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Re: Whalewatcher photos

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  • cecbell
    As I understand it and for what little it may be worth: Indices like SA/D and SA/Wetted Area ratios have to be taken in context to be meaningful. These are
    Message 1 of 21 , Jul 25, 2010
      As I understand it and for what little it may be worth:
      Indices like SA/D and SA/Wetted Area ratios have to be taken in context
      to be meaningful. These are only useful in light and ghosting conditions
      when raw sail area is more influential than sail and hull shapes. At
      higher wind and boat speeds, sail planform and hull design
      characteristics have to taken into account. There's no need for a whole
      new set of SA/D guidelines if you don't try to apply it out of context.
      The reason I mentioned SA/D ratio in the earlier post was because of
      comments I saw about WW's performance in what I took to be light
      conditions.

      I wouldn't be so inclined to pin a definition of efficiency down to one
      thing because "efficiency" depends so much on what the goal is. If
      you're trying to make boats of good capacity and performance available
      to those who want them but might not otherwise be able to afford it,
      then Drive/Cost ratio seems to be a very good way to look at it. I sorta
      like a Drive/Hassle definition, where hassle is in terms of setting up
      and sailing. It might not always be the cheapest but it can make things
      enough easier so that you go sailing more often than otherwise. I think
      your idea of Drive/Heeling ratio overlaps this somewhat since it's
      definitely a hassle when you have to go to extremes (either in cost or
      athletics or both) to provide extra righting moment for a tall rig.
      Unfortunately, the physics of the situation are unforgiving. What we
      gain in one area we have to give up in another. Another measure of
      efficiency is the Lift/Drag ratio. If the boat needs to be able to sail
      high, then L/D has to be improved. There's no getting around it. Which
      is why racing boats tend toward the high aspect ratio tall rigs and high
      A/R fin keels on slippery hulls--none of which is cheap or easy--lousy
      Drive/Cost ratio. But these sails, by design, lack drag, and lift is not
      necessarily drive. As the boat falls off to beam and broad reaches, sail
      drag lines up better with the desired track and adds to the drive. A low
      sail plan, with its poorer L/D, does this much better than the marconi
      sail and has better drive for the area--but only in this direction. And
      how high do we have to sail anyway? The 80 - 20 rule seems to apply. If
      we can get 80 percent of what we want at 20 percent of the cost, not
      bad. Fire up the motor when we have to make up the difference.


      --- In bolger@yahoogroups.com, "Doug" <oarman89@...> wrote:
      >
      > I think that the conventional wisdom on appropriate sail
      area/displacement ratios is applicable only for marconi rigged sloops,
      since those are by far the dominant configuration today. Because of the
      high aspect ratios of these rigs, if the sail area gets large, the
      center of effort gets high and the heeling moment quickly becomes
      excessive. This in turn leads to deep draft, high ballast ratios, and
      extreme design elements like ballast bulbs on the end of fin keels. If
      you want the speed that comes from high SA/D ratio, in the fashionable
      rigs, you need a pretty extreme hull design.
      >
      > The beauty of the quadrilateral and split rigs that Mr. Bolger favored
      is that the sail area gets spread out horizontally rather than
      vertically, whether on long booms or on yards, or both. The center of
      effort stays low so the heeling moment stays low. Not as much ballast is
      needed. Wading draft is practical. One can carry plenty of sail area for
      ghosting, without the complication of flying a spinnaker. On any given
      hull, you can carry much more sail area in the low rigs than in the
      marconi sloops. A whole new set of SA/D guidelines is needed for the low
      rigs. It would be a nice little research project to mine this data out
      of the PB&F design catalog. Easily reefed rigs line the Chinese gaff
      will have even higher ratios.
      >
      > Mr. Bolger's writings often promoted drive to cost ratio as a measure
      of rig efficiency. I think the better measure of rig efficiency is drive
      to unit heeling moment ratio.
      >
      > Doug
      >
      > --- In bolger@yahoogroups.com, "cecbell" cecbell@ wrote:
      > > Ted Brewer offers the
      > > characterization that values over 20 are high and typical of
      ultra-light
      > > racers, class racers and daysailers. By comparison, coastal cruisers
      > > tend to be lower, around 16 - 17, and racing yachts about 17 - 19.
      >
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