Mariner’s Web

The whole of the sail-plan is greater than the sum of it’s parts

This is a variation on a well known phrase as it applies to fore and aft sails and sloop rigs. The goal of an accomplished sail trimmer is this greater sum. The interaction of the headsail and mainsail trim in combination can result in a significant improvement in the driving force of the total sailplan.  The force may be further improved with another foresail and the cutter rig with the caveat that you will eventual reach a point of diminishing returns. The effectiveness of a cutter rig is dependent upon many factors such as sail and rig design and apparent wind direction and velocity.

There are some general sail trim considerations when two or more sails are interacting in the same flowfield:

• Circulation of Air Flow – Each sail has its own circulation with a headsail slowing the flow on the mainsail in the area known as the “slot”.
• Slot – The area of overlap (or near overlap) between the headsail and mainsail.  The art of sail trim requires trimming all sails for the appropriate shape and position for sail interaction in the slot.
• Angle of Attack – The mainsail will experience downwash (a heading wind) from the headsail, resulting in the ability to trim it to a higher angle of attack with less chance of stalling or flow separation.  This is good news for the main sail which is the key to balancing the helm under varying conditions.
• Twist -  In general each sail will be trimmed with the same degree of twist as indicated by the leech profile.  This may require several adjustments such as sheet lead position, sheet tension, traveler position, etc.
• Backwinding -  Downwash may be seen as backwinding on the mainsail luff and can be used as a trim indicator.  Some degree of backwinding is acceptable for medium to heavy air and it should be evenly distributed up the sail.  In heavy air it will become more pronounced as you begin depowering and flying the main “by the battens”.
• Backstay tension – An increase in tension will increase mast rake which results in: less headsail sag and draft, less draft in the mainsail, and in general a flatter sail plan.

Good sail trimmers are good communicators, fine tuning each control (avoiding the urge to cleat), monitoring sail angle, depth, draft position,  and sail and rig tension to provide for the proper shape, twist, and telltale flow.  But the gifted sail trimmer will strive for the “greater whole“.

Flowfield and wind speed around a Sloop Rig