Wind Chime wrote:Judy,
I read all your posts and look forward to your comments, because I know you make sails for a living and you are also a small yacht sailor.
I do question your advice about not using a boom kicker.
We have a boom kicker and in my opinion it works extremely well here in the Pacific Northwest where we have a lot of inconsistent flukey soft winds.
I know this post is talking more about strong winds, but there a lot of days using the boom kicker holds the boom parallel with the deck, this in turn, does not flatten the sail out by the weight of the boom pulling the sail and leech down. Consequently we end up with a better sail shape, with the depth and position of the draft in the sail able to generate more power in light wind.
Darry
I wouldn't use a topping lift or boom kicker to hold the boom up. The leech will have too little tension, and the top of the mainsail will twist off excessively. The boat won't point.
Regards,\
Judy
Hi Darry,
In light wind, you are doing the right thing by letting the boom end rise, and putting some twist in the sail. In certain conditions, like very light winds, it's the right thing to do. It's also often a good thing to do when sailing on a reach in strong and gusty winds. But that's not a good sail shape for pointing in high winds.
There is a wind speed gradient from the top to bottom of the mast. Near the water, the wind is slower. The higher you go, the faster the wind. When you sail at an angle across the true wind, it seems like the wind is apparently coming from a different angle at the top of the mast than at the bottom. We call this the apparent wind. Sails have to be shaped to match the apparent wind. (*see notes on apparent wind below)
In light winds, (but not in heavy winds), there is a great deal of "twist" when you compare the wind angle of the apparent wind near the water to the wind angle 30 feet above the water. You want to match the attack angle of the sail (twist) to the angle of the apparent wind. If you have tell tales on the ends of the battens, you'll know whether or not you've got the sail shaped correctly to match the twist in the apparent wind from top to bottom of the mast. In light winds, you might use the topping lift or the boom kicker to put more twist in the sail.
How do we control the twist? By allowing the boom end to rise, which reduces leech tension. In 2-4 knots, a boom kicker is strong enough to lift the boom end. In more wind than that the boomkicker will be overpowered by the settings on the mainsheet, vang and traveler. You can also use the topping lift to lift the boom end. But most sailors use the interplay of traveler, vang and mainsheet to constantly adjust leech tension and thereby control the twist in the sail.
If you're sailing in stronger than 3-4 knots velocity of true wind, the apparent wind is twisted only a very few degrees differently at the water level vs the masthead.
If you are trying to point upwind in high winds, you don't want to have excessive twist in the sail. You want to match the twist in the sail to the twist in the wind angle. If you have tell tales on the ends of the battens, you will be able to see if you've done a good job matching the twist in the sail to the twist in the wind angle.
If you're on a reach in high winds, especially with strong gusts, putting excessive twist in the sails is one way to luff the top in the weaker moments and avoid stalling (heeling) in the gusts, so there are some points of sail and wind conditions where you may want a lot of twist. But in high winds, the boomkicker isn't strong enough to accomplish that. The most effective way to do that is by using the interplay between traveler and mainsheet and vang. Topping lift is a substitute, but less easy to adjust on the fly.
Leech tension's significance is that it controls the amount of twist in the sail from bottom to top. It has a very tiny effect on draft depth or position (fore and aft position). Once you have gone from some leech tension to no leech tension (i.e increased the twist) there is no effect on the depth or position of the draft. You control the position of the draft, fore and aft, by the interplay between the outhaul and the luff tension.
Fair winds,
Judy B
Notes: (from
http://www.uk.northsails.com/RADUploads ... s-Work.pdf)
Apparent Wind
Apparent wind is the wind velocity experienced by the sails on a moving boat. This is the wind speed
and direction that can be directly measured (felt) from the boat while it is moving. It is a combination of
the true wind and the wind generated by the motion of the boat. The figure shows how these two wind
components are added to create the apparent wind.

Notice that the apparent wind vector at the bottom of the rig, where the true wind speed is slower, is
shorter (slower) and angled from a more forward direction, than the apparent wind vector at the top of
the rig, where the true wind speed is faster. The true wind is coming from a single direction in this
example, but varies in speed with height due to the earth's boundary layer. This variation in true wind
speed not only causes the variation of apparent wind speed with height, but also its variation in angle.
This is because all of the mast and sail are moving at the same speed and in the same direction as the
boat across the moving air. Since the wind solely due to the movement of the boat is identical at all
heights, the apparent wind speed and direction resulting from its addition to different true wind speeds at
various heights is different.