Tuning Trailer Yachts Part 2
Adjustments generally available for mainsail tuning are mansheet, boom vang, luff cunningham, outhaul and traveller (Figure 6).
Because of the unsupported top- mast section, mainsheet tension is critical. It is important to put a mark on the mainsheet at what you consider to be about the right tension for moderate airs, then work around that mark. A little more tension will be needed in fresh airs, a lot less (sometimes up to 600mm less) in very light airs.
As the mainsail of a trailer yacht is usually of fairly low aspect ratio it is easy to overtrim the mainsheet, tightening the leech and reducing boatspeed. A good way of assessing this is to sight up the sail from underneath the mainboom to the top batten and make sure this is not above a line parallel with the boom (Figure 7) - this, of course, being not too easy to do in fresh airs! In the very light and fresh, the top batten should be falling off this line between 10 and 20 degrees.
Leech tell-tales are generally placed on the main at each end of the batten pockets. These consist of strips of light nylon spinnaker cloth approximately 25mm wide x 230mm long (Figure 8). They are a good indicator of mainsheet tension as if one or more are not flowing (except in very light airs), too much tension is indicated.
Here also boom vang tension is applicable in light airs as it is not much use having the mainsheet well eased if the vang is too tight. In the very light, 25mm on the boom vang can be critical.
The most important (and difficult) tell-tale to set is at the second batten down. This and the top one are the first affected by too much leech tension, whether from mainsheet or boom vang. However, the second is also usually the intersection of the jib with the mast and the amount of twist in the upper part of the jib can also affect it. If the others are flowing and this is not, try easing the jib sheet or shifting the lead aft to twist the upper part of the jib a little more.
A wool tell-tale is attached halfway across the main just above or below the top batten and by a combination of this and the respective leech ribbon it is easy to tell if the airflow is stalled either on the wind or reaching under main and jib. Adjustment is dictated by mainsheet and vang tension, luff tension and traveller position (Figure 9)
It is important to have a stiff bottom batten in the main. If it wasn't for the weight, I would pour concrete into this pocket... well, perhaps not quite, but that is the degree of stiffness needed. Once when about to sail on a friend's boat and discovering a flexible bottom batten, I managed to find a reasonably straight tree branch which I taped to the batten, preferring the bulk to a soft batten. This helps give a clean exit to the bottom part of the leech. To help the top of the main twist off, the top batten wants to be soft at the for\'ard end and stiff at the aft end.
On most trailer yachts these are through-battens, but the same applies... in fact to an even greater degree. If the batten is shaped as an even curve, when the mast bends the draft becomes too far aft, which is why the batten must be shaped with the major bend in the first third. The second batten can be quite soft at the for'ard end but should be reasonably stiff at the aft end. This batten doesn't take much loading. The third batten should be stiff throughout its length, with the for'ard end tapered to give a smooth transition from sail to batten (Figure 10)
A wire strop and block and tackle is generally used but it is worthwhile to have the tackle part as short as possible to keep stretch to a minimum . (Figure 11) If there is room available and it does not clutter the top unnecessarily , the tackle can be double-ended so that adjustment can be made from either side of the boat. This can be particularly useful spinnaker reaching in the fresh when a quick ease of the vang can prevent a broach.
The boom vang should be tight enough to keep the leech in a fairly straight line off the wind. However, it pays not to be too tight in the light so as to give lift to the sail and allow slow moving air to exhaust off the leech. In a fresher breeze the flexing of the mast and weight of wind will allow this in any case and so it can be tight. If the vang is left on when rounding the leeward mark, particularly in a dying breeze, be careful that it does not end up too tight, thus tightening the leech of the main more than required.