|Topic Review (Newest First)|
|02-16-2008 09:00 AM|
Just to clarify my earlier comments a bit more: We frequently DO raise the traveller well above the centerline for windward work. My comments above were discussing the idea of bringing the boom across the fore-'n'-aft centerline.
Occasionally we will bring the boom above the centerline, but only when we want to slow the boat by deliberately over-trimming (for instance, when sidling slowly up through a crowded anchorage under sail preparing to drop the anchor.)
Jeff's note above suggests some people do as you do with positive results, so maybe there are variations in boat design and rig geometry that make it advantageous to do so? Live and learn.
|02-15-2008 08:28 PM|
You may call that discussion wordy, but I'll call it AWESOME!!!!
Thank you very much.
|02-15-2008 08:25 PM|
Thanks guys that was very helpful. It looks like I'll keep trying it and figure out what it is I'm doing that works or doesn't.
JohnR you may be right about the slot. I usually get to this state by pointing the boat allowing it to speed up , trim the sails, point it a little higher, trim the sails, etc., until we seem just perfect - then I start screwing around. The main thing I'm trimming is the genoa and it is possible I throw off the slot at that point.
Nolatom since this is a lightish air thing for me I'm pretty sure I have the outhaul eased and the main sorta full. I think I loosen the mainsheet a little and move the traveler over and then tighten the mainsheet to see if I can bump up the knotmeter a little. It usually gets a couple of tenths. If the wind is too fresh we heel too much and stay the same or slow a little.
I have a cruiser mentality but I appreciate keeping up or passing even with the dink in tow. But I have never studied the nuances that racers deal with. Looks like this year I'll be getting some lessons on J-24's and all its going to cost me are Tuesday evenings and a few beers.
|02-15-2008 08:24 PM|
This is piece that I wrote on Gradiant Wind and Twist. Its a bit wordy but it explains what you are experiencing.
The phenomina that you are describing is called "twist". Controling twist is a critical part of optomising performance. Some degree of twist in light to moderate winds is almost always a good thing.
To begin with, if you look at a sail in section (cut horizontally through the sail) it is a wing. Even very efficient wings have an "incident angle" and "slip angle". In other words a wing (or a sail)needs to be placed at an angle to the wind to work. For a given wind and any given sail and any given boat in a given condition, at any given spot on the sail, there is an optimum angle of attack in order to achieve the best performance.
What further complicates all of this is that the wind at the top of the sail is actually different than the wind at the bottom of the sail. Called 'gradiant effect', in light to moderate winds the wind speed typically increases the higher you get above the surface of the water.
Visualize this effect this way, there is friction between air and water and between air and air. Because of this friction at the surface of the water there is a (barrier)layer of air that does not move at all relative to the water. Next to this layer of air is another layer of air that moves slowly over this stationary barrier layer. That layer feels the friction of the barrier layer and the friction of the layer above it that is motavated by the ambient wind. Each higher layer moves a bit more quickly comapred to the layer below until at some point up in the air there is a point at which the air moves at the speed of the ambient winds and does not feel the affect of the barrier.
In light air, this affect can be tens of feet deep. At very much higher wind speeds this whole gradiant effect, barrier to free flowing wind, is only a couple inches deep. We usually sail where the effect is somewhere in between but typically taller than the average mast height even in moderate winds.
In a sailboat, this means that the boat feels more true wind at the masthead than it does at the deck. Because of the way that apparent wind works, the higher wind speed at the masthead produces higher apparent winds at the masthead that are also more abeam to the boat than the apparent winds that are felt lower in the sail. Getting back to your question, twist allows the sail to have differing attack angles as you move up the sail, each at a proper angle of attack relative to the apparent wind that it is passing through.
If you eliminated twist in light to moderate conditions, some of the sail will be over trimmed or some of the sail will be under trimmed for the conditions. In light to moderate winds the traveler should be brought to windward, and the vang eased, allowing more twist. On some classes the boom may actually need to be brought slightly to windward in these conditions in order to be able to maintain both flow and pointing ability. This is less of an airbrake than it might seems since boats generally have a greater leeway angle in light winds.
Of course as windspeeds increase, gradient wind effect decreases and so as wind increases in speed, twist should be reduced. This is done by lowering the traveller and increasing halyard, outhaul and sheet tension. In a really strong breeze the sail needs a comparatively flat camber and angle of attack and so the sail should be bladed out. This means maximum halyard tension, outhaul tension, backstay tension, mainsheet tension. To further reduce the angle of attack the traveller is dropped as well. This will decrease weather helm and heeling.
Jibs have twist as well. Twist in jibs is controlled by the jib sheet lead angles. Moving the jibsheet car aft tightens the lower sail and increases twist in the sail, moving the track forward pulls down on the leech and so increases twist. On jibs you increase twist in really light air to open the slot and in really heavy air to reduce heeling.
The clues to proper amount of twist comes from the teltales. On mainsails the leech teletales at the battens provide the best information. All of the teletales up the leech should be flying when the sail is set properly. When there is inadequate twist the teletale at the head will be stalled and sucked back into the sail.
On jibs, the luff teletales should all be flying and all of the teletales hould 'break' evenly. On small jibs (with battens), leech teletales are very helpful with sail trim as well.
One of the problems with battenless sails is that it is much harder to control twist without developing leech flutter. That problem as much as the smaller sail area is what kills performance in in-mast furling sails in lighter conditions.
|02-15-2008 07:43 PM|
It's more a question of how flat, how full, and how far in you want your mainsail, a question that can only be solved by trying it out. If you haul your traveler to weather, you can ease off a little on the mainsheet, giving you a slightly "fuller" main, with more "twist", than otherwise. It may help pointing, if you need it to stay above the leeward guy's backwind, though it may diminish speed somewhat.
Pointing versus footing has been the balancing act on a beat for centuries. Welcome to it. If you're windward, you try to point so you stay out of leeward boat's backwind. If you're leeward, you try to foot so you don't get blanketed by windward (general rule here, there are exceptions in particular cases).
I tell beginners to leave the traveler centered. Later, I'll tell them when to pull it to windward, or ease it to leeward.
Sounds like you're "learning by doing", which is the best way..
|02-15-2008 06:56 PM|
I will not argue with your success, but it is generally considered disadvantageous to bring the boom above the fore-'n'-aft centerline of the boat (normally it is not even brought all the way to to the centerline).
This is because you will end up with quite a bit of hook in the sail, such that some of the breeze may be hitting the sail for the first time on the windward side near the trailing edges. This is bad for the "flow", since you want wind to pass over the leading edge and retain a laminar flow along both sides of the sail, then exit smoothly along the leech for maximum aerodynamic efficiency.
I would suggest you get some empirical readings via gps to see if the advantage you perceived was real or wishful. You may have gained an advantage, but it may have been for other reasons (such as the slot was too tight between the mainsail and the genoa, and over trimming the main with the traveller reduced some of that drag...)
|02-15-2008 06:24 PM|
Close hauled traveler use
This might be interesting and there are people here who will probably know what I was doing and whether or not it is a good idea.
In light to moderate winds when we are close hauled and possibly being challenged by another boat I will sometimes bring my mainsail's traveler fairly far across to windward. This brings the boom across the centerline to windward. It usually gets me a few tenths of a knot and allows me to point a bit higher.
In stronger winds this is a bad idea as it induces heel and weather helm. But it seems to give us a nice edge. Should I keep this in my bag of tricks or am I delusional.
BTW I sail a 1981 Lippincott sloop about 8600 lbs with 4K in ballast, main is about 215 ft sq.