Spreaders
Setting up Spreaders
The spreaders affect how much the middle of the mast bends. It's important to match your spreaders and their sweep aft to the luff round that is cut into your sail.
A good sailmaker can give you a recommended measurement but you can get an idea yourself if they can't help you, or if you have an old sail.
Pictures coming soon
How to Measure Spreaders
Spreader Length
Measured from the mast wall to the shroud (add pic above)
Spreader Offset
Measure from the mainsail track to a thin line tied between the shrouds (add pic above)
Note. If there is any movement in the spreaders you will need to pull them aft slightly when measuring.
Setting the Spreaders
Make sure the spreader bracket is tight to the mast. If a rivet is loose or the bracket can move around at all, replace the rivets with new ones.
Start in a fairly neutral position. Hold your trapeze wire to the deck where the shroud enters. Look up each shroud from the chainplate to the hounds (attachment point on mast), the spreaders should not deflect the wire fore or aft but should deflect outboard approx 1½” to start with. This is easily spotted while holding the trapeze wire.
The length of your spreaders mainly controls the mast bend side to side. Lighter crews can use shorter spreaders aiming for about 1" deflection which allows the tip to fall to leeward spilling excess wind. Heavier can lengthen the spreaders which will hold the mast tip longer, keeping the rig powered.
Prebend
There's often a misconception that prebend is built into the mast by the manufacturer. Prebend is actually induced by loading the spreaders when rig tension is applied. We call it prebend because we bend the mast before we hit the water or load the sails.
You can measure prebend by applying tension to the rig and then holding your main halyard to the back of the mast, just above the gooseneck. Measure the distance from the back of the mast, at the spreader bracket, to your main halyard.
As a starting point your prebend number is half of your mainsail luff curve. To measure the luff curve, a two person job but three is better, put your main flat out on the ground. First measure up the luff from the tack, 2125mm and mark the luff. Hold a thin rope on the luff of the sail at the head and bring it down to the tack of the sail. You then measure from your luff mark to the the rope. Now that you have the luff curve measurement you need to set your prebend to half of this number to start with.
If you sweep your spreaders aft you will add prebend flattening the middle of the mainsail and reducing power. Sweep forward will increase mainsail depth and add power.
Quick Notes
Spreaders back - increases prebend to depower. Spreaders forward - decreases prebend, more power.
Shorten spreaders if overpowered, lengthen if underpowered.