers, stem, other strip wood.
Western Red Cedar is favourite. It splits very easily, and needs
to be coated from ply surface to ply surface with glass.
'Decorative' stripwood - Gunwhales etc.
Spruce or mahogany. Spruce is lighter, mahogany is more durable,
but can be very heavy. At the most a thin capping is all you should
use. Really good clear pine may be almost as light as spruce and
is easier to get hold of. Getting suitable wood can be a problem,
and be very careful about adding weight. If you are not worried
about getting a Grand Piano appearance then use cedar and glass
coat it.
Some people have even used balsa wood coated in glass for glass
coated stripwood, but of course in this case the wood is doing
no useful work, and serious quantities of balsa wood get very
expensive.
Building jig.
Formers are best made from something like 3/8" building grade
plywood. Slightly damaged sheets can often be got cheaper. Strip
wood can be anything that's reasonably solid and cheap. If you're
using second-hand wood make sure you get every nail out!
Glues.
Use Epoxies. SP320 is the best for the sort of fibre re-inforced
ply construction we are talking about here, SP106 or West 105
are cheaper.
Tools.
A
staple gun is essential for fastening plywood down. It is cheaper
and lighter to tack down with staples and remove them afterwards
than to use expensive marine-grade fasteners and leave them in.
Otherwise normal carpentry tools are fine, but you can never have
enough G cramps. If you have less than about 15 get borrowing!
Advice is always helpful, but make sure you understand it completely!
Be a bit careful of people who have built 5 Enterprises. They
will probably tell you that such and such a bit has to be that
thick or glued on that way. It may well end up too heavy and not
strong enough! If advice does not seem to add up pay attention
to your own inner murmurings. One excellent piece of advice is
to have a comfortable 'thinking chair' in which you can put your
feet up with a cup of tea - or something else relaxing - and look
and think hard about the current problem. Thinking time is rarely
wasted - especially if it means you don't have to do something
twice!
I will assume that you are building a self draining boat, to
be quite honest if you don't you will curse yourself when you
start sailing it!
Any place where two bits of ply join at an angle needs a generous
lightweight fillet, preferably with glass reinforcement running
1" or more from either side of the join. Beware weight. Use
the lightest possible filler. The glass over the join should be
cut on the bias at 45 degrees - it will go round corners more
easily and is twice as strong.
Wooden boats are like banana skins. The longitudinal seams
will split open unless they are sufficiently braced across the
join. This is mainly achieved by transverse bulkheads. Locate
these about halfway between the mast and the bow, under the mast,
at the rear of the daggerboard case, under the mainsheet position,
and between the mainsheet and the transom. The transom itself
forms the final bulkhead. This one needs to be 6mm wherever rudder
fittings will attach, and really solidly glassed in. Cherubs are
notorious for rudder fitting problems. It is better to have a
smallish drain vent in the transom rather than a completely open
one, both for strength and for sailing. A longitudinal bulkhead
will run from bow to stern. Don't forget to arrange for the minimum
3 buoyancy tanks.
Absolutely vital is a really good join at the centerline and
chines on the outer skin. Some people cold-mould the bottom out
of thin ply strips, and this gives a really strong boat if you
have the time and skill. In the days before false floors two layers
of 3mm ply for the bottom skin were recommended, and in this case
the joints were staggered.
A single of 4mm ply is recommended. A lot of modern designs are
very flat floored, and it might be possible to get away with a
single sheet, but this will very much depend on the amount of
rocker. Normally you will need a joint in the middle. For the
reinforcement you could put stringers on the inside, (and in any
case one light stringer will probably be needed to keep the profile
correct), but a much better way of stiffening up and supporting
the outer skin is run the inside fibre coating over epoxy fillets
onto the hog and chines. This provides a really good support with
a minimum of hard spots. The hull skin should also be glass coated
on the outside, and this should be done last of all, running from
the underside of one gunwale to the other.
You will need to scarph joint two pieces of wood for the bottom
panels as twelve foot 6" lengths of marine ply are rarely
available, and this is best done on a flat surface before putting
the panels on the boat. You may well find that vertical strips
need to be cut in the skin on the centre line near the bow because
of double curvature. If you have a really complex shape you can
make it up from all sorts of odds and bits. In any case, once
the boat has been turned up, you will need to add a lot of stiffness
in this area, which takes a real pounding going upwind in a chop.
Probably the most suitable method is to use two layers of uni-directional
carbon criss-crossed diagonally from chine to hog, and then a
layer of glass over that. Try to fillet and shape the hog to let
the glass go in one piece from chine to chine. This area used
to be a common point of failure in the old days, and is one where
the use of modern materials really pays off. Extend this extra
reinforcement to about six inches behind the daggerboard case.
The glass coating needs to be made with a very low resin to glass
ratio. Beware of adding weight! The way to achieve this is to
use a heat gun to warm up the resin and a roller to really spread
it out. Don't be tempted to warm up your pot full of epoxy though,
it will go off in no time at all, leaving you with a beautifully
moulded epoxy casting! It is wise to keep your resin in a tray
once it is mixed as it will not go off as quickly.
This is an important issue. With the sort of rig tension used
these days the peak loads going down the mast probably approach
tons! Exactly how you step the mast is up to you, but the favourite
location is on the false floor. It is conventional to support
the mast with a space frame. This is not the sort of alloy contraption
you find on some boats, but a ply/carbon/strip wood construction
integrated with the main bulkheads in this area. Longitudinally
it runs from the keel under the mast up to the top of the bow,
from there up to the prodder anchor point, back down to the mast
foot, and from the mast foot back down to the hog. Maybe also
back to the post. Laterally it goes from the mast foot and the
hog out to the shroud anchorage points, the front bulkhead being
angled back on both sides. Arrange it so that the mast sits on
the crossing point of the longitudinal and transverse bulkheads.
You will need a really strong point for the prodder. It is advisable
to have a foredeck on a wooden boat as it does a lot to stiffen
up the front of the boat and support the rig loads. The basis
for the prodder support can be the beam down the middle of the
foredeck, which meets the upward extension of the space frame.
Put a transverse bulkhead across too, but make it very light as
it will take very little load (except when the crew crawls across
the foredeck to disentangle the kite. The bulkhead supporting
the foredeck will also be angled, but should run from the shrouds
to just in front of the mast.
With the typical Cherub rig of lower shrouds and prodder you will
not need to support the mast at deck level, but you might want
to put a bit of strength there in case fashions change.
Fortunately the false floor does a lot of the support for the
daggerboard case. If you have a stringer each side of the daggerboard
case to support the false floor they can support the sides of
the daggerboard case. You may well want to bring the daggerboard
case up a bit higher as the foundation of a tunnel. This keeps
control lines out of the way, and gets the bowsprit out of the
way when retracted, and can be used to stow the kite. The tunnel
is probably best made from two layers of 3mm ply to give stiffness.
You will have a variety of bulkheads to mount the false floor
on as detailed before. Naturally all those which are not required
to be watertight will be reduced to webs. These should have slim
battens mounted on the top. You should also have stringers running
down in the main stamping area. Work out exactly where the battens
etc. are going to run, and put an extra layer of glass on the
underside extending 25mm each side of where the bulkhead will
touch. This will reduce the hard spot considerably. The floor
should be 3mm ply with a layer of 100g kevlar on the underside.
On the topside put a layer of 105g glass, and run it up over the
fillet where the sidetank joins. This will stiffen it up, add
wear resistance, give you a non-slip surface, and make holes much
more unlikely. While on the subject of false floors, you will
no doubt need hatches in them, but never put them where they can
be stepped on. The hatch causes a hard spot which will crack.
Right in front of the transom is a good place, and it also gives
good access to bolt on rudder fittings.
If you have a reasonably deep false floor you can probably
do without side tanks. However I would still advise side decks
as they stiffen up the gunwales so much. The join between tank
side and floor is a common point of failure, give it a good fillet
and glass reinforcement. You may as well have some kind of side
tank , because the box section gives so much extra stiffness,
but you may well want to run sheets and things through forward.
Probably 200 hours or so with experience, and maybe approaching
300 for the first time builder. Cost of materials depends vastly
on what you use and where you can get it. As of 1991 you would
probably spend between UK£300 and UK£600 pounds on
the hull, which is appreciably cheaper than foam sandwich.
Finally - don't forget: If in doubt - think. If still in doubt,
ask a few people and think again. "Think twice and cut once
is a very appropriate maxim. Have fun!
Jim Champ, adapted from Dick Jarrett. Andy Paterson of Bloodaxe Boats, who
is by far the most experienced builder of glass/ply Cherubs gave
me a great deal of help in preparing this piece, in particular
in the choice of materials and other details of fibre-ply sandwich
construction.
Photos (c) Bloodaxe
Boats, Jim Champ, Bloodaxe
Boats.
If you wish to have fuller construction details for building
a ply sandwich boat then it would be worth considering purchasing
a set of plans from Bloodaxe
Boats, which contain much more detail than is practical to
include here.
If you would like details on the methods traditionally used to
construct ply Cherubs then the best source I am aware of are the
drawings & construction plans for Farr Design #48 from Farr Yacht Design
Ltd.
For further information on this page email the Webmaster