Another Question About Tri Hulls

Discussion in 'Multihulls' started by Wavewacker, Oct 25, 2011.

  1. Wavewacker
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    Wavewacker Senior Member

    Wondering again, why can't a palnning ama be lowered for motoring and lifted allowing better sail performance with a displacement main hull? Could the amas lift the bow of the main hull enough to a flat arae for planning? Think small....
     
  2. daiquiri
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    daiquiri Engineering and Design

    Perhaps you meant to say it the other way round? Amas for sailing and no amas for motoring?
     
  3. Wavewacker
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    Wavewacker Senior Member

    Ok, I guess it could be the other way around as well. Thought was that since a motorsailor is so inefficient, why can't the hulls be adjusted to provide a more efficient craft?

    Since hydraulic rams are really pretty inexpensive, seems a ram could be used to lower the amas or raise the main hull....of if small enough maybe this could be done manually.

    :confused:
     
  4. Doug Lord
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    Doug Lord Flight Ready

    Ian Farriers tri's are designed with a planing main hull under sail and high L/B amas.
    A planing motor sailing tri, using a main hull designed for it, could power up with the amas clear of the water* since ,at rest, the amas could be barely touching the water. It could probably be designed to sail well and power well-and be more likely to do both well than is a monohull motorsailer.

    * very small dedicated or slightly modified ama foils(used for sailing) could be used to stabilize the boat under power reducing any tendency to roll from ama to ama.

    You might ask Kris here: http://www.boatdesign.net/forums/multihulls/new-sail-tnt-34-a-31643.html how he envisions the "motorcat" to function under power alone-it seems designed with a lot internal combustion power.
     
  5. Wavewacker
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    Wavewacker Senior Member

    :D So, I'm not crazy after all, it is done....

    And can the amas carry much weight, for storage and perhaps fuel/water while doing this?

    Thanks
     
  6. Doug Lord
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    Doug Lord Flight Ready

    -------------------
    No. Weight is the enemy of multihull performance whether under sail or power.
    Overloading a multi defined as a "cruiser" is all too common because of the temptation of all that room. Resist it.
     
  7. daiquiri
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    daiquiri Engineering and Design

    In the displacement and fast displacement modes, it can be and has been done. The trick doesn't lie in the planning-assistance by the amas, but in the wave reduction or cancellation due to destructive interference ( http://en.wikipedia.org/wiki/Interference_(wave_propagation) ) of wave systems generated by the main and lateral hulls. For example, see this video and note how small is the wavetrain left behind that ship: http://www.youtube.com/watch?v=5RgfwrXevNU

    I think Leo Lazauskas could tell us pretty many things about wave interference, since he has done some interesting theoretical work about it: http://www.cyberiad.net/library/pdf/tl98.pdf
    I am also sure that Ad Hoc or DMacPherson could do the same, in view of their practical experience with fast multihulls and observations from towing-tank tests.

    The previous mechanism works well (as the above video shows) for very slender hulls, but if you want to extend it to planing amas, the results would probably be positive only at planing speeds and rather disappointing at displacment speeds. That's because planing amas need width in order to efficiently generate a hydrodynamic lift.

    A wide (planing) surface has a higher hydrodynamic friction coefficient than a narrow one (for the same total wetted area) and hence a wide ama also has a higher friction drag than a narrow one. That fact can be mathematically demonstrated for a simple surface like a flat plate, but I wouldn't bother you with that now.
    If the two amas (a wide and a slender one) have the same displacement (or buoyancy) too, then a wide one will also have a higher wave drag and a bigger "hump" on it's drag curve. That's because a wide ama will have a much lower slenderness ratio than a thinner one.

    The above facts are the main physical reason why planing hulls are generally efficient only at planing speeds, and much less at displacement speeds. And that's also why you don't want a planing ama for a sailboat intended to cruise efficiently at displacement speeds. You might take it again into consideration if you decide to design a challenger for Hydroptere or SailRocket. ;)

    Cheers
     
  8. Doug Lord
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    Doug Lord Flight Ready

    I agree with Slavi- a planing ama* may work on small high speed trimarans particularly if foil assisted. But on a motor sailer trimaran I'd keep the amas thin-16/1 L/B or thinner. It's another story on the main hull-it can be designed to plane either under sail or under power alowing a wider main hull than would otherwise be possible.

    * Antrim disagrees-he says his planing amas-that are also fairly long and thin with widish transoms- plane and are an asset to his trimaran design because of it.
    Just check out Antrims design info here:

    http://www.antrimdesign.com/trimarans/erin/
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Define inefficient.

    Daiquiri has given you a good synopsis of the issues to consider, hence the question Once you define what YOU consider is inefficient, you shall meet others with a differing view. And that's the point. A hull if it works, satisfies the SOR, IS efficient. In the fact that it meets the objectives of the SOR.

    If you take said hull and investigate its hydrodynamic properties, alone, you shall easily find it to be at variance with your expectations. A successful design balances all the requirements of the SOR. To "optimise" as some call it a single parameter of a design may seem a laudable objective, but just means that you're not mindful of the other objectives with SHALL be at odds with the hull shape you have selected.

    To coin an engineers axiom: if it ain't broke, don't fix it.

    In other words, if the hull satisfies the SOR, why worry that one attribute/parameter of the whole design is not perfect, so what?

    Thus, an efficient DESIGN is the objective to satisfy the SOR, not an efficient HULL. The hull alone does not define the whole design.
     
  10. Wavewacker
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    Wavewacker Senior Member

    I understand...I think, and will study the sites above. efficiency then is in the eye of the beholder I suppose, if it is acceptable.

    I'm no going to be too demanding, ignorance is bliss you know, my thinking or huntch was to raise and lower amas to change from a sailing displacement hull to one that get over the hump of the displacement hull form. Likehaving a sailboat and fitting amas with a larger outboard. I think raising the main hull out of the water is asking too much initially, lowering a planing hulls and putting them under power seems that the main would then rise, having less effects....

    I understand wave propogation in terms of radio frequencies which basically I suppose is the same application, might be off, but I think I have it...kinda.

    Another issue is a rotating ama that might be a displacement hull and when turned 90 degrees (at its side) offers a planing hull.

    I sailed my kayak and used some pool worm floats playing with an ama to stabilize it. It was on a U shapped tube with the floats slipped over the straight ends that trailed aft and the cross of the U was across the kayak, I could raise and lower them by tiltine the U up or down. I found that with more wind and dipping them dipper I'd go faster than if they were barely in the water.

    I have no idea why, but just thought by dipping the amas and changing the water surface most relied upon would provide better performance.

    I have no particular boat in mind here.
     
  11. daiquiri
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    daiquiri Engineering and Design

    I humbly disagree with Antrim when thay call that ama "planing". It has a transom stern, but the slenderness ratio of the ama doesn't indicate that a sufficient dynamic lift vs. buoyancy ratio can be attained to call it planing. A semi-displacement at best, imho.
    Unless it trims up so much during sailing that it essentially becomes a surfboard, with just the flat aft part immersed in water.
     
  12. Doug Lord
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    Doug Lord Flight Ready

    ====================
    I designed and built a rough conceptual model using a stepped hull in planing mode- the thing rotates 180 degrees. The idea was just to test the feasibility of the geometry.

    Pictures L to R-1) displacement hull, 2) planing hull(after 180 degree rotation)

    click on image:
     

    Attached Files:

  13. Wavewacker
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    Wavewacker Senior Member

    EXCELLANT! How does the rotating ama hull test out? :cool:

    Reading the definition of the two wave types requires an understanding of the terms used to define them, LOL, so I didn't get it totally.

    In high school I wanted to be an engineer, my counselor told me my math was not sufficient to take that route, I guess that's why I became a business type, majoring in finance/accounting and economics....LOL! But I have an imagination.

    Thanks guys!

    I saw a small tri in the gallery, and "expedition" tri with retracable wheels, heavily builtin glass and a full canvas top, looked like an exciting little machine. I was wonder why not put a small motor on it (it had solar) and if you could get it to plane it would be awsome. The rig is for sale for 75K, a little much for a 15/16 footer I thought.

    I see to my wave propgation as to interference was not on target, but understand that better now.....

    So, is that why some tris have amas that are not the same length? I saw one that one was narrow and long but the other was much fatter and shorter. Was this to cancell out the wave forms produced? I woun't ask for the math to calculate that!
     
  14. daiquiri
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    daiquiri Engineering and Design

    If you are talking about leisure trimaran sailboats, I doubt that designer has bothered to rigorously analyze the wave interference. The hull and ama's spacing and dimensions were more likely determined from hydrostatic considerations, internal accomodation requirements, from experience, from previous similar boats or by some inevitable rule of thumb.
    Deeper hydrodynamic analysis and towing-tank tests are done for financially more important projects - navy ships, fast ferries, high-end competition boats like those for A. Cup etc.
     

  15. Wavewacker
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    Wavewacker Senior Member

    Yes, just the small stuff that individuals might use. But if some design would take off like a Hobbie I could see the financial viability of testing and research. But that's not where I was going at all. :D
     
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