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Technical Manual

Basic tuning

At first sight the apparent simplicity of the single sailed una rig gives the impression that there is little involved in tuning it. However, the lack of a foresail and shrouds creates almost as many problems as appear with the combination of two sails.

The basic aerodynamic difference concerns the positions on the sail at which the attached airflow over the leeward side breaks away. Without the benefit of a jib to redirect the flow along the leeward side, the 'break point' occurs relatively early. Coupled with this is the fact that without a foresail the airflow is not diverted up stream of the mainsail and the angle which the apparent wind makes with the centreline of the boat is therefore greater.

To accommodate these airflow characteristics, O.K. sails are cut with the maximum draft of the sail positioned at approximately 50% of the chord. This helps the laminar flow to adhere to the leeward side for a greater proportion of the chord length. However, this does cause the resultant force generated by the sail to be angled back and consequently by the drag induced by leeway is increased. The solution is to set the sail at the greater angle to the centreline of the boat by setting the traveller well to leeward. This increases the rig's forward driving component to an efficient level.

This fact establishes the first basic rule for sail setting in the O.K., that is, never stall the boat by sheeting the sail too close to the centreline.

The approximate position for the clew can also be defined by the fact that in a moderate to fresh breeze the boom end should be level with the deck. In most cases the mainsheet system will be 'two blocked' at this point. The most common explanation for this setting is that with the boom set as low as possible, the deck has the maximum 'end plate' effect on the sail, that is, the deck prevents excessive pressure loss under the boom. However, I have yet to see conclusive proof of this effect and the reason may well have as much to do with the adjustment to the rake of the mast. Suffice to say that in practice the method works and therefore deserves to be adhered to.

We have already seen that the position of the clew is quite rigidly defined for windward work. The attitude of the rig can then be set to suit the particular crew weight by raking the mast either forward in order to tighten the leech (to gain power), or aft in order to lose power. The range of adjustment here is quite small since when raking the mast aft a point is reached where the leech does not stand well enough to retain an acceptable pointing ability. Too much forward rake means that creases appear diagonally across the sail when the boom is sheeted hard down.

Having established the correct rake the only remaining 'coarse' adjustment is to locate the entire rig in its optimum position fore and aft. Theoretically the rig should be moved well forward in order to reduce weather helm, however, in practice this does not appear to be a crucial setting. A good starting point is to position the forward edge of the mast deck bearing ring 70-72cm from the inter section of the deck and bow.

Fine Tuning

Once the rig has been set up in the boat using these basic 'coarse' adjustments, all that remains is for the helmsman to use 'fine' adjustments to alter the basic sailing shape designed by the sailmaker, to suit the prevailing conditions. The controls available are the cunningham, kicker, traveller, and mainsheet. In addition, some sails are designed to make use of an adjustable outhaul and even inhaul.

In light airs the sail should be flattened as much as possible, although this is difficult to achieve without creating undue leech tension. In practice a compromise is reached, which gives a sail with the flow well forward and a very slightly hooked leech. The kicker should be used to apply the necessary leech tension. The mainsheet should not add any leech tension, and the traveller should be set to take account of the sea condition. The flatter the water, the further in the sail may be trimmed, but remember not to stall the rig. The cunningham should not be needed at all.

In moderate wind speed maximum power is required from the rig. The mainsheet should be almost 'two-blocked' so that no power is lost through the leech falling off. The outhaul can be used to give extra camber (where the sail design allows for this adjustment). The cunningham can be tensioned too as to take the wrinkles out of the sail. As ever, the position of the traveller is governed by the wave conditions, generally speaking it should be well out board, never more than 300mm from the side-deck. Off the wind, the kicker should be used to keep the leech as tight as possible without stalling out. (A tell-tale on the top batten is a useful indicator.)

As the breeze rises, the rig should be depowered so that the helmsman's weight can hold the boat upright. The clew should be on the black band. The mainsheet should be 'two-blocked' so that the mast bend is taking as much luff curve out of the sail as possible, thus flattening it. The cunningham should be hard down both to stop the draft being blown aft and to open out the top of the leech.

The traveller should be let right off. Downwind the kicker should be eased to allow the boat to be sailed flat. Also it is useful to ease the kicker so that the boom end does not catch in any waves. Easing the kicker can cause problems when running since the lack of leech tension allows the top of the sail to fall forward of the mast and apply a force tending to push the mast to windward. Without the benefit of a jib and spinnaker to balance the rig downwind, the O.K. is intrinsically unstable. Once the leech is allowed forward of the mast, control becomes very difficult and invariably results in a capsize.

Since the boat is sailed to windward with the mast bent and the boom on the deck, it is not advisable to lose power in a gust by easing the mainsheet. The traveller can be used up to a point, but in practice the most effective means of spilling wind is to have the rig 'work' automatically. This involves combining a mast with suitable bend characteristics with a matching sail. The secret is to find a mast which will provide maximum power in a moderate breeze, but which can be bent further to flatten the sail for stronger winds. The mast should be fairly stiff from heel to just above the gooseneck. This ensures that sufficient leech tension can be obtained. Over the full length of the sail the mast should bend in a fairly even arc fore and aft allowing for efficient sail shape control. As far as athwarthships bend is concerned, the top section of the mast should bend to fall off in the gust although care must be taken to ensure that the mast stands upright over the bottom section so the pointing ability is not impaired. Popular theory suggests that lightweight sailors should use masts which bend more than those used by heavy weights. However, it has been found that this is not as critical as was once thought. A well cut sail should be adaptable enough to be suitable for most crew weights.

In even higher wind strengths the boat must simply be feathered through the gust upwind and the mast left to do its job. In particularly choppy conditions it does sometimes pay to ease the mainsheet and twist the sail off. This allows the boat to power through the waves on a lower heading.

At present, competitive O.K. masts and sails are available from a variety of manufacturers. Rigs should be chosen for both durability and suitability to individual sailing technique. After that, it's up to the guy on the helm!


These are very old articles, taken from the British association's original technical leaflet. They date from the 1960's, but contain some useful information - particularly for those sailing with wooden masts!

The 70-72cm measurement for the deck bearing ring is too great for a metal mast. 65cm is closer to current usage.

Oddly enough, the comments about the stiff/soft mast in the third paragraph from the end are echoed in the current, summer 1998, edition of 'Finnfare'.