UK-Cherub Class

Get Your Heart Racing


Building your own Ply/Glass Cherub.

Disclaimer 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.

Introduction.

2601-1980xxxx-paterson2-erikbloodaxe.jpg 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.

Design.

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.

Preparation.

2628-1980xxxxb.jpgHaving 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 ery 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!

Construction.

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!

Cherub Specifics.

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!

Ply joints.

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.

Bulkheads.

custardtruck-20021203b.jpgWooden 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.

Outer Skin.

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.

Mast support & Space Frame.

custardtruck-20021203d.jpgThis 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.

Daggerboard case.

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.

False floor.

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.

Side tanks

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.

Time.

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 © 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.


tech/wood.txt · Last modified: 2013/06/25 15:55 (external edit)