Proa design

Discussion in 'Multihulls' started by Tcubed, Sep 13, 2008.

  1. keith66
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    keith66 Senior Member

    I imagine that having to reef or shunt at sea by running up & down the length of a skinny proa to fist sails would be a quick way to get drowned!
    Our 36 footer the Voodoo child had a jib at each end set on a wykeham martin furling gear, very simple & instant to use. When shunting we bore away & let the main go and she would sit stopped with the outrigger to windward it was just a matter of furling one jib, unfurling the other and then powering up the mainsail and away she would go. The standing lug main set on a midships mast generated the horsepower the jibs just balanced it out, i will try & find some photos of her it was a good few years ago now!
     
  2. Tcubed
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    Tcubed Boat Designer

    I look forward to your numerical forestay sag analysis, mainstay.

    In the argument of rigs for proas one must define what the priorities are. If avoiding going on deck is a priority, i would not recommend the rig i have drawn. Actually, if that were my priority i would be designing for myself a motorboat. No matter how you slice it, there will always be a lot of physical work and getting wet, etc, with sailing.

    For me, i'm used to going on deck for all sail handling issues, so it is not a priority. And i would say that handling a jib set well inboard on a narrow hull (~1.2 M beam @ outer f'stay, ~1.9 M beam @ inner f'stay, ~2 M beam @ front of cabin ) is pretty cushy after years of going out twelve feet of bowsprit.

    Shunting is pretty much like you describe it mainstay, except there is no going belowdecks at any point of this manoeuvre. And the halyards all go to base of the mast. This has worked for centuries. I have never had problems with raising and lowering jibs. I would put the sheets to the hatch, though, because trim adjustments are much more frequent than anything else, and also so they're within easy reach for quick action in the case of a sudden squall, etc.

    The process for dropping a jib is like this;
    The sheet is eased if it is too tight to allow the jib to go all the way down its stay.
    The halyard is untied (the very tip is always secured) and the downhaul-if there is one- is pulled smartly.
    Go forwards and get the lashings on.
    If there was no downhaul this last step should read ;
    Go forward, pull the jib the rest of the way down and get the lashings on.

    If the jib does touch the water, so what? It's clipped on to the stay so it will not fill up. It just skims along the surface, waiting for you to tie it down.

    My views on roller furlers can be seen at post #49.

    For reefing it would be as you describe as well Mainstay, and of course there's no climbing out to the end of the boom. The clew lines come in to the mast, all very standard slab reefing. This can be done very quickly, contrary to what some people think.

    The rig you describe does not convince me that it would be all that easy to reef, i'm afraid. These types of sails can be made to be very efficient, though.

    Keith, you describe <<running up and down..>>. I imagine you don't mean that literally. I consider that when sailing you must be hanging onto something at all times, or at least wedged up against something, or life expectancy becomes very short. Ommited from my drawing are all the grab rails that run the full length of the boat. I almost never tie myself to the boat so lots and lots of sturdy handholds is very important to me.

    Anyone know a simple method of drawing grab rails in in maxsurf or rhino? It's pretty much pointless for the design, but nice for the rendering. There are, i think, a lot of things for which good old pencil and paper is still the quickest and easiest tool for design and drafting....
     
  3. rob denney
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    rob denney Senior Member

    G'day Ken,

    Would love to see all the pictures and details you have for Voodoo Child.

    Absolutely agree about the folly of going out to the ends of the boat to shunt. Of course it can be done, and it may be less difficult than a bowsprit, but it is still wet and dangerous. I have found the jib is not required for balance, now just have a single main. Works well, saves a lot of money, weight and windage and shunting is very simple.

    No matter how simple a stayed rig is to shunt, it is not as simple as an unstayed one, which bends in strong winds so automatically puts in the first reef for you. And shakes it out when the breeze eases. Further reefs are far simpler with a rig that weathercocks and a boat that stops to make it easy.

    regards,

    Rob
     
  4. Taikaha
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    Taikaha New Member

  5. Taikaha
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    Taikaha New Member

  6. MAINSTAY
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    MAINSTAY Junior Member

    Tcubed
    Hope for your contributions in an analysis aimed at determining stay sag I've started in the Boat Design Forum.
    Larry
     
  7. sigurd
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    sigurd Pompuous Pangolin

    Tcubed,

    Congratulations on very beautiful design. I hope it will not be as heavy as Rob says. Then it will be fast in light weather. But in heavy weather it would be faster if you turn it around, your lw hull is so wide, it meets any chop and slows... 20-30kt, meets wave, driven down hard by rig, stops quickly, but does the rig stop as well? I mean, there are several ways to attain longitudal authority, but a thinner boat does not need as much in the first place because it stops slower! If there is a need for more bouyancy in the bows, make them tall, not wide, is my opinion.

    There is a big race tri with tall "canoe bows". Wish I could find a pic but I can't remember its name.

    The point that you make on being suspended between two crests, and losing yaw authority, and broach, may be important. Me I have only broached in monos, which is not a problem unless the spinakker tears. Not so for your proa. So if you designed your rig to be so slow to release that you are trying to make a failsafe hull to compensate, then you should ditch it and find a better rig.
    A free flying kite is one example.

    Cutting away the fore and aft foot may give your rudders less to work against, but you already designed the rudders to have part of the chord non-pivoting (as far as I understand), and that you have to lift the fore one, so those parts doesn't add up. It will slow you down a bit. You get a blunter, shorter boat in waves. As with the flare, you get more hobbyhorsing. That old thing about reducing wetted surface in the light, by rocker, I have almost no faith in. How many % are we talking? And what is your aero, wave and friction drag at that speed?

    The flare, overhang and tall ends of trad surf boats (proas, canoes, kajaks, viking ships) are bow-lifters, but remember that these boats are not tolerant of dipping the bows much, unlike closed boats with the crew to ww. Also they had less yaw authority than what is possible with two rudders.
    I suggest getting beam- or side-hung rudders for crash worthiness, authority, quick and easy shunting and finally because you can put hydrofoils on them and still retract. I believe trimming a long skinny hull correctly in pitch will account for a lot of speed. I once picked up a huge, heavy log from the water and laid it across the bows. My all up weight increased maybe 20% but my speed increased. After that I put windsurfer fins on the 6hp, and gained a knot or more. But I have those silly transom thingys back there, getting them up and another half meter bow into the water, making the waterline longer and pointier, is probably what did it. A masted rig will trim you down (kite and motor pitches me up), but having control over it cannot hurt, and will help against digging into the surf.
    To lift half the 3.5 ton boat, CL 1, at 15kt you need about 0.6 m2, 20kt 0.35m2, 25kt 0.22m2, 30kt 0.15m2. So there is a good amount of pitch control even for not so huge surfaces fore and aft. I would like to mention a model in the proafile yahoo group which steered by tilting the foils laterally, since you seem to be adamant about the WSA. That may also be less prone to ventilation than a rudder, on the other hand it could be complex to implement. Another way is ofcourse to invert them (from inverted T to T) in the light. For a normal rig the seakeeping of a proa like yours would probably be great, but you seem to want to push the area a bit (as do I), which can lead to pitch issues.

    What rob said about the pod. It is like a short fractional bouyancy ama with no control surfaces, and a huge lateral area when immersed - except it will be slamming like hell when not capsizing you. And relying on it for safety... I'd rather not say my opinion. I could be wrong though. I'd rather have a water sensor to a rig trim line for dumping power, if I had a boat that could not tolerate inversion.

    Another thing, whatever rig you choose, unless you fly a kite, see if you can close the gap completely to deck. This will reduce your induced drag. Tom Speer wrote about it somewhere here, and there was a spreadsheet even.

    "I have always been very concerned with windage in all my designs[...]"

    Then no doubt you are aware of that there is a very big difference between the drag of rounded corners and sharp ones, as in your sketches. I think there is something about it here, and the fellow writes a bit about the history of multi hull shapes and stuff. http://www.john-shuttleworth.com/Articles.html

    I don't fathom why you think a ww cabin would have more windage than a lw one. rather the opposite, since harry ww hull is shorter. Your ama has less projected area than his lw hull though but hinging an argument on this doesn't add up when you concider the CD of your LW hull.

    In short, I think your boat will be great to sail, and may be a great kiteboat, it even has a rig for when the wind is less than 8kt or so. As for the design decisions, you know best ofcourse, it is your boat.

    "Anyone know a simple method of drawing grab rails in in maxsurf or rhino?"

    Rhino: Make a curve and make a pipe/tube around it?

    To MAINSTAY: "To reef, I imagine I would:
    1. Lower the pivot, and secure the loose brace."

    How can you lower the pivot with a leeward stayed mast? you'd have it sliding along the shrouds? Sounds sketchy. This is like a gibbons rig, am I right?

    EDIT: your ama is too long. It is like putting a fourth wheel on a trike, and then having to make extra suspension to account for the extra loads and accelerations.
     
  8. Tcubed
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    Tcubed Boat Designer

    Thanks . And thanks for a well thought out post .

    The leeward hull ha en lwl/wlbeam of 12.33 at design wind and 13.7 at rest. I think those are pretty good slenderness ratios. It looks wide due to the flare. So i don't think it will have problems but maybe it would be better still to try and increase slenderness even more. I will experiment a bit with the flare and see how that can be optimized. The flare gives tremendous reserve of buoyancy which means it goes through less immersion changes due to the conditions. The total displacement is 3360 kg of 2180 kg is in the ama so the main hull is only 1180 kg. Compared to tris these are fairly low (but not crazy low) figures.

    The rudder is now on struts sticking out of the ww side of the main hull. It is balanced and the whole thing can swing through 180 deg plus the normal range of angles for steering. I'm not sure what you mean about the rudders.

    The rocker thing i am not sure about. I'm pretty final about the length, and the weight of each hull. The question remains on how to best optimize. Like you say the rocker may not be worthwhile for the saving in wetted area. I am not just trying to reduce wetted area in light airs (which is always a problem for any bi hull arrangement) but also have variable prismatic. It is quite interesting to play with.

    I don't see why you say flare will create 'hobbyhorsing' as it is a powerful heave damper. Rocker does induce pitching though , that is true.

    The gap between sail and deck . You are completely right and in fact that gap is not how it should be. However there are two problems with closing the gap; One is that it can only be closed for a portion of the foot due to the cabin that drops down . This could seemingly be solved by making a flush deck but that does not work either as the sail is at an angle so nescessarily it goes off the edge of the boat at some point. The pod helps here by creating a kind of endplate, but there are certainly limits to how big it can be made (and a lot of reasons to not make it so big either).
    The other reason is that having the clew too low is dangerous. The reason is that when flying/almost flying a hull it must obey the same kind of 'rules' as a dinghy. If a gust suddenly hits and heels the boat to a large angle , when the sheet is released the boom hits the water and the rushing water prevents the sail from going out. Therefore the clew must be high or a boomless sail used but i cannot see how a boomless sail could be used here without sacrificing efficiency or going to a (too) narrow sail.

    There certainly are two schools of thought on the sponson issue. The way i see it , it works a you say a kind of 'third' hull that creates a bump in the righting moment curves and a sort of 'buffer' against letting the sail capsize the boat. It is not meant to make it uncapsizable of course.
    As for tripping , i know what you mean, but the shape of it is such as to minimize this effect (flare and immersed volume before the edge goes underwater. ) Also by the time tripping becomes an issue -survival conditions- i imagine that the conditions are already such that no sail can be carried and it would be lying to sea anchors. The exact shape of the sponson is not yet finalized, in fact nothing in this design is yet finalized. As for the pounding there is no reason why it should pound heavily if it is correctly designed.

    Your point about the square corners and windage is a very good one. The reason i drew them square was because another factor of consideration in the design is ease of construction and i was intending to build the cabin and sponson of plywood on stringers type of thing but i might change that so i can round out some of those corners.

    The reason i say it is less windage for the cabin in lw hull is because that hull needs to be buoyant anyways. Imagine you were to design a racing proa that did not need any accomodations at all. The windward hull becomes not much more than heavy 'log' and the leeward hull still needs enough buoyancy to hold its own weight plus the weight of the heavier ww hull and go over the waves easily, so it is naturally the more voluminous hull. So by adding just a little volume to lw hull it becomes livable, whereas to make ww hull livable means making it much more voluminous.

    You say the ww hull is too long - when you see that it displaces almost twice as much as the leeward hull do you still think it is too long?

    What does WSA mean? [...adamant about WSA..]

    Unfortunately i'm without rhino till the weekend , or i would show the better pics in rhino.

    I do expect a certain amount of trimming to happen of course and will be analyzing this further once i get to the detailed number crunching stage of the events. I will also make a 10 to 1 model to double check it does behave as anticipated. This is also one of the reasons i did not put a very high aspect rig on it.

    A foil at the stern pulling down would cure bow down attitude but one has to calculate what its drag is (plus the comlications, weight and addition to apparent displacement etc) . In fact it is something i experimented with on my models. Models have enormous righting moments for their length so when the sheets are eased the bow goes down and a foil at the stern prevents this. In the models case it was so extreme (almost submarining from a reach to downwind) that a foil was very much worthwhile. In this case i'm not so sure , but it's worth thinking about.

    Pictures

    Profile to show sheer line and fairly high bows,
    sections with sponson (so far) and static waterline and waterline at design wind,
    Newicks' Greewich propane showing wing water clearance.
     

    Attached Files:

  9. sigurd
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    sigurd Pompuous Pangolin

    To be more clear, I feel that the traditional flare is probably appropriate for the leeward cabin boat (as is hard edges for plywood construction), but that it is going to slow down the boat when it gets immersed - the static waterline becomes more irrelevant since the wl is moving up and down. So I think that the unflared lw hull of harrys will be faster (and I argued that it was not neceserily more prone to pitchpole, due to less abrupt stop, and that reserve bouyancy can come from height), and that the eventual extra windage will not make up for it.

    If I were to make a masted race boat, then I would first optimise the lw hull, and likely it would be much like a double ended race tri ama. Not much flare there. But also impossible to step a mast on (except with compression members to the akas), due to the low height and narrow beam. Modifying it for an unstayed mast would not require beam or flare, so that is what I'd do - ending up with a harry-like lw hull.

    Then I would make the log voluminous enough to carry the cargo, and I think I would end up with a shorter and taller log than yours, based on intuition and experience from the twitchy movement of my cat. Basically with a short ama you don't get pitch torque input from the ama immersion - the vaka can do its thing. I would not make it flexible in pitch, risking resonant movement.
    Same goes for the inertia, in the tall one the weight can be more centered.
    I think it may get less WSA (wetted surface area) than the long one too, when there is some pressure in the rig - the long one is more all in or all out of the water, whereas the tall one is more gradually rising - a bit difficult to explain what I mean, but basically flying the ama is not stable, you will be trimming or steering to keep it there. When the long one dips, the whole ama surface will dip (I exaggerate), whereas the tall one's volume and WSA will more gradually change.

    Then I would ask myself where to put the volume of people space. Clearly, there is already volume in the vaka, but how much must I compromise its first purpose (going fast) in order to live there? That is the most subjective question. If I had to add much flare close to the waterline, then effectively I had a wider boat in seas. The ama would ride much higher in inclement weather so flare there could be wider and deeper without seeing much water. What I would end up with, I am not sure - maybe like yours, maybe like harry lw hull but a tad wider high up between the beams, maybe with the living space to ww.

    As for the hobbyhorsing of a flared vs. a piercing, taller hull. I may be wrong on that point. I base my belief more on dreaming than experience there, I think. But let me try to explain.
    Exaggerated, the tall bow slices into the wave, continues to slice, gradually the up force increases, at the maximum upward acceleration the center of bouyancy of that bow is close to the center of gravity of that vaka. So the motion will be more heaving and less pitching than the flared one.
    Flared hulls, in my imagination, tend to over-react - the maximum pitch upward speed occurs after you really need it - the bow rise too fast and just at the top of a wave where there is a trough behind. So the boat is not properly in phase with the sea.
    I am pretty sure that even if this assumption is correct for some hullshapes, driving force vectors and weight distributions, it will be wrong or irrelevant for others.

    As for the rudders. Ok you changed them to sit on axles sideways through the hull. That is what I would do as well.
    Incidentally you then already have them pivoting in the right direction for AoA control of the eventual foils (but would require good bearings and more actuation power than flapped foils).

    The pod: If you intend to fly the ama, then I think you are already just a heavy gust from immersing it, which would lift your rudders. How close is immersion of the roof then, I don't know. When immersed, the roof may be your only submerged lateral area. On the other hand there may not be much pressure in the sail at that point. The old fractional bouyancy tri amas were dangerous, according to Shuttleworth.
     
  10. sigurd
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    sigurd Pompuous Pangolin

    This is the one I mentioned, steering by tilting the foils.
    [​IMG]
     
  11. Tcubed
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    Tcubed Boat Designer

    Dynamic response to the sea surface is one of the most complex aspects of boat design. And very interesting too.

    There are essentially three factors at play ; the rotational moment of inertia, the curve of righting moments and the damping .

    This in turn can be separately analyzed for each of the three rotational axes; pitch (transversal axis) , roll (longitudinal axis) and yaw (vertical axis).

    Furthermore , there is also the linear responses in sway, surge and heave but we'll leave that to one side for now.

    Ideally, for pitch response, one wants the boat to instantaneously adapt to the sea surface, without wasting energy creating unnecessary disturbances to the water.

    The most important thing here is to reduce rotational moment of inertia about the transversal axis (rmoita). This reduces the amount of energy that goes into accelerating the ends of the boat up and down again with each wave.

    Then there is the damping . Long , shallow shapes have a lot of pitch damping anyways due to the straightness of the buttock lines. Flare increases heave stiffness so i would associate it with greater pitch damping.

    Length, or really the longitudinal righting moment, versus the rmoita will determine the pitching period , whilst damping determines the decay.

    Making the ama longer will increase the period of its pitching. For each and every hull there is a wavelength that will coincide with its pitching period and thus induce resonance. At this point damping becomes all the more important as it is the only thing that keeps cyclic motion from growing uncontrolably. Fortunately it is actually impossible to design a boat with zero damping. However what is of concern here is how to maximize damping without compromising other aspects of the design.

    Sigurd, your last post is missing the link i think.
     
  12. sigurd
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    sigurd Pompuous Pangolin

    http://f1.grp.yahoofs.com/v1/4OiDSX...bpYeB0oEk/Hydrofoil proa model/Foiledproa.jpg

    here. Maybe you have to be a member of proafile to see it. I can send it to your email.

    I like your rig, but even if your boom is prevented by water, you could let out the outhaul? Also, with the windsurf boom I don't know why you need the track on the pod.
    How is the upper yard, what do you call it?, held to windward?

    I think your rig could do with a little more complexity for the long trips with nothing to do but trim sails, so what about trying to incorporate topsails or wingtip feathers? variable camber double skin? It would be a ******* to trim the tips in leeward angle, AoA and fore aft rake, but if you got it right you could possibly get rid of half the induced drag?
     
  13. sigurd
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    sigurd Pompuous Pangolin

    Edit: I see the Propane, its akas will ofcourse slam like mad, but fortunately nobody sleeps there!

    These are the sort of pictures I am comparing your lw hull to, that is why I am saying it is wide: John Shuttleworth's Naia: [​IMG]
    [​IMG]
    [​IMG]
     
  14. Tcubed
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    Tcubed Boat Designer

    I do not understand what you mean with pitch torque input.

    What i have decided is to let the hulls conform to the water surface independently. I guess you mean if they are rigidly connected. I have yet to understand why white man's multihulls are almost all like this, as each hull is going through a different piece of water. To make them rigidly connected means they are perpetually fighting one another and the entire boat takes an average path , so neither hull is ever well aligned to the water surface. Plus the forces involved are large and so the akas have to be very strong, therefore heavier.

    In the case of a proa i would think it is even more important than for a catamaran as the hulls are quite different. In this particular case they are both about the same waterline length but that is about the only similarity as the WH weighs almost twice as much as the LH and is much less voluminous. Initially the WH had a total volume specific density of 0.5 but then i expanded it a little bit so it' about 0.35 so it would be easier to pack in the cago. By contrast the LH is something like 0.09.

    I expect the WH to not pitch very much at all and fly from wave crest to crest, sometimes going right through a wave and out the other side, whilst the LH will closely follow the water surface.

    ***

    I imagine that in practice i will aim to sail with the ama displacing 10 or 15 % of its weight. If i'm sailing on my own i would lower this to about 50 or 60 % while i'm asleep. If asleep with my wife on watch i might increase this to 40 or 50%.

    As far as complexity goes, no that is about as complex as i want to go even for long passages.

    My goal for speed is to be able to get to Venezuela in a day (24 h) providing there is a reasonable wind like 15 knts or more.

    Your link gives me 'document not found'.
     

  15. sigurd
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    sigurd Pompuous Pangolin

    Pitch torque input is probably a wrong term. What I mean is that if the buoyancy of the ww hull is hypothetically just a ball between the akas, the pitch of the ama will not be affected by waves, and it will not have any rotational inertia in the pitxh axis. Thus the whole boat's pitch will be ruled entirely by the vaka. I am thinking about my 6m cat, which has a sort of twitchy motion from the requirements of the two hulls to conform to the water. It is not entirely rigid - but I'm not sure if that helps, or makes it worse.
    I drew a 2ton hull to see what you are talking about, I am used to thinking of lighter boats. I think 30cm beam is ok for 6-7m boats, 60cm by 10-12m gives 2 ton for a reasonable depth. Not as small as I thought, maybe 15m (your length, more or less?) is in the sweetest spot after all - it is sharp enough to go through waves without noticing much, as you say. Have you had a look at Leo Lazauskas' pages on slender hulls?
    It will be fun to watch the long, flexibly connected ama. Is the pitch swinging of it damped also when flying?
     
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