thoughts on ama drag : outrigger design

Discussion in 'Multihulls' started by fabrice, Feb 8, 2010.

  1. fabrice
    Joined: Mar 2009
    Posts: 54
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Poitiers, France

    fabrice Junior Member

    I'm trying to design an outrigger canoe that will fit my goals.

    My hull seems OK, but I'm a bit confused by ama designs I can see on factory outriggers.

    Basically two ways to go :
    1) long and slender, looks like a little canoe, with not much rocker.
    2) shorter, rounder, banana shaped. (extremly short waterline)

    1) seems accurate if I plan to paddle fast : less drag beyond 3 knots, progressive drag curves.

    2) seems dramatically "overdragging" beyond 2 knots !!!

    The reason for the banana shape success these days, invoked by a designer I spoke with : "The ama's water entry should be as far back from the propulsion as possible.
    The "air" portion is meant only for buoyancy purpose if needed when leaning.
    When you design a straight ama, it's directional ability makes the hull turn toward it"

    well, my drag prediction curves tell another story : 10 times the drag at 3 knots, and a big vertical curve at 5 knots.

    Could somebody help me putting this right ?
     
  2. jmolan
    Joined: Dec 2008
    Posts: 65
    Likes: 2, Points: 8, Legacy Rep: 65
    Location: Mexico/Oregon/Alaska

    jmolan Junior Member

  3. marshmat
    Joined: Apr 2005
    Posts: 4,127
    Likes: 146, Points: 0, Legacy Rep: 2043
    Location: Ontario

    marshmat Senior Member

    From what I've seen (mostly photos, videos and sailors' reports, as such craft are very rare around her), those "banana" outriggers are generally held near the surface, not supporting much weight.

    If kept to windward (Pacific proa style), the banana should work reasonably well, as the crew can shift their weight around to get the appropriate righting moment with the outrigger just skimming the surface. The banana shape then gives it a somewhat progressive feel as it contacts the water, rather than the whole thing touching down at once.

    If the ama will be kept to leeward (Atlantic proa), then obviously it will be carrying much of the boat's weight when you're under sail. In this case, you'd want a minimal drag shape optimized for your planned cruising speed and for the fraction of the boat's weight carried on the ama. If you have a "banana" to leeward, the crew will have to hike out to windward to keep the boat level, and the ama is just used to protect against capsize.

    A trimaran is in essentially the same situation as the Atlantic proa, and its amas tend to be designed accordingly.
     
  4. garydierking
    Joined: Sep 2004
    Posts: 135
    Likes: 17, Points: 18, Legacy Rep: 174
    Location: New Zealand

    garydierking Senior Member

    Ama drag

    Are you planning to paddle or sail?
     
  5. fabrice
    Joined: Mar 2009
    Posts: 54
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Poitiers, France

    fabrice Junior Member

    mostly paddled to begin with, maybe sailed later.
    There's a legend behind the ama in the outrigger community. I read very often that it's supposed to soar on waters.
    Sounds phoney to me, as the center hull is always round and more than tippy, you have to rely on the ama's buoyancy !

    The evidence lies in the canoe's tendency to turn toward the ama in normal forward strokes. That's the worst problem when paddling.

    The question becomes then : what is the most accurate way to minimize ama's "attraction" ?
    less ama drag ? long ama ? short ama ?
    ama's water entry pulled back ?
    A bit of ama pitch (ama not parrallel to hull, but slightly nearer on front ?)
     
  6. Chris Ostlind

    Chris Ostlind Previous Member


    Likely to require a totally different ama shape when you get to that point in development.



    It really depends on what kind of waters you'll be paddling. Calmer waters, such as bays, or lagoons, do not need a sizeable ama. The purpose of the form will be more for providing a stabilizing effect. This is a lot like you see when the really skinny rowing hulls are brought to a stop. The rower(s) extend the oars and let them rest on the surface of the water. The oars have very little buoyancy, but they provide a bracing style of stabilization to the boat and keep it from rolling over. Calm water amas behave similarly.

    Rough water amas are different in that they do provide some degree of buoyancy, while doing the same stabilization thing as described above. The really good OC1 paddlers try to keep the ama out of the water as much as possible in calmer water, simply because the boat will be faster without the added drag.



    This set of questions has everything to do with how you will be using the boat. An all-out speed paddling machine will need different ama shapes than will a casual boat which operates at a lower speed regime. A rough water boat will require different ama shapes than will one for quiet waters. You'll also have to consider ama pitch as mounted, the shape of the form, the operating speed envelope, the entry and exit shapes relative to flow and the element you describe... toe-in, which may, or may not, be required, as it can contribute to more drag than it cures... if done to excess.
     
  7. Guest625101138

    Guest625101138 Previous Member

    A single outriggers on a canoe is quite different to twin outriggers on a trimaran. With the later they should be set just on the water so there is no appreciable contact in calm water. Ideally they could be height adjustable to be set for the conditions.

    With sailing the outriggers can be set to carry the entire displacement when heeled and they should be as long as the main hull and slender for fastest performance.

    For a stabilised monohull you want iutriggers that generate a lot of lift with little immersion but will also easily punch through waves.


    Rick W
     
  8. fabrice
    Joined: Mar 2009
    Posts: 54
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Poitiers, France

    fabrice Junior Member

    Chris you're definitely right.

    I want a casual boat for the family (3 paddlers), as fast as possible while being comfortable.
    Able to paddle in 4 / 5 bf wind, 2 / 2,5 m waves at sea but still comfortable on a large calm river or lake.

    I reckon the ama should be quite different under sail, the idea being making two sets :
    paddling : one ama, shape and iatos lenght matching the purpose.
    sailing : two amas, another shape, another iatos length.

    Rick : kinetic forces involved in paddling are deceivingly poor compared to sail force. (maybe 1/10 HP each paddler).
    I get the intuitive feeling an ama as long as the hull will do no good to the design, in paddling conditions.
    Not to mention every commercial design aiming at the opposite direction.

    As for the idea I read recently : pushing the ama's water entry behind the main propulsion force helps the hull going straightforward : Is there any data upon the subject ?
     
  9. Chris Ostlind

    Chris Ostlind Previous Member


    You'll need to design for the roughest conditions you expect to face. Obviously, this would be the ocean in the wind/wave situations you indicate. You'll need a more buoyant ama than a pure, flat water boat. You'll also need to favor the buoyancy forward a bit more. These boats will surf quite easily and there's nothing quite like a fast ride down a wave face with a nasty, ama-under flip at the bottom of the wave.





    There are plenty of sailing outrigger style canoes that only have one ama, rather than two. I've also seen quite a few single ama boats rigged with a smaller, safety ama on the off side. These are generally more sporty sailing outrigger boats where the crew is active and they are looking for every ounce of weight reduction to gain speed. Overall, I think that your plan is solid for a family boat that you'd like to use in many ways.




    I have not seen any data on that topic. There's a guy who infrequently visits this forum by the name of Wayne Iwamoto. Wayne lives in Newport Beach, California and has designed all kinds of different outriggers over the years. You can reach him with a private message through this post: http://www.boatdesign.net/forums/multihulls/q-gil-gils-catamaran-28495.html

    Another really good resource for anything outrigger in nature is Billy Whitford at the Newport Aquatic Center (NAC). Billy was the steersman on a winning, 6-man outrigger entry in the Moloka'i Hoe race across the 41 mile Ka 'iwi Channel back in the early 80's. http://www.ohcra.com/molokaihoe/molokaihoe.htm He is now the Director at the NAC. He may be tough to get to respond, but keep bugging him. http://www.newportaquaticcenter.com/
     
  10. fabrice
    Joined: Mar 2009
    Posts: 54
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Poitiers, France

    fabrice Junior Member

    thank you so much chris for your help and informations !

    The idea I was speaking of was seen on a french forum, on a topic by a former va'a designer and builder.
     
  11. Guest625101138

    Guest625101138 Previous Member

    Fabrice
    I have a good idea of the power level involved. I have designed pedal powered boats for myself and quite a few others.

    I have an OC1 canoe converted to pedal drive and I can hold about 9.5kph on it compared with 10.5kph on an optimum hull for my power and weight that has twin outriggers. I have attached pictures of both.

    There is significant advantage in having balanced outriggers if you are paddling as all the weight can be carried on the main hull. I prefer the term stabilisers rather than outriggers because they work best when they are not immersed - just surface skimming.

    The best outriggers are quite long for sailing or paddling. The loading conditions are different though. Aft set outriggers are best for pedalling/paddling. For sail you would have them as long as the main hull if you want to get the best speed.

    As a paddling boat the outriggers are stabilisers. It is preferable to have the centre of drag aft of the CoG for yaw stability.

    In the limit as a sailing boat the lee outrigger becomes the only loaded hull so it needs to have buoyancy and dynamic lift forward of the sail centre of pressure.

    The conditions between paddling and sailing are quite different.

    Rick W
     

    Attached Files:

  12. fabrice
    Joined: Mar 2009
    Posts: 54
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Poitiers, France

    fabrice Junior Member

    Hi and thanks a lot rick for all the good stuff here.
    If I understand you well, you can take a pedal run a bit faster in your tri than in the OC, but not by much.
    seen from the water line point of view, the OC stabilizer doesn't look any longer than the tri ones, does it ?

    Your point on yaw stability is quite inspiring :idea: and makes more sense than the former idea I was speaking about on ama drag.
     
  13. Guest625101138

    Guest625101138 Previous Member

    The speed difference may not seem much but it translates to the OC1 having about 30% more drag at the same speed. This is due to the short immersed length of the outrigger.

    The outrigger on the OC1 is longer than the outriggers on the other boat but it is always immersed and actual immersed length is quite short. It increases overall drag substantially. OC1s are rated much slower than K1s or surf skis in handicap events.

    If you have outriggers in a paddled craft you should use two of them to get the fastest arrangement. Set them up so they do not touch the water in level trim. For the fastest boat the main hull will be too narrow to sit inside.

    Rick W
     
  14. fabrice
    Joined: Mar 2009
    Posts: 54
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Poitiers, France

    fabrice Junior Member

    http://www.huki.com/store/index.php?page=Gull_Wing

    How about that ?
    Now that's a tri.

    I used to fiddle about my wife's kayak, 2 triangular buyoancy bags and a PVC frame (sorry no pics of that thing, it was quite ugly but extremely accurate despite some windage).
    Same idea, same goal, and worked well.

    Are we really improving the traditional outrigger canoe ?
    why not ? :D
     

  15. Guest625101138

    Guest625101138 Previous Member

    Fabrice
    This link shows the current 24hour world distance record holder on water:
    http://www.adventuresofgreg.com/HPB/2008_09_15_archive.html

    The stabilisers on the boat just surface skim. The main hull has a BWL of 230mm. It is about the fastest hull you can make for a reasonably fit male.

    A good sailing tri would have vastly different outriggers.

    Note that the Huki hull has the outriggers set above the water. That hull has a much greater BWL than the fastest hull. It has some inherent stability so the outriggers are not required to provide initial stability.

    As I noted earlier OC1s are quite slow by comparison with the fastest kayaks and surfskis or stabilised monohull.

    Rick W
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.