Mainsail Design Notes

Mainsails are straight forward in their construction. Quoting is simply based on the vessels P and E design specifications.


We normally quick quote sails for expected wind speeds of 12 to 20 knots. Cloth weight is very important to overall sail performance. Racers generally prefer light weight cloth and Charter Boats generally use heavier cloth to compensate for inexperienced sailors. Too heavy a cloth will sacrifice boat speed and windward performance. If you sail in winds 15 to 25 knots you should go up an ounce. Winds greater than that, go up 2 ounces. Please note that this is based on average sailing, not just a windy day.

Both the luff and the foot of the sail need to be taught to produce great sail performance. A sail that is 2 inches too long will sail poorly. A sail that is 2 inches too short will sail perfectly. Also, be wary of trying for too much luff. Remember that the mainsail has a headboard with a width of about 8 inches. You don't want that headboard interfering with the backstay. Some vessels have black bands to specify how high the designer wants the maximum luff. This is particularly true with wood masts that often have the backstay attach directly to the mast as opposed to a T-shaped mast head fitting on aluminum spars. Normally, we consider maxluff to be 8" below the halyard sheave.

Normally a sail will stretch about 1.5 percent over the useful life of the sail. Thus, if you stated that maximum luff was 40 feet (480 inches) you could assume the sail would stretch 7.2 inches over it's life. We would normally design a sail that was 39' 5"  for the max luff of 40 feet. Measurements are always sail edge to sail edge. Never attempt to use pin to pin.

On a mainsail, keeping the luff and foot taught, requires a correctly located tack pin on the sail. That pin is specified by the M1 and M2 measurement on the measurement form. That position is to the center of the tack pin. Sailmakers often characterize that position as tack cut-back and tack cut-up. Please pull this measurement to insure peak sail performance. This measurement is never standard and varies from vessel to vessel in the same model year. It is the easiest of all measurements to take. It is to the "center of the pin". If the A measurement is too great, the sail will require a "jack line". The Jack Line readjusts the luff and foot sail loads. Normally, sails with more than about 5" on the A measurement require a Jack Line.

The distance from the mast to the backstay tells the sailmaker how to design the roach for your sail. Always include that measurement so that your sail can have optimum roach. The backstay measurement is taken at boom level from the aft face of the mast (not the end of the boom) to the backstay.

Normally, a mast has some rake to it. The tack angle refers to the angle created at the junction of the mast and boom. If a mast had 1 degree of rake, and the sailor wanted the boom parallel to the waterline, the mainsail would need a tack angle of 89 degrees. If the mast had 2 degrees of rake, the tack angle might be 88 degrees. However, as the vessel sails, the main fills with wind and the boom rises a bit, thus when sailing, an 89 degree tack angle might be more like 87.

Vessels that have dodgers often require a reduced tack angle, such as 86 degrees. If you have a dodger to clear, the best way to be insure your sail is correct is to pull a leech measurement with the boom in the lowest safe position to clear the dodger. The measurement is from the fully hoisted sail shackle position to the boom at the outside edge of the sail just past the clew, at boom level, while the boom is in its lowest safe position. If not specified, we would design a sail with 88.5 degrees of tack angle. Most mainsails have tack angles of 88.5 degrees.

Reef positions are normally positioned by the sail design software. If you wish to specify a reef height, that is normally specified as a height above the boom.

Most common problems with mains are too long a luff or foot and/or poor tack pin measurements. Always pull your luff measurement with the sail shackle on the halyard.

When we supply the computer drawing of your sail you will have an opportunity to double check the luff and foot and adjust if necessary. Every properly designed sail must go through this basic process. It is these measurements that generate that extra knot you want.

We normally supply luff and foot slides and slugs for no charge. If you wish free slides and slugs, you must specify the ones you wish.

If you have questions, feel free to email us at sails@fareastsails.com