από Geoff Moore
The J/24 Mainsail is the easiest sail to trim because it is so visible. Just being visible, however, doesn't mean it trims itself. Shroud tension, mast step position, and backstay have to be adjusted properly to insure you get maximum performance in all conditions.
Lets break it down into two parts. First there are those controls that affect the mainsail through mast bend. Second there are those controls that affect only the flexible membrane itself.
The ability of the mast to bend allows us to change the overall depth of the main. The more bend the flatter the sail gets until we reach a point where the bend is more than the luff curve of the sail can absorb. At that point the
mainsail starts to develop diagonal wrinkles and we say the sail is "turned inside out", or "overbent".
Depth in a mainsail can be incrementally reduced by pulling the backstay. The point where the sail turns inside out is a great visual aide to maximum mast bend. Any bend beyond that point is detrimental to the overall
performance. That means you stop pulling on the backstay, and/or the lower shrouds need more tension.
But what about lighter air?
Since the mast can never be held perfectly straight in heavy air, the luff of the mainsail has to have a certain amount of positive curve to it. So the exact opposite to "overbending" happens in light air. The luff curve in the
sail has too much curve in it relative to the mast, or the mast is too straight. This is much harder to detect than an inside out wrinkle, but it is every bit as disastrous to performance. The visual indicator of this problem is simply
a mainsail with too much draft too far forward. The way we get around this in light air is to "prebend" the mast.
All the tuning guides will tell you where to put your mast butt, and how tight to tension your shrouds. These are the critical factors in setting up your prebend. Tuning guides are great reference tools, but they do have serious limitations. They don't take into account any minor variations in your boat, or any changes in your sails as they age. Having a mental image of what looks right is very important.
Knowing when and how much backstay to pull on is also difficult to describe. This is because there are very few visual clews. In practice most
racing is done with backstay all the way off, or all the way on. When conditions warrant backstay contro, the clews are usually rudder pressure and heel.
Cunningham affects draft position. A tighter cunningham pulls the draft forward. Mast bend moves the draft aft. Therefore, more cunningham is needed to pull it back into position. Likewise, as sails age the draft moves aft so more cunningham will be needed. Most sailors don't recognize that bolt ropes shrink much faster than the sail. So it is important to measure cunningham control by visually checking draft placement rather than by
how hard you pull on it. As the mainsail approaches the point of "turning inside out" increased cunningham will smooth out the sail so that you can use even more backstay.
The outhaul affects the lower third of the sail. It is important because it adjusts the exit angle of the lower batten. If it is eased too much the bottom part of the main will stall, and you will not be able to keep up in light air.
Conversely, if the bottom batten is too open the J/24 with its short headstay is very hard to keep balanced. The boat sails better with as much power aft in the boat as possible. The best place to view this is standing to windward of the mast looking aft while someone eases and trims the
outhaul. Here again, and older sail will need more outhaul.
The vang, traveler and mainsheet all work together to control twist. Adjusting twist is the fastest way the skipper can affect the balance, or feel of the boat. Less twist should result in more rudder pressure. In light air this is very difficult to detect. In heavy air the traveler is not long enough or easily adjusted enough, so we switch to vang sheeting. Vang sheeting sets a certain twist by holding the boom down hard while the main is adjusted
to keep the boat balanced. Lots of vang also helps by bending the lower part of the mast, therefore opening the slot and decreasing backwind. If there is ever a doubt over whether to vang sheet or traveler sheet, remember a foil with absolutely no twist is more efficient than a twisted foil. A twisted foil is more forgiving, however. (you don't see too many twisted aircraft wings, or even keels for that matter)
In general a sailor coming from another class will find that the J/24 mainsail can be sheeted tighter than they expect.
**You can tell if your boom is centered by looking at the part of you mainsheet leaving the ratchet block. When it is straight up, the boom is centered. You can see it from the rail.
***The best way to check your top batten angle is to compare it to the mast head crane. Since the crane is always pointed straight back you don't have get under the boom to sight it.
* Geoff Moore *
* Shore Sails Ltd *
* 7 Merton Road *
* Newport, RI 02840 *
* 401-849-7700 *
* fax 401-849-7952 *
* firstname.lastname@example.org *