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PostPosted: Sat Jul 30, 2005 9:30 pm 
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Joined: Sat Jul 30, 2005 9:17 pm
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Is there a general recommendation for diamond wire tension on an 18sx? The assembly manual makes no note of how tight they should be.

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PostPosted: Sun Jul 31, 2005 4:06 pm 
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Joined: Mon Nov 22, 2004 7:18 pm
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Location: League City, Texas, USA
I owned and raced a Hobie 18SX for a couple of years (with reasonable success). I don't think I ever found recommended diamond wire tension on the internet for the SX - so I just used what had worked on the Hobie 18 as passed down to me by our local H18 guru:

Put tape marks at 1ft, 2ft, 3ft up the mast from the bottom of the diamond wires. For a medium weight crew we used the following adjustements:

Light air: 1ft mark
Medium air: 2ft mark
Heavy air: 3 ft mark

Adjust the diamond wire tension until you can push the wires in on both sides to almost touch the mast at the desired mark (without using excessive force).

Here is the theory behind this: Loose diamond wires (e.g. 3ft setting) let the mast bend more when the mast is rotated out and the down-haul and main are cranked on (heavy air sailing). Bending the mast flattens the sail which reduces power without incurring drag.

In light air you want the power so the diamond wires are tensioned more - making the mast stiffer and less likely to bend on the minor axis (from side to side).

This is old school cat technology - more modern boats run much higher diamond wire tension (over 900 lbs sometimes) to prebend the mast backwards. They also have much more powerful down-haul and main sheet systems (up to 16:1 down-haul and 10:1 main) which lets them bend the mast backwards in the major axis. This flattens the sails the same way I described - but without having to rotate the mast outwards which creates drag and turbulence at the entry of the sail.

I read in Rick White's excellent "Cat Sailing for the 90's" that he had experimented with pre-bending a Hobie 18 mast with some success. I'm not sure if this was with a comp tip though. I tried running my rig pre-bent but the mast section seems to be much stiffer than the comp tip - so when you crank on the down-haul and mainsheet it just bends the comp tip back without bending the mast and flattening the middle of the main. In other words, prebend did not work for me on the 18SX.

Chris.

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