Tacking angles?

[Tom Lathrop]

If the wind direction remains constant, apparent wind will move aft in a puff and forward in a lull. So, if your steer to a constant apparent wind you will be lifted in a puff and forced ot bear off in a lull. Of course the wind seldom cooperates in a completely predictable manner so it is left to the skipper to determine the best reaction at any given moment, which is why the performance of identical boats varies so much on the race course.

On a reach when we are usually looking for power, the quickest way to the next mark is to bear off in a puff and steer up in a lull. That maximizes the average wind speed and boat speed. If planing is possible, that changes things again and it is usually best to get on plane whether it takes you up or down. If you are day sailing, just do what is most fun for you.

There are no absolutes in this and waves as well as position of other boats add other dimensions to decision making. Its the chess game nature of sailing that makes it so captivating and why some excell and some never seem to get the hang of it. Modern instruments can do much of the decision making, but where is the challenge in that?

[Howard]

On the displacement boats I have sailed, assuming light to moderate winds where a boat can and will speed up or slow down in response to puffs and lulls. In a puff, the boat speeds up and the apparent wind shifts forward. As soon as that happens, my sails will start to luff. I can either take in sheet to trim the sails to the new apparent wind to get them drawing again, or I can pull my tiller to weather to bear off until the sails fill and start pulling again. Unless you want to fiddle with sail trim all day, the easy way is to bear off.....at least as far as the tiller is concerned. The opposite happens in a lull.

I have noticed the same thing with my asymmetrical spinaker in light air. On a beam reach, any breeze at all will get the boat moving, but the apparent wind then shifts forward and the sail collapses. I can either bear off or trim it in to keep it drawing. Keep doing that and eventually you get into a situation where you are trimmed in all you can and the boat is still sailing almost dead downwind. You notice the same thing when motor sailing in light air. The only sail that won't luff is one that is sheeted in tight and you may still have to bear off to keep it from luffing.

Years ago, I built a self steering wind vane for my 17 foot sloop. I mostly single hand and wanted to be free from the tiller for long legs of several hours or more. The most interesting outcome of that was how good at steering those things are. They never get tired and mess up. One particular event comes to mind. Wind speed was around around 12 knots but the variation was below 10 and above 15. Enough wind we were pushing hull speed. I was sailing with a pack of 5 similar boats and we were all headed about 10 miles down the the lower Patuxent River. I set the vane gear to a close hauled setting and let it do all the work. In a puff, the boat would heel and the wind vane would apply a lot of weather helm.....more so than I would have as the boat was heeling from 15 to over 20......I would probably have allowed her to luff up......but the vane steers to apparent wind....so that was it's response. But there was also a noticeable increase in speed and for some strange reason, she always pointed to a higher compass setting too. In the lulls, it applied lee helm and it would steer a higher course. Bottom line is we ran off and left the fleet, pointing higher and going faster. I got to the windward mark......a navigation marker at a channel bend......a good half an hour before the rest of the fleet. I hove to and waited for the rest of the fleet to arrive. I got above the windward mark on one tack.....some of them made at least two. Again, nothing I did......I was just along for the ride. But it was a good lesson in how to helm a boat!

[MisterMoon]

The CS17 is pretty helpless downwind in light air.

I think just about if not all boats are helpless downwind in light air. It is faster to "tack downwind" (gybe back and forth). It varies a good bit as to what is the best TWA between designs. But for light winds most fall in the range of 150º -135º from true wind.

Yeah, but boats with taller rigs are less helpless. Maybe a log canoe style topsail is the answer?

Um.... Maybe not...

[Tom Lathrop]

Howard, Notice the first phrase of the first sentence in my post. What you observe will certainly be the case sometimes and sometimes not. In a puff, usually the wind shifts aft first and then goes back forward as (if) the boat speeds up and the final result should be no change in relative wind direction on the sails. Puffs are seldom just an increase in the wind and are usually accompanied with a change in direction. This is often no more than a change in the vertical wind sheer bringing stronger wind down to the surface. Wind sheer is much less noticeable in stronger winds. Wind direction is seldom the same at the surface and higher up and this must often be noticed and taken into account in sail trim to get the best advantage by adding twist to the sails.

In any case, it is the task of the person(s) steering the boat and trimming sails to figure out what is hapenning to the wind and make adjustments accordingly. There is no one best solution that always works.

[Howard]

I should probably clarify one thing. When sailing a heading to the day's median wind speed, the first thing that happens (assuming no change in wind direction) when a puff hits is the apparent wind speed does shift aft, as Tom suggest. In response to the greater force of the higher wind speed, before the boat has had a chance to speed up, the boat heels over. With the center of effort already aft of the center of lateral resistance, the boat naturally wants to weathercock into the wind. That, plus the dynamic forces of the hull both try to turn the boat to weather (a good thing). Weather helm, of course, counteracts this to maintain the heading.

I have always understood this to be part of the reason for the belief that boats are said to head up in a puff and fall off in a lull. But once the boat responds to the additional wind with more speed, then the apparent wind does shift forward.

Less understood is the idea that wind direction (at least frontal winds) is seldom constant and oscillates in direction, and the direction it comes from may differ between puffs and lulls:

http://www.sailingbreezes.com/sailing_breezes_current/articles/april02/dell.htm

What this does not mention is that gusts from frontal winds do not blow from a constant direction as a sea breeze might, but tend to oscillate to the side....meaning if you are sailing on a starboard tack, and a gust hits, it will be a lifting wind (northern hemisphere). If you don't think so, go down to a body of water sometime on a day when the winds are gusting and notice the direction of the normal winds and where the gusts come from. Most likely, it will be from your right.

[Howard]

Sorry......post above got hung up and is a few clicks out of phase.

Tom's reference to wind direction being different at the surface than higher up is a tricky one that never made any sense to me and if it was so, would that not also matter on what tack you were on?

The only thing that has ever made sense to me (and I could be dead wrong about this) as far as sail twist up and down the mast is concerned is that wind speed differs up and down the mast, and twist is required for a sail just as it is for propellers on airplanes and wind generators. The tip swings faster than the hub, so to get the air to come across the blade so it is pulling, the twist has to occur. Wind speed increases as you get away from the water surface and the friction and turbulence at the surface.

In an extreme example, wind direction does change with altitude. That is how they steer hot air balloons. They go up and down.......but that is several hundred feet or more.....not the few feet of difference we see on a mast. Or, as I say, I could be dead wrong.

I know in light air, tall rigs always wax me. Aside from the differences in lift of their high aspect rigs, they get wind and start moving when my sails are limp and hanging. First few times that happened I was left wondering where they were getting their air? We were sitting side by side and they were moving and I wasn't. You can't see it, but it's up there.

[Howard]

BTW, I found this site not long ago. It was developed for Radio Controlled (RC) ships, but in my experience, many of these theories and principles are the same if the "ship" is 4 feet or 40.

http://www.onemetre.net/Design/Design.htm

http://www.onemetre.net/Race/Race.htm

I find the racing stuff to be interesting, even though I'm not an avid, die hard racer. I find that getting somewhere to beat a tide change or to my destination in 5 hours under sail vs. 7 hours having motored the last 5 miles to be just as rewarding as winning any race. The same techniques that a racer would use still apply to me and sailing a boat well is simply part of the fun and enjoyment of sailing. Tricking the wind into taking me where I want to go.

[Tom Lathrop]

This is getting kind of deep. The twist in airplane propellers has nothing to do with wind twist with height above water. Propellers twist in order to maintain a constant pitch vs diameter. Same thing with any kind of propeller. In a constant wind there will be twist in the APPARENT wind with height. This is due to friction at the surface that slows the wind near the surface. Apparent wind is the vector sum of wind speed and boat speed. Therefore if the boat speed is constant, apparent wind will be more forward down low and more aft higher up the sail, calling for some degree of twinst in the sail. More twist in light wind and less in high wind due to lesser vector effect of boat speed in high wind. Its just natural and not a figment of imagination.

There is a lot to learn but you should not overthink it. Don't depend on anecdotes and look more at basics.

Edited to add APPARENT, which was erroniously left out.

[Howard]

Ah......apparent wind (and thus direction) changes up and down the mast due to wind speed gradients up and down the mast. That makes sense. Most places I've seen this described have always left out the "apparent" part, leaving one to conclude a change in the actual vector, which never did make any sense to me.

[Tom Lathrop]

Ah......apparent wind (and thus direction) changes up and down the mast due to wind speed gradients up and down the mast. That makes sense. Most places I've seen this described have always left out the "apparent" part, leaving one to conclude a change in the actual vector, which never did make any sense to me.

Actually Howard, the true wind can sheer with height. In fact for true wind to be in different directions at height (as you said in the baloon analogy), it must sheer. Lots of things can cause this, like local topology (river channels, hills), localized thermal conditions (warmer or colder surface water), etc. It is very common in summer in light air although change in apparent wind is usually the greater issue. Those that recognize the condition and trim their sails accordingly can make big gains in a race when most boats are moving slowly. Anyone wanting to become thoroughly confused can read Stewart Walker on the subject of "vertically unstable air".

Years of one design racing on inland lakes forced the learing and practice of these methods in order to be successful. Real situations are usually too complex for anyone to be right all the time. The best you can hope for is to be right more often than your competitors.

[Hirilonde]

This discussion helps describe why tell tales are so important. I let them tell me what I need to know. Them along with boat trim and the feel of the helm. If I can keep my boat heeled (or not) as is it's best trim to weather, my sails acting like the foils they are and my boat staying as close to the wind as is efficient then I must be doing something right. There is no time to get philosophical about what is going on and then decide what to do. Knowing how the boat will react to different kinds of puffs is good for learning, but rarely do you rationalize decisions based on knowing the details of what is happening. By seeing and feeling the information the boat, helm and sails/tell tales send me I make my decisions. If I am good at this, I will make the right decision more than not and execute the best action (sail trim, heading, weight distribution).

Actually, with practice, you don't even need to see at all

http://www.blindsailing.org/index.php