on the design of ultra slim yachts

Discussion in 'Boat Design' started by dionysis, Dec 16, 2003.

  1. Blether
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    Blether Junior Member

    Hi,

    >>So far no one has explained why the boat won't work<<

    I too've been reading with interest. Dionysis, I'm surprised to see you've suddenly dropped the canting keel - though I did wonder where you'd be carrying the fuel to work this and the movable ballast. Without it, you're back to Gonzo's "plank on edge" - i.e. long, narrow, deep - certainly in the fore-aft vertical plane, where it counts in terms of motion through waves upwind and downwind. The only difference is you've removed a triangle of fin/hull forward and abaft of what's now the fin keel. [Sorry, Paul B, I'm ignoring you :) ].

    Your hull starts off as half a cat, but having a deep keel, has a big difference in moment of inertia in that plane.

    What's your thinking on:

    A) Engineering in the mast/keel/hull joint area. I picture you barreling off the back of a southern ocean wave and burying the bow into the face of the next. Two scenarios - (1) the mast and/or keel shear away where they meet the hull, or if a single member, slice forward through it; (2) the hull shears immediately before the mast foot/keel root as the vessel peforms something between a pitchpole and a broach.

    If you don't go any lower than, say, 135 degrees from true wind, won't ultra-slim be on her ear under mast alone ?

    B) The 'plank on edge' boats were described as wet and tender. (Now you've no canting keel) what is your feeling about these potential failings in your design ? With no flat width letting the hull surfboard, and scarce reserve buoyancy amidships, she'll be a wave-piercer all right, and unlike a catamaran, anyone above decks will be in the middle of the water sweeping over. As for tenderness, how far will she heel in different steady winds ? When hit by gusts ?

    C) What will be the gap between the deck and the foot of the sail(s), to keep them out of the water ?

    D) Livability & practicality. In heavy weather, will she be sailed entirely from below ? How will tasks like reefing, emergency rig repair and external maintenance be carried out ? How many crew will she carry on long passages ? What will be her range, given store-carrying capacity ?
     
  2. dionysis
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    dionysis Senior Member

    what I think

    Blether, this is the way I was thinking:

    All up weight ~ 18,000 lbs, displacement to length ~ 30. Lighter than equivalently sized cats and trys by a long shot. Shape optimises monocoque hull strength.

    Self standing mast (a-la wyliecat) height ~ 80 ft, sailarea ~ 1,200 square ft ~ 500 - 600 lbs weight, hence inertia of mast sail not very large - certainly not as large as a stayed mast/boom etc. Mast canted aft considerably, so forward shearing force vector considerably minimised; mast bury is about 7 ft, this is good. Flying off wave and spearing the next wave - what is wrong with that? There is no brickwall effect: just pierce as much as momentum requires then rise to wave; we are talking about a boat 84 ft long. What is the difference between this behaviour and either a mono or multihull behaviour in the same conditions? In fact the shape allows much better behaviour compared. I cannot see that there would be any engineering problems - righting moment is ~ 70,000 ft lbs: this is very small (~ 1/3rd ) for an equivalent yacht.

    The problem is 4 tons of lead ~ 13ft below the hull on a stainless steel foil. My answer is to fix the foil at decktop and bottom of hull, again about 7 ft bury - still need to ask, how is this different from an open 60 yacht? It is acually better.

    Being stopped by a wave at speed is not a problem - the hull is shaped in such a way that it will always come to a stop gradually never suddenly, since it is so sharp.

    My worry is coming off a high wave and doing a belly flop, but even then the boat is so narrow, the worse damage would be noise.

    Since the righting moment is small - this requires an efficient (highlift to drag = single sail cat/windsurfer like configuration) sailplan, which will drive an efficient hull. Which, if any hull is easily driven this is. Nevertheless unless you add more weight to keel she would be a little tender:

    I calculate resistance at hull speed to be about 700 lbs, thrust when pinched in closehauled condition, needs about 20 knots apparent - this is design wind about 16 knots - side force about 2000 lbs, times 35 ft for average center of effort = 70,000 ft lbs = 30 degrees heel.

    Everytime I look at mono boats in smooth or rough conditions going to weather, they are heeled at 30 degrees or more. So again nothing special here.

    Winsurfer style sail, clew very high off deck: reaching or running with clew to leeward ~ 30 degees heel ok before dipping boom. So no worries there either.

    As I said in the beginning of this thread, liveability is cramped - this is the only hesitation - perfect for racing, but a little cramped for cruising - which was why I designed this boat in the first place, for my own use.

    Reefing etc, all done from the cockpit - no different than for any boat - there is a need to design a walkway along the upper sides of the deck (since the deck is round) , but this is the only provision for walking around on deck. I cannot see any problem with getting about.

    Accomodation is for two permanently, another two can come along for the ride - berths available. Standing headroom ~ 30 ft long ~ 6 ft wide on average. Imagine a 30 ft boat's accomodation in width, and an 80 ft accomodation in length. I could cope with this kind of space, but my otherhalf wants all the modern and stylish things.

    Canting keel is good idea, especially if you want to go into shallower areas - good too if you want to not heel so much, you can optimise canting keel so that the boat is only 8 or 9 ft deep, but the problems are you need to provide separate sideforce = foil/s - you need to provide hydraulics etc for moving keel - not attractive - too complicated.

    My idea was to use winches to winch the keel, and this is great, but the idea of the longitudinal centre being taken up by machinery = separating hull into for and aft compartments, hmm... not so good, but still ok, but sideforce a problem. Canting keel more vulnerable in grounding.

    Anyway still thinking about the design, trying to furnish the 70,000 ftlb righting moment while limiting draft: that is the main problem.

    I hope this answers some of your questions Blether.

    cheers
     
  3. Unregistered

    Unregistered Guest

    Have you thought about using the free-standing aero rig, or bergstlrom (Cant spell it).
    This would give you the advantages of the sloop configuration, without the huge bending loads in the hull that break AC boats from time to time?
    Just a thought
    Matt
     
  4. dionysis
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    dionysis Senior Member

    on the aero rig

    What makes a difference to the stress experienced by a freestanding rig at its root, is the efficiency of the sailplan: the higher the lift to drag of the sailplan, the lower the amount of heel, for given thrust, hence the lower stress.

    From what I understand the aerorig is efficient on a reach and a run, but not so efficient closehauled. This boat has adequate righting moment when reaching and running, but needs effciency when beating.

    Still the aerorig is a good all round sailplan. I don't think it's viable for this design.

    cheers
     
  5. Blether
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    Blether Junior Member

    Hi again.

    For an overall picture, I've made a biro-aided design (BAD (tm)) sketch - like this ? (Sail shown flying to leeward, or too small - take your pick).

    Raking the mast is a nice way to mitigate the shock loads it'll experience in piercing waves, if you can engineer it to handle the compression loads that are no longer in column - some flex required ?

    >>The problem is 4 tons of lead ~ 13ft below the hull... how is this different from an open 60 yacht?

    - the leverage that the hull has on the joint, but needn't be insurmountable.

    >>As I said in the beginning of this thread, liveability is cramped...
    >>...Reefing etc, all done from the cockpit - no different than for any boat

    - sorry, I was thinking not so much of accomodation, but the fact that if you're piercing waves, you've got, not torrents of white water, but solid blue water consistently sweeping the decks and cockpit - the stuff that breaks fittings. How long can one person stand being out in that ? When the sail tears, can it be dowsed ? Even in the cockpit (is this roller reefing ? I'm guessing we're not reaching up to handle any tack cringles), this aspect *is* different from other boats.

    ---

    My gut feel is she'll not make a sustained-buildup heavy weather boat because she's too extreme to sail and to stay in one piece in chaotic wave conditions of that severity - and so not a long-distance boat.

    On the other hand, I'm asking myself whether, if her accomodation was a ballasted, freely-rotating sleeve within the hull, you might not be able to call being horizontal the whole time a valid storm tactic and a clever way of reducing top-hamper.

    Taking a keel extended as far as deck level, don't you have plenty of depth for a fin telescoping inside a sheath ?

    The conditions for crew above decks in general need careful thought, but even if she doesn't work over long distance, she would be some machine. I hope you can achieve the required strength within the weight you plan.

    When can we see the one-man prototype ? :)
     

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    Last edited: Jan 22, 2004
  6. dionysis
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    dionysis Senior Member

    Yes that's the idea - mast a little further aft and not so canted, keel a lot slimmer and rides in sheath like you say.

    This boat is not heavy, so it won't submarine. Large racing cats in rough conditions submarine (take green water ) a lot mainly because they have half of their displacement 40 ft to weather developing a yawing/pitching moment that buries the lee bow.

    First a 1 metre model.
     
  7. Re "Large racing cats in rough conditions submarine (take green water ) a lot mainly because they have half of their displacement 40 ft to weather developing a yawing/pitching moment that buries the lee bow."

    That may be true but can I ask - if it's only pitching/yawing from the weather hull, why do slim boats like International Canoes, small beach cats and Moths (without foil rudders) all nosedive a lot in flat water? Is the problem not simply the fact that ultra-slim sections don't develop enough dynamic lift?

    Many of these boats also have very rounded foredecks to reduce the effects of nosediving, but that leads to major problems working on the deck. I know the tri guys live on narrow foredecks, but they have wide tramps further aft and they are an insane bunch of ironmen. Would a device of horizontal stanchions that you mention be able to withstand being dragged through water at 25 knots? The drag on a cylindrical stanchion is very high, isn't it? We lose them on conventional slower boats quite often, and they aren't 90' long and 15' under water. And the experience in cats is that anything that increases drag on a submerged bow will lead to tripping, so isn't having a high-drag bunch of stanchions underwater going to exaggerate the nosediving that the rounded foredeck is intended to ease?

    If current big boats sail with their bow down a lot (as they do) even with the flat sections up forward to develop lift, how will the ultra-slim boat keep its bow up at all? Boats like the IRC 80 Nicorette already sail upwind with green water over the bow a surprising amount of the time, and as mentioned by someone else when you get to this sort of speed, water over the deck becomes a significant problem and danger.

    The ultra-slim concept has been tried offshore, in a boat called Seltrust Endeavour (I think) when it did the Parmelia (UK to Australia '80) and then Spirit of Pentax in the first BOC and (I think) in the unsuccesful attempt to circumnavigate twice non-stop. She was a 55 foot schooner, about 7' in beam. She was also, from all reports, extremely uncomfortable and dog slow upwind and she retired from the BOC after a knockdown.

    There was a similar but larger boat, the schooner Sundowner of about 130', built about '72 with the basic structure of a steel tube about 11' in beam, with flat decks added on top. I think she now lives in Tasmania, Australia. Once again, from reports in the press and AFAIK results in a Tall Ships Race, she sails on her ear and barks big-time - apart from reaching OK in moderate winds AFAIK.

    re the 1 metre model - somewhere onthe net, there is an analysis of why the ultra-slim route championed in model yachts by Graham Bantock will not work in full-size boats. Finally, there is some evidence (from the Moth class and perhaps from the Farrier tris, which work very well at a higher b/l ratio) that 10:1 is the "magic number" for efficiency.

    Interesting idea, though.
     
  8. dionysis
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    dionysis Senior Member

    Thanks for the thoughtfull input Another Guest,
    I will start at the top:

    Yes and no, these boats all have large sail areas, with vertical CE moments around the same as their longitudinal inertias - this may have a lot to do with the nose diving.
    Yes, it is a problem - the horizontal stantion idea was a first go at providing deckspace - the stantions need not be circular by the way, perhaps a simpler approach, but one that I don't like because it spoils the aerodynamic efficiency of the hull, is to just put solid wings either side. This is a design challenge that needs more thought. My preference is to not have anything at all and just have strategic hatches where there are needed. Still it is a problem.
    In the first instance, this hull is about the most efficient (aero and hydro dynamically) as that you can get - = a lot smaller sailplan = smaller pitching moment.

    You cannot get away from taking green water over the bow in an ocean racing boat since you are driving the boat so hard.At least with this hull, whatever green water you take will flow off a lot quicker and with far less obstruction than with any flat topped hull.
    What can I say - schooner!- all that top hamper, and not very weatherly to boot. No comparison. Ditto for Sundowner. Although stiffness is a challenge for sure.

    Perhaps the one-metre model argument that you mention is this
    Behemoth
    article. It is a good read and I will read it again. Yes, stiffness is obviously an issue. It is not just a matter of stiffness though, since the sail configuration is critical. It is here where a lot can be done. I am working on it.

    Cheers
     
  9. xarax

    xarax Previous Member

    Hydrodynamically faster means faster overall?

    As I understand it, your strategy is essentially this: take the cleanest possible hydrodynamic shape, find how heavy ballast can carry, put this at the end of a fin as long as (structurally) possible, and see what happens! There is no doubt that it will work, and a large part of the dirty "rating rules" history was nothing more than an effort to prevent similar developments. She, the ship, was meant to be female, fat and round, not thin and long. (One can argue to what extent all these rating rules restrictions were necessary because of the structural properties of the traditional building materials.)
    My question is this: How do we know that, making the hull a little fatter, the ballast a little heavier, and the sail area a little bigger, a faster boat wouldn t emerge?
     
  10. dionysis
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    dionysis Senior Member

    Well put xarax.

    It is easy to find out the answer to your question.

    Calculate the overall drag of a wider boat at design hullspeed say; including that due to induced, heeling and leeway and parasitic, calculate the thrust of the sails given by available righting moment in the close hauled, reaching and running conditions; balance thrust and drag, and see if the boat is faster.

    cheers
     
  11. MikeJohns
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    MikeJohns Senior Member

  12. dionysis
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    dionysis Senior Member

    Thanks Mike, an interesting read.

    I have not run the numbers on open 60 designs - although I have read that they can do over 20 knots, and that they slam alot upwind. And lets face it with all that sail spread, no wonder they are fast.

    People complain about these new designs and forget that they are being driven so hard. No wonder they pound, and break up occasionaly.

    Ellen MacArthur's comment comes to mind. When asked what it was like: "Oh,...there were a lot of ups and downs..." (paraphrased)

    SailDesign's points about pitching and green water over the bow in waves, have to be reckoned with. I think these things are relative and are a "problem" while racing.

    There are so many aspects to take into account - too heavy for the water plane area and the bow submarines, but then you get less heave and an easier motion, too light and the bow most probably will pound and get stopped. I agree with saildesign - you have to take the middle way and get the proportions right.

    I wonder what boats will be like in 100 years.

    I am in the process of building a model to get the CE - CLR balance right and investigate it's behaviour in waves, although I dare say it will be difficult to get the pitch inertia of the model to mirror the real thing adequately.

    We shall see.
     
  13. D'ARTOIS
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    D'ARTOIS Senior Member

    Even if this design will float, the movements under sail will be in such a way, so violent, that the sheer existence on the vessel will make living on it next to unsupportable.
    You may model a boat anyway you like, if you cannot sail it - it will remain a model. Therefore there are modelboats.
    Nevertheless, good thinking - it sharps the mind!
     
  14. dionysis
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    dionysis Senior Member

    Thanks D'Artois, but why do you say the motion would be violent?
     

  15. xarax

    xarax Previous Member

    Active horizontal foils would be needed.

    With the absence of horizontal surfaces of substantial area, I think that you will encounter severe pitching problems. A possible solution might be a T-tail with active horizontal foil control, like an inverted airplane tail.
     
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