UK-Cherub Class

Get Your Heart Racing


Thinking of building a Cherub?

If you are thinking of building a Cherub it can be confusing getting to grips with all the different parts and stages of the build. This website contains a wealth of information on most of the techniques required; however it can be difficult navigating around the index and finding what you need. This page is intended as an overview of the build process together with links to more in-depth articles. This article will mostly look at building a modern Cherub using foam sandwich construction, other methods are possible and articles covering different techniques have been linked. Some of the articles are linked to more than once as they often cover several sections.

Note: This page is being used as a handy reference to the articles that we have and to work out subjects that need expanding, links to articles that exist are in green and those that have yet to be created are red. It is in a very early stage at the moment, all members please help to fill it out.

Building a Cherub can be broadly divided into several stages:

  • Designing
  • Mould
  • Hull
    • Under Deck Structure
    • False Floor and foredeck
    • Various Semi Structural Items and hard points
    • Fairing and Painting
  • Fitting Out
  • The Rig and Spars
  • Foils

Designing

The Cherub is a development class with quite open rules so there is the oppertunity to build your boat to your own design, it is not the only way as there are a number of proven designs out there and a small fee to the designer will let you build the design and probably get advice for the build along with the design.

To get some idea about the principles involved in the design it is worth reading the Designing Cherubs as well as some of the other pages we have covering the subject.

Further Reading

Mould

In order to build a hull you need a design and then a jig or mould built to that design. There are two main types of moulds the male mould and the female mould. With the female mould the boat is built inside the mould and with the inside skin of the mould conforming exactly to the outside skin of the boat. This is the method normally used with production boats and can produce good results with very little fairing needed on the outside prior to painting, however this type of mould takes much longer to build and needs careful preparation before use. The most commonly used type of mould used in the Cherub fleet is the male mould. Here the hull is built on the outside of the mould with the outside skin of the mould being the same size as the inside skin of the hull. This means that the size of the mould needs to be adjusted to the thickness of the foam used. Also the outside skin will need more fairing to finish the hull off, however many people consider that it is possible to build a lighter hull this way.

If you are going to vacuum consolidate your hull as you lay up then the mould must be built in such a way that it is airtight and you have a platform around the edge of the mould that you can stick the vacuum bag to. The mould is normally built by placing thin strips of wood over a series of frames, the wood strips are cut to be as close to each other as possible with filler in any gaps that are left. It is quite common to cover the mould in a layer of glass cloth for extra strength. This is then faired and covered in a release wax so that it is possible to get the hull off! There are a number of moulds floating around in the class at the moment and a simple option is to borrow one of those and build a boat on it. This saves the time and cost of building a mould yourself although you do have to arrange to pick up and return or store the mould when you have finished with it. If you do not feel that you have the expertise for building the mould and hull shell then there is always the option of getting this part of the hull built by a professional and then completing the bare shell yourself. This can be quite a good option as you know that the hull a major part of the boat has been properly built and is light weight this may enable you to complete the rest of the hull without having to invest in a vacuum pump.

  • A Jig which the foam is attached to and then the outside skin is laminated. Once set the shell is removed from the jig and the inside skin laminated.

A Jig2675-1999xxxxa.jpg

  • A Male mould, the inside skin is laminated, then the foam then the outside skin.

A Male Mould2688-20051112a.jpg

  • A Plug and Female mould, the plug is created in the shape of the boat, a female mould is then made on the plug, which is removed. The hull can then be built in the female mould, with the outside skin laminated first, followed by the foam and inside skin.

A Plug A Plug with the female mould removed

Hull

The Hull is built using a jig or mould to give it the designed shape and to support it during the build process. The Hull is either build on the outside of a jig or male mould or the inside of a female mould. When building over a male mould you will laminate the inner skin, possibly using a vacuum bag to help consolidate, then the foam is cut and shaped, this is probably the most time consuming stage as this will dictate the fairness and precise shape of the finished hull. Once the foam is in place the outer skin can be put on. At this point it is ready to come off the mould. Building in a female mould can be slightly quicker as the outside of your hull will already be fair when it is removed from the mould, however the mould takes much longer to build and prepare. 2688-20051216a.jpg

This can be quite a good stage to get a professional to build to, it saves you the problems of building or transporting a mould and you know that a major part of your boat has been properly done.

Tips

  • Put peel ply onto the mould in the positions where you are planning to attach bulkheads, fittings or intend painting this can be peeled off after the hull is taken from the mould leaving a clean area with no release wax for further building.
  • Some types of foam can be softened using a heat gun allowing them to be bent into shape more easily.
  • To shape some foam around a particularly tight corner, it can be scored with a knife in a series of lines along the direction of the bend.

Under Deck Structure

This primarily consists of the bulkheads and spine together with the centreboard case. The layout of the bulkheads is partially governed by the requirement in the rules to have 3 separate buoyancy tanks.

The two main layouts are

  • One tank forward of the shroud bulkheads and one each side of a solid spine aft of the shrouds
  • Three tanks across the boat with one forward of the shroud bulkhead, one between the shroud bulkhead and one in the middle of the cockpit and a tank between the mid bulkhead and the transom.

2676-1999xxxxb.jpg 2685-20070418p.jpg 2686-20050220a.jpg

Bulkheads that are not used to seal buoyancy tanks can have large parts cut away to leave a lattice work to provide the support where needed. As the bulkheads are just flat panels they are quite easy to build but if you are building without a vacuum pump it may be a good idea to buy these pre laminated by a boat builder. All that is required is to cut them to shape and fit them. The closer the fit of the bulkhead to the hull the less filler that is required for a strong joint, helping to reduce weight. A good fit can be made using plywood off cuts as templates, these are cut and sanded to shape, this shape is transferred to the bulkhead panel which is cut out and sanded to a tight fit.

The bulkheads are typically just filleted to the hull, again using a small fillet is good for weight. It is often worthwhile reinforcing the joints of shroud bulkhead and around the mast step with some cloth to spread the load. The Centreboard case also needs to go in below the floor, it is obviously important to get this aligned along the length of the boat on the centreline, it also needs to be perpendicular to the bottom of the boat. This is definitely a case of measure twice and cut once. The centreboard case needs to be securely fastened to the bottom of the boat and be thick at the back as repairing a damaged case after the boat is built can be a nightmare.

Tips

  • Make a pattern for the bulkheads out of scrap ply, then draw around where the bulkhead is to be cut and sand to an exact fit.
  • When placing the bulkheads ensure that the positions line up with the position of various structural components of the boat.
  • When gluing the centreboard case into the boat put the board in the case (covered in tape) this ensures that the case is not distorted and that the board will go in and out.

Crew Deck and Foredeck

The Crew Deck or False floor goes on top of the Bulkheads and provides a lot of the strength in the boat and also somewhere to stand while sailing the shape and height can make a difference in how the boat is to sail, how quickly the water drains and where the various parts fit. The false floor could be made from flat sheet , however it is probably best to build it with some curve in it. This can be done using a mould to support the deck while it is build but also using the bulkheads to support it and give shape. To build a mould-less deck, cut the foam for the deck to size so that when weighted down it will fit neatly to the top of the bulkheads. With the deck flat on the ground laminate what will become the underside, leave for long enough for the resin to green but not go hard and lift the deck onto the boat, weighing it down with as many weights as you need to get it flat. The boat should be protected with polythene or parcel tape so that you can remove the deck after it has cured, it may also be worth fitting temporary battens to the sides of the boat between the bulkheads to help with the support. Once cured the deck will hold its shape so can be removed, trimmed and fitted properly, then the top surface can be laminated.

A foredeck is optional, some boats have them and some do not, it is largely personal preference whether a foredeck is fitted, however if one is not used then some other way must be found replace the stiffness that the foredeck provides. There are a number of boats that use a mini foredeck to support some of the shroud loads and locate the jib track.

Major Structural Items

Consisting of the larger more heavily loaded parts of the boat, these include;

  • Snout and Bowsprit support
  • Shroud and Forestay Supports
  • Wing Bars
  • Gantry

These parts of the boat are all quite highly loaded yet still need to be built light and integrated carefully into the hull. The integration is where the biggest gains in structural weight and overall stiffness can be made. You need to think of the loads these items are likely to see not just in normal usage but also in unusual situations such as grounding, collision and rigging failure. You need to visualise what these loads are doing and how they are passed into the hull, how they are trying to fold the hull and what structure you have to prevent this. In many cases you can move some structure about a bit so that structural items can serve more than one purpose. With the advent of the t-foil the gantry needs to be much stronger than before and it was already a heavily loaded part of the boat the attachment of the gantry into the hull structure is more important than before. The Shroud and forestay points are obviously important to get right, many boats have used shackles or D rings effectively tied into place using unidirectional Carbon. This is a good technique but sufficient layers, particularly over the metal of the D-ring, must be used to ensure the point load is spread out into the structure.

Minor Structural Items

Consisting of smaller lighter components in the boat consisting of but not limited to;

  • Spinnaker Chute
  • Centre Console
  • Kick Bars
  • Hard points for fittings

The design of all these is incredibly variable and the best way to work out what you want is to have a good look at peoples boats and ideally sail in as many different designs and styles as possible. In general when attaching these items you need to try and ensure that the fibres run in the best direction to take the load and that there is enough material around the attachment points to effectively spread the loads away from the point of attachment. 2675-1999xxxxe.jpg 2688-20060216d.jpg 2686-20060403d.jpg There are as many ways of attaching fittings as there are fittings, with foam sandwich construction it is not a good idea to screw fittings in place as they will quickly pull free. Simply bolting fittings is also problematic as the bolts can easily crush the foam and pull through the thin laminate.

Fairing and Painting

Can make or break you boat, It is important to fair the hull skin so that it provides a nice smooth bump free surface for the water to flow across, however using too much filler will result in a heavy boat and sanding all your laminate off will result in a weak boat. Fairing the inside of the boat is even more difficult, the corners and edges are particularly tricky to sand and there is a penalty in leaving too much filler on the boat, but no speed advantage once you have the main rough bits smoothed out. The lightest filler possible should be used and it can be tinted with different colours to help you see where you are in the sanding. Cherubs are typically built without gel coat as it is heavy and can only be used when building in a female mould, this means that they need to be painted. There is quite a tradition of wild paint jobs in the Cherub class so there need not be any restriction on your imagination when deciding the colour scheme. Cherubs have been successfully painted with everything from standard exterior gloss to metallic car paint to two pack polyurethane. They have also been badly painted in most of those things as well. The key to getting good results is ensuring that the substrate is sound and clean, each layer is compatible with the one below and that it is applied in clean dust free conditions in accordance with the manufacturer’s instructions.

Further Reading

Fitting Out

The fitting out stage is great fun as you can see your boat getting closer and closer to the water, it also takes more time than you expect as the list of different jobs to be done is surprisingly long. Typically cherubs are fitted out as simply as possible, however the systems you need still add up and fitting them into a small boat it is easy to make it look crowded. Controls that are needed are;

  • Mainsail
    • Kicking strap, gnav or temple vang
    • Cunningham
    • Outhaul
  • Jib
    • Self Tacker Track
    • Jib Sheets
  • Spinnaker
  • Sheets
  • Halyard and retrieval line
  • Pole outhaul
  • Tack line
  • Trapeze lines with elastic takeup
  • T-Foil control

To get all those to work you need quite a lot of fittings and string, you also need to attach them to your boat, this can be tricky with a foam sandwich construction as it is poor at dealing with point loads. Screws will not last and even bolts will pull through unless large washers are used. Some methods used for attaching fittings are outlined below;

  • Plywood core – During construction the core is locally replaced with plywood. This gives a strong area to bolt or even screw fittings to, however overtime the plywood will absorb water and rot.
  • Core Replacement – The holes for the fitting are drilled through the top skin of the sandwich only, then using an allen key in a drill the foam is crushed around the holes, the voids are filled with filler and the fitting can be bolted in place.
  • Carbon/glass Backing plates – In areas where access to both sides of the structure is possible you can cut out a section of 3-4 mm thick carbon or glass plate slightly larger than the fitting to be attached, this spreads the load over a larger area.
  • Fronting Plates – A backing plate is made up and the fitting bolted to it, the bolts are then cut to length and the nuts bonded to the backing plate. This plate with nuts on is then bonded to the deck of the boat and glassed over the top. As the nuts are now captive the fitting can be removed and replaced as necessary. This is particularly on the false floor of a boat where you cannot get to the inside.
  • Tying – Self explanatory but with the number of blocks that are designed specifically for tying in place it might be worth designing in some points that they can be tied to rather than adding weight by bolting a fitting in place and tying to that.

Further Reading

The Rig and Spars

For the rig and sails you can do as much or as little as possible, many people build their own booms and spinnaker poles and a few people even build their own masts and sails. Home built masts used to be extremely common in the Cherub class, however the production masts have come down in price and improved in quality to a point where buying one in is a good option. For those on a budget repairing a broken mast from another class can be a good option but will not be as good as a custom built one. For the rig to work well together it is important that the sails match the weight of the crew, the boat, the bend of the mast and the way the mast is rigged so it is worthwhile talking to a sail maker early in the project to work out the optimal layout for you.

Further Reading

Foils

There are a good many people who will build excellent foils for you and a few who are closely connected to the class, however building something like a rudder or centreboard is an excellent introduction to high tech composites and often a good starting place to see how you enjoy the epoxy fumes and dust involved in a boat build. As said before the centreboard needs to be done before the false floor goes in so that you can build the centreboard case around it. The rudder can wait till the end of the build but having a completed boat just waiting for something to steer it with is not much fun.

Further Reading

And Finally

Other bits you may need at some point include a road trailer with launching trolley, a cover to protect your boat from the elements and some foil bags to keep them from damage. There are quite a few different articles on the website describing most of the build techniques many of them are linked to from this article but the rest can be found in the Index of Technical articles Cherub sailors are always keen to help with advice for building projects and the class runs regular sticky weekends where there is informal instruction and parts are built for newcomers to the class there is also a technical help network.


tech/thinking_of_building.txt · Last modified: 2017/06/19 14:24 by jp233