How it works
A Steersman platform is fitted to each side of the cockpit, on the coamings. They each have a lock which either keeps the platform fixed, or allows it to swing. Each jib sheet winch is either mounted on its platform, or immediately in front of it. When setting the jib for a particular point of sail, the jib sheet is tensioned up as normal on the appropriate winch, with the platform in the locked position; the spring is tensioned up using a jamb-cleat which is also mounted on each platform, and the lock is then released. The spring is then fine-tuned, again using the jamb-cleat, until a steady course is achieved.
SPRING TENSION VARIATION
For different points of sailing, the wind pressure in the sails change dramatically, from a high pressure when sailing close hauled to a low pressure on a broad reach or running. The shock cord spring works with this large variation in the following way.
The spring consists of a length of shock cord fed through a double block at the stern end (usually attached to the pushpit), and a single block at the forward end which together with the ‘cord-fixed-end’, and the ‘cord-free-end’ are attached to the winch platform. The ‘cord-free-end’ is fed through the jam cleat to adjust the tension. This means that there are 4 shock cords which make up the spring. To reduce the spring tension, the single block and ‘cord fixed-end’ can be moved to a ‘keeper eye’ fitted in the base of the platform. This then provides a variation in spring size from four cords down to one. For lighter winds, this whole arrangement is abandoned, and a single lighter shock cord is fitted in its place.
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Up wind sailing
When sailing up wind if the boat luffs up, the tell-tails change showing that the pressure in the sails has dropped, the jib sheet slackens slightly, causing the shock cord spring to pull the winch platform aft, steering the boat downwind, until she is back on course. Conversely, if the boat veers downwind, then the pressure in the sails increases, again shown by the tell-tails, the tension in the jib sheet increases pulling the platform forward, steering the boat up back up wind and again, back on course.
With the wind on the quarter, the jib is slightly masked by the mainsail. This has the effect of reversing the rules from up wind sailing. Here, if the boat veers downwind, the wind pressure in the jib drops, and if the boat luffs up, then the wind pressure in the jib increases. Consequently, for tiller steered boats the jib sheet must now be fed through a block at the stern end of the boat and then led forward to the winch, and the shock cord spring is fitted to the forward side of the winch platform.
For wheel steered boats, the steering pole is simply moved from the lower half of the wheel to the upper half.
There is also transition band somewhere down wind of beam reach when either set up can be used.
Wind Dead Astern
For goose-wing sailing, the system reverts back to normal – jib sheet fed direct to the winch, and the spring fitted to the aft end of the winch platform. However, if the coarse deviates more than about 10 degrees, the forces reverse, and can cause an uncontrolled gybe. (NOT RECOMMENDED)