|Topic Review (Newest First)|
|09-25-2006 08:35 PM|
Thanks all for the responses.
I cannot tell you how helpful it has been. I was out on the Chesapeake on Saturday, and it was windy enough to give me more heel than I was comfortable with. I will use this new information to allow me to sail in more difficult weather.
Thanks all again.
|09-25-2006 10:14 AM|
Jeff, thanks for a very good overview for those learning about sail trim, and a good refresher/reminder for those with experience.
|09-25-2006 08:09 AM|
You have gotten a good set of answers here, This question comes up from time to time and below is my response to an earlier dicussion that might provide a more comprehensive view on sail trim vis a vis using the traveler:
In a general sense, there are a lot of factors in powering up or down a sail but the two biggies are the depth of its camber and the angle of attack. The deeper the camber (curvature of the sail) the more drive the sail develops, but along with drive (which is the forward component of the force generated) with increased camber comes a higher side force as well. This sideforce causes heeling and leeway.
In really light air, the air flow lacks sufficient energy to flow around a sail with too deep a camber. The sails want to be comparatively flat and have a moderately small angle of attack.
As the wind picks up, you can introduce deeper camber which is how you 'power up' a sail. In a general sense this powering up involves an easing of halyards, outhauls,and backstays.
As the wind continues to build, drive and subsequently speed increases as well, but at some point hydrodynamic drag (the drag caused by the boat passing through the water) becomes the limit on speed, and at this point additional drive is not necessary. As this point is being approached heeling becomes excessive. As the boat approaches this point the sails need to be depowered. To depower halyards, outhauls, and backstays are tightened. This pulls fabric out of the body of the sail, flattening curvature of the sail. The sail produces less forward drive, but it also produces less side force.
The second aspect of this discussion is angle of attack. For any given wind and sailshape, at any point on the sail, there is a proper angle of attack. If the angle of attack is too flat, the sail luffs, and if the angle of attack is too steep, the sail generates less lift and more drag and greater sideforces causing more heeling and leeway.
Because of gradiant wind effect, (slower air near the water than higher in the air due to the friction between the water surface and the air above) in light air, the apparent wind angle felt by the sail will be different at the head of the sail than at the foot. The apparent wind at the foot of the sail, will appear to be more forward than the air at the masthead. To allow the sail to have a proper angle of attack twist is introduced into the sail so that the upper part of the sail has a different angle of attack than the bottom of the sail. Here is where the traveler, backstay, and the boom vang come into play.
By bringing the traveller to windward, the pull of the mainsheet becomes more horizontal than vertical. In doing so, the boom is held inward toward the centerline, but the boom is allowed to lift a little, and that lifting eases the tension on the leech of the sail allowing more twist to develop.
As the wind builds, gradient effect generally becomes insigificant, so the whole leading edge of the sail wants the same angle of attack and in general, that angle of attack needs to be much flatter than it would be in moderate winds. To unify the angle of attack, the traveller is lowered to leeward and the mainsheet tightened, which increases the downward force on the leech of the sail. This increased leech tension removes the twist from the sail. As the wind builds the angle of attack can further be lessened by lowering traveler further to leeward. As you bear off on a reach, the traveller can be further lowered to maintain the proper angle of attack without powering up the sail, but at some point the sail needs to be eased broader off than the length of the traveller can permit while still generating the proper downward force, at that point the Vang takes over the main role in controling twist and the sheet then simply controls the overall angle of attack of the sail.
Simply easing the mainsheet in a strong breeze does allow the head of the sail to twist off and reduce heel, but it comes at a price. In easing the sheet the boom rises and allows more fabric into the body of the sail increasing power just when you need to reduce power, and also in order to obtain enough drive, the lower portion of the sail is overtrimmed developing a lot more weather helm than would occur with proper sail that has been properly bladed out (meaning flattened and with a smaller angle if attack).
The backstay tension (especially on a fractional rig) can be used to depower the rig further. On any rig, even one with a stiff mast, tensioning the backstay removes sag from the forestay and is doing so, draws fabric out of the sail in a horizontal direction, flattening the jib and depowering it. As the forestay is tightened the mast moves aft and that also changes the relationship between the jibsheet lead and the head of the sail, allowing the leech of the jib to open slightly, reducing the angle of attack of the upper portion of the sail. On a boat with a bendy rig, and more dramatically and controllably on a fractionally rigged boat, as backstay tension increases the mast bows forward, in doing so it also draws fabric out of the mainsail, depowering the sail in the same manner that tightening the forestay flattens and depowers the jib. Also similar to the jib, the masthead moves aft as the backstay is tightened and that opens the leech slightly at the head of the sail, easing the angle of attack and further reducing heeling, weather helm and leeway.
There is a tendancy to dismiss this as 'racer stuff' but these kinds of subtle sail trim adjustments can make for a much more comfortable and controlable passage as well as adding significantly to the speed of the boat.
Lastly, I really disagree with the idea that "if your sails are basically trimmed correctly, and you are making the kind of fine adjustments that a skilled racer would use, those fine adjustments can't be measured by your knotmeter". Small adjustments to backstay or traveller positions can tremendously reduce weather helm and heel angles. On my prior 28 footer, these fine tuning items were good for a half knot or more, and on bigger boats or in higher winds, these kinds of minor adjustments can yield enormous gains in speed, tracking, and comfort.
|09-25-2006 12:37 AM|
In any boat where the mainsail is a significant portion of the sailpower, the traveller can in some ways be looked upon as the gas pedal.
When everything "looks" OK, sail shape, draft, and the boat feels powered up but boat speed is off, easing the traveller to leeward (down) a few inches will often result in an immediate improvement in speed and easing of weather helm, which, if excessive, also hurts boatspeed.
This effect is very dramatic on fractional rigs, less so sometimes on large "J" masthead boats.
In heavy air easing the traveller reduces the angle of attack of the sail and reduces the likelihood of stalling. It is not unusual, nor necessarily slow, to carry a slight backwind bubble along the luff of the main in those conditions.
|09-24-2006 07:09 PM|
This is a super over-simplification but here goes:
Traveller takes the boom left/right, mainsheet takes it up/down.
If you've sailed a smaller boat without a traveller, you probably used the mainsheet to move the boom left and right and didn't worry too much about up or down. You can use a combination of both to achieve this all important twist thing that folks mentioned above and is worth reading about and practicing alot.
In the meantime, trimming with the traveller to move the boom left and right will probably be the most familiar maneuver, in effect.
I am newbie too, so if I am dead wrong any corrections from saltier ones will help me too!
|09-24-2006 04:14 PM|
When you lower the traveler, the shape of the main is maintained. If instead you ease the main-sheet, the shape of the main changes quite a bit because you are not only letting the boom out but the back end of the boom rises. When the boom rises, the leech (trailing edge) of the main to opens inducing what is called twist. The leach at the top of the main becomes much more open than the leach towards the bottom of the sail. If you want twist, great. If not, Doe!
Say you are sailing directly to your desired destination. You worked hard and finally got all the tell-tales from the top to the bottom streaming back on the main. The wind shifts ten degrees towards the back of the boat causing the tell-tales to curl around to the back of the sail. Just ease the traveler until the tell-tales are again pointing back. If you instead ease the main-sheet, you cause twist allowing the top part of the main to open further than the lower part. You will notice that the top tell-tales recover but the lower ones are still curling around the back.
If you continue to ease the main-sheet until the lowest tell-tale streams, the top tell-tales will not be streaming. You might use this "excessive" twist setting when sailing in wavy conditions. As the boat weaves its way through the swells, some part of the main sail will be providing drive. As the boat turns away from the wind, the top tell-tale is streaming. As the boat turns towards the wind the lower tell-tales will be flowing.
If you sail on a boat that has a boom-vang, but no traveler (or you decide not to use the traveler), you can use it to prevent the boom from rising when you ease the main-sheet. I think they call this technique "vang sheeting".
When the traveler is positioned directly below the boom, the primary effect of easing or tightening the main-sheet is that the boom rises or lowers. When the boom is all the way out, say perpendicular to the boat, the effect of easing or tightening the main-sheet is that the boom moves out or in. As the boom comes in, the effect of the main-sheet the transition from "in and out" to "up and down".
When sailing down wind with the boom all the way out, tighten the boom-vang to reduce some of the twist of the main.
As you learn more and more about the various control lines, you find that each usually has a primary effect and many side effects.
Why might easing the traveler be better then easing the main-sheet? If you boat does not have a back-stay, you may be depowering your main but you will also be powering up your jib (if you have one) and in certain respects, also may be powering up your main. The back stay goes from the top of your mast to the back end of your boat. When it is tightened, it removes power-inducing sag from the forestay. Also, it compresses the mast causing the mast to bow forward which helps to depower by flattening the main. Boom-vang pressure will also bend the mast.
Not only does positioning the traveler under the boom allow for the boom to lower when the main-sheet is tightened, but it also bends the mast and tightens the forestay.
If you have no backstay or boom-vang applied when you ease the mainsheet, the main and the jib get a more powerful shape. If you lower the traveler instead, you maintain the mast-bending pressure and forestay tension.
Also, by limiting the twist of the main by lowering the traveler, I think you also maintain a larger slot between the trailing edge of the jib and the front backside of the main. If you just ease the main-sheet, the slot may be more restricted.
|09-24-2006 12:21 PM|
For upwind sailing-
Traditonal logic is that keeping the traveller high on a light air day with the mainsheet eased out a little will give the main an aerodynamic twist. This also gives the main a little belly to catch the wind. Never have the boom higher than the centerline of the boat, though. 'Loose is fast' and easing the backstay and cunningham helps, too.
In heavy wind lowering the traveller and bringing in the sheet as much as possible (but don't forget to ease it in puffs) will flatten the sail and spill wind out of it which de-powers the main. Tightening the backstay and cunningham help flatten the sail, too. Also, like PBzeer said, it'll help the helmsman with weather helm.
Downwind you want the traveller low and the backstay, etc. eased. The main is no longer being pulled in an aerodynamic lift put the wind is pushing the boat.
|09-24-2006 11:40 AM|
|PBzeer||Using the traveller will also reduce weather helm.|
|09-24-2006 11:07 AM|
Simply put, when you ease the main sheet, you do depower, but the boom goes up as well as out so the main bags. This is not the ideal shape. When you ease the traveller, you can still sheet the boom down to keep it flat. By moving the boom outboard, you depower but you have better sail shape and the boat will point higher and balance better.
|09-24-2006 10:28 AM|
What is the point of a traveller?
I am obviously new to sailing. I have asked this question several times and can't seem to get an answer.
When I ask the purpose of a traveller, I am told it is used to depower the main. While I understand that concept, I don't see why Sheeting out the main shee will not accomplish the same thing. Why would someone move the traveller rather than sheet out the main sail?