How do I 'read' the "Standard Draft Polars" chart

Discussion in 'Boat Design' started by Squidly-Diddly, Nov 22, 2008.

  1. Squidly-Diddly
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    Squidly-Diddly Senior Member

  2. Tcubed
    Joined: Sep 2008
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    Tcubed Boat Designer

    The lines represent the locus of the boat's velocity vector as it goes through the various headings.

    Notice each line has a number, this is the line for that number windspeed in knots ( at ten meters altitude according to them )

    The polar has different circles representing boatspeed. The center represents speed = zero. The further away from the center, the faster, it is marked in increments. The heading is the angle from the wind, straight up is straight upwind. Notice none of the lines reach the straight up axis since the sailboat cannot sail straight upwind.

    Notice the lines are not continuous. They get to a certain angle off the wind and stop. Then another line starting from about a reach but with the same windspeed number carries on and goes all the way down to the axis that goes straight down, dead downwind. This represents different sail combinations (spinnaker or no spinnaker, etc.).

    Standard draft just means the polar is plotted for the better performing model, the one with the standard draft. The option with a shorter keel will have a slightly slower plot

    Hope that explains it for you.

    My question would be are these real, tested, actual sailing speeds plotted or are they just what a velocity prediction program computer software predicted. You can be sure that if they are the latter (and they probably are), they are going to be somewhat optimistic over real world figures where there are real waves and real control issues.
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