Is this true? A light displacement boat will handle much better in waves if twist is set properly? I think it goes like this. The top of the mast pitches forward, its relative speed increases, so AW moves forward. The top of the mast pitches backwards, its relative speed decreases, so AW moves aft. If twist is set, so the top of the sail is powered when the AW is aft, that helps pull the mast forward counteracting the pitch backwards. This setting will also luff easily when the AW moves forward, thereby adding no power to a mast that is already whipping forward.
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(Armchair sail theorist alert) Most of what you say seems quite reasonable. My one thought would be that the pitching of the boat through waves involves relatively larger forces than the variation of forces resulting from the changes in AW and AW angle. If that is true, then the moderating effect on pitching would not be that big. You do mention a "light displacement" boat. But larger boats, with larger displacements, and taller rigs (the important factor) will have a more dramatic effect on the pitching of the mast. I suppose once the boat length starts to exceed the wave length by some factor, then pitching will be reduced. Ross Garret (I think that's his name) has a great book on the Symmetry of Sailing, and I was just reading a section on this exact subject last night. He provides an example where the AW at the CE point of the sail might vary from ~7-~20 knots (with a correspondingly large variation in wind angle too) for some sample boat, wind and wave conditions. He does discuss an interesting fact where it takes a certain amount of time for maximum lift be be achieved when the AW wind angle changes. The amount of time can be calculated directly from a few things (wind speed, chord length, etc). I guess the idea is that a pitching mast/sail will have a hard time generating, or attaining maximum lift. It's important to keep your boat moving through waves, and having your sails luffing for a portion of the time, might sacrifice too much power. The other approach would be have your sail adjusted so they never luff. Then the sails would vary between pulling hard, and over sheeted and maybe somewhat stalled. As Ross Garret discusses, depending on how long the sail is somewhat stalled for, and how long it takes for the air flow to adjust to the change in AW, might mean that you get more power than having your sails luffing some of the time. Food for thought. Chris
(Armchair sail theorist alert) Most of what you say seems quite reasonable. My one thought would be that the pitching of the boat through waves involves relatively larger forces than the variation of forces resulting from the changes in AW and AW angle. If that is true, then the moderating effect on pitching would not be that big. You do mention a "light displacement" boat. But larger boats, with larger displacements, and taller rigs (the important factor) will have a more dramatic effect on the pitching of the mast. I suppose once the boat length starts to exceed the wave length by some factor, then pitching will be reduced. Ross Garret (I think that's his name) has a great book on the Symmetry of Sailing, and I was just reading a section on this exact subject last night. He provides an example where the AW at the CE point of the sail might vary from ~7-~20 knots (with a correspondingly large variation in wind angle too) for some sample boat, wind and wave conditions. He does discuss an interesting fact where it takes a certain amount of time for maximum lift be be achieved when the AW wind angle changes. The amount of time can be calculated directly from a few things (wind speed, chord length, etc). I guess the idea is that a pitching mast/sail will have a hard time generating, or attaining maximum lift. It's important to keep your boat moving through waves, and having your sails luffing for a portion of the time, might sacrifice too much power. The other approach would be have your sail adjusted so they never luff. Then the sails would vary between pulling hard, and over sheeted and maybe somewhat stalled. As Ross Garret discusses, depending on how long the sail is somewhat stalled for, and how long it takes for the air flow to adjust to the change in AW, might mean that you get more power than having your sails luffing some of the time. Food for thought. Chris
This last time pounding through the chop
We had the 135 sheeted in pretty tight and the Main eased to the edge of luffing in order to stay more upright than 35 degrees and to reduce weather helm. The boat power through the waves fine and only lost speed with the last, big set. I was pointing pretty high in order to keep some sea room until we got to the channel. TWS varied from 22-30 and apparent was in the neighborhood of 28-38 knots. We had the 135 furled to about 90% and one reef in the Main.
We had the 135 sheeted in pretty tight and the Main eased to the edge of luffing in order to stay more upright than 35 degrees and to reduce weather helm. The boat power through the waves fine and only lost speed with the last, big set. I was pointing pretty high in order to keep some sea room until we got to the channel. TWS varied from 22-30 and apparent was in the neighborhood of 28-38 knots. We had the 135 furled to about 90% and one reef in the Main.
Depends on the boat ......
I like to S-turn (slightly) up and down waves, so that the max. 'power up' occurs just as the bow is 'reemerging' and is lifting to meet the oncoming wave (just at the exact moment you need more power for the 'ride uphill'), and the sail 'just' begins to stall out at the top of wave. If you can S-steer (by following a FULL set of telltales on jib/main, etc. ... especially a set of 'steering telltales' on the jib), you can keep the windflow attachment and angle of attack quite constant and without all that 'noise' going on at the 'top'. For a lightweight displacement or planing hull you dont need all that much 'twist' aloft ... only with heavier crew (rail beef) or heavier boat do you need more 'shoulder' and more twist; even so. less twist aloft will be more 'stable'. For most conditions, I find by keeping the second batten from the top parallel to the centerline will usually get me the fastest beat and I usually S-steer the boat to keep the wind angles constant, your mileage may differ. Since you are going down the backside of a wave faster and you dont want to stuff the bow into the next wave the boat will have the apparent wind more aft (due to the speed) and that angle will put you at an angle to the bottom of the trough (less bow immersion) In contrast when going 'up' you need to power up for going up and needs a little bit of wind on the weatherside (and the tell tales on BOTH sides streaming straight back but the 'slow down' during the 'up the wave face' will allow a higher point as the apparent wind goes forward (because of the slow down) and gives you a 'lift' ... and if you dont S-turn to meet the shift the boat will be caught powered up. This all depends on how much the boat is pitching (or hobbyhorsing if the boats ends are 'heavy' or overloaded) I get a lot of practice time for steep chop sailing/racing on the shallow Chesapeake (crab crusher) and on large shallow lakes (scow).
I like to S-turn (slightly) up and down waves, so that the max. 'power up' occurs just as the bow is 'reemerging' and is lifting to meet the oncoming wave (just at the exact moment you need more power for the 'ride uphill'), and the sail 'just' begins to stall out at the top of wave. If you can S-steer (by following a FULL set of telltales on jib/main, etc. ... especially a set of 'steering telltales' on the jib), you can keep the windflow attachment and angle of attack quite constant and without all that 'noise' going on at the 'top'. For a lightweight displacement or planing hull you dont need all that much 'twist' aloft ... only with heavier crew (rail beef) or heavier boat do you need more 'shoulder' and more twist; even so. less twist aloft will be more 'stable'. For most conditions, I find by keeping the second batten from the top parallel to the centerline will usually get me the fastest beat and I usually S-steer the boat to keep the wind angles constant, your mileage may differ. Since you are going down the backside of a wave faster and you dont want to stuff the bow into the next wave the boat will have the apparent wind more aft (due to the speed) and that angle will put you at an angle to the bottom of the trough (less bow immersion) In contrast when going 'up' you need to power up for going up and needs a little bit of wind on the weatherside (and the tell tales on BOTH sides streaming straight back but the 'slow down' during the 'up the wave face' will allow a higher point as the apparent wind goes forward (because of the slow down) and gives you a 'lift' ... and if you dont S-turn to meet the shift the boat will be caught powered up. This all depends on how much the boat is pitching (or hobbyhorsing if the boats ends are 'heavy' or overloaded) I get a lot of practice time for steep chop sailing/racing on the shallow Chesapeake (crab crusher) and on large shallow lakes (scow).
I think you have it right
but I said it both ways in my senior moments, I hope you'll forgive me. I haven't raced in waves for some time now. The point is that when the top is luffing the bottom is driving, and when the top is driving the bottom is stalled. At every point in between some part of the sail is always driving, so you never lose drive in the sail. The normal trim would have a straight, flat sail trimmed to the nominal wind, which totally luffs as the mast goes forward and totally stalls as the mast comes aft. Driving correctly only as the mast passes through the nominal point. The continuing drive of some part of the sail with a slight twist, is much more efficient than the on and off drive (more off than on) of the straight sail. This applies to heavy displacement boats as well to an even greater degree because their bow dips deeper in the trough and raises higher over the crests. As Chris said this change in the height of the drive off the water has little affect on the pitching, because the pitching is caused by the wave, and the force of wind doesn't match the force of water. Remember this is a moderate twist so that some part of of the sail is always properly trimmed to the changing AW due to waves. It is not the more extreme twist you can use in the main to quickly lower the center of effort to reduce heel rather than dump the whole sail by letting the traveler. This more extreme twisting is similar to scandalizing a gaff main by dropping the peak of the gaff. It's an old technique that buys time before putting in a reef.
but I said it both ways in my senior moments, I hope you'll forgive me. I haven't raced in waves for some time now. The point is that when the top is luffing the bottom is driving, and when the top is driving the bottom is stalled. At every point in between some part of the sail is always driving, so you never lose drive in the sail. The normal trim would have a straight, flat sail trimmed to the nominal wind, which totally luffs as the mast goes forward and totally stalls as the mast comes aft. Driving correctly only as the mast passes through the nominal point. The continuing drive of some part of the sail with a slight twist, is much more efficient than the on and off drive (more off than on) of the straight sail. This applies to heavy displacement boats as well to an even greater degree because their bow dips deeper in the trough and raises higher over the crests. As Chris said this change in the height of the drive off the water has little affect on the pitching, because the pitching is caused by the wave, and the force of wind doesn't match the force of water. Remember this is a moderate twist so that some part of of the sail is always properly trimmed to the changing AW due to waves. It is not the more extreme twist you can use in the main to quickly lower the center of effort to reduce heel rather than dump the whole sail by letting the traveler. This more extreme twisting is similar to scandalizing a gaff main by dropping the peak of the gaff. It's an old technique that buys time before putting in a reef.
AW Wave thanks
Thanks all. Lots to digest. The knowledge gained with age can be of great value to the less experienced.
Thanks all. Lots to digest. The knowledge gained with age can be of great value to the less experienced.
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