M242 Mast Tuning Guide


This document is a guide for basic mast tuning on the Martin 242. Any references to numbers are based on the cut of the Ullman Sail but the technique will be the same for any mainsail. 

A.     Boat Preparation

Before starting, make sure:

  • All halyards are loose
  • Boom vang is loose
  • Backstay is loose
  • Boom is resting on deck
  • Shrouds are loose – turnbuckles should be almost fully unscrewed.

Note: Depending on the type of turnbuckle you use, this is a good opportunity to make sure the threads are evenly fed into the turnbuckle. We suggest you completely undo turnbuckles and start at zero turns.

Tip: paint one side of all turnbuckles black. This will help you keep your place when tightening or loosening the shrouds.

Tools Needed:

  1. Wooden mast wedges and something to trim the wood (e.g. a hacksaw)
  2. Power drill with counter sink and 5/32 bit
  3. #10 x 5/8 Flat head wood or sheet metal screw
  4. Mast boot material
  5. Adjustable wrench
  6. Screwdriver
  7. 12’ Tape Measure
  8. 50’ Tape Measure (optional)
  9. Loos Gauge

B. Mast Preparation

1. Centering the mast at deck level

The first step in tuning your rig is to center the mast at the deck. Do not expect the mast to look centered in the collar.

Note: Not centering the mast properly will result in an incorrect mast shape which will only get worse when tension is applied to the shrouds. You will notice this incorrect balance first when you cannot achieve consistent starboard versus port shroud tension. The result can be inconsistency in sail shape, speed and point from tack to tack.

Make sure you have wood blocks or any other preferred method of locking the mast at the deck handy.

a. Apply enough tension so the shrouds are not dangling but not tight in any way. To do this, once the turnbuckles are evenly threaded, make sure the same amount of thread is showing for the uppers and lowers on each side. Start applying the same number of half turns to the corresponding turnbuckles on each side. Do this until there is consistent (light) tension on all four shrouds.

b. Using a 12’ tape measure, measure from each lower shroud to the center of the back of the mast.

Note: Make sure to measure from the same shroud height on each side.

Tip: A good place is just above the vang lug on the mast.

c. Continue to measure from the shrouds as you place blocks around the mast holding it at the same distance from both shrouds.

d. Once the mast is centered and the blocks are securely in place, mark the position of each block around the mast. This will greatly facilitate this procedure next time the mast is removed.

Tip: It is good to minimize mast prebend so the sail is not too flat. To accomplish this, the mast should be positioned (blocked) as far aft as possible on the deck opening.

2. Centering the mast at the top

The second step is to center the top of the mast. If the above procedure has been done correctly the top of the mast should be close to center.

Note: Not having the mast top centered before starting the tuneup procedure can also cause inconsistent starboard versus port shroud tension as well as inconsistency in sail shape, speed and pointing ability from tack to tack. This is usually noticed by differences in the shape at the top of the mainsail from tack to tack.

a. Attach a tape measure to the main halyard and pull up enough to take a reading on both sides. The reading should be taken using the same tension to the halyard at the aft most U bolts on both sides.

b. Adjust the upper shrouds to bring the mast to center. This is accomplished by tightening one and loosening the other.

Tip: Half turns on the turnbuckles usually work best. Marking the outside of the turnbuckles black will help you keep track of the half turns

c. Once the mast top is centered, look up the back of the mast. The mast should be straight. Any curvature is usually caused by uneven tension on the lower shrouds and should be minimal. If an abnormal curvature is observed, check the lower shrouds and make sure one side is not over tightened. Adjust the lower shrouds appropriately to remove the curvature. Make sure to adjust both sides by tightening one and loosening the other. Half turns on the turnbuckles usually work best.

d. Once the mast is visually straight, take tension gauge readings of all the shrouds. If the tension is too loose to allow any reading, apply four (4) half turns to each turnbuckle until a reading is possible. The tension between corresponding shrouds should be similar. Any differences can be corrected by adjusting the corresponding shrouds. Make sure to adjust both sides by tightening one and loosening the other. Half turns on the turnbuckles usually work best.


  • Mast centered at the deck and top
  • Consistent shroud tension on the corresponding shrouds

C. Base Setting

A base setting is a combination of upper and lower shroud tension that optimizes sail shape for given average regional conditions. For example, a base setting in San Francisco will not be the same as one in Marina del Rey (at least not during the summer). Base settings in the same region will differ based on sailing styles, weight on the boat and sail manufacturer. It is important to develop a feel for a base setting that will suit your sailing style.

1. Getting to base setting

For a Martin 242 in Marina del Rey, a starting point for a base setting can be 44 on the upper shrouds and 38 on the lower shrouds as measured using inexpensive Loos Tension Gauge. When your rig is centered and you have consistent tension on the shrouds. You are ready to set your base tension.

a. Put eight (8) half turns on the uppers and four (4) on the lowers.

b. Take a tension reading and continue applying and even number of half turns on uppers/lowers until you have reached the desired setting for your region.


  • Rig is centered
  • Shrouds have even tension
  • Desired base setting on Tension Gauge

D. On the Water Tuning – Advanced Rig Tune

Check your fleet rules regarding adjustment on the race course. The goal of adjusting the rig on the water is to power or depower the sail plan as conditions change. Knowing how much to go one way or the other when adjusting the rig takes practice, observation and mostly, time on the water. Decide on what change in conditions would cause you to make an adjustment in your rig tension. For some sailors it may be 2 knots; for you it may be a 5 knot increase or decrease.

1. When adjusting the rig on the water:

Always apply the same number of (half) turns to the corresponding shroud on both sides. A minimum adjustment is four (4) half turns on the uppers and two (2) on the lowers. Any less in either direction will not likely create any change in the shape of the sails.

Important things to look at when making rig adjustments:

a. Sail shape

Frequently look at the mainsail while sailing so you get a feeling for the shape. This way you’ll be able to notice the changes in sail shape after adjusting the shrouds. Note the effects of adjusting the upper and lower shrouds

b. Leeward shrouds (while sailing upwind)

There are different schools of thought on how these should look while sailing upwind. (Dangling or tight)

c. Speed, power and point

Speed test with other boats to get a feel for you speed, power through waves and pointing ability. If the groove is too wide or too narrow:

  1. Adjust main halyard
  2. Adjust main sheet
  3. Adjust jib halyard
  4. Adjust jib sheet
  5. Adjust rig tension

E. Every race day morning before leaving the dock …

  • Take a tension reading to get a feeling as to what the previous day’s changes were.
  • Set the rig back to base setting.
  • Make sure the top of the mast is centered.
  • Look up the back of the mast for unusual curvature. Adjust appropriately.
  • Take a tension reading and readjust as necessary.

Tip: It is helpful to have the same person always adjusting the rig.

Tip: Write down the adjustments as they are made. This can be done on deck and then transferred to a notebook later.