T-rudder for pitch damping/control on a catamaran?

Discussion in 'Boat Design' started by Red Dwarf, Mar 13, 2013.

  1. Red Dwarf
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    Red Dwarf Senior Member

    I have been doing some research on T foil rudders. There are many references to using T rudders on moths and small cats to lift the stern and then with weight movement balance etc.

    I can not find any reference to a T foiled rudder being used for active pitch damping and control. I am not referring to a hydrofoil craft but to a normal semi-displacement hull. The T rudder would be actively controlled just as normal stabilizers (like Wesmar or Naiad) are actively controlled.

    I want to know if the use of a active controlled T rudders could eliminate hobby-horse on a catamaran and in general make the ride more comfortable. I am also curious if the use of T rudders could help reduce or eliminate the "vomit comet" pitch/roll coupling action.

    Not really looking for any speed improvement just comfort. Of course I do not want to make the seakeeping in a following sea worse. That would be a deal breaker.

    I would appreciate any examples of this being done and how it worked, good or bad.
  2. Ad Hoc
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    Ad Hoc Naval Architect

    You would need active flaps for the real control. You would be able to dampen the motion, with active flaps on the "T" part, rather than the rudder part, per se. But how much depends upon the vessels' natural roll/pitch/heave motions, and how stiff the vessel is and then what sea conditions you expect to encounter to see how effective it is in such conditions and if any benefit.

    BMcF may have more data to provide, being the guru on motion control :D

    You may find the paper "The effect of Antipitching Fins on Ship Motions" by M.A. Abkoalsowitz of use, even thought it is based upon monohulls.
  3. gonzo
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    gonzo Senior Member

    The first time I sailed a boat with a large wing keel, the easy motion really surprised me. It was a Sadler 34.
  4. Red Dwarf
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    Red Dwarf Senior Member

    Thanks for the article, I will get it and add it to my collection.

    Yes, I am looking at only moving the horizontal part of the T, either the whole foil or an element at the rear of the foil. The rudder will function as a normal rudder.

    With hydraulics and sensors you could actuate the T-foils just as well as conventional roll stabilizers that are placed midship. Since the T-foils are at the stern I don't know how effective they will be at countering roll.

    Here are a few documents I have found. They all say T-foils are effective at making pitch changes but none are on a conventional hull and actively controlled using a closed loop with sensors and actuators.

    This links to an interesting article regarding T-foil rudders. http://www.swaylocks.com/node/1005698

    Attached Files:

  5. Doug Lord
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    Doug Lord Flight Ready


    You may have seen this but just in case here you go. Its an article by John Ellway on rudder t-foils with either skipper control or just preset.
    It's interesting to note that t-foil rudders were used in the Moth class for years before they started flying to prevent pitchpole and damp pitching.
    Even though they are now foilers, the t-foil rudders on the AC 72 cats are not adjustable while sailing but work just fine.
    I can't find it but you probably know that Paul Bieker designed an adjustable t-foil rudder for the I-14 and placed it where it can ,apparently,"recover energy" from the flow around the boat, but he also put a t-foil on a displacement 30-40 footer that I can't find any reference to now. I remember reading that when the boat heeled there was some crosscoupling between the rudder and t-foil.
    Almost all new A Class cats are using small fixed t-foils or "L" foils to help control pitch with the curved lifting daggerboard foils now in wide spread use.
    Just found that Dan Leech, who has designed race winning R class foilers is designing the Leech 750 that uses a rudder t-foil on a sportboat. Many sportboats are sailied flat like dinghies so the crosscoupling problem may not surface: http://www.leechboats.com/news.cfm
    click on the image-

    UPDATE:I eliminated the t-foil rudder article since I see you have it already

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  6. Red Dwarf
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    Red Dwarf Senior Member

    Nice picture.:) I am interested in using the T-foil rudders on a power catamaran to improve ride and seakeeping. Mainly I am interested in reducing the two worst aspects of a catamarans ride which is hobby-horsing and the corkscrew "vomit-comet" motion.

    I found a paper that covers exactly what I am interested in. The only problem is it is more academic than applied science.

    "Research of Combined Control Scheme for Fast Catamaran Motion Control
    Using T-foils and Interceptors"

    I don't see any reason the findings wouldn't apply to a power catamaran in the 10-20 knot range. Now if I can only find someone that has actually built and tested the concept put forth in the paper.

    Here is a excerpt from the documents conclusion:
    In this paper, a bad seakeeping performance in moderate
    to rough sea conditions, the drawback of the fast
    catamaran ferry is discovered and analyzed. Then existing
    stabilizers and methodologies for the purpose of seakeeping improvement are carefully investigated.
    An important fact is pointed out that the seakeeping
    amelioration should include both longitudinal and transverse
    motions improvement, which means that the
    roll stabilizer - fins and heave/pitch stabilizers - T-foil
    and interceptor, are simultaneously required.
    But in this paper, authors find out that for catamarans,
    which have wide breadth, the longitudinal motions
    stabilizers T-foils and interceptors can also generate
    enough roll moment to counteract the wave stimulated.
    Based on this discovery, an ingenious 3-DOF
    motions autonomous control system is built up. This
    system can freely make switch to control the longitudinal
    motions or roll motion.
    In this paper the hydrodynamic characteristics of the catamaran, T-foils and interceptors are also studied. A
    post-compensation decoupling method is proposed to
    decouple the coupled longitudinal motions. A practical
    control algorithm - PD controller is employed in
    the control system.
    Numerical simulation results are also presented in
    this paper, which show that the designed system can
    satisfactorily control the longitudinal and roll motions
    of the fast catamaran.
    In our future work, the control model will be realized
    and towing experiments will be carried out.

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  7. Ad Hoc
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    Ad Hoc Naval Architect

    The effectiveness on roll is, as noted above, dictated by the hulls principal dimensions, in this case the waterplane inertia. The wider apart and the lower the L/B ratio shall inevitably result in a very "stiff" and thus stable hull form. Consequently the ability to control its motions become more difficult since more force is required to counter the roll. So, as noted on another thread, the smaller the waterplane area the better for controlling motions more easily.
  8. Red Dwarf
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    Red Dwarf Senior Member

    Ad Hoc-So for example, for this catamaran I have modeled hulls from 2.5ft of draft to 4ft of draft. All have the same displacement and all would work regarding systems, SOR, etc. Is it a no brainer to go with the 4 ft draft as it has the smallest waterplane area?
  9. Ad Hoc
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    Ad Hoc Naval Architect

    I don't see any catamaran?

    You will have to review the change in WPA with increasing draft. Also don't forget the restoring moment will change with a deeper draft too, since this is proportional to the displacement.

    No such thing as a free lunch!
  10. BMcF
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    BMcF Senior Member

    Transom mounted active foils have been demonstrated by many. That solution has been found less practicle than others.
  11. Red Dwarf
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    Red Dwarf Senior Member

    Thanks. Since I am looking at 10-20knot speeds I think that means a fin placed midship for roll, just like other stabilizers. I'm still looking at options for pitch. I think the low speed rules out interceptors.

    I have access to complete CNC shop with a 20 ft 5 axis router. I plan on cutting some 1/10 scale models out of 25 lb urethane foam and doing some testing. I have programmed numerous different 9 axis autopilots as used on RC multicopters so the AP stuff is easy.

  12. superyachtmobil
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    superyachtmobil Junior Member

    We experimented on a 30 m displacement ship on a long passage and we determined that to control pitching, you needed to control the Bow, when you are able to control the Bow relative to the wave pattern, then the pitching is dramatically reduced. Most seas tended to have an erratic wave amplitude with cycles being being spread out from about 60 seconds - right out to 10 - 20 minutes. We determined that any T-foil rudders would not provide any real benefit to pitching dampening to a mono hull displacement ship, but with a small cat, it would.
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