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Technical Manual

Thoughts On The Renovation Of An OK

by David Boyes, Don Andrews and Richard Jaques

The second hand market always offers many inexpensive boats. This is clearly one reason why many younger people find the class attractive.

This article is intended as an aid to those of you who have bought a cheap second hand O.K. (say for £200 or less) and want to increase its competitiveness. It should be possible to do this quite cheaply provided you are prepared to do the work yourself.

The hull shape of the O.K. is clearly defined by the class rules and therefore the boats' performance will be only marginally affected by hull shape. Any significant lack of speed will almost certainly be largely due to an outdated rig. That is not to say that your hull won't benefit from some attention : clearly it will.

1. The Rig

This is probably the most critical factor. Wooden rigs are no longer competitive, particularly on off-wind legs. A metal mast and sail to fit will cost you about £350 new: they can be obtained secondhand from the used boat list for upwards of £100. It is probably true to say that any metal rig is better than any wooden rig for most people in most conditions. - The moral of the story is to look for a cheap secondhand boat with a metal rig if you possibly can.

Usually a secondhand rig does not include the boom. There is nothing wrong with using a wooden one. It will need metal reinforcement plates at the pin, and any control lines will have to be external, but they will be none the worse for that.

Do make sure that the take-off points for the mainsheet permit the sheet to pull down vertically when the traveller is as far out as it can go. This means the two take-off points are about 3 inches apart. On many old rigs they went up to l2 inches or even l5 inches for some reason.

As for controls, keep them strong and simple. If in doubt, do without. Dick Batt used to go like a train with minimum controls, a tradition still maintained by Denmark's Stig Westergaard, D1234. Probably the simplest and most effective kicker system is the lever illustrated: it won't strain your intellect to work out how to make one out of aluminium alloy and stainless bolts. at very little cost. Incidentally, overdeck controls work just as well as the more fashionable underdeck ones: see the NcKrill/Gore article for a simple and effective system, though it recommends a kicker winch in place of the lever.

Older boats had their masts stepped well aft, because of the flexibility of wooden spars. The simplest way to accommodate a more forward position, say 700 mm aft of the stem, is to rig the mast more upright. Really all you are trying to achieve is a neutral helm neither weather nor lee and this is influenced not only by mast position, but also by the cut of the sail. Sail upwind, sat out flat, with the rig adjusted so that the boat sails in a straight line with the tiller down the centre line. If you have to pull the tiller towards you, move the mast forward, or the reverse if you have to push it away from you.

You may have some difficulty with getting the 270mm bottom band measurement right with a metal rig in a boat with an upper mast step bolted above the deck. Don't forget that all that is required is that the top of the band is 265-275mm above the deck (not the step), and that the top of the boom does not come below that line. It can come above it, and the performance loss would be minimal. The top mast band comes 5400mm above the bottom one, and so leaves stacks of room for all the sails I've ever measured.

Ideally, you may wish to relocate both mast steps. This should locate the heel of the mast about 3300-3320mm forward of the transom, though Joe Whitwell recommended 3430mm in one of his articles.

It is possible to put in a new bottom step through the hole you cut for the flush deck step mounting, but it is a bit like decorating the hall through the letter box. The job is a little easier if you cut a circle for a hatch cover near the top step so that you can put left and right hands through separate holes.

I'm told the force exerted on the bottom step can be up to 0,5 Ton (this is in F5, steady state), so make sure it's solid. The forward edge of the mast ring at the top step is ideally about 680-700mm aft of the stem.

Most older boats will have a deck bearing bolted on to the top of the deck. It is far neater if a flush fitting bearing can be fitted here. Although this is not essential, metal masts are now fitted with a top bearing and gooseneck to suit a flush fitting deck bearing. Shockcord may be used to prevent the varying sized chocks from jumping out, but backing plates should be bolted to the top bearing and pass under the deck beams to prevent it from lifting.

2. Weight

You can only expect to make marginal gains here. Stripping and re painting will help a bit, but it requires a lot of effort for rather little effect except to appearance and perhaps skin friction. You can save weight on ancillaries, though. Richard's alloy tiller is a nice example. Fittings are lighter now than they were twenty years ago, and can be replaced over a period of time.

Probably the most dramatic gains accrue from installing lightweight foils. 18mm plywood centreboards weigh about 11lbs. One laminated from Western Red Cedar and cased in glass cloth and WEST weighs about 6-7lbs: a marked increase in buoyancy in the middle of the boat. Similar benefit can be obtained by making up a fixed rudder in the same construction as the centreboard, and in both cases you can up-date the aerofoil shape at the same time.

Don't forget the slot in the centreboard makes installation much easier than the hole found on the older boards. (Believe me, you'll only want to remove and replace that bolt once.)

Only have a lifting rudder if it is necessary for local conditions as it can never be as strong or as light as a fixed blade. In order to counteract the forces on the pintles, bolt them as far apart vertically as possible. A watertight hatch in the transom allows regular inspection.

Replacing the foils is not all that cheap. New, from a well known, supplier, you are into the £250-300 mark. But even home made (see John Bottomley's article of Spring 1982) you are into £70-80 for materials and fittings. Expensive, but probably worth the effort, and easier to make than you would think.

3. The Hull

The easiest way to improve the hull is to spend a pound or two on some padding for the side decks. Probably the insulating mat (e.g. Karrimat) sold in camping shops for putting under a sleeping bag is the most easily obtainable, and one mat provides ample material. Cut it in bits, stick it with contact adhesive, and cover it with sailcloth screwed under the carlin and gunwale. Alternatively, use the method described in the Gore/McKrill article, only slightly more complex. The padding, not compressed, can rise to 40mm above the sheerline, and the side decks can be up to 240mm wide. See the rules for exact dimensions: you may not exceed those on the diagrams, except that you can have l0mm of uncompressed padding over the sheerline.

You can also add to the comfort by introducing an inner carlin: early boats were usually built without. Don't make it too steep. About 45' seems about right. Then set your toe straps to suit your build. Again, see Gore/McKrill for details.

The maximum depth for the carlin is 80mm below the sheerline, all of which helps to reduce the intake of water in to the cockpit when you roll tack, in addition to minimising the load on the calves. A possible construction plan is included.

If your hull is well overweight, or if a lot of work needs doing on it you might consider re-decking. In the past people have managed to reduce overall hull weight considerably by carrying out this task. It gives an opportunity for more thorough drying cut of the hull and of course the replacement of old wood by new is bound make a difference. Whilst the deck is off you may notice other areas where weight could be reduced. Perhaps the original builders were too generous with wood for the deck beams, in-whales, gunwhales and so on.

However, this job does have its drawbacks. It is fairly costly, running into £70-80 for materials and fittings. Removing the old deck may damage the hull, as it is probably fixed with glue and ring barb nails. It is probably worth the effort, but do not think you can do it in a weekend.

Finally, check out all your fittings. Are they strong enough, light enough, and securely enough fixed, preferably with bolts rather than screws? Does everything work? If not, do something about it, and your investment of time and money will be repaid in trouble free competitive sailing.



The prices quoted applied in Britain in the early 1980's.
The comments about the superiority of metal masts over wood are probably justified in terms of competitiveness, but some enthusiasts like wood, and good luck to them. Don't expect a sail to be transferable from wood to metal, or the reverse. These days, of course, experiments are permitted under certain circumstances with carbon fibre, the very latest technology.

The kicker or vang drawn in this article is now pretty well universal. From my researches, I believe it originated in the USA in the 1970's.

The recommended distance from the stem to the front of the mast ring is now nearer 650mm than 700mm.

There are as many ideas about side decks as there are OK sailors. Generally, the comfort of wide side decks is preferred, but I understand some sailors use quite narrow ones, and reduce the discomfort by using stiffened hiking shorts. They think they can sit out further.