pitching control

Discussion in 'Multihulls' started by pitbull, Mar 28, 2019.

  1. Clarkey
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    Clarkey Senior Member

    I think that Gary Dierking has also expressed similar sentiments about his sailing outriggers - that flare invites too much reaction to wave action.
     
  2. Doug Lord
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    Doug Lord Flight Ready

    Here's a video showing Gitana 17 with her reverse bows, planing amas and lifting foils doing exactly what Verdier designed her to do-go thru waves with no tendency to pitchpole. She has never pitchpoled in two years of high speed sailing at sea.
    There is one clear cut shot at 4:23 in showing the port bow submerging up to the forward crossarm with barely any reduction in speed with the bow almost immediately re-emerging.
    The way that worked is due to the planing ama shape and to a large degree because of the large ama lifting foil coupled with the main foil( a T-foil on the main hull)-both ahead of the mast, both working with two rudder t-foils to control pitch.
    Another shot-not so clear-at 4:55 in.......

     
  3. Dejay
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    Dejay Senior Newbie

    So if you go for a double ended and symmetrical fore/aft hull you would get less dampening to the pitching motion and less resistance against squatting.

    How efficient is it to counter this with a type of horizontal foil near the aft of the ship like in the picture doug linked? Maybe attached to the outboarder or something like a pod drive on the stern of the boat?

    Could a foil actually be more efficient (in terms of propulsive power) than using the hull shape to dampen pitching and squatting? It might be more efficient to resist the pitching motion in the first place than dampening it.

    My reason for considering a double ended hull is an easier to build mold like the harry proa intelligent infusion but for a power catamaran. So not really "needed" but it would make construction and assembly much easier.

    PS: Also thanks @W17 designer for all the excellent info your website! That construction method summary is really the best overview I've found on the web.
     
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  4. Doug Lord
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    Doug Lord Flight Ready

    =====================================
    Foils can definitely control pitching in most cases-the downsides are that they will stickout on one or both sides and that they need forward motion to work well. But they can be an excellent solution.
    Take a look at this thread -particularly post 30- for some ideas on hull shaping for high performance boats-not limited to foilers:
    Hollow Sections aft on ULTIME Trimaran Amas https://www.boatdesign.net/threads/hollow-sections-aft-on-ultime-trimaran-amas.60102/
     
    Last edited: May 13, 2019
  5. Dejay
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    Dejay Senior Newbie

    I figure at speed they would create a bit of counter lift to pitching motions. The change of angle would be minimal though. But even at rest it should add quite a bit of both resistance and dampening. And if it's attached to your outboarder it could be easily adjusted or even removed when not needed.
     
  6. W17 designer
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    W17 designer Senior Member

    Just to note that I agree with Doug that adding foils can certainly help control pitch and there's a note to that effect in my 2013/4 articles. For racing, foils can even overcome much of the negatives of reverse bows, even if I am not personally a fan.
    But when considering pitching, it's important to relate the boat length to the wave height when comparing photos or videos. Although it varies with speed and boat design, my experience has been that pitching 'typically' only starts to show up when the wave height reaches about 1/15th of the boat length. So for the Gitane 17 that would be when the waves reach about 3.7ft (1.13m) ... which is certainly more than what appears in the video, so I am not sure it's something we can judge pitch resistance by, despite her smooth riding. For comparison, I have sailed my W17 in short waves of about 1 meter ( at a S/L^0.5=2) without excessive pitching, and that would be equal to about 3.5m waves (@15k) for the Gitane 17 ... a considerably tougher test I would say. Just something to consider.

    PS: Not surc if BoatDesign or the original poster can correct the typo in the Thread name but would be great if so, as if someone searches for a thread like this on 'pitching' they may not find it when it appears as it presently does .... as 'piching' :(
     
  7. Dejay
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    Dejay Senior Newbie

    Maybe the admin can pich in
     
  8. trip the light fandango
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    trip the light fandango Senior Member

    .
    .The idea of a fixed fin on the bow[as described by W17, developed in the 50s] that will help recover from submerging provided the craft is moving forward, has a small impact on other aspects of the practicality of the boat . But ideally they would just fly up when the bow submerges and drop back to a closer flush with the hull in between actions :- open when the craft begins pitching, water pressure venturi operated maybe ..?..
    There's a place or argument for a "hot" button that could be pushed with mechanically actuating ..cams, that set the foils to loose, or angled to the surface .
    Enjoying the thread, all interesting stuff
    , Haven't got to the original bows article yet, thanks again.W17 designer,
     
    Last edited: May 12, 2019
  9. Dolfiman
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    Dolfiman Senior Member

    Very interesting article on bow types, may be one can add this one here below ( of the steamer Ss Delphine) to your typology, combining a reverse bow for the lower half and a conventional vee for the upper half, the lower and the upper ends being on the same vertical : is that type of bow interesting for the pitching issue?, previously experimented for multihulls ?
    SS Delphine : Résultats Google de recherche d'images correspondant à https://keyassets.timeincuk.net/inspirewp/live/wp-content/uploads/sites/23/2015/02/del7.jpg https://images.app.goo.gl/QcrrLfdBHXxZtZQ2A
    As regard the Fig. 2 of your W17 article, I would be interested in having the sources for the residuary drag curves in relation with the L/B between 5 to 15.
     
  10. Dejay
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    Dejay Senior Newbie

    Also are the units for that speed/√length graph in knots / √feet?
    Never mind, it is, as explained in the article :)
     
  11. Doug Lord
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    Doug Lord Flight Ready

    ------------------------------------------
    W17, Gitana 17 is a little over a hundred feet LOA(100/15=6.6'). Her experience has been sailing(flying) in up to 3m waves at 30 to 40 knots.
    In last Novembers Route du Rhum, she sailed at 40 knots in waves up to 4m(13') for the first time until the bow section of the stb ama broke off. What was learned is that the unique foil system(similar to the Fire Arrow Foil System) allowed higher speeds in bigger waves than were anticipated in the structural engineering of the boat. The foil system proved its remarkable capability while pointing out an engineering(or building) shortcoming. The foil system worked well in what were really tough conditions. That fact was a revelation to a lot of people . Some would have thought that if anything was going to fail it would be the foil system-instead the hull structure failed*. But in many ways that experience was a blessing in disguise because it showed the real potential of Gitana 17 and her foil system.

    * a similar but worse disaster occurred to Banque Populaire** in the same Route du Rhum: the entire stb ama broke off resulting in a capsize and total destruction of the boat.
    ** using a nearly identical foil system
     
  12. W17 designer
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    W17 designer Senior Member

    Sorry Doug ... I should have checked the length ...I had just auto-assumed 17 was for 17m ;-)
    This would however make my point about 'relative wave height' for image comparison even more justified as 100/15=6.7ft waves. The boat is certainly quite an amazing machine.
    You must have felt that system was an endorsement of your Fire Arrow. There's no doubt that foils can help achieve a lot of things, but as they are vulnerable to overload and damage, they are not for everyone. I do however think that smaller 'assist' foils will become more common on fast cruising boats .. noting though, that they do require speed to work ;)
     
    Last edited: May 12, 2019
  13. Doug Lord
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    Doug Lord Flight Ready

    I think you're 100% right about foil assist. I'm sure they'll also be more and more race boats using full flying foil systems as well as some cruiser racers like the Eagle Class Cat: Eagle Class 53 Catamaran Foiler https://www.boatdesign.net/threads/eagle-class-53-catamaran-foiler.60758/
    They will offer foil assist as well as full foiling.
    The video of Gitana 17 ,besides the beauty of the boat on foils, also illustrates exactly how a well designed wave piercing ama works-I think Verdier nailed it.
     
  14. W17 designer
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    W17 designer Senior Member

    I would like to see a video of this 100ft boat (with foils) sailing and pitching in 3m waves @ 30kts ... before the ama bow broke off of course ;) That's a L/wave ht. of about 10, so equal to 1.7ft waves for my small W17. That's an almost weekly occurrence for me. My 'relative' speed (to Gitane's 30k) would then be 12.3k which is quite doable also.
    So my personal conclusion to all this, is that it's a lot easier to get impressive figures for the larger boats, as when scaled down, they are really performing closer to what we might consider 'a brisk normal'. But the relative impact risks (at 30-40 kts) are clearly augmented ... underlined by the ama eventually snapping. This is not uncommon for these long, fast boats. As amas become ultra slim and lean, their smaller X-section and limited strength makes them vulnerable. I actually brought attention to this in a 2012 article on ... 'How materials are changing design’, (scroll down about 3/4 of the page). https://smalltridesign.com/Trimaran-Articles/Trimaran-Design-Materials.html

    BTW: for those interested (Trip the light?), I just found a B&W photo of an early British catamaran that solved nose diving with fixed bow deflectors. See this article about 80% down. https://smalltridesign.com/Trimaran-Articles/AmericasCup/AmericasCup2013.html
     
    Last edited: May 12, 2019

  15. W17 designer
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    W17 designer Senior Member

    Dolfiman sent this: May be one can add this one here below ( of the steamer Ss Delphine) to your typology, combining a reverse bow for the lower half and a conventional vee for the upper half, the lower and the upper ends being on the same vertical : is that type of bow interesting for the pitching issue?, previously experimented for multihulls ?
    SS Delphine : Résultats Google de recherche d'images correspondant à https://keyassets.timeincuk.net/inspirewp/live/wp-content/uploads/sites/23/2015/02/del7.jpg https://images.app.goo.gl/QcrrLfdBHXxZtZQ2A

    The SS Delphine was designed in the 1920's for use on the Great Lakes. (A pet project of the Dodge brothers after selling their shares in Ford ;). While this concave bow shape should work as well as the convex AXE bow re pitching, there is always the risk that it will scoop up kelp and lobster lines etc., so just less practical. For the SS.Delphine it makes more sense, as on the lakes during the winter, the lower part of the bow would act as a small icebreaker to lift up and break thin surface ice. .... a totally different need to a sailing, pitching catamaran ;-)
     
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