The Pitchpoling Myth

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

  1. Richard Woods
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    Richard Woods Woods Designs

    I read it everywhere: If you make a catamaran wider it will pitch pole

    Now where is the evidence to back that up? It seems to me that this is
    something someone wrote once years ago and since then everyone has just
    blindly repeated the dogma.

    Probably they do so because at first sight it sounds logical. If a catamaran
    is made wider it becomes more stable sideways. Thus proportionately it must
    become less stable fore and aft, all other factors being equal.

    But A) is that true? B) how many (besides me) have tried making a catamaran
    wider to see what happened?

    The original writer was, I suspect, a promoter of early narrow English boats
    (like Prout and Sailcraft) worried about newer, wider designs. So it is
    ironic that one of the first pitchpoles was of a very narrow, low freeboard
    Prout 27 in Germany.

    In practice catamarans tend to capsize diagonally, not cartwheel end over
    end. Indeed if they did go end over end then obviously the hull spacing would be irrelevant.

    So it is the diagonal distance from windward stern to lee bow that is
    important. Clearly then, as a boat is made wider this distance increases and
    so it becomes more stable overall.

    My 24ft Strider design has a 22ft WL and normal hull CL spacing of 10.6ft
    giving an overall beam of about 14ft (so when it was designed over 25 years
    ago it was considered wide). In 1986 I built an experimental Strider with a 14ft CL spacing.

    Compared to it's WL length that is wide! In fact it looks scarily so on paper, still wide in the boatyard but looks great on the water.

    A number of these extra wide versions have been built since then. None have
    pitchpoled or capsized. Indeed I have always thought that these wider boats
    sailed better and were more stable than the narrower ones.

    I guess if there was any truth in the rumour that wide boats pitchpole then
    catamarans would gradually be getting narrower. Instead they are getting
    wider. Even the last generation of Prouts were wider than earlier versions.

    So I say it again.

    It is a myth to say that just making a catamaran wider means it will pitchpole.
    There are many more important factors that determine whether a boat will
    pitchpole or not than just the hull spacing.

    Any sensible (ie constructive) comments are welcome!

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  2. Crag Cay
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    Crag Cay Senior Member

    As this is the origin of so much 'wisdom' within sailing, I can't see any reason why it wouldn't be the source of the pitchpoling theory.

    It would seem the force couple that causes a diagonal pitch pole wouldn't be hard to replicate with a tethered cat. I would be amazed if a wider boat was easier to upset.
     
  3. RHough
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    RHough Retro Dude

    Just making it wider won't cause it to pitchpole.

    Just shooting from the hip, it is the ratio of force needed to pitch pole (or as you more correctly state capsize diagonally forward) to the force needed to capsize to leeward. The transverse RM is limited by width. If the size of the rig is increased to take advantage of the higher RM the available force is closer to the force needed to sail the leeward bow under and trip the boat.

    A 8x20 boat has a 8 ft righting arm, and a 21.5 ft diag arm. About a 2.6:1 safety margin.

    A 16x20 boat has a 16 ft righting arm and a 25.6 ft diag arm. A 1.6:1 safety margin.

    If the rig remains the same you get a 20% increase in pitchpole resistance, and double the lateral capsize resistance.

    However, the extra separation also doubles the rotational inertia around the lee bow. The 20% increase in the diagonal arm does not compensate for the added inertia.

    If you go ahead and put a bigger rig on the boat, you have a combination of a smaller safety margin and have done nothing to compensate for the added inertia. The boat will be more likely to pitchpole with the greater hull spacing.

    The big offshore tris are close to square, the lack of longitudinal stability and the increased chance of pitch pole are limiting factors when the rotational inertia is considered.

    Going from 10.6 x 22 to 14 x 22 is not so radical. The righting arm is 32% greater, the diag in 6% greater, the ratio of righting arm to daig arm is 20% lower. If the rig is not large enough to make lateral capsize a concern, the simple change in beam should not make pitchpole a concern. If the rig was made bigger to use the 32% extra RM, the leeward bow would get buried more easily and pitchpole becomes more likely.

    Does that make sense?
     
  4. terhohalme
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    terhohalme BEng Boat Technology

    You are right. It is a myth. Making catamaran wider doesn't make her more sentitive to pich pooling, if keeping the sail area as it was.
     
  5. RHough
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    RHough Retro Dude

    Which begs the question, if the rig is the right size for the beam and RM, what is gained by increasing the beam? Greater pod/deck/tramp area?
     
  6. terhohalme
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    terhohalme BEng Boat Technology

    Yep, the world is relative.

    I'd like to think, the rig must be the right size for both the transversal RM and the longitudinal RM. Perhaps having the transversal and longitudinal inertia (second moment of waterplane) almost the same? This leads to LWL/BCB = 2, about. (LWL = length waterline, BCB = beam between centerlines or bouyancies)
     
  7. Richard Woods
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    Richard Woods Woods Designs

    Also a big reduction in drag from wave interference between the hulls. Also less slamming under the bridgedeck.

    The wide Strider is 24ft long, 22ft on WL and 17ft overall beam. So it is wider than many (older) trimarans. Wider than the original Telstar 26 for example.

    Richard Woods
     
  8. Alan M.
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    Alan M. Senior Member

    If you only make the cat wider, and don't change anything else, it is no more likely to pitchpole. But if you use the extra righting moment and specify a bigger rig, (which many designers do) that's a different story.

    Probably a better way of looking at it is, for any given beam, a longer boat is less likely to pitchpole.
     
  9. bill broome
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    bill broome Senior Member

    surely bow configuration plays a role? a spoon bow like a surf ski would tend to skid out from a pitch pole. this question is just an extreme form of the 'diving' question that is inherent in multihull design.

    if a wider boat has a bigger rig, it has a bigger driving force tending to bury the lee bow. so one must balance the various objectives in the usual way that makes boat design interesting, and frustrating.
     
  10. RHough
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    RHough Retro Dude

    I notice that the Strider Turbo has the wider beam and also an increase in sail area from 25 m^2 to 34 m^2 ... did the height of the CE change also? That is a 36% increase in area, and only a 32% increase in RM, correct? If the CE height increased also, then the heeling moment increased more than the 36% greater sail area.

    How does the boat handle with the 34 m^2 rig on the narrow beam? ;) What windspeed does it take to get a hull up? That would reduce wave interaction to zero. ;)

    I would think that a boat that does not fly a hull regularly also is a low pitchpole risk.
     
    Last edited: Aug 19, 2008
  11. Petros
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    Petros Senior Member

    Richard,

    I think you are correct, depending on what of the many variables you are changing. The first time I ran across this idea was on some NA's web site explaining that in their new design high-speed cat it had the "ideal" ratio of length to beam of 2 to 1 (40 foot long to 20 foot wide). I could not figure out what they were talking about, it did not make any sense there was any such thing.

    I went over in my mind all the variables and could not see any inherent advantage to this "ideal" 2:1 L/B. I figured it was just marketing of their new design. Later I have seen it explained elsewhere on this forum that to take advantage of the wider beam you would want a larger sail, but then the forward driving force tends to push the lee bow into the water causing a pitch pole. But this would vary widely with both rig design (how high the aerodynamic center is on the sail plan), the L/D ratio of the rig and the bow and forward hull design (resistance to diving). Most racing cats have really narrow bows, but ISTM a clever designer could put a lot of flair in the bows so the water normally sees a narrow bow, but as pushed down it develop more righting moment to resist pitch poling when pushed hard.

    Merely making the beam wider will increase stablity, but it will also add weight. If you do not need it any wider for the given sail plan, why add the extra width? IT seems to me that it just comes down the the amount of safe margin you are willing to tolerant.

    I do not think there is any such thing as an "ideal" length to beam ratio, it just depends on what the designers intent is and what kind of trade offs the designer is willing to make. And I am sure there are as many opions about that as there are designers out there.
     
  12. terhohalme
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    terhohalme BEng Boat Technology

    Richard
    Is there any data of the accident of Prout 27 or the boat in internet? I found very lite data of Prout 27.
     
  13. SheetWise
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    SheetWise All Beach -- No Water.

    This does make sense (intuitively) if you take it to extremes -- for example, if beam was 3x or 4x the length.

    I mention this only as an exercise -- at what point do you believe that beam to length has exceeded safe limits? Is there a maximum? Do you believe there's an optimum ratio?

    Curious minds want to know ...
     
  14. Richard Woods
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    Richard Woods Woods Designs

    The Prout 27 was the Haxted Argo (spelling?) running into the Elbe against the tide in the mid 1960's.

    Very well reported at the time and also for the next 20 years.

    Try the AYRS publications for more details, or any pre 1980 book on cruising catamarans.

    Richard Woods

    www.sailingcatamarans.com
     

  15. catsketcher
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    catsketcher Senior Member

    It does have some historical backup

    Hello all

    I think the case is as Rhough said, wider boats usually get more sail. However it is pretty important not to make the boat too wide because of feedback to the sailors.

    I had a Twiggy tri in my 20s which was almost square. It had a pretty powerful rig for a 31 footer- especially with its thin bows. Working, reaching or broad reaching it was very stable and I would be very happy with its ability to carry sail. If I was very powered up in these points of sail and then went square then she would go very bow down.

    I once got hit by a squall in Sydney Harbour. I was on my own and within a minute I went from having fun to - Oh my God - type sailing. My dinghy and mono background had me bear away to a square while I tried to work out what to do. The bow depressed so far as to scoop up every chop and my deck was almost green with water. I was very concerned and realised pretty quickly that this wasn't working. When I rounded up the load went onto the float and the bows popped up and off she went happily.

    In my view it is important that a mulit have LESS sideways stability than fore and aft and diagonal. This is very tricky to work out as real fore and aft stability is a product of both lever arms and the drag of an immersed bow. However I would not like to sail on a boat that had an assymetric stability profile.

    The biggest reason is that sailors can do something about easing sheets and reducing sail when the multi lifts a hull upwind or reaching. Downwind - when the bow digs in all they can do is hope they didn't put up too much sail and cross their fingers. So don't go too wide - what that is is up to you but I think your boat should be less stable reaching and working than on a square run.

    cheers

    Phil Thompson
     
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