Question on definitions for upwind angle

Discussion in 'Sailboats' started by gs41escapade, Oct 13, 2009.

  1. gs41escapade
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    gs41escapade New Member

    The typical value of upwind angle quoted for 'most boats' is 45 degrees, less for sportboats and ACC, more for multi-hulls, etc.

    My question is, is this 'apparent wind angle' (AWA), 'true wind angle' (TWA), or what I will call 'windward progress angle' (WPA - meaning factoring in leeway angle as well).

    The reason I ask is that with my GPS, I can measure WPA from the recorded tracks, and am trying to figure out my actual versus expected performance.

    Either I'm doing OK (but not great), or I'm doing terrible and need to fix things.

    Thanks for any insights.
     
  2. Paul B

    Paul B Previous Member



    Generally that is TWA.

    For a modern fin keel boat 45 degrees is about as far off the wind as you want to be in TWA when sailing upwind in normal conditions, tacking through 90 degrees. Most of the time you would be closer than that. Even 30+ year old production boats like the Ranger 26, Catalina 27, or Cal 27 will tack in less than 90 degrees TWA with good sails and someone who knows how to sail.
     
  3. jehardiman
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    jehardiman Senior Member

    But it is important not to get too hung up on mimimizing TWA. What you really want to optimize on is "Velocity Made Good" (VMG) which is the speed you are actually advancing straight into the wind.

    Look at most polar plots and you will see that maximum VMG does not occur at minimum TWA. If you pinch up too much VMG goes down because boat speed decreases faster than distance sailed, conversely if you foot off too much you may go faster over bottom but not as fast upwind as you have to sail a longer distance.

    The actual true and apparent wind angle for maximum VMG will vary on wind conditions and sail choice.
     
    Last edited: Oct 13, 2009
  4. Paul B

    Paul B Previous Member

    My comment above was factoring in VMG. On almost any boat you could sheet a jib very close to the CL and sail a very small TWA, but go nowhere fast. I was not saying that.

    However, there are times when you do want to maximize your pointing ability at the cost of VMG.

    If there is a current line and pinching will allow you to stay in the slack water it can be beneficial.

    If you are racing and can avoid making 2 extra tacks as you aprroach the weather mark you do it.

    If you are racing and round the leeward mark just behind another boat you may wish to go into point mode for a time to hold your lane if you are on the favored tack.
     
  5. kim s
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    kim s Junior Member

    wind angle

    If youcan ignore what the chart plotter is saying IE yournot being sett into danger, the best way to check is with a compass. as said earlier , most boats should tack through 90 degrees.
    The gps is showing a course that includes too many variables to really judge. Amongst these are Leeway, tide and sea state.
    If you can tack through 90degrees, then I should be happy with that and relax and ENJOY.
    I used to have a 7.5m light discplacement boat and in flat water and steady wind , I could tack through 85 deg. in a lumpy sea state, it was better to bear off 5 deg so the wave length increased slightly so I did not slam and stop. Again, the best way is to watch the VMG . Most gps will give you that. Its quite an eye opener if you have not played with them before.
    To get the boat itself going in flat water -again ignore the gps. use the log. This shows exactly what the boat is doing throught the WATER. you can then play with sheeting positions to optimise the sail shapes.
    In lumpy conditions, I tend to undersheet the headsail to give the boat more drive and trim the main for weather/lee helm. I found that quite often people tend to crank everything in too tight and complain " I just ould not get the boat going".

    kim
     
  6. gonzo
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    gonzo Senior Member

    The GPS has to be hooked up to the wind indicator at the very least. Otherwise you are getting an angle to the average or calculated wind.
     
  7. gs41escapade
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    gs41escapade New Member

    Thanks for confirming my suspicions. It's time for new sails -- the current main is at least 6 years old (my time) and probably much, much older than that.
     
  8. ancient kayaker
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    ancient kayaker aka Terry Haines

    The tacking angle seems to be a good measure of upwind performance. It should be twice the TWA and using it as the measure will ensure that the boat has at least enough speed to complete the tack.

    What do the good boats achieve, those that are really good in this respect that is, and what are the design factors that help them to do this? Do those design factors tend to impact negatively on their other performance aspects?
     
  9. Joakim
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    Joakim Senior Member

    This depends very much on boat in question. A Cruiser-Racer or any other boat that sails well on a beat should be able to tack less than 90 degrees viewed from GPS. This applies to 30 year old boats as well.

    The angle gets smaller with increasing wind. Below 8 kn many boats struggle to the 90 degrees tack, but at 12+ kn many boats are able to tack 80 degrees or even below. AWA should be under 30 degrees, even 25 degrees on a beat.
     
  10. tspeer
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    tspeer Senior Member

    Most people report what they observe, which is based on the heading of the boat. But the correct value would be what you're calling the WPA.

    In principle, the heading of the boat is irrelevant, but in practice it is hard to measure the leeway angle and there is a limit due to the stall angle of attack of the keel. I think you have to look at leeway differently from the sailor's point of view vs the designer's point of view. The sailor wants to minimize leeway for best performance, while the designer wants to increase leeway to moderate values to increase performance.

    The WPA, or what I would call the course, has a fundamental relationship that involves 45 degrees. If you measure apparent wind angle between the course through the water and the apparent wind direction (making it the sum of the apparent wind angle measured from the boat's centerline and the leeway angle), for a high-performance craft that maintains an approximately constant apparent wind angle for a range of courses to the true wind, the best Vmg is obtained when sailing (WPA) at 45 degrees plus half the apparent wind angle to the true wind. Most boats do not have the performance to achieve this and thus sail higher angles. But it does illustrate that 45 deg is not arbitrary.
     
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  11. matoi
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    matoi Junior Member

    What would approximately be the desired leeway angle for modern fin keels, and what would approximately be the desired leeway angle for a flat centerboard (Laser for example) ?

    Thank you. Best regards,

    Mato
     
  12. philSweet
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    philSweet Senior Member

    Regarding Terry's question about the design factors that lead to tight pointing ability- The ability of some boats to point well is partly an artifact of the boat's design, not what drove the design. Big fin keels delay stall. They also reduce leeway.
    Match racers need to win tacking/luffing duels at low speed. This often drove the fin design. The boat that can force his opponent to stall first effectively makes him go sail elsewhere on the racecourse.
     
    Last edited: Aug 28, 2010

  13. ancient kayaker
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    ancient kayaker aka Terry Haines

    Minimizing Tacking Angle

    Momentum would assist tacking in light airs but would inhibit agility - speed before the tack would be a better substitute.

    The large fin area you note would also suggest large sail area. High aerodynamic and hydrodynamic efficiency are a given, leading to high aspect ratio of fin and sail together with minimum turbulence from the hull due to leeway, also minimum air drag of superstructure.

    Hm ... tall, hard to handle rig, deep fin that limits marinas choice and too little underdeck space for comfort - this is not going to be the perfect cruiser is it?
     
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