M20 Tuning Guide (vintage)

This tuning guide is courtesy John Spargo, who worked with Melges in the 1970s. It is most appropriate for the aluminum spars on so many of our vintage boats.


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.


4 thoughts on “M20 Tuning Guide (vintage)”

  1. It’s great to see a new scow fleet forming. I see you’ve had input from Crear, Caulfield & Spargo. All good sources of info and history. Keep it up!

  2. I recently acquired a 1963 Melges M20 fiberglass hull number 32. Can you tell me where I can obtain original instructions? I couldn’t seem to find them on the current Melges website.


  3. I noticed you should be an expert regarding the M-20 I have a question as I recently acquired an M-20 for good old fun sailing which I haven’t been able to do for over 40 years. I keep reading about how this boat likes to submarine in “big” waves. By chance could you define “big” in feet. It also sounds like “big” means something different when basically flat on the water versus healed over 25 degrees or so.


    1. Hi Robert,
      First, congrats on joining the fun!
      Wave series that can lift the stern and drive the bow down are the culprit. This can happen going dead down wind on a lake with rollers, but also a series of very large wake waves can do it. I find waves greater than 18 inches can create the pitch but they also need to be spread far enough apart… so maybe 20 or more feet, crest to crest.
      The symmetrical spinnaker has been replaced by an asymmetrical on the I20, which encourages reaching and heeling downwind. Both those techniques will keep you out of trouble.
      If I see the nose dive… I push the stick(assuming I’m on windward deck) to heel the boat and dump any water that is on the bow. Easy, unless another boat is upwind.

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