How to Tune a Catamaran Rig: A Professional’s Guide
This is Shayne tuning the rig on a Belize 43 in Antigua — a Canadian boat called Passat whose owners had their wires replaced in the States, cruised the Caribbean, and now needed everything dialled back in after the initial stretch. The fundamentals here apply to any catamaran with a stayed rig.
What you’re looking for
Start by looking up the back of the mast track. Not the front, not the side — the back. This is the quickest way to see what the rig is actually doing. On Passat the bottom panel had developed a curve to starboard from sailing predominantly on one tack down the coast. That curve needed to come out before anything else.
The track itself is only as reliable as the person who bolted it on. Use it as a starting reference, then confirm by looking up a groove or feature that’s extruded into the mast — something that runs continuously from base to masthead without interruption. Follow it up with your eye and look for bends, wiggles, and inconsistencies.
Straightening the mast
To remove a curve to starboard in the lower panel, tighten the port lower diamond. The diamond wire pulls on the spreader, the spreader pushes the mast, the curve comes out. Simple enough in principle — make half to two turns at a time, go back up the mast, check, repeat. On Passat the lower panel came straight with two turns, then a half turn on the upper diamond finished the job.
On a boat with aft-swept spreaders, tightening a diamond wire will also push more pre-bend into the mast. Check the pre-bend after every adjustment.
Setting pre-bend
Pre-bend is the forward curve of the mast between the spreaders. You measure it by running a line from the masthead to the mast base — the gap between the line and the mast at the spreaders gives you the pre-bend. Allow for the offset where the line exits the sheave at the top.
The target is one to two percent of the mast length. On a 15-metre mast that’s 150 to 300mm of bend at the mid-panel, where the bend should be greatest. If the mast is too straight you need more pre-bend — loosen the forward diamonds slightly and let the aft load push the mast forward. If it’s bending too much, tighten the forward diamonds to push the spreader and restrain the bend.
Rig tension
On a double-diamond rig be careful to adjust upper and lower diamonds evenly — if the lower diamond is too soft and the upper too tight you’ll see excessive bend in the bottom panel and not enough above, rather than a consistent curve from base to masthead.
The rule of thumb for catamaran rig tension: sailing upwind, fully powered up, the leeward shroud should just go loose as the boat moves through a wave. On a performance multihull like a Gunboat that’s around 12 knots of wind. On a heavier production boat like a Fountaine Pajot it’s closer to 15 knots.
If the leeward shroud is staying bone tight the rig is over-tensioned — you’re loading the platform without any benefit. If it’s going very slack the rig is too soft and the mast will bounce excessively. Find the point where it just releases under load and you’re in the sweet spot.
Mast rake
Rake is how much the mast leans aft. Check the manufacturer’s specification first. If you can’t find it, look at the mast butt — on a properly raked rig the base of the mast should sit flat on the mast step with no gap at front or back. A gap at the front means too much rake — shorten the forestay. A gap at the back means not enough rake — lengthen it. Pre-bend affects this slightly so if you’re seeing a gap after adjusting pre-bend, check the rake again before assuming something is wrong.
The front beam and martingale
Look down the beam from one hull to the other. You want to see it sitting flat to very slightly bent downward — five millimetres maximum of downward deflection. That small downward pre-bend tells you the martingale is doing its job and the front end is locked up properly under rig tension.
If the martingale is too loose to tension by hand — which can happen when the forestay is loaded and transferring load through the whole system — ease the side stays first. That takes the rig tension off the forestay, which frees up the martingale so you can tension it properly, then re-tighten the side stays.
After tuning
The initial tune is not the final tune. After another 200 miles of sailing the wires will continue to settle and the rig will need a check and probably a small tweak. That’s normal. Build it into your routine — check the rig at the start of each season and after any significant passage.
If you want to discuss rig tuning for your specific boat, the technical forum is the place — join the conversation here.