Our boat is 32 feet and has a symmetrical spinnaker. My crew and I have an ongoing debate about twing lines, which they feel are not necessary, but I consider useful, especially during heavy-air jibes. Can you help settle this issue?
Dan Dickison responds:
Thanks for your question; it's an interesting one. Twing lines can be an effective adjustment on spinnakers, whether symmetrical or asymmetrical. On symmetrical spinnakers like yours, they're used for two principal purposes. The first is to adjust the amount of twist in the leech of the spinnaker. You can do this by tensioning or releasing the leeward twing line. This deflects the angle of the spinnaker sheet and consequently alters the twist. Essentially you might want to close down the twist in moderate or light air, and open it up in big breeze, depending upon the cut of the spinnaker.
The second main purpose for twing lines is to help steady out the spinnaker when you're sailing in choppy conditions. The boat makes better speed when the pressure on the spinsheet is steady, but the erratic waves and chop tend to bounce the spinnaker around, so you can counteract those by applying a little tension on the leeward twing. It has the effect of nominally over-sheeting the sail and thus keeping it from luffing.
And you're right about the advantage of having twing lines rigged for controling the spinnaker when you're jibing in heavy air. It's not unheard of for a bowperson to have a bad jibe and miss the pole. When this happens, it's nice to rely on the twings to help keep the spinnaker under control until the bow person can recover and get the guy back in the pole end.
I hope this information helps you settle your dispute with the crew. Remember, it's better to go out and prove this on the water than to take my word for it. Here's wishing you good luck in your racing.