UK-Cherub Class

Get Your Heart Racing


Building a Rudder Stock

Most Cherubs seem to get a number of rudder stocks over the course of their lives, as its one of the most popular breakages. It is also a good first project as provided you do it before your old one breaks you can still sail while you are building it.if it does not work you can always go back to the old rudder and stock.

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 composite structure without someone experienced in the materials to contact for advice.

Design

The “Dagger” or “Cassette” type of rudder/stock has a number of advantages over the fixed rudder or conventional lifting rudder. It is the construction of the dagger rudder that is described here. The Basic design has a sleave that closely fits the shape of the rudder blade that the rudder slides into. With a full length rudder pin that goes down through a hole in the tiller and another at the bottom of the stock.

The number one cause of trouble is an attempt to attach standard metal fittings the light weight glass or carbon structure of the boat or stock. It is supprizing how highly loaded the rudder blade is when the boat is going along and this puts huge loads through the stock and back to the boat all concentrated through the pintle and gudjion. Using the standard aluminium fittings gives problems with finding solid structure to bolt through and corrosion. The best method is to use a full length pin and construct the attachment bearing surfaces from carbon and glass.

The Sleeve

Start by moulding the sleeve around the rudder blade. Its difficult to take too many precautions against getting epoxy on the blade - lots of work to remove. Cover the top half of the blade with parcel tape, with it overlapping the trailing edge of the blade, and apply release agent or candle wax. Suspend the board with the leading edge horizontal and uppermost. Laminate the case using an appropriate layup (100 g/m² glass + 2 x 200 g/m² carbon or 4*200g/m2 glass would be fine, allowing the fibre to overlap past the trailing edge. When its cured, trim the case to about 2mm oversize. Split the case from the foil, and add several layers of 18mm masking tape plus parcel tape to the trailing edge of the foil to make a thick edge to the foil. Replace the blade in the case, fill the open joint at the trailing edge, and laminate strips of glass around 20mm wide over the joint, with kevlar at the bottom (all held in place with masking tape). Its probably also worth putting a few layers of glass extra at the leading edge, as it tends to get a lot of bashes as you pull the blade in and out. When cured remove the sleeve, remove all the packing from the blade and make sure it slides very easily through the sleeve. If you can't slide the sleeve off the blade at all, or if it seems stiff on the blade, then cut it neatly down the trailing edge and try again with more layers of masking tape! The sleeve will need to reach from the top surface of your stock to the lower edge of the bottom fitting. These means about 75mm greater than the distance between fittings, but make it bigger and trim later!

Lining the Sleave

Some people like to line the sleave with a soft slidey material to protect the blade from scratches and make it eiser to pull up and down. Materials that can work are

  • loops side of velcro
  • Felt or thin carpet
  • ProGrip (Padded but not slidy)
  • Jap Tape

These can help make the blade fit more snugly but if they start to come off inside the stock they are a pain to replace.

The Bottom Fitting.

This is a favourite breakage point, and needs to be stronger than you'd possibly believe! Dave Roe reckons that the peak sideways load on the rudder stock is in the close order of magnitude of 180Kg or 400lbs. If you want a mental picture of this imagine the boat sitting on its side with 80 (Eighty) bricks piled up on the top of the blade. Start by laminating up a solid carbon plate, about 4mm thick, about 75mm longer than the section length of the rudder blade, and 75mm wider than its thickness. This needs a great many layers of carbon cloth, and make sure that the fibres are aligned both 45/45 and 0/90 degrees to the blade. This is a great time to use up all those odd bits that have been accumulating in the plastic bag of offcuts in the garage. If you are worried about point loads from the plate acting on the foil or are planing to use a T-Foil rudder, then glue a piece of 8mm foam onto this, and then cover the other side of the foam in a couple more layers of carbon cloth the thickness will help the bottom plate resist the upward forces from a T-Foil. Andy Paterson finds this unnecessary for a standard rudder. Anyway, either way we now have a substantial black plate. Cut a hole in this so that it fits over the sleeve at 90 degrees to the foil with (of course) equal overlap. You may wish to have the plate angling up about 15 degrees fore and aft to clear the stern wave, but this depends a lot on your rudder gantry. Between the two you probably want the aft end of the plate about 100mm above static waterline. Make sure that the sleeve fits freely - its very easy to squash it so that the blade won't go through. When you are really happy with the fit (check three times!) then glue the bracket onto the sleeve. Use a nice strong filler mix - there will be load on this so use some microfibres and silica along with the light stuff. I suggest you wrap the foil in plastic again and put it through the sleeve to make sure it still slides freely while the glue is setting. Give a nice generous fillet with your favourite light filler between sleeve and plate, and put a layer of glass over the fillet - it all helps distribute the load evenly onto the blade.

The Tiller

There are a lot of ways of doing this. I suppose the best, especially if you happen to have a handy bit of carbon tube lying around, is to make the tiller from a piece of tapered carbon tube, cut a slot for the sleeve, and glue it in a similar way to the plate. You need to be quite careful about the tube you pick, especially about the resulting thickness at the leading edge of the blade where a lot of tube is cut away. I guess a lot depends on how much extra carbon you put on. My normal method is to make a rectangular section core - on the last one I laminated up several layers of foam, ending up with a core about 50mm deep and 75mm wide at the pin, tapering to about 20mm by 75mm at the trailing edge, and 50mm by 25mm at the extension. This gives a shape something like this. Don't forget to put some high density foam or a wooden pad in the tip where you will be bolting on the tiller extension. Andy does it slightly differently. He uses exclusively HD foam and makes it 20mm thick tapering to 10mm and 60mm wide, tapering to 10mm, with an oval section rather than rectangular. Opinion varies greatly how long the tiller should be. Basically the shorter it is the more room you (or your helmsman) will have round the knees when hanging desperately off the back corner on a nasty pitchpoling 2 sail reach, but the greater the loads will be. I think my stock on Halo Jones is about the shortest, 540mm from pivot to tiller extension, and most are probably nearer 700mm than 600.

Now its time to get the fibres on. My memory is very vague about what I used. I *Think* I used a layer of 200 g carbon cloth, aligned 45/45 on all four faces, plus two layers of unidirectional carbon on the sides, plus another couple of layers of carbon cloth top and bottom around the pin/leading edge area where the big loads are. Finally I gave the whole thing a couple of layers of 200g glass top and bottom, and overlapping the sides to keep all the carbon in place. As ever compress the layup - for this a Workmate and parcel tape is helpful. Andy's recommended method is to laminate 1 layer + 1/2 layer uni-directional carbon (300g/m²) tape x 100mm , plus 210g/m² glass (spiral wrap of 50mm wide strip). This spiral wrap seems to work well and squeezes the excess resin out and holds the carbon down). Anyway, once you have done that you can glue the tiller onto the sleeve. Basically the same technique, but this is your last chance to check that you have the distance between the two pieces correct, so measure lots of times. I tend to have a small fillet at the top, so the sleeve extends slightly above the tiller, the idea being to distribute the load from the top skin.

Pivot

Finally its time to put the pivot in. Ideally you will have some pultruded glass tube exactly the right size for the pin. Believe it or not this is available! If not then make your own by wrapping cloth round a very well waxed pin, but you'll probably need to ream it out after the layup has shrunk on curing. Either way drill through your tiller and bottom late with a drill big enough for the glass tube. If the bottom plate is solid, not partially cored, then you don't need the bottom tube, just make the hole big enough for the pin. There are spectacular loads coming through the pin, and it needs to be very strong. I dig out the foam core next to the skin top and bottom of the hole and create a void that can be filled with filler. If the core is high density foam then this is unnecessary. Glue in the tube., again with a very strong fibre and silica rich filler. It is essential to have the pin in while its setting to keep the top and bottom lined up, and equally essential to be very careful to make sure the pin is exactly in the middle and exactly parallel to the rudder blade. When the pivot tube has set put some more carbon round it to make sure that the loads are thoroughly transmitted to the skin, not the core. Get a good bit of reinforcement just at this point, because its a real trouble area. The bottom plate can be trimmed to a more streamlined shape if you like, and certainly round off the corners. You will probably need to shape it rather more round the pin to make sure it doesn't hit anything. Have a good look at the “stop points” on maximum steering. You don't want the sleeve to get bashed if you jam the tiller hard over. Make sure that something substantial takes the thumps when you jerk the tiller hard towards you in a desperate attempt to bear away. If necessary build a couple of carbon stops. Finally fill, fair and paint as you deem necessary, then bolt or screw on the tiller extension. Now you can go sailing again!

Based on original by Jim Champ July 1999 (with considerable input from Andy Paterson).


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