Length of ama on outrigger canoe?

Discussion in 'Boat Design' started by dogfuel, Sep 14, 2008.

  1. dogfuel
    Joined: Jul 2008
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    dogfuel Junior Member

    Bear with me ... I'm just beginning to figure things out...

    Why are the amas on small outrigger canoes so long? Seem to typical be 75-100% of hull length.

    It seems to shorter ama (still keeping a 16/1+) length/beam would put much less stress on the structure and still serve needed purposes ... not to mention be better to tack.

    Any thoughts?
     
  2. masalai
    Joined: Oct 2007
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    masalai masalai

    Not on a canoe - a good canoe is for paddling and should not need an ama... a Voyaging canoe is usually a "catamaran" or of "twin hull form" for stability and carries some form of sail assist - See what you can google from wikipaedia of the Polynesian & Melanesian peoples of the Pacific....

    Where an "outrigger" is used it is often just a shaped log. and their length / beam ratio related to the availability of the log and the mass of the fattest person....
     
  3. dogfuel
    Joined: Jul 2008
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    dogfuel Junior Member

    ama length

    Thanks for the reply - I should have clarified that I was asking about a sailing outrigger canoe. The purpose and need for the second hull is clear, just wondering why the need to be long as I've seen on almost all examples.

    Looks like most amas are 75-100% vaka length - Seems to the ignorant (me) that 50% main hull length (with the same displacement) or so would provide the stability (and weight if needed) and put a lot less stress on the craft, so allowing a lighter build. Also seems easier to tack and to right if turtled.

    J
     
  4. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    Hi DG.

    The length of the Ama is often a question of cost in time and materials, certainly for traditional builders. And, as Mas so succinctly put it, how big an arse does it have to support. Some outriggers can be even longer than the main hull.
    There is a current discussion on Proas (which are just outrigger canoes with attitude) ,for example, at

    http://www.boatdesign.net/forums/showthread.php?p=227967

    where all the configurations are being discussed, and some interesting figures are available.

    One of the design considerations is which side of the sailing canoe will be leeward, (I gotta keep remembering ... ama is the windward hull, vaka is the leeward hull), or will it be called on to do both.

    The longer the outrigger, the less drag it will be when "pushed" into the water as a Vaka, but if it is to windward, (Ama) then that becomes less of an issue.

    A long outrigger can make turning more difficult when paddling or sailing as well, so you can have a tradeoff between sailing speed and manouvering.

    I suppose there would be no problem having two or three sizes that you could mount depending on boat use and weather. Try doing that with a catamaran :)

    Are you building at the moment? Pictures most welcome if so.

    Good luck with the project.
     
  5. dogfuel
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    dogfuel Junior Member

    outrigger sailing canoe

    I should have some pictures soon ... not really building yet but working out and testing stress on a piece to rapidly reconfigure a small (18') thin canoe hull based on Gary Dierking's book from Crab Claw rig, to windsufer rig, to two windsurfer rigs to shunting proa (proa is just for kicks, not at all practical for me).

    Recofiguration using 80/20 extrusions as tracks and their linear motion components - could be a stupid waste of time but I'm having fun.
     
  6. dsuursoo
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    dsuursoo Senior Member

    it's also an interaction between bow waves, among other things.
     
  7. Chris Ostlind

    Chris Ostlind Previous Member

    There are probably a dozen different issues to bring together on this topic if one wishes to arrive at a correct answer.

    How fast is the boat designed to go?

    How much weight will it carry?

    How big of a sail area are you suggesting for the boat?

    What sort of overall utility will you expect from the boat?

    What is the budget for building and maintaining the boat?

    What kinds of materials do you wish to build the boat from?

    What sorts of safety margins are you looking for in the boat?

    Will you and any crew be acting as live ballast while sailing, or is this a static sailing situation where the boat needs to be able to, generally, take care of itself as to balance and stability?

    And the list goes on and on...

    These questions need to be answered before you can arrive at a suitable solution as to, how long the ama and what volume, in the design.

    Long amas are not necessarily difficult to turn, depending on design, so do not restrict yourself to a singular approach in this regard.

    Short amas, can also be difficult to turn if they are not designed correctly and they bring with them a decided disadvantage, at times, when it comes to resistance to pitchpoling.

    In short, the entire boat needs to be viewed as a whole structure. It can not be isolated as distinctly separate elements if you want it to perform over a wide array of design applications.

    Chris
    www.lunadadesign.com
     
  8. Guest625101138

    Guest625101138 Previous Member

    To be effective the outrigger has to carry load. This either provides weight for righting moment or buoyancy for righting moment. The more it is loaded the better the stability.

    A hull carrying load will develop wave drag. If it has reasonable displacement it will add a lot of drag unless it does not have reasonable length. You might only have 10% of the total displacement on the outrigger but it could contribute 30% plus of the drag.

    I do not like single outriggers. They are required to carry load all the time unless you operate on the scary edge. In a following see as you roll over a wave or it passes you by, a short outrigger will drop into the trough and set you in a roll. The closer it is to the length of the main hull the less you roll.

    I much prefer a trimaran set up. The amas are balanced and you can set them above the normal water level. This means all the wight is carried efficiently on the centre hull providing you can shift weight to keep it balanced. The outriggers only act as safety net.

    If you intend to really press the boat and carry a lot of load on the amas then long slender ones with ample buoyancy to support the whole displacement will give good performance.

    Short dumpy amas are like a brake as soon as they dip into the water.

    If the ama does not have enough buoyancy to carry the total displacement you can roll reasonably easily once it is submerged. It is important there is enough buoyancy in the bow to prevent nose dive. Almost impossible to recover once this happens.

    Rick W
     
  9. dogfuel
    Joined: Jul 2008
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    dogfuel Junior Member

    Thanks for all the insight and comments ... this is a little simple plywood boat boat (18') - most of it decked - to be driven by either a 9.5m2 windsurfer rig, an appropriately sized crab claw, maybe two smaller windsurfer rigs as a ketch. Mast step and collar moves and locks over about a four foot track and an second mast has a step fixed in the bow. Leeboards mount on either hull over about a three foot track and I main try one wih a horizontal foil at the bottom to lift or pull depending on whether the small hull is windward or leeward (btw, on an outrigger canoe, is the small hull always an ama or do the names change on a tack?)

    I'll probably build on 14' and one 8' ama (or "little" hull depending on the answer to my above question) and observe the things everyone mentioned first hand.

    Thanks again for all the comments - I don't know much about boat design (obviously) or sailing for that matter but my wife is ill and I spend a lot of time at home hanging out with a five year old ... thought a reconfigurable boat would be fun and educational ...I'll let you know
    Maybe 2 - 400 lbs of live balast and sailed just for fun
     
  10. Jan-Einar
    Joined: Mar 2009
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    Jan-Einar Junior Member

    Small trimaran vs. outrigger canoe

    Hi Rick,

    I feel I have to comment on your statement
    The ´flying the ama(s in this case)` mode of sailing on a trimaran that you suggest is beautiful, stylish and fast sailing.
    Agreed.
    Only it is a little too acrobatic and demanding for the occasional sailors on a ´I love to sail and not too slow.´attitude.
    Like me :cool:
    The outrigger canoe with ´just´one ama does offer not that much speed but a more forgiving arrangement regarding trim and attention to manoeuvres but is still faster than conventional touring dinghies.
    Since I did spend quite some time singlehandling the dutch Volksboot (VB) 480 dinghy I learned to appreciate that in a boat.

    And there are two more things on the outrigger sailing canoe that attract me and other lazy guys, I suppose:
    - When cartopping (We talk about light craft, don´t we?) or trailering in disassembled condition there is just one ama to install before sailing! ;)
    - On a small trimaran the amas need to be so low in displacement they do not prevent capsizing in a bullet-proof way OR the ama-main hull (vaka)-ama arrangement needs to be strong enough to withstand the situation vaka in a trogh, amas on wave tops or vice versa. Think about long term effects and material fatigue! You need something strong there. If that is to be light it will be expensive.
    On an outrigger canoe with just two ´heights´of water surface to bridge the boat just rolls but does not stress the crossmembers (akas) with all it´s weight.​

    So my understanding of small craft is:
    Speed no matter what: trimaran
    Easy handling, transport, low cost with good speed: sailing outrigger canoe

    regards,
    Jan-Einar
     

  11. peter radclyffe
    Joined: Mar 2009
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    peter radclyffe Senior Member

    If the ama is the second hull, it may dig in if its too short
     
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