Sailing upwind against the current

Discussion in 'Laser Talk' started by tpfaok, Feb 15, 2007.

  1. tpfaok

    tpfaok New Member

    Likes Received:
    0
    Trophy Points:
    0
    How do you suggest I sail upwind to the windward mark when there is an adverse current? The strength of the current is the same at both sides of the course.
    Should I head more towards the opposing current to keep the exposure of the hull to the current to a minimal?
    E.g. should I sail more on closed-haul than a closed-reach if the hull is less exposed to the current on a close-hauled c.f. on a closed-reach? I.e. to minimise the leeway even thu' the boat has more speed on closed-reach than on closed haul?

  2. TonyB

    TonyB Member

    Likes Received:
    0
    Trophy Points:
    16
    It makes no difference either way. It's hard to explain, but the current is carrying the boat to leeward rather than pushing it, and will therefore take the boat to leeward at the same speed regardless of the angle of the boat to the current.
  3. HECS

    HECS New Member

    Likes Received:
    0
    Trophy Points:
    0
    Imagine the water is like a rug, being pulled across the floor.

    How you angle yourself on the rug does not affect the speed at which you and the rug move across the floor.

    Angling the hull across a flow, when you are floating freely in that flow, will not affect the way the flow moves the hull any more than it does when the water is still.

    Hmmm. Not a good explanation, but trust me on this one.....
  4. Deimos

    Deimos Member

    Likes Received:
    6
    Trophy Points:
    18
    You say the current strength is the same across the course but what about its direction ? Sailing with a current can affect the apparent wind and thus might make a difference if the velocity (speed and direction) is not the same across the course. The reason for the change in the apparent wind is; imagine you are drifting with the current with no boat speed and in zero wind - you will feel a wind the same speed as the current in exactly the opposite direction. Thus you may find one tack lifted by this effect (lee bowing the current). However (as others have said) this will only impact on your tactics if there are changes in current across the course (current speed or direction).

    Ian
  5. Rob B

    Rob B D12 Secretary for 2013

    Likes Received:
    1
    Trophy Points:
    38
    Shallow water has less current in it. To avoid current sail on the shallow side of the course unless wind pressure trumps going out of your way to aviod current. If it is the same across the course everyone else has to deal with it too. So, sail the best course as you would without current and just remember the current will affect your laylines. One tack the current will help on the other it will hurt. Which tack has the advantage depends on the set up of the course, wind direction, mark position and so on. Try to stay on the advantage tack the most.
  6. RainMan

    RainMan New Member

    Likes Received:
    0
    Trophy Points:
    0
    Not mentioned is the speed of the current, is this .5 knot current or a 5 knot current? Is it coming from a river mouth or a tidal current? I.e. San Francisco bay. Pretty much stay as close to shore/ walls/piers/misc disturbances as is safe. If it’s a small current .75 knot or less than play wind more strongly, 5 knot more wind/ better angle than sail in the current. Best plan might be make a picnic day with the significant other and watch what the boats racing are doing.
  7. Georg W.F.

    Georg W.F. New Member

    Likes Received:
    0
    Trophy Points:
    0
    I think your question might have originated from a phenomenon known as the lee-bow effect. People used to argue that if you point higher you get lee-bowed by the current. However, as pointed out by others, current does not work this way: you are pushed in the same direction no matter how you are lined up in that current.
    So, the theory of the lee-bow effect is still confusing people, and current is confusing. Looking at how you move in comparison to other boats and in comparison to the land is very helpful.
    G
  8. computeroman2

    computeroman2 Member

    Likes Received:
    0
    Trophy Points:
    16
    There are two phenomenons that occur with current; the two are often confused as they can act simultaneously. First, in a body of water with an even width, current flows faster in deeper water. If the width changes, e.g. a bottleneck at the mouth of a harbor, the current will flow faster through the shallower bottleneck.

    That aside, if the current is in fact uniform, as mentioned already the lee-bow effect doesn't really matter- everyone is uniformly pushed down the course. What will matter is boatspeed- if you're going strait into the current, you have the current working to directly kill your boatspeed. If you sail across the current, you don't lose forward speed. So unless the wind is directly up-current, try to find the tack that is more perpendicular to the current flow.
  9. tpfaok

    tpfaok New Member

    Likes Received:
    0
    Trophy Points:
    0
    How should I sail if the wind (10 knots) and the tidal current (7 knots) are in almost the same direction?
    Am I correct that sailing perpendicular to the current flow is not an option as the current will head me off from the windward mark?
    Am I better off sailing closed-haul or closed-reach? The former is shortest course to sail to the windward mark while there is more boat speed for the latter.
  10. TonyB

    TonyB Member

    Likes Received:
    0
    Trophy Points:
    16
    It really truly makes no difference.

    Sailing perpendicular to the current will take you forwards and sideways, and sailing directly into the current will take you backwards. The overall impact on your VMG will be the same in either case.

    If you don't believe me, draw yourself up some vector diagrams and you will see what I mean. As long as speed and direction of the current is fixed, you will be on the same ladder rung (ie VMG) after the same amount of sailing time regardless of the direction you go. Try it for some different current speeds and directions and the results will be the same. I knew that one day I would find a use for the vector maths I studied all those years ago!

    Whether you foot or pinch also doesn't matter. The current will still have the same effect on you either way. HECS' analogy in an earlier post of a rug being pulled across the floor is a good one.

    And all of the above is irrelevant if the current is moving at seven knots. A laser will not make ground to windward on either tack in an adverse current of that strength, unless you attach an outboard.
  11. computeroman2

    computeroman2 Member

    Likes Received:
    0
    Trophy Points:
    16
    I also personally find a 7-knot current a little ludicris. However I do think sailing perpendicularly vs. into the current makes a difference. I had a miserable race last year in a river with a uniform 3-knot current. The wind was only 6 knots or so and angled so that one tack was dead into the current and the other was exactly perpendicular. If I sailed strait into the current, I completely stopped after ten or twenty seconds and lost flow over the boards. Sailing crosscurrent was much faster- I didn't experience a lot of leeway because the current created more lift over the CB and made leeway minimal. So if you have a choice, I say sail across the current as often as possible.
  12. TonyB

    TonyB Member

    Likes Received:
    0
    Trophy Points:
    16
    I take your point. Sailing directly into a three knot current and making no SOG means that you have three knots of flow parallel to your foils. Freeing off will alter the angle of flow and generate some lift. So there is a third component to consider in the vector diagram.

    If you are sailing across the current you will move over the ground faster but will also be losing a lot more ground to leeward. Ultimately you won't be moving towards the top mark any faster (centreboard generated lift aside).

    Regardless, if you are sailing to windward you are going to have to sail on both tacks at some stage and if the current is consistent across the course there is no benefit in taking one tack or the other first. Which means that you should ignore the current as a strategic factor.
  13. SAILWRITER

    SAILWRITER Member

    Likes Received:
    0
    Trophy Points:
    8
    The only way to get extra lift from current is to anchor or stand on the bottom! Just as you are zipping at over 1000 miles an hour eastward while standing on the earth without feeling it, if you look down at the water while sailing in current, you will not see the water move. Just sail the boat and adjust for lay lines.
  14. TonyB

    TonyB Member

    Likes Received:
    0
    Trophy Points:
    16
    Sailing at three knots directly into a three knot current is the same thing as anchoring or standing on the bottom - you stay in one spot, therefore the flow is visible and directly parallel to your boat and also your foils. Sail at a slight angle to the flow and you will get flow across your foils to generate lift.

    I did get in backwards in my last post though - you would need to pinch rather than foot to get the flow going the right way around your foils to generate lift. I doubt that than the minimal lift you would gain would not offset the reduction in speed from pinching.

    And anyway, like I said before, you are going to have to sail on both tacks eventually to get to the windward mark and it makes no difference which way you go first. So ignore the current and just sail to the wind and the fleet.
  15. SAILWRITER

    SAILWRITER Member

    Likes Received:
    0
    Trophy Points:
    8
    I give up! Dave Perry has been trying to explain this for years. Yet there are still sailors who want to believe that they get extra lift when they are drifting along with current. Less apparent wind, yes. No effect on the foils. Ask somebody who sails at College of Charleston, the current capital of North America, if they change their style of sailing for current. What am I saying.....I have to race these guys. Go ahead! pinch!
  16. fosq

    fosq New Member

    Likes Received:
    0
    Trophy Points:
    0
    If you're making zero speed over ground on one tack on a beat to windward, it doesn't matter how fast you're going on the other tack, you'll never get to the windward mark.
    In order to generate lift over your blades, you need an angle of attack. The boat must be making some leeway in order to generate lift. If you're making three knots upwind against a three knot current (stopped over ground) heading up does not change the angle of attack because you're also changing the direction the boat is going. If you anchored in a strong current and turned the boat in either direction, you would change the angle of attack and generate lift. When you're sailing in that same current, any flow over the blades is generated by you're own power. If you didn't generate power to move forward, you would drift backwards and there would be no flow. When you head up you only change the direction the boat is going, you don't change the direction relative to the current...the current vector remains the same and you are always being set in the same direction with it.
  17. TonyB

    TonyB Member

    Likes Received:
    0
    Trophy Points:
    16
    Umm, yeah... I've read "Winning in One Designs" too.

    In that book and elsewhere, Dave Perry disproves two different fallacies. The first is that there is a difference in current induced apparent wind from one tack to another. The second is that if you pinch when sailing directly into current, the current will hit your lee bow and push you to weather. The so called "lee bow effect". If you read through my posts, I am saying the same thing as Dave.

    I am also saying something else which he does not cover, as far as I can see. That is that when I am sailing directly into current, with zero or better SOG, the current is flowing past my boat at a rate of (SOG_of_boat + speed_of_current). The important point is that my boat (and centreboard) don't know and don't care whether it is the boat or the water moving or a combination - all it can see is the water flowing relative to the foil.

    It is the flow of the water relative to the speed and angle of attack of the foil that induces lift. So, if I maintain a steady SOG while changing the angle of attack of my centreboard, it holds that I can generate some degree of lift. If I can turn into the current and wind (ie pinch) without reducing my SOG, I will lift relative to someone who continues to sail directly into the current. Of course, as I said in an earlier post, it is unlikely that I will be able to pinch and maintain my SOG and the loss of speed will probably not be offset by the gain in height. However, that doesn't mean that the effect does not exist - just that it may not be worth using.

    I may be wrong, but nothing that Dave Perry says disproves it. We are talking about different things. If you, Dave Perry, the College of Charleston, a passing physicist or anyone else can tell me where my reasoning is wrong, please share.
  18. SAILWRITER

    SAILWRITER Member

    Likes Received:
    0
    Trophy Points:
    8
    Point taken. Without delving into word semantics, if you pinch just enough to get to beyond straight up current, the boat will appear to be drawn to that direction. Vector of forces thing. There are certainly times when that is tactically great! You may make a mark otherwise missed without two more tacks, You will definately need two more tacks if the bow is below the straight up current.
    BUT, it may be faster to put the bow down, go the extra distance faster and make those two tacks.
    Gotta go do RC duty for midwinters. Let's see whether the hot-shots change their sailing style for current....
  19. mlemieux1978

    mlemieux1978 New Member

    Likes Received:
    0
    Trophy Points:
    0
    People talk about this as though there is a definitive answer. there isn't. both arguements are correct. what it boils down to is VMG (velocity made good towards the mark). as we all sail the same boat, in theory as the conditions change we will all do the same thing. the difficulty is recognizing when those conditions change and having the ability to shift gears. with the laser i submit that pinching in current is not the answer unless there is a significant amount of breeze, at which point you will probably be pinching some of the time anyway. There really isn't any way to really do the math while sailing a laser. Local knowledge is the only answer. The college of Charleston argument brings this point home. The current from the Cooper and Wando Rivers are not going to be the same over the whole course. i know this well enough since i still traverse these waters on a monthly basis with a tug and barge, so i have a pretty good understanding of what the current is doing there in most conditions and stages of tide. the sailor must decide if height towards the mark will pay off if the distance sailed is less at a slower speed rather than increased speed through the water to get across the tidal flow to the shallow water where the effects of the current are less. an exact prediction of how much current you have in a location is impossible to predict a meaningful amount of time in advance of a race due to the effects of wind across the waters surface, and the wind velocity and direction are constantly changing, so arguing to determine if there is an end-all technique for this vagary of the sport will continue forever. I bet if you watch the whole fleet for the first day and then the gold fleet on the last day of any regatta you can name you will notice a distict difference in how any boat is sailed.(except maybe the locals) knowing what to look for on a certain patch of water and ahving the knowledge of both theories will make you a better sailor.
  20. Georg W.F.

    Georg W.F. New Member

    Likes Received:
    0
    Trophy Points:
    0
    The above is all pretty confusing. I think that you should stop talking about vmg. The vmg is ALWAYs independent of current. You always have to sail a certain angle to the wind in certain wind conditions in order to go fast.
    What changes with current is NOT the VMG, but the apparent wind.
    1. If wind and current are running in the same direction and you sail upwind, the apparent wind will increase (because you go faster and create wind).
    2. if wind and current run in opposite direction and you sail upwind, you go slower and hence the apparent wind will decrease.
    3. if current runs accross the course one tack will create more apparent wind than the other.
    You will always deal with this apparent wind. Do not pinch: that is exactly what Perry argues against! So, sail the wind that you experience and treat it as if there is not a current. Forget about the rest, such as vmg and pinching, it is all hogwash!

    Georg

Share This Page