M-20 Scow Tuning Guide

Knowledge is power. We see this in every sport throughout the world. Racing sailboats is much different from the other sporting events. Sailing requires tuning for different wind and water conditions. Many of these tuning adjustments are very small, yet critical.

We have outlined for your information tuning information for many different boats that we race on a regular basis. The measurements achieved have been tested through countless hours on the water in a variety of conditions. What is truly unique with this booklet is the fact that we have simplified the tuning process for all of these classes in order to make the process easy for our customers. You will be able to achieve newfound speed in your class. These measurements coupled with the fastest one-design sails in the world will give you the knowledge for speed. In sailboat racing this is a combination for power and speed!

North Sails will continue to bring you the very best in sailing service and technology. Additional knowledge can be learned from the North Smart Book. Our objective is to allow you to set the pace in your racing class.

Mast Rake

In order to set proper shroud tension, the order in which to proceed is as follows:

  1. Attach a measuring tape to the main halyard and hoist to the peak (the upper black band) and lock the halyard ball into the latch at this point. Measure to the intersection of the deck at the transom 28′-4″. Make the shrouds snug when the mast is raked in this manner.
  2. In light and medium winds set the rake using the jib halyard so that the rake measured in the same manner as in number one is 28′ 4″. This is your sailing mast position.
  3. In heavy air the rake should be 28′ 2″, when sailing to windward you will find the boom to be quite close to the deck, the main sheet blocks on the boom and traveler will be near to touching.
  4. If you are using a magic box put marks on the spar at the junction of the magic box wire and the jib halyard as a reference point.
  5. If you are using the ball and latch or the sharks teeth, make notes on the spar to correspond with the rake measurements given above.
  6. Make certain that when you measure rake that the mast is not bending.

Mast Bend

Most new mains require a considerable amount of mast bend to set right. After setting the rake as described above, tighten the backstay until the measuring tape shows 27′ 10″. This is a good starting point for bend. Generally in medium airs when you sheet the boom to the center of the boat, tighten the backstay until the upper batten is 12″ to 18″ off the backstay. Trimming harder tightens the leech as does easing the backstay. In no event sail with the leech closer than 12″ to the backstay at the top batten.

Other Mast Adjustments

Your spreaders should be 17″ long when measured from the mast to the shroud and should be angled so as to deflect the shroud forward of a straight line from tang to chain plate by 1 1/2″. This can be accomplished by laying a board across the spreaders from tip to tip and adjusting the spreader angles so that the distance from this board to the mast is 2 1/2″ to 3″.

Cut wood chocks to fit into the mast well alongside the mast so that absolutely no side bend is permitted at deck level.

Sail Adjustments

Jib: 0 – 7 Knots

  1. Attach sheets to 2nd hole or corner in clewboard.
  2. Tension luff only barely enough to eliminate horizontal wrinkles.
  3. Jib car from centerline 14″
  4. Sheet tension: sheet until the third batten from the top is parallel to the centerline of the boat.

Jib: 8 – 15 Knots

  1. Attach sheets to the second hole (corner).
  2. Tension luff to eliminate horizontal wrinkles.
  3. Jib car 15″ off centerline.
  4. Sheet tension: sheet until the second batten from the top is parallel to the centerline of the boat.

Jib: 16 – 30 Knots

  1. Attach sheets to third hole.
  2. Tension luff to eliminate horizontal wrinkles.
  3. Jib car 15″ from centerline in rough water, but eased out as necessary to avoid backwinding the main if you are in smooth water and have eased the main traveler.
  4. Sheet tension: sheet until the third batten from the top is parallel to the centerline of the boat.

Main: 0 – 7 Knots

  1. Sheet tension: Sight top batten parallel to centerline of boat, or about 12″ to 18″ off backstay.
  2. Traveler carried 12″ to windward at 0 mph, to 3″ to windward in 7 mph.
  3. Vang only snug.
  4. Very soft Cunningham tension.
  5. Outhaul in 1 1/2″ from black band.

Main: 8 – 15 Knots

  1. Sheet Tension: More firm. Sighting top batten, maintain it 12″ off backstay.
  2. Traveler on centerline.
  3. Firm boom vang tension.
  4. Cunningham eliminate horizontal wrinkles.
  5. Outhaul 3/4″ in from black band.

Main: 16 – 30 Knots

  1. Sheet Tension: Very firm. Top batten 18″ off backstay.
  2. Traveler on center to all the way out in big puffs.
  3. Very firm vang.
  4. Very firm Cunningham.
  5. Outhaul to black band.

All these generalizations are norms and averages that have proven fast over many years. Some experimentation by your part may be necessary to fine tune your particular rig and sailing style.

Good luck with your new sails and please feel free to call us with any questions you may have.

Sail Care

Your North Sails are constructed out of the best materials on the market today. We make sure of this by testing every roll of cloth we use. Through proper care and maintenance your sails will give you the performance you have come to expect from a North Sail.

The most important factor for a long life for your sails is to watch them for signs of wear and tear in high load and chafe areas. Be sure to wash the sails off with fresh water and dry the sails thoroughly before storing. A dry, mild climate is best. Excessive heart can cause problems with the sails due to the possibility of shrinkage. It is best to roll the mainsail and jib.


When hoisting and lowering the sail try to minimize the amount of creasing or wrinkling of the sail. Every time the sail gains a crease the cloth breaks down that much faster. Always have someone contain the leech and luff during these procedures.

The battens can be left in the sail without any problems. Be sure to roll the sail down the leech so that the battens will not twist. This could cause damage to the battens.


When rolling the jib keep the battens perpendicular to the leech. Pay special attention to the battens and batten pockets for wear and tear.


The spinnaker is fairly straight forward. Be sure to repair all tears and pulled stitches. Folding the sail when storing is best.

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