Seawind 1600 Owner's Manual page 60

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The spreader diamonds control the mast pre-bend and ensure that it cannot bend sideways.
The mast is initially set up with approximately 150mm of pre-bend, that is, the mast has a
gentle bend along its length so that at the midpoint it is approximately 150mm from an
imaginary straight line between each end of the mast. This is achieved by first tensioning the
outer diamonds. With the correct pre-bend achieved, continue to adjust the spreader wires to
get the mast straight in a sideways plane. Second, take up the inner diamonds equally until
they are firm and sharing the load. For advanced tuning this pre-bend should match the cut of
the mainsail.
Having set pre-bend you now need to look at the stay tension. The forestay length has been
determined to allow the top of mast to have an aft rake of approximately 20ins (500mm). This
rake affects the overall balance of centers of effort for the boat. Take up the outer side stays
on both sides evenly, changing each side a few turns at a time. Do not over tension; take them
up evenly until the stays feel firm.
NOTE: At standing height you should be able to rotate the stay in no more than a 100mm
diameter
Last, take up the lower stays evenly until they are firm against the pre-bend provided by the
spreader diamonds. Their purpose is to contribute to holding the mast in column with the pre-
bend and stop it from "pumping".
Now go sailing. The correct rig tension can be best checked at sea. In a reasonable sail to
windward (say, in 15 knots or more), the lee stay will normally go slack but not flop around.
This is acceptable. If it does "flop" around take up the slack with a few turns (count the
number of turns). Tack and take up the same amount on the opposite side so as to balance the
tension on each side.
Once you have completed all the above steps, the rig should be well set up and should not
need further adjusting. However, one last check is worthwhile. When sailing hard to
windward (say, in 15 knots), go to the base of the mast & look up its length. It should be
perfectly straight in its side-ways plane all the way up to the jib halyard exit. Above this point,
the mast is unsupported & it is normal that this top part will fall away to leeward. In a fore &
aft plane, the mast will be bending slightly aft but not S bending or pumping.
TROUBLESHOOTING
If shroud turnbuckles are at full extent or main sheet blocks too close – take up forestay
adjustment
Mainsail wearing on shrouds – have a sailmaker repair rubbing strips. Sail with main not
touching shrouds by making more use of very high main roach not twisting out by not having
main sheet fully out, main sheet on harder and tacking down wind.