How do I adjust the mast rake and rig tension on a Monsun 31?
My Hallberg-Rassy Monsun 31 has quite heavy weather helm when close-hauled, and the leeward shrouds look loose in a breeze. How should I set the mast rake and rig tension, and what should I measure before adjusting the bottlescrews?
Answer
On a Monsun 31, set the mast straight athwartships, establish a modest aft rake by adjusting forestay length, then tension the shrouds evenly and check under sail. Aim for light weather helm and leeward shrouds that stay tensioned when close-hauled, rather than chasing an unverified factory rake measurement.
Inspect fittings and record settings
Inspect the parts carrying the rig: chainplates, their attachments below deck, mast support, spreader roots, wire terminals and bottlescrews. Tensioning a rig with a moving chainplate or settling mast support will temporarily hide the fault and may make it worse.
Monsun 31s may have replacement spars, furlers and standing rigging. Identify what is fitted before using another owner's settings, and record wire diameters, bottlescrew positions and any mast maker's identification.
- Look for broken strands, cracked swages, bent clevis pins and seized threads.
- Check that bottlescrews have adequate thread engagement at both ends.
- Photograph the fittings and measure the exposed threads before changing anything.
Use a steel tape, notebook and wire tension gauge, such as a Loos gauge sized for the wire's diameter and construction. There is no useful universal bottlescrew torque: lubrication and thread condition affect how turning force translates into wire tension.
Measure rake and adjust forestay length
Measure afloat in calm conditions, with normal cruising gear aboard, minimal heel and no crew moving about. Hang a weight from the main halyard and measure its horizontal distance aft of the mast at a repeatable height, keeping it clear of the boom. Record the boat's trim, as changes in loading can alter the apparent rake.
Treat this measurement as a baseline, not an absolute specification. A small aft rake is a sensible starting point, but a precise millimetre figure requires the spar dimensions, measurement height and correct rig information.
Shortening the forestay brings the mast more upright and generally reduces weather helm; lengthening it generally increases aft rake and weather helm. Ease the backstay and relevant shrouds before making a controlled adjustment, then restore tension and measure again.
Support the mast independently before disconnecting any stay. Have a rigger handle adjustments concealed inside an unfamiliar furler.
Mast-foot position also affects balance where an adjustable track is fitted, but do not assume your Monsun's step allows movement. Structural alterations to the step fall outside routine tuning.