Tuesday, January 24, 2017

Wind Accross Current

In an instructor clinic last year, Steve McBride and I were explaining how current affects the ripples on the water and so we naturally transitioned into the affect of current wind on apparent wind.  We talked about wind against current and wind with current and started to move on.  Naturally Abby asked what happens to the wind and waves if there is cross current.  Since we didn't want to get into trigonometry, we parked that question, but we never ended up getting back to it.  So at long last here is my answer, although as usual with this blog I got carried away with it...

Wind With Current

Starting from square one, If you are standing still on a moving walkway at the airport inside where there is no true wind, you still feel air hitting your face because the moving walkway is pulling you into the still air.  Relative to you there is what we could call 'walkway wind'.  This walkway wind comes in the opposite direction as your motion.  Current can be thought of as a moving walk way or conveyor belt.  In this post, I have called the wind felt due to current 'current wind' and I will try never to use the word current to mean 'now', just moving water.

In this situation the surface of the moving water only feels a very light three knot breeze and so it is likely to have very small ripples and so it would look lighter in colour like a calm patch.  When you sail, you sail by the apparent wind, so this really is a calm patch as far as your sail is concerned and if the whole course has the same constant current, it is more than a calm patch, it is a calm day.  In this case, the current only really matters at the laylines (of the marks and finish) and at the starts.  

Avoid undershooting the windward mark and try sneaking in on people at the leeward mark because everyone will tend to leave too much room around the leeward mark.  At the start there is likely to be huge line sag (take a transit).  Because you can't sail straight upwind, boats on starboard will tend to creep towards the port of the line as they try to stay up near the line causing pile-ups at the pin and openings at the boat end of the line.

Wind Against Current
Next let's take the situation where the wind and current are going in opposite directions.
This time the water is being pulled up into the wind so the surface of the water feels extra wind and so it will be choppier.  As far as the water at the surface is concerned there really are 10 knots of wind, so the water will be quite choppy.  Without a reference point, the only way that you may be able to distinguish current wind from true wind is that current, especially a change in current, often makes the water more turbulent which can make the water look particularly jagged and irregular.  We sometimes call it piranha water because in some cases it almost looks like piranhas are trying to poke up from the surface.  Once you get used to looking at turbulent water, you can often recognize current, but just because you can't obviously tell that there is turbulence doesn't mean that there is no current.

If the current extends across the whole course evenly, it doesn't affect strategy except, once again at the laylines and starts.  At the windward mark it is a good plan to undershoot the windward layline.  At the Gorge (Columbia Gorge Racing Association at Cascade Locks, Oregon) there is strong windward current because it is a large river.  In races and in training, I have undershot the windward layline extremely in the three boat length circle, but then just I could just go head to wind, retain my rights and wait for the 3+kt or so current to pull me up to the mark (if tacking would mean having to duck boats).  This way people can only pass me if they have drastically overshot the mark and can sail past my bow.  Of course it would be faster to anticipate the current properly and hit the layline perfectly, but usually everyone overshoots the mark by a lot and the Gorge often hosts big regattas.  When there is strong windward current it is important to brush up on your rule 18.3!  There are a lot of places to be gained.

At the leeward mark the danger is hitting the mark.  I have sailed almost completely past a mark in light wind with windward current, only to have the back windward corner of my transom hit the mark as my boat was pulled sideways upwind.  

At the start line it is important to be cautious in windward current.  My strategy is to get a good transit and the find a part of the line without much congestion.  When boats are close to each other they tend to get caught up with each other and get swept way over the line.  The Gorge is my favorite example of windward current, there is also sometimes upwind current in Victoria and Vancouver or West Vancouver, but it depends on the tide, so it is less reliable and often not as strong as the Gorge is literally a river flowing upwind.  While playing around with downspeed pre-start maneuvers in windward current, people often get excited when they realize that by lifting their dagger boards a certain amount, they can get the drag of the wind on their flapping rig to perfectly cancel the upwind thrust of the current.  However with your rig forward on your boat and your rudder aft on your boat, this strategy will tend to pivot your bow off the wind until you are on a beam reach or below.  If you are all alone this might be okay, but in a crowd it is a problem, you are asking to be luffed.  As soon as you need to avoid someone (and most people will be to leeward of you), you need to drop your dagger board to maneuver properly and then you get sucked over the line.  

The strategy that I have had more success with is to point straight head to wind and then try to avoid touching my boom and to avoid tacking.  At the Gorge there is usually plenty of wind to push you astern, so if you point your boat far enough upwind that your boom and so the drag of your sail flap inside the back corner of your boat, and then if you angle your tiller straight or slightly to windward, the drag on your sail causes you to drift straight backwards with attached flow on your dagger board and rudder.  What is more, unless you touch your boom or pass head to wind, you have right of way over most other boats.  You are on starboard and it would be hard to luff you since you are already pretty far head to wind.  If you accidentally tack, you are subject to Rule 13 and have to go back below close hauled and reset.  If you touch your boom then you are subject to Rule 22.3 "A boat moving astern or sideways to windward through the water by backing a sail shall keep clear of one that is not".  However if you are moving astern due simply to good clean drag of wind on your rig and you have not touched your boom (even with your shoulder the jury tells me) you retain your rights.  Unfortunately that may be a hard one to explain to the person that you reverse into.  Yes, technically they hit you from clear astern while you were minding your own business, but you better be able to prove that you didn't back your sail.  It is probably a good idea to warn them ahead of time: "watch out clear astern boat, I haven't backed my sail!"

The really cool thing about sitting on the line heat to wind, moving backwards is that to start going again it is relatively easy.  You gently push the tiller away from you to reverse onto a tight reach, sheet in and hike.  It feels weird because the flow on your foils has to switch directions, but it powers up quickly and unless someone is to leeward and bow out on you, you have a good shot at a good acceleration.

Cross Current

That was quite a segue, but at last we have arrived at the final scenario: current flowing across the wind direction.
The angle of the resulting apparent wind depends on the relative strengths of the wind and the current.  4 knots is pretty extreme, but imagining it helps you recognize what happens on a smaller scale with less cross current.  Also inspite of my best efforts, I found that my 7 knot arrow is too short or my 4 knot arrow is too short, but the apparent wind angle doesn't look too far off 30 degrees.

This math behind the 8.1 knots comes from the Pythagorean Theorem based on the triangle drawn above.  The true wind and the current wind don't have to be perpendicular, but if they aren't then you need even more trigonometry.

The angle calculation that gave us 29.7 degrees comes from the trigonometric function 'tangent'Arctangent is the inverse of tangent and it is just used to solve the equation.

Bla bla bla that is math not sailing...

What does it mean for sailing?

Again, let's assume the cross current is constant thorough the course and moving from course-right to course-left as shown above.  This often comes up at West Vancouver Yacht Club where the course is set up in deep, fast moving water if they can get their marks to hold.

If the race committee ignores the current when setting the course, then in the above cases with our 7 knots of true wind and 4 knots of cross current, the course would look quite skewed to the sailors and to anyone else who is not anchored.  Below I have made a slightly less extreme scenario with the apparent wind only skewed fifteen degrees left of the true wind.  Even so, as the diagram shows with the current pushing left you would hardly be able to make the pin.  A good race committee would realign the start line to take this into account, but this is tricky for them because sitting on the anchored race committee boat, their wind instruments do not pick up the current wind.

However, if the race committee squared the start line to the apparent wind by factoring in the current wind, the current will not only be pushing along the line, it will also be pushing up across the line which is dangerous for starting over early as we talked about.  Below is a diagram of the course squared to the apparent wind, notice the current's angle (because you certainly will when you are trying to sit on the start line). 

To deal with this current direction, I recommend taking transits and executing conservative starts.  There will most likely be several black flag starts and a pile-up at the pin.

The next question is where is the windward mark?  In my diagrams I have the start lines square to the rhumb line, but this is actually quite unlikely.  The race committee may or may not start with the start line square to the rhumb line, but they will probably move it around once the fleet starts getting general recalls.  

If the race committee set the course based on readings from an anchored boat in this current, the windward mark will be skewed off to the right.

Also, the starboard layline moves away to windward and the port layline comes down to leeward.  On the upwind, the boats will be pointing at about 40 degrees to the apparent wind as I have shown with the yellow boat in the above diagrams, but their 'course made good' through the water will be heavily skewed by the current.  You can imagine a boat sailing normally relative to the apparent wind, but then it is also being pulled sideways by a conveyor belt while the marks are not.

In the above diagram, there will end up being significantly more time spent on port than on starboard upwind, so when in doubt you should be on port.  You might just want to tack onto starboard if there is a particularly nice right shift.  In fact, even if the race committee squares the windward mark to the current-induced apparent wind, part of the current will be pushing everyone left and the laylines will still be affected, but because you now have a component of windward current added into the bargain, the laylines will come up sooner. 

As you near the port layline (being sucked left, that is the one you will probably hit), take a transit through the windward mark to see whether you are near being swept past the windward mark even though your bow points beneath it.  Remember, as always, it is very risky to approach the windward mark on port within the three boat length circle.  

The downwind leg will also be skewed, even more so if they have tried to compensate for the skew on the upwind and have nearly even upwind sailing time on port and starboard for the upwind leg.  

Since Laser downwind angles are pretty deep, you have to have thought out where you need to go before the downwind starts and to get a visual on the leeward mark.  If the race course was not set to the true wind rather than the apparent wind the mark will be farther to sailor's right than anticipated which is okay because you will be pushed right by the current anyway, so don't start by pointing at the mark or else half way down the course you will have to be pointing back the other way.  

If the course is set to the apparent wind then the leeward mark will look like it is in the right place off the bat, but if you forget about it while jockeying for position and trying to go fast, you will find that it has migrated sailor's left.  People forget that they are on the conveyor belt, so they sail straight down wind when they really want to by the lee or broad reaching slightly into the current so that their course made good is towards the mark.  Again if there is a transit available sighting through the leeward mark can help you keep track of whether or you are tracking towards the mark or slipping sideways.
Below I have drawn out the diagrams for cross current coming from course left to course right with the situation that the race committee has set the course either to the true wind or to the apparent wind.
In this case you would be able to lay the mark from the barge (maybe even from the pin with the current) except that you won't be able to star the race because everyone will be crowding the barge and actually being pushed up into it.  The poor race committee will almost certainly shift the course more like the diagram below.  However keep in mind that cross current is often fairly weak.  Imagine the diagram with half as much left-to-right cross current.  If it is a relatively small fleet and the race committee is good at identifying boats over the line, it could well be sailed.  Watch out for the barge when starting, think about the laylines, and try very hard not to hit the leeward mark.

This looks like a much more reasonable course, but still be careful about getting sucked up into the race committee boat and being pushed over the line by the current, as well as hitting the marks.  The leeward mark would be easy to accidentally hit, as I said before, but I should also mention the windward mark.  Unless you make a point of sailing past it before bearing off, you could easily hit the mark with the back of the boat as you exit the mark when there is cross current from either direction.

It would take too long and may be overly confusing to talk about all of the other permutations and combinations of scenarios where current is strong enough relative to the true wind for it to be significant, but I encourage you to get out some props: toy boats, arrows, maybe a sheet of paper that you can pull with the boats on it but not the marks to simulate current.  Also if you have a favorite scenario, particularly if it is common at a major event you have been to let me know and I could make another post about it playing through the scenarios at the critical points.

It is tempting to study these examples and come away with supposed rules of thumb, but the reality of cross current is that it is often localized, changing with time, or at different strengths in different places.  So it is generally true, in say an unpredictable gently oscillating breeze, that you want to sail the long tack first, and with cross current that means sailing into the current first, however when you sail into strong current, you don't make much ground.  Not making much ground is fine if the current is equal throughout the course for the whole time you are on the leg, but if the current is less somewhere else, or if somewhere else on the course the current is moving at a different angle, then you actually want to sail sideways across the strong current to get out of it and if you have to sail into the current, do so where it is weakest.  

The next layer of complexity is that currents are often weak relative to the effects of the wind, so all of this may be going on in the background but the person who wins may have completely neglected the current effects to focus on wind strategy.  Here is an example: there is a quarter knot of current relief on the right but there is a big persistent left shift.  The people who sailed right may have sailed a faster absolute speed because they were slowed down less by the current, but it could be that the people on the left on the inside of the big shift got to sail 50 boat lengths less distance and so they came out ahead in spite of the current.

Another point where people can tend to go wrong with cross current is that if you are sailing upwind with a cross current, it is tempting, especially if there is a shore that is moving in your peripheral, to fall into the fallacy that if you are footing you are just on a treadmill, but if you are pinching you make more ground upwind.  However, so long as the current is constant throughout the course, every sailor has to sail over the same amount of water that will pass across the course, so the part of your movement that is into the current is inevitable.  Don't pinch.  If you sail your fastest angles, at some point you can tack and cross the pincher.  You will do best if you can sail your fastest angles for as much of the course as possible.  That sounds obvious, but to execute it, it requires thinking ahead so that you don't spend the last part of your upwind or downwind leg on a beam reach trying to correct your course back to the mark. 
Happy side-sliding!  Let me know if any of you have stories about current in specific scenarios: building, dying, turning, current; localized current, tide lines, whirlpools, weighing off various wind strategies against current strategies.  I can't cover it all, but it might be nice to post someone's current story if there is interest.

Many of my craziest current experiences have come in and around Oak Bay.  Here is a link to Bob Britten's huge store or local knowledge about Oak Bay.  The last few movies include some super interesting current-related scenarios.

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