Fully planing amas for small Tri - opinions?

Discussion in 'Multihulls' started by frosh, Jul 18, 2006.

  1. frosh
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    frosh Senior Member

    I am part way through the design and construction of a 5.6 meter tri for "off the beach use" with amas resembing a miniaturized M20 lake scow. http://www.alaska.net/~fritzf/Boats/M20/M20.htm

    All up displacement including crew around 250kg and ama buoyancy a little over 200kg each. Amas to be 2.3 meters long 60 cm wide with flat bottom, the same length to beam ratio as the 20ft. M scow.
    Main hull is optimised for speed having a length/beam ratio of 14 and well capable of planing or sailing quickly in displacement mode.

    Amas will be canted to sit flat on the water at slight heel angle.

    Crew will be hiking on windward ama with one on trapeze to provide sufficient RM for most of the wind conditions encountered in Perth Australia. (around 20 knots)

    Power to weight ratio compared to an M scow will be around 85% higher, for mainsail and jib. In addition an assymetric spinnaker will be carried.

    Dynamic lift of the ama will also augment the RM from the crew weight. (At least that is the expectation, and I don't expect the ama to be much immersed at speed).

    What opinions of performance in various conditions would be expected?

    Also do you think that conventional amas that are slim and around the length of the main hull and with round bottoms would be better for high speed sailing in protected but windy waters?

    I know that this has a superficial resemblance to the Bethwaite HSP but proportions of main hull are different, and amas are much larger. :confused:
     
  2. Chris Ostlind

    Chris Ostlind Previous Member

    Hi Frosh,

    Do you have any analysis of the various possible configurations? e.g. Wetted surface drag comparisons of the named ama configurations, resistance to pitchpole, functionality beyond the chosen sailing grounds, etc.

    It would seem that you are going to be fighting a real battle in the lower speeds with all the wetted surface drag. Maybe that can be overcome with pure power and a very active crew who can keep the form well up on the surface, I don't know.

    I would take a wild one here and simply say that the only way to really get a handle on this potential is to actually do what you are doing. Build the boat and then getting some serious water time to discover its potential looks to be supremely informative.

    I recently did this design of a 5.2 meter tri for fast, solo beach style sailing. While there's nothing really experimental about the boat, it will be easy to sail quickly and yield predictable results and a lot of fun on the water for whomever chooses to build one.

    I woudl ike to see your boat on the water, Frosh, and look forward to some great photos of your effort in action.
     

    Attached Files:

  3. Doug Lord

    Doug Lord Guest

    "Fully planing Ama's"

    Sounds like an exciting project ,Frosh. I'm not sure the "C scow" shape is ideal for an ama on a highspeed small tri-you might consider the Parlier bow shape on his planing cat-just to reduce possible high shock loads. I think the "planing ama" idea is excellent particularly if you solve the pitch control problems that will occur if the ama is capable of planing with the main hull flying; will it be designed to fly the main hull? I'd look at the dynamic lift from the ama carefully to see if, at some point, it could support the weight of the whole boat-whether you want it to or not. I think the planing ama's have a significant edge over high beam to length ratio hulls in stronger winds if high speed is the goal. Check into Parliers writing about his planing cat and the theory behind it if you haven't already. Your relatively small ama's remind me a bit of Parlier's stepped hulls with the aft section cut off- you'll have to be real aware of pitch control the more the ama is loaded while planing at high speed....
    I would guess your thinking is to keep the ama(s) clear of the water at least until it is capable of planing? That is if you ever have lite air in Perth!
    Good luck and hope you'll post some sketches and pictures soon....
     
  4. frosh
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    frosh Senior Member

    Thanks! Chris and Doug for your valued feedback. I had already almost completed one ama in foam/carbon around the same shape as the International Moth current shape. I couldnt get motivated to build the second one. On reflection it was because it would not be pushing the boundaries going this route, and I would rather be doing something fairly radical. I intend to keep the ama "just kissing" the surface in lighter breezes and progressively allow more weight from RM to transfer to leeward ama as the wind strength increases. I have owned and sailed a few scow Moths in the past and my experience is that in stronger winds off the wind these hulls plane very freely and go like "stink" with seemingly little form drag and not all that much skin friction drag (which is much less of a factor in higher wind).
    The "million dollar question" is how much dynamic lift will be produced in somewhat choppy water. I think you are right Chris; I have to build it and then find out by actual water time this coming Summer. I don't think calculations will all that much help.
    I am aware of Parlier's Hydroplaneur, but this suits much finer hulls and I have my heart set on being able to plane on a mini scow, and support at least 50% of the total displacement. By maintaining some contact with the water for the main hull, I hope to avoid major pitching. The amas and rig are set well back which I believe will also help minimize nose diving.
    I will post some pictures and sketches when I can soon. :)
     
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  5. frosh
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    frosh Senior Member

    I am posting a rendering of the main hull and the "mini scow" amas done on Freeship. Hoping that the files appear OK.
     

    Attached Files:

  6. Chris Ostlind

    Chris Ostlind Previous Member

    Interesting output from FreeShip

    What is the fore/aft relationship of the amas to the vaka hull?

    It would seem to me that the placement of these rather short forms will be absolutely critical to overall sailing capabilities, especially with the avialable power as you suggest. Your wire monkey is going to be busy until the boat achieves escape velocity.

    Can the amas rotate around the aka beam axis, or are they fixed?

    Is there a potential for adjusting the ama placement fore and aft for different sailing sessions? Or is everything pretty locked-in due to shroud placement, etc?

    Yeah, I know, lot's of questions

    Chris
     
  7. frosh
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    frosh Senior Member

    Hi Chris, Main hull came out very accurate, ama was difficult and should have an almost flat bottom with very slight V and vertical topsides.

    Position of ama will be fixed by two full width 100% pro manufactured carbon tubes. Therefore it might be difficult but not impossible to tune the angle of attack of the ama bottoms, but I was not planning to do so.

    The amas will be located from around 1100mm from stern to 3450mm, which corresponds fairly much with CG of the vaka when sailing.

    Shrouds will be attached to outside of amas so no adjustment here either.

    It has to be an educated guess. If it very unsatisfactory I will build new amas and try that. I am committed to seeing this thru to a conclusion where the amas are providing considerable dynamic lift hence contributing to RM.
     
  8. Chris Ostlind

    Chris Ostlind Previous Member

    Complete Boat Rendering

    How exciting to be full of thoughts with all the little stuff to pull together on this new example.

    Do you have a full rendering of any sort of the complete design as you see it?
     
  9. catsketcher
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    catsketcher Senior Member

    Leeward float redundancy

    Hello all,

    The little power tri is an interesting concept. I have often thought of a tri that uses only extreme beam and small floats to get going. I am already building another folding cat so the tri will have to wait.

    Trimaran crossbeams have to be strong and heavy because they act like cantilever beams buried in the main hull. I wondered if you take the HSP route and go away from bouyancy to leeward and go like a skiff and have weight to windward whether you will be faster. Small floats give you water bumpers so you don't flip all the time but by going this way you get away from cantilevers with their attendant high loads to compression beams which are much less loaded. The weight to windward is counteracted by the rig force (assuming yuo have a side stay) and these then load the beam in compression. Sort of like a Pacific Proa that can tack. In my drawings I had the leeward float out of the water like the HSP to lessen drag so float shape isn't too much of an issue.

    Having sailed skiffs I wouldn't think the thing would be too hard to sail with a nice easy to adjust rig and it would be much lighter with small floats and crossbeams than the alternative tri. I think it would have really low drag and be more interesting to sail than a normal tri.

    cheers

    Phil Thompson

    www.foldingcats.com
    info@foldingcats.com
     
  10. Doug Lord

    Doug Lord Guest

    planing ama's

    Frosh, you would probably do yourself a favor if you design the attachment of the planing hulls to the crossarms so that the pitch attitude of the ama vs the main hull is easily adjustable. That the planing hull is at the correct pitch attitude when under load and going fast is very important to keep drag down and hard to calcu-guess correctly in this kind of arrangement. Also, it is likely that the ideal relative pitch attitude may be a bit different for different speeds.
     
  11. frosh
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    frosh Senior Member

    Phil, thanks for your comments. My main hull is already completed and being around the same proportion as a skinny Moth (probably even slimmer) has no hull stability at all. For two sailors to keep such a hull at a fairly constant heel angle with a reduntant leeward float (emergency only) with two or three sails flying would require two experienced Moth sailors, or better, which we are not. Performance wise you probably are right, but the concept becomes extremely demanding to master. I don't think that I will lose a lot of speed with my leeward ama fully planing, as it adds RM therefore sail carrying power. It also overcomes the extreme tippiness of the vaka as the load on the ama can vary from very little to maybe 100kg + with no real change in heel angle, and little increase in drag.
    Doug, I hear what you are saying, and I will think hard about this before constructing the crossbeam to ama connection. I'm just concerned about weakening what is the probably the most stressed part of a tri, and also exactly how to do it. :)
     
  12. PhilT
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    PhilT New Member

    Torsion

    Hello again,

    Frosh I think that stipulating the correct attitude of the floats may be harder than thought. Two reasonably small crossbeams will "suffer" torsion if the CP of the float is not directly between them (if they are both identical). Twist will probably occur pretty readily when the float starts to lift. The CP or CB (depending on planing or displacement mode) will change with every wave. certainly my 31 ft Twiggy would wobble its float every wave until I tightened her underwires and made her into one article.

    I would think this could be made to work for you as the twisting of the crossbeams could be keep the angle of attack positive. I don't know how big the beams are but if they are small tubes with an underwire you could play around with packing under the ain hull connection and the underwire tension to get your angle of attack changes. I think that going sailing will tell you what is needed so building in a little room for error may help.

    Then again from the Twiggy experience I really liked it when I had tighter underwires and the boat functioned as a whole. It did freak me a out a little when all three hulls seemed to want to do their own thing. Underwires and a tight rig led out to the underwire attachment, proved a solid and strong arrangement.

    One last thing about stability. In medium winds I think you would be surprised at how easy it is to keep a boat under its rig. In skiffs in no wind it is very hard to keep three fat blokes from tipping the thing over. In 15 knots it is very easy to sail the thing and keep it balanced - use sheet and steering to keep the hull under the rig. Lots of fun and easy too.

    cheers

    Phil Thompson
     

  13. frosh
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    frosh Senior Member

    Phil, I think that the carbon crossbeams are large enough and also the overhang on each side of the vaka is fairly short that there will be almost no twist of the ama except maybe under very steep chop. No underwires are really possible as I have no mounting points. As the rear crossbeam will mount almost at the transom of the amas any twist due to water action will push the bow of the bow upwards a little which is probably no big problem.
    Regarding the angle of attack which has been alluded to by Chris and Doug as well, it is definitely a problem to go adjustable. I think that I will try for the same line for the planing surface of the ama as the rear one-third of the vaka and try it on the water. The beams are not buried into the main hull and can be fairly easily be individually lifted if I find it necessary later.
    My experience sailing this boat in its previous mode as a tacking proa, indicated that once moving quickly we could balance for long periods with only the main hull in the water, but there was not enough stability to launch a kite, and we never attempted it. The change to tri configuration is mainly to allow us to more than double sail area off the wind.
    Where my tri will differ to a production 18 ft. catamaran is that all hulls will plane, and the boat fully rigged should weigh in around 100 kg rather than 180kg. :)
     
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