Spreader tuning rules of thumb :

I'm coming to the believe that the following rules of thumb apply to the tuning of the spreaders and diamond wires :

-1- Decrease prebend if you want better pointing and more power in the lighter and medium conditions

-2- Increase prebend if your sail is overpowered too soon, or when the boat still feels sluggish when the sails are clearly full with power (pulling on sheets). (here you're sails are too full and too draggy to acquire high speeds despite being filled with drive)

-3- When the boat is still flighty when the prebend is set for good pointing (and good speed in very stable windstrengths) than increase the spreader rake decreasing the diamond wire tension to maintain the same amount of prebend. This will introduce more "pumping action" in the gusts where the loading up of the diamond wires will bend the mast more in the gusts leading to a momentary flatter sail. This will translate into accelleration. You should be able to feel the difference if you have a better setting.

-4- Decrease the spreader rake if the boat is timid in all gusty conditions even when experiencing significant gusts. You have too much pumping action.

-5- Also decrease spreader rake when the tension on the diamond wires drop below a minimum value. You must never sail with diamonds that go slack during sailing (luff diamond wire). This bends the mast too far into the jib slot and closes it off hurting your pointing. Also you will ten stand up the top to much now and will make the mainsail to much bound up (limited) in spilling air in the top.

-6- Set diamond wire tension to suit the optimal spreader rake and optimal prebend for specific mainsail and your way of sailing with it. Forget about any claims that tight diamond make the top fall away better in strong winds (and the other way around) It is better to reduce mastrotation in strong winds to get the same effect. Getting prebend and spreaderrake (pumping action) just right is (in my opinion) noticeably more important than having a particular amount of diamond wire tension.

-7- If you really run into problem with diamond wire tension (minimal amount) than shorter your spreader arms so you can tighten the diamond wires again wihtout changing the optimal spreader rake or optimal prebend.

-8- If you put too much tension on your diamond wires for the given optimal spreader rake and prebend than lengthen your spreader arms and adjust the wire tension accordingly.



I believe you can trade-off downhaul tension for increased spreader rake in gusty conditions. If spreader rake isn't large enough you tend to compensate this in gusty conditions by pulling harder on the downhaul. This hurts pointing and will make you underpowered in the luffs. Increasing spreaderrake will tend introduce more pumping in the gusts allowing you to run with less downhaul tension which should improve pointing and keep you powered-up in the luff thus improveing speed as well.

Prebend, I'm finding, is relatively strongly related to used mainsheet tension which in turn is strongly dependent on the crew weight on board. Less weight means less sheet tension to fly a hull and thus you'll need more prebend to make the mainsail have the correct twist profile and draft with the lower sheet load. If you are running heavy and are increasing sheet tension to bring in the squaretop to keep yourself out on the wires than having too little prebend will flatted out the middle part of the sail too soon. Increase prebend then to off-set this.

In principle your should find the optimal prebend for a given set of mainsheet tension and downhaul tension that your mainsail likes. Mainsheet tends to bend the mast all over its length more than downhaul tension does. Downhaul tension tends to flatten the top more than the middle part of the mast. So you'll first have to find the optimal ratio between mainsheet tension and downhaul and find the best prebend to that. As everything is linked to eachother you can't get this right in one try but you'll have to experiment with a few prebend settings and choose the best one and test some more around that "best point".


Wouter