MAINSAIL TRIM: Make the Most of your Mainsail Controls
3/17/2008

Use these rules of thumb for mainsail trim as a starting point, then experiment with small changes to see what works best for your boat.

MAINSHEET & TRAVELER

The mainsheet and traveler are the two main controls that help you trim the mainsail upwind to maximum efficiency.  It is very important to keep the boom on the centerline of the boat and to keep the top batten parallel to the boom.  Check to see if the leech telltale on the top batten is flowing.  If it’s not, you need to twist the sail off more by easing the mainsheet and pulling the traveler further to windward.

One thing to remember is that more leech tension closes the leech, thus allowing the boat to point higher.  However, if you trim past the point of stall – where the top telltale stops flying – the boat will start to slow down.  Therefore, keep an eye on that top telltale to make sure you are not overtrimmed.

As the boat begins to be overpowered, the traveler should be eased down to reduce weather helm and keep the boat at less than 25 degrees of heel.  As the traveler is eased, you will develop backwind.  This is not a problem.  In puffy wind, the traveler should be played aggressively.  Choppy water usually requires a little more twist in the leech than flat water, so you should pull the traveler higher and ease the sheet. 

While sailing upwind in underpowered conditions, the outhaul should be adjusted to keep the sail’s foot shelf half-open.  When all crew members are on the weather rail, the shelf should be closed.  If you have to ease the traveler down frequently in the puffs, you should put in a flattening reef (if possible).  The flattener is a ring in the leech just above the clew. It is independent of the cunningham and will help remove a great deal of fullness from the bottom quarter of the sail.  Initially, do not fully tension the reef, but rather increase the tension as more flattening is required.  Off the wind, both the outhaul and the flattener should be eased to fully open the foot shelf.

CUNNINGHAM & HALYARD

The cunningham and main halyard adjust the luff tension which affects the position of draft – the maximum fullness -- in your main.  As a general rule, the draft should be 50% aft from the luff. In underpowered conditions, you should have horizontal wrinkles on the luff to allow the draft to stay in its designed position.  As the wind increases, the draft will move aft due to mastbend and cloth stretch, so you need to add luff tension to hold it at 50%.  As you get overpowered, you should just barely remove the wrinkles.  As the wind continues to increase, add cunningham to keep the draft at around 50%. In choppy water, the draft should be a bit further forward (40-45%) for better acceleration. In very flat water and good breeze, the draft can be allowed to slide aft to 60%. Off the wind, be sure to ease the cunningham right off.

BACKSTAY

Whatever your rig configuration, you need to make your main flatter as the wind increases. Choppier water requires a bit fuller sail for a given wind strength. You should also mark all adjustments so you can consistently reproduce fast settings. Off the wind, the permanent backstay should be eased right off.  The only exception to this is while you are power reaching – use about half of the full backstay tension to depower the sail and support the rig.

In traditional masthead rigs with no running backstays, the permanent backstay controls both mastbend and headstay sag. Mastbend is the primary adjuster of mainsail fullness.  As wind increases, so should backstay tension. We strongly recommend that you have a powerful and easy way to adjust the backstay.

For a masthead rig with running backstays, these ‘runners’ are used to control mastbend and also have an effect on headstay sag.  At a given permanent backstay tension, more "runner" will straighten the mast (making the main fuller) and remove headstay sag.

On a fractional rig, the permanent backstay controls mastbend, while the runner (attached at the hounds) controls the headstay sag. A fractional rig may also have a checkstay which controls lower mastbend similar to the way runners on a masthead rig work.

BOOM VANG

If you have a powerful vang, you can use it to induce lower mastbend if you want to flatten the main entry low down. In general, this is not necessary on most big boats.  Off the wind, the best rule is to keep the top batten parallel to the boom. When close or beam reaching, you can also watch the top telltale to make sure it continues to flow.  On a power reach when you are on the edge of a broach, keep a hand on the vang to dump it off if the rudder starts to feel like it is stalling.  And downwind in big waves and wind, you can help stop oscillation by "overvanging" and hooking the leech.

For more detailed information on trimming your Ullman main, contact your Ullman loft.