Tuning Guide – Keith Whittemore

Posted by on Apr 26, 2012 in Tuning | Comments Off on Tuning Guide – Keith Whittemore

Per your request here is how we set up a J-24. Basically it is exactly the same as the North Sails tuning guide with the additions of mast butt adjustment and how we set up our rig tension.

First to be clear on the absolute basics:

  • Mast blocked (“J”) measurement at max (2925 mm from stem fitting)
  • Head stay at max length (8670mm)
  • Spreaders at minimum length (760mm from mast)
  • Mast at minimum height (Band no more than 400mm above sheer)
  • Spreaders at 155mm to 165mm sweep from back of mast (we like 165mm as we sail in a lot of light air and if you usually sail in a lot of wind we would suggest 155mm)
  • Spreader sweep even by sighting up the shrouds
  • Install a mast butt adjuster from Hall Spars. Halls web site is www.hallspars.com. Do not use just a standard turnbuckle as it will “gaul” up and not work after you try and move the Matt Butt under load. We installed a piece of 2mm Nylon sheet under our mast step to get it to be able to slide under higher loads.


Rig set up:

  1. With mast installed and free to move side to side, lowers loose and the uppers at 18 on old style Loos gauge we use a wire from the Genoa halyard down to the rails and/or chain plates to see that the mast is straight up in the boat. (Don’t forget to check to see that your spreaders are the same height where they meet the upper shrouds) If you want to get very fancy you can do this out of the water and use a transit to see that the keel, hull and mast all line up correctly.
  2. Move the mast side to side in the partner until it is absolutely straight.  Block the mast from side to side movement in this location.  (As you tune the rig do not be afraid to “shake” the mast and rig around a lot to be sure all parts are settled in and not hanging up anywhere)
  3. Using your mast butt adjuster move the mast butt for and aft until the mast is absolutely straight for and aft. This is what we call our “neutral” position and it is at zero pre-bend where the mast lines up from the tip, hounds, partner and step exactly. Permanently mark your mast step at this “neutral” position.
  4. Using your mast butt adjuster pull the mast 1” aft from the “neutral” position and drill a hole.
  5. Then keep moving the mast butt aft and drill additional holes at 1-1/4”, 1-1/2”, 1-3/4” and 2” back from “neutral” (five holes total)
  6. With the mast butt in the 1-1/2” aft hole tighten the Uppers to 24 on the Loos gauge.
  7. Tighten the lowers to 21 on Loos gauge.
  8. Check that the spreaders are at 160 mm to 165mm sweep.
  9. Check that the spreaders each have the same sweep.

(This mast step set up is quite a bit different from what North has in their tuning guide but we believe that the J-24’s are all different enough that to get a consistent “neutral” position on a boat-to-boat basis that this is a more accurate way of locating it than just checking head stay sag)


Masts butt settings:

  •              Zero to 4 knots of wind – 2” aft of neutral (we rarely use this position)
  •              2 to 8 knots of wind –   1-3/4” aft of neutral (this is our default as again me sail in a lot of light air)
  •              8 to 15 knots of wind – 1-1/2” aft of neutral
  •              15 to 22 knots of wind – 1-1/4” aft of neutral
  •              22 plus knots of wind – 1” aft of neutral (We never go forward of the 1” position)

Shroud tension:

We do this exactly the same way for all wind conditions except when it is extremely light where we keep the shrouds very loose and we know that is just plain going to be very windy and then we set the rig at the North tuning guide max wind numbers at the dock and leave it.

What we do is set up the boat going to windward with all sails trimmed and crew weight in the right place and “feel” the leeward shrouds. When set up as above we adjust the shrouds so that there is about 2” of easy flex before you feel any firmness in the shrouds. If they are floppy tighten them and if they feel snug loosen them.  We try to keep the uppers and lowers even. (Note: We always do this setting up in “lulls” as we believe it is critical to be optimized for the light spots on the first beat)

We have spent a lot of time checking our settings against the North tuning guide settings and find them pretty close to the published numbers.  The advantages are you do not need to guess how much wind there is, the adjustments are infinite and exactly tuned to the real wind velocity. (Not to mention not having to take you’re main down between races to check the rig)

Couple of refinements and notes to the strategy:

  • In very very light air shrouds need to be floppy.
  • In lighter air the lowers should be slightly looser than the uppers to induce pre-bend.
  • In heavier air the lowers should be a slightly tighter than the uppers to prevent over-bend.
  • In chop go looser
  • In flat water you can be a bit tighter
  •   Too loose is always faster than too tight

The goal that we are trying to accompolish with this set up is good power for:

  • Lulls
  • While accelerating
  • Chop

But also, as is necessary and then to be able to de-power and go high by going to a very tight mainsheet to tighten up the head stay when the conditions allow.

Genoa Trim:

Unless over powered we set up the Genoa so that when you pull it all the way in the Genoa touches the shrouds at the chain plate and spreader tips at the same time.

We trim the Genoa at 3” to 6” off the spreaders and only go all the way in for a few minutes when we must point and have some speed that can be bled off.

Final note:  Rig tension and sail set up is critical for speed in a J-24 but, Mast Butt location is not.  There are plenty of very fast J’s that never move their Mast Butts.  I would strongly suggest that you set the Butt at 1’1/2” aft and work on getting speed through rig tension before playing with the Butt adjustment. (And if you forget to take the damn thing off it can get you tossed)