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Debrief: IC37 Rig Optimization and Equalization Project

by Mike Marshall & Steve Liebel 5 Nov 2023 09:16 PST
IC37 fleet sailing © IC37 Class Association

In the spring of 2023 the IC37 Class partnered with North Sails to embark on a project to optimize and equalize the rigging setups of the IC37 fleet.

The following is an explanation and description of the revolutionary process that took place to create the most even One-Design fleet in the world.

To undertake the IC-37 rig tuning project, we had to first understand the issues. As is the accepted standard among one-design fleets, the rigs were set to shroud tensions, headstay lengths, J-dimensions, and a mast butt position to combine these measurements into a "dock tune" for the boats. While this process generally works, it is not actually measuring what the mast and main see as equal between the boats: the actual mast bend. If any one "dock tune" measurements have a high error tolerance, that error is magnified in the achieved pre-bend in the mast leading to inequality on the water. We also found some variations in markings in manufacturing, especially prominent in where the mast buts would be placed.

Equipped with this knowledge, we used the new standard Southern Spars rig tuning process on every one of the boats using the same mast jack (30 or so times repeatedly). Our target was to have the same headstay lengths, J-dimension, shroud tensions, and pre-bends - where the only variable would be the mast butt position. Mathematically, masts that have the same laminate have to react the same within the tolerance of error if the pre-bend and shroud tensions are the same. The outcome was a set of masts that have the same rake, shroud tensions, and a pre-bend. The pre-bend error before the work was 50% and after the rig-tuning project was 3% or less.

Previous tune notes:

  1. Most of the boats that we checked were lifting off the jack at 20% load more than the mast was designed for.
  2. The range of pre-bends on the masts at the top spreaders had a 50% error range.

Our process:

  1. The boats were first tested for a lift-off pressure with the V1s only having tension. The V1s were adjusted until the lift off pressure was achieved. The D1s and D2s were loose. The same jack was used for every boat. All halyards and backstays were loose.
  2. The next lift-off pressure was with the V1s and the D1s on. Again the D2s were still loose.
  3. The final lift off target came with the V1s, D1s and D2s.
  4. The prebend at the bottom spreader was measured compared to the target.
  5. If that pre-bend was incorrect, the mast butt was adjusted in the appropriate direction.
  6. Very IMPORTANT - The entire process was restarted from step 1 with the new butt position and the V1s lift-off pressure. Steps 1 through 6 were repeated until the correct pre-bend at the bottom spreader was achieved.
  7. The pre-bend at the top spreader was confirmed.

The final results were that all boats had pre-bends that were within a 3% error. Then out on the water you could see the same setup profile of each boat. This was confirmed by class coach Moose McClintock.

Frequently Asked Questions:

Q:Is the tune perfect for everyone's opinion?
A: Not necessarily, but all the boats are IDENTICAL. Next step will be this winter in Florida, we will work to optimize the rigs working off the identical pre-bends.

Q: Is more runner load required?
A: On some of the boats with 130 mm or more pre-bend at the top spreader, there will be a noticeable change in the runner load. For other boats, with the lower bend numbers, they might not see a difference.

Q: Are the rigs looser than before?
A: Yes, they are correct for the lift-off pressure that the mast was designed for.

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