Multi speed/length relationship?

Discussion in 'Multihulls' started by RHough, May 17, 2008.

  1. RHough
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    RHough Retro Dude

    First, thanks for posting.
    Second, thanks for pointing out that the SA-D or Bruce Number are not all of the story.
    Third, to have a match race, the boats must be close enough in performance for tactics and boat handling to become the difference. If one of the 90 footers is 10% faster than the other it can hide behind the line, start 12 minutes late and still win.

    The real question is, what chance does Alinghi's 40 foot Cat have against BMWO's 90 foot (Cat or Tri)? If they were monos, I'd guess the 90 footer would be about 40% faster.

    What I'd like to learn is how to guesstimate speed potential for multi's.
     
  2. Chris Ostlind

    Chris Ostlind Previous Member


    I'm sorry, but I seem to recall a match race back in '88 when a catamaran of such superior speed simply sailed out from under a much bigger mono, by simply bearing off and flying away. 20% greater distance sailed at twice the boat speed = a drubbing of high order.

    Short of running the cat over and breaking her up, there was absolutely nothing that could be done to stop the event from transpiring. Pure speed

    Were you speaking of another type of match race?
     
  3. idkfa
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    idkfa Senior Member

    FAIL - Sailing backwards???? The variation in the boats (per generation) is small... Barring straight-line speed, any other (under)performance I would to attribute to the helm/crew and not the design. The variation there is much larger.

    Pure speed would allow recovery from penalty(s) and a loose cover, so one did not chance errors covering.

    Is this only your opinion or can you quote some other source?
     
  4. jehardiman
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    jehardiman Senior Member

    Ok, there has been some truth in these threads and a lot of drivvel..

    1) There is no such thing as 'A limiting hull speed". Vessels will always move as fast at the available power will push them against the resistance curve. Note that the resistance curve is comprised of pressure resistance (i.e. wavemaking), skin friction, air drag, and sea state effects.

    2) The pressure resistance curve for a single hull has humps and hollows based upon L/B ratios and forebody/afterbody shape. The variations in resistance cuvre shape/value are due to the interaction of the shape generated pressure waves. Some shapes, such as infinite planks have no humps and hollows in thier pressure resistance curves. See Wigley hull analysis.

    3) The pressure resistance curve for multi hulls has humps and hollows based upon the hull beam/seperation ratio as well as the individual demi-hull pressure resistance. Here, additional variations in resistance cuvre shape/value are due to the interaction of the individual demi-hull pressure waves. See the work by Michell, Tasaki, or Fry.

    4) The size of a sailing cat is not a factor on it's speed because the inherant waterplane righting moment is very large compared with it's displacement. More important to absolute speed is the power to displacement ratio that can be achieved. This tends to drive to "over square" vessels (i.e. the beam is greater than the length) limited only by the ability of the hulls and/or crew to prevent pitchpoling.
     
  5. RHough
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    RHough Retro Dude

    Ok ... not buying this. A 100' wide 10' long cat would be as fast as a longer boat? If the prevention of pitchpoling is a design criteria and length helps, then length is a speed producing factor. Speed of picthpoled boat = 0. :)

    I get that the power available for sailing craft goes up with the square of size, while the displacement goes up with the cube of size. Given equal materials and construction, going bigger reduces the power/weight ratio (if I have that right), so for any state of the art in material and construction, there is a practical limit beyond which gains in speed are unlikely. There is also a practical limit on the small side, somehow I don't see a ten foot cat being faster than a 20 foot cat.

    There is obviously no 'rule of thumb' for the speed - size relationship for multis or someone would have posted it ... :)

    How about a formula that is used to rate different multi-hull designs for racing? How are handicaps arrived at? Just SA-D? Portsmouth numbers seem to show that bigger = faster (as it is with monos).
     
  6. jehardiman
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    jehardiman Senior Member

    Believe....
    [​IMG]

    And for a full discussion of size and weight and how they relate to cat speed and ability, re-read my comments in this thread (ignoring the whole foiler side conversation).

    http://www.boatdesign.net/forums/showthread.php?t=13511
     
  7. TeddyDiver
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    TeddyDiver Gollywobbler

    No you don't have that right. And you can't just compare 10' and 20' boats without knowing all the other facts too-
     
  8. Richard Woods
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    Richard Woods Woods Designs

    I probably wrote my message, above, too late at night to make sense to anyone.

    One of the key prestart techniques in match racing is to go head to wind. Both boats end up doing this and then, obviously, in time they start drifting backwards. My understanding from reading magazines, rather than from being on board or talking to any of the crew, is that the UK boat didn't do that as well as other boats. Something to do with the rudder shape I believe.

    I said "Failed" because it didn't win the Cup.

    I believe a 2 masted boat under the Deed of Gift can be longer than a single masted boat, and I further believe there is no size limit to the second mast. In other words it can be very short.

    A proper match race does need boats to be of similar speed. Otherwise it is a drag race with two entries. Unfortunately it seems highly unlikely that the boats being built for the next Cup will have such similar speeds and thus there won't be the competition that we would like to see.

    The really interesting thing will be whether the sailors will want to carry on sailing multihulls after the next event, or just follow the money men and revert to the strange shaped, slow monohulls produced so far.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  9. RHough
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    RHough Retro Dude

    You said:
    YPE is 'square' about 30'x30' if size length is not a factor then it would be faster if it was smaller? :)

    As usual it is next to impossible to get a straight answer or I am incapable of asking just the right question.

    Sorry to have wasted everyone's time. I thought the question was clear enough but I guess not ... or people want to argue and dance around without trying to be helpful.

    C'ya
     
  10. Doug Lord

    Doug Lord Guest

    Oversquare

    The most well known "over-square" boat around is Hydroptere-LOA 59' , Beam 80'......
     
  11. jehardiman
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    jehardiman Senior Member

    Your question was about hull length. YPE "hulls" are only about 10' long. Thier arrangement may be 30' LOA, but the "hulls" are short. This is what I meant when I said "limited only by the ability of the hulls and/or crew to prevent pitchpoling". There are many other ways to this...such as foil ballasted kite pod boats, and other ingenious contraptions.

    In real hydrodynamic terms, there is no shape independent connection between maximum speed and "hull" length...like a hydoplane on step or SWATH, or SES...all different.
     
    Last edited: May 20, 2008
  12. RHough
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    RHough Retro Dude

    See I knew it was my fault for asking the wrong question.

    The spacing (length) of YPE gives it the longitudinal stability to reach high speeds. This is much the same as one long hull for the same reason. YPE did it by making the ends of the hull do all the work and cutting out the middle.

    You know as well as I do, that is not what I was asking.

    If their are two similar catamarans, both have D/L ratios of 15 and SA-D ratios of 100, one is 45 feet long (LWL), the other is 90 feet long (LWL). Both have the same L/B ratios in the hulls and the same overall L/B ratio. Hull forms and prismatic are the same, neither one planes, and anything else that you can come up with is the same. The only difference is the LWL. They both have the same type rig, etc. etc.

    Will the 90 foot boat be faster than the 45 foot boat?

    If not, then a 22 foot boat with the same ratios will be as fast also, as will a 120 foot boat.

    If that is true, then the only size consideration is load carrying ability so you have enough food and water to keep the crew alive during the race.

    There must be some formulas that are used to predict speed potential of multi's. I don't think that SA-D (power to weight / Bruce Number) tells the whole story. If it does, I'm just too stupid to see it and all the ratings that I've ever seen that tend to show that longer boats are faster than shorter boats are no more than black magic and wild guesses.
     
  13. Richard Woods
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    Richard Woods Woods Designs

    I posted it elsewhere, but do so again here.

    Try using the attached spreadsheet of the Texel Rating system. It is the best quick and easy performance indicator that I know.

    Whether it will cope with the America Cup boats I don't know.

    Even if it did you'd still have to define "fast". In a straight line? In a "trench" like windsurfers use? For 24 hours offshore? Close to the beach on one tack like Sail Rocket and Yellow Pages? Short tacking up a twisting river?

    I think your initial question was a bit too simplistic, which is why you didn't get the answers you wanted.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     

    Attached Files:

  14. oldsailor7
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    oldsailor7 Senior Member

    RH.

    What you say is quite correct.

    However there are other factors at work which, in general, make bigger boats easier to go faster.

    Such as --the ability to handle big waves and rough water better, plus the percentage of the overall weight of the boat, which is provided by the weight of the crew and their equipment, is less.

    Imagine a WETA with a crew of five. :eek:

    Also bigger boats have bigger sails and bigger sails are more efficient, just like the bigger wings on airliners. I have already mentioned the advantage of taller rigs, which can take extra advantage of the wind gradient.

    However Tris like the Corsair 28R, and Cats like the 30 ft "Raw to the Core", are currently making nonsense of much larger multi's.

    Monohulls don't even rate. :D
     

  15. jehardiman
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    jehardiman Senior Member

    The problem is in the "etc,etc." I would not design for maximum speed a length constrained 22 foot cat to the same criteria I would use to design a 120 foot cat. Water, area, and volume and therefor drag, load, and power do not scale lineraly!

    If you were to build two geosims, one twice as large as the other, and compare them at the same speed, then one would be operating at a Fn of SQRT(2) to the other. Additionally, the Sa/D CANNOT remain the same without doubling the sail area of the larger boat, and then additionally the smaller will have twice the wetted surface per unit of displacement, while the larger will have 4 times the ability to carry sail. So realisticly, doubling the size of a cat reduces wetted surface to displacement by half, allows the sail area to displacement to double, and increases the load on the hull by a factor of 16.

    Yes, there are, but they are based in comparisons of "like sized" vessels and first principals looking at the ability to carry sail (i.e. available power). But you are never going to find an "rule of thumb" like speed = X*LWL...except as it applies to boats of a particular size, of a particular shape, rigged a particular way, designed to a particular set of rules/criteria.
     
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