The Pitchpoling Myth

Discussion in 'Multihulls' started by Richard Woods, Aug 18, 2008.

  1. ropf
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    ropf Junior Member

    Hello,

    in my mind the main topic is interesting enough to stay on it. Dependent of the kind of cat the pitchpoiling appears completely different. If we talk in the same time about old style narrow cats, modern crusing cats, slightly wider modern performance cats, overpowered beachcats and the nearly square lago di garda racers - this is total misleading.

    It's even not enough to express the stability in single numbers for different directions. We need plots of the righting moments vs pitch and heel angles - compared for different length/beam ratios. Once we got some basic clarify here we may disscuss the influence of different wind directions, wave interaction, crew trim, hull shapes, dynamic effects...

    If somebody provides a functional model (freehsip) of a typical cruising cat hull I will do the calculations and some graphical represenatation.

    regards
    ropf
     
  2. sail.scow
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    sail.scow Rrrrrrrrrrrrr!

    Me too.

    The topic gets the brain thinking. It would be interesting to see some of the theory quantified.

    Personally I am happy just thinking it through, and I hope the other contributors don't mind my musings. Sort of thinking out loud.

    How about transferring the moment from the CP to the leeward hull somewhere? It would work out like placing an equivalent weight onto the deck.
    Like using a Hobie, and placing a weight on it.
    A static representation of the moment.
    • SA, wind speed, educated guess at the CP
    • estimation of total force at CP times the height of the CP above the float or hull where we want to put the weight=Nm
    • transfer that moment down vertically from the CP to the height of float or tramp
    • that moment divided by gravity=kg.
    There is a weight we can place, on a spot on the hull or tramp. If CP is over the water, then it's another step to move the moment inboard until it lands on the vessel.
    • a horizontal line from CB to CP with a point 'g' where we want to place the weight on that line
    • distance CB-g divided by CP-CB, times kg
    Any thoughts on that?
     

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    Last edited: Aug 29, 2008
  3. masalai
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    masalai masalai

    sail.scow, said: "So the interesting thing is blending those forces into a vessel that can meet different criteria."

    The prime design problem is rooted firmly in the intelligence (lack thereof) of the skipper & crew in allowing / causing a capsize (unless their mission was "destructive testing).... :D:D:D:D:D
     
  4. rayaldridge
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    rayaldridge Senior Member

    I'm not sure I buy this. Here's the problem for me: as wave height increases, so does length. A wave high enough to pound is so long from crest to crest that it ordinarily isn't a matter of the two hulls being on either side of a breaking crest, and besides, the boat would have to be running parallel to the crests in order for this to happen.

    However, anyone who's sailed on a cat with narrow hull spacing can probably remember seeing the water humping up between the hulls, even in flat conditions, if the boat is going along pretty fast. We used to own an old Wharram, which had fairly narrow spacing, and slat decks, and you could see the phenomenon Richard alludes to-- water was forced up as the hull waves collided.

    It may be that this problem is primarily associated with older cats with the spacing that used to be considered acceptable. There may be a point when increasing beam does affect pounding negatively, but as a general rule, I think the closer the hulls, the higher the clearance should be.

    Ray

    http://slidercat.com
     
  5. rayaldridge
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    rayaldridge Senior Member

    Me?

    [​IMG]

    Ray

    http://slidercat.com
     
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  6. Spiv
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    Spiv Ancient Mariner

    Gregor Tarjan has probably more miles on cats than all of us put together, as well as Americas Cup racing. He is a USCG licensed captain and member of SNAME.
    In his great book "Catamarans, every sailor's guide" he has a section on Diagonal Stability & Beam-to-length Ratio that I have scanned for you all to read.

    You will see that he agrees with Richard Woods wave interference factor, but not wave slamming.

    Rubbishing anyone on this forum does not conciliate to discussion, I have great respect for Richard Wood, he has made a lot of worthwhile contributions on many threads on this forum, as well as to the world of Catamarans in general and I certainly appreciate the time he spends posting here.

    We all sell something, even if just our credibility and I would buy a design from Richard.
     

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  7. rayaldridge
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    rayaldridge Senior Member

  8. Richard Woods
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    Richard Woods Woods Designs

    I apologise for my sarcasm of a couple of days ago, sorry to have caused offense.

    Rhough. I always read your messages as they are sensible. That is why some time ago I asked what boat you sailed and where

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  9. Alan M.
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    Alan M. Senior Member

    I've only sailed on half a dozen or so cats, and all of them would have had a LWL/BWL of better than 10:1, so I can only base my experience on that. But in my experience the cats I've been on hardly produced any bow waves at all, even at high speeds. (which is why cats can sail at much better than theoretical hull speed.)

    Here there's a video of a relatively narrow beamed 55' powercat at 30 knots. There is footage taken under the bridgedeck. http://www.alluramarine.com.au/

    As for already existing waves hitting the bridgedeck, it seem fairly obvious to me that for a fixed bridgedeck clearance a wider beam would allow a less steep wave to hit. If you had 500mm clearance and hulls 4m apart, a wave gradient of 1 in 4 would touch. If the hulls were 6m apart a gradient of 1 in 6 would touch.
     
  10. RHough
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    RHough Retro Dude

    No worries mate. It just seemed out of character for you. Your wealth of experience is something we all benefit from.

    I started sailing after I helped my father built a Penguin, then raced Stars and what ever I could get a crew place on, A-Scows, and got my first multi experience racing a Couger Cat ... horizioned the fleet in light air then they left us for dead when the breeze filled in. :(
    In my 20's I sailed Lasers and crewed on monos.
    There was the lost decade married to a non-sailer ... so a bit of crewing and a Bic sailbord to keep me sane.
    Now I have a "Classic" Catalina 30 ... that no matter what you do to her is a 5-6 knot ******* ... 12.4 knots in 40 true is *NOT* fun on that boat ... I'm back to having a Laser and a Hobie 18 for sailing, got a tatse of real mutli saiing racing a Corsair Sprint 750 in Mexico ... 18+ knots in a 24ft boat *was* fun!

    Cheers,

    Randy
     
  11. Richard Atkin
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    Richard Atkin atn_atkin@hotmail.com

    I just read all that stuff posted by Spiv. (nevermind any copyright laws Spiv :D:D). Maybe Gregor Tarjan does know what he's talking about, but he didn't explain much in those pages. Maybe I read it too fast.
    Tarjan says that if you keep the same clearance but widen the beam, you end up with the wrong beam to clearance ratio which will lead to slamming. No explanation.

    Alan M seems to explain it a bit further, but to be honest, I just don't get it.
    He says " If you had 500mm clearance and hulls 4m apart, a wave gradient of 1 in 4 would touch. If the hulls were 6m apart a gradient of 1 in 6 would touch."
    I don't understand that.

    Apologies to Richard Woods for wandering off the main topic....could we include both topics in this thread? cos slamming and pitching are so important to hull spacing.
     
  12. Spiv
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    Spiv Ancient Mariner

    Richard,
    I am sure Gregor is only to happy about me giving a plug on his great book; some of the readers of this thread might even end up buying his book....

    Another very experienced author is Charles Kanter with over 100,000 miles of sailing, delivering, surveying and critiquing 1000+ boats. In his book "Cruising Catamaran Communique" he talks at lenght about slamming, pitching and LBR.

    He says a LWL/BMAX of 2 makes a 'Balanced" cat, diverting from that ancient Polynesian rule can create more problem that it is trying to cure.
    He says wave interference is reduced, but wave pounding is increased (same as Tarjan), so you must disproportionally increase bridgedeck clearance.
    He also says that the biggest problem is that the transverse stability can eventually be compromised, leading to burying the lee bow.

    I have given this some thought and realised that even if one builds a cat with larger beam and does not increase the sail area, he could run into some problem:
    Immagin you are going with 70 degree apparent wind of 25knots.
    The wide beams allows you to carry more canvas than if you had a narrower beam, you are happy with your high speed and feel safe and secure.:):)
    Now your route has to change and the same apparent wind comes from 120 degrees and you start burying your lee bow.:(:(

    So, I would say, make your cat as wide as practical, but keep it in the 39~42% BCL/LWL ratio advocated by Tarjan. (where BCL= Beam between Center Lines and LWL= Length Water Line)
    Additionally, make the bridgedeck as high as possible 14~18% of BCL, using the higher ratio for larger boats.
     
  13. CT 249
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    CT 249 Senior Member

    Excuse me Masa, how are you so sure that you have more multi experience than other people here?
     
  14. Doug Lord

    Doug Lord Guest

    Has anyone sailed a Stealth F16 with rudder t-foils or another cat similarly equipped? Can you discuss the improvement in handling or lack thereof from the use of these foils on a catamaran?
     

  15. Richard Woods
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    Richard Woods Woods Designs

    That is an interesting point. I haven't sailed a Stealth but nearly 30 years ago I was working for Derek Kelsall and we designed a trimaran with foils on the rudder and daggerboard. In its first major sail it capsized in a pitchpole. We understood that was because the rudder foil provided too much lift as it surfed down a big wave.

    However when I raced radio controlled multihulls we found that they NEEDED a foil on the rudder to hold the stern down or they would pitchpole very easily.

    So not all boats behave the same

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
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