Hull Drag of Small Dingies/Skiffs

Discussion in 'Sailboats' started by Matth, Apr 23, 2007.

  1. Matth
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    Matth Questionable

    If anyone has the means and inclination to do similar tow testing on other small hulls, I would be very interested in hearing your results. I am currently trying to choose a new hull for my no-heel rig, and after yesterday's session, it seems more important than ever to choose something with low drag.

    I would be most interested in results with Lasers, Tasars, 29ers, Hobie cats and other smallish cats, but by all means, post whatever you find!
     
  2. sigurd
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    sigurd Pompuous Pangolin

    tell more about the rig?
     
  3. Matth
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    Matth Questionable

    I've been hesitating to put the design out there, but here it is. I've attached an explanatory powerpoint presentation, plus a short video clip taken from some recent testing.

    Basically, I believe I've got the major problems of no-heel boats solved: tacking, weather/lee helm, efficient sail presentation to the wind, simplicity of control (mainsheet and rudder), and simplicity of construction.

    There are of course many other possible configurations of the basic design besides the one shown.

    I look forward to your comments... with some trepidation.
     

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  4. MalSmith
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    MalSmith Boat designing looney

    Matth,

    Up until this point the thread has mostly been about the performance of dinghy or non skiff style hulls. How does this subject relate to your rig design? Your powerpoint presentation makes claims of potential high performance from your simple rig concept. However the sail area/displacement ratio of your prototype shown in the video is pretty low and I would not expect any spectacular performance. As soon as you start to scale up the rig to get the sought after performance, you will run into all of the weight, structural and control problems that every other zero heeling moment rig designer has had to face. If the goal is moderate performance with zero heel, then I guess your configuration works.

    For evidence of some of my own attempts, see http://www.members.optusnet.com.au/mal.smith1/Vproa.html :eek:

    Mal.
     
  5. sigurd
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    sigurd Pompuous Pangolin

    Mal, I did ask. I think both of your rigs look quite nice and practical, at least at a small scale. The jibe in the video looks very nice and effortless. I remember the one with the VP model starting to take off too! Very funny it was.
    But for good weight and performance, complexity and cost at larger scale, look at peter lynn's arrangement for his mono kite boat. It lines up the kite pull with somewhere near the middle of the rudders' depth, independently of the kite location, far as I can understand.

    Matth, did you consider a narrow hull or two?
    If you want to look into it, E.O. Tuck and L. Lazauskas has some nice pages which I am currently reading, about their drag. http://www.cyberiad.net/multihulls.htm
    For theoretical planing stuff, Savitsky and Shuford are two fellows that I intend to read some time. There are some spreadsheets here which I didn't try yet.

    But, maybe you want to find an open development class, to verify your rig against optimised boats? Moths or IC's maybe?

    Edit: Oh, by the way, around the last link there is another no heel rig model.
     
  6. PI Design
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    PI Design Senior Member

    I haven't been able to watch the video yet, but nice presentation. You've clearly done a lot of research.
     
  7. Matth
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    Matth Questionable

    Thanks for the comments. I'm sorry that the presentation doesn't go into more detail about how the rig works; I will add some more description and illustrations and re-post, in a new thread.

    The prototype in the video has only a 5.0 sq.m sail. I am working on putting a 76sq.ft on next (same as a Laser, for better comparison). As the sail gets bigger, it is harder to counterbalance the weight of the rig. If my weight isn't enough, then I'll attach a small floater hull (windsurf board) at the end of the horizontal member--think SailRocket but small and sluggish.

    I would prefer to stay with a pure monohull, or a cat with the hulls in fixed alignment--Thanks for the links Sigurd, I will dig into those. I think the best solution for the rig counterbalancing will be to use a counterweight rotating with the rig but on the opposite side of the central pivot/mast. This could either be a lump of metal/water tank (on cruisers) or else a seat for a crewman (see pic attached) on a smaller high-performance skiff where any extra ballast would be unwelcome. This could be a small cat hull too.

    Having a counterweight would remove some of the bending on the support structure and put things in a more pure compression. Also would mean that any leaning/pitching/heeling angles would not make the rig want to swing around the central pivot.

    The pic attached and the prototype in the video have a complex rig support structure, but since making those I am leaning more towards a simple vertical mast and a spinnaker pole, like the setup shown in the presentation. The extra lines visible in the presentation picture (and video) are bungee cords to keep the sail from drooping inwards under its own weight in light wind.

    The pic shows a wingsail with a mainsheet system at the bottom that allows the wing to swing right around without fouling the mainsheet, so it allows various tacks and gybes with the wing rotating inwards or outwards, and the rig swinging around the front or the back of the hull. The boat in the video has a simpler setup in which the sail can only be tacked by rotating inwards and the rig is limited to the forward 180 degrees of rotation.

    Hmm--I seem to have said a lot here--my apologies, and again, I will put it all in a new thread.

    Regards,
    Matt
     

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  8. frosh
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    frosh Senior Member

    I like the innovative thinking for this rig. On a boat of this size, if performance is really important, definitely use the crew as a counterweight. Carrying any form of ballast for balance will become a pain especially during transitions, and will significantly slow the craft down, not to mention making it sluggish to respond to changing wind or sea conditions.:)
     
  9. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Could you explain why wavemaking drag is exponential and skin drag varies linearly?
     
  10. Paul Scott
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    Paul Scott Senior Member

    Mal, every time I see your ideas, it gets me back into thinking about insane craft.

    Thanks

    Paul
     
  11. messabout
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    messabout Senior Member

    Leo;
    That explanation will be lengthy. Please look for Marchaj books. He does the explanation rather well.

    Matt:
    Some time ago I did some garden variety towing tests with a whole gaggle of quarter scale models. I was careful with scale weight, relative speeds, etc. I made an effort to get the models away from the tow boat, out of the influence of wakes and prop wash by towing from a boomed out pole with a bending beam type drag scale.(actually a fishing rod and calibrated flat reference plate)

    The whole exercise was fun and bewildering at the same time. As in your experiment, the water was like mollasses. Some of the best results came from what I considered a least likely candidate. It was a Bolger Gypsy. That boat deliverd curiously good results in speed envelopes less than about S/L = 1.75 After that it seemed to hit a wall where the drag increased out of proportion to increased speed. I mean way out of proportion to the V^2 concept. It would plane if I went fast enough but it did not diminish the pull on the tow rig when it popped up. Other models displayed similar behavior. I experimented with weight placement to change trim angles with some difference noted. Changing the point where the tow line was attached had some effect as well. A Laser like model worked better than the Gypsy in the transition range of speeds but less well at lower speeds. Go figure......

    I experimented with and without boards and rudders in place. I noted a pretty big difference in drag with those parts installed. I can only speculate on the reasons. Maybe seperation or eddy making that could not be observed from my vantage point. All the models were polished to a fare thee well in an effort to minimize skin drag. (1200 wet or dry and finished with 3M Finesse-it.) The only thing I proved was that I don't know what I don't know.

    A model of a Tornado cat hull was a mystery maker too. It performed with lower drag numbers and did not hit the wall untill well after S/L 2. That one had to be tested with a weighted board in place because it was only a single hull. The whole exercise was fun but not particularly enlightening. I figure the Gypsy boat was some sort of inexplicable anomoly.

    For a revolutionary rig like yours, an attractive experimental choice would be to scrounge an old Proctor or Nethercott IC. We already know something about the performance potential of those things. That would give your rig a fair test with which to compare performance.
     
  12. sigurd
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    sigurd Pompuous Pangolin

    messabout; check out Lazauskas' pages that I linked to. Well written, long and goes into detail about wavemaking and friction.
    Re. your tests, maybe the appendices steered? The Europe dinghy is a beast to tow, it steers here and there.

    Matth, nice pic there. What do you use to make it? I use rhino but never bothered to put a person there, would be nice to have some times. Is it a lot of work to change its way of sitting/standing?

    Will go look for the rig thread. Thanks!

    edit: ah, can't wait, or my clipboard will forget what I was going to say.

    If you don't know it already, look for Tspeer's post in the "windward heel - upwind best of all?" thread. He shows the simple formula for vertical lift of a canted rig.

    When I see your rig I, as usual, start thinking of proas. Check out the Cantharry in the photo section of groups.yahoo.harryproa.
     
  13. Paul Scott
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    Paul Scott Senior Member

    As long as we're talking about drag numbers, anybody have any thoughts on how good Vacanti's wave/friction drag numbers are?

    Paul
     
  14. MalSmith
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    MalSmith Boat designing looney

    Magic Hullforms

    Please bear in mind when talking about low drag hulls that there is no such thing as a low drag hull per se. For instance when comparing displacement hulls to planing hulls, for hulls of the same LWL and displacement, the narrower, deeper, small transomed displacement hull will give lower drag at speeds below 'hull speed'. The broad transomed flatter planing hull will giver lower drag at speeds above 'hull speed'. At some point the resitance curves of each hull wil cross over, and maybe more than once. The point is to look realistically at the available thrust and pick the hullform which gives the lowest resistance at the most likely operating speed. If cost is no option and there are no size restrictions, make the hull as long and narrow and as light as possible. If you can always guarantee high thrust, then maybe a small flat planing hull is the way to go, but it wil not give you good all round performance.

    Mal.
     

  15. Matth
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    Matth Questionable

    Back to hull drag: I've been thinking more about the analogy that I mentioned earlier in the thread (and no-one commented on--perhaps it's dumb?) of a boat moving from displacement to planing mode being like an airplane accelerating down the runway with the wings taking more and more of the weight until the plane takes off.
    The wavemaking drag of the hull would be analogous to the induced drag of the wings--it is negligible at low taxiing speeds, and builds proportional to V^2 (Cl and aspect ratio etc would be constant up to takeoff). The peak of the 'hump' would be at takeoff, and as the plane continues to accelerate the induced drag decreases as the Cl goes down (constant lift means lower Cl at higher speed).
    The boat analogy would be that wavemaking drag is negligible at well below hull speed, builds exponentially up to a peak at the 'hump', then reduces at higher speeds.
    So I wondered, how does a 'no-hump' hull shape work, and does this analogy give any insight into that?
    A hull tends to 'jack up' at hump speed, and flattens out again at higher speeds. This is, I guess, the angle-of attack of the hull, and the higher it is, the higher the Cl -- and the higher the wavemaking/induced drag.
    For the plane, the wheels are set so that the wings are at a steep AoA while taxiing, so that the wings produce maximum lift and the plane takes off as soon as possible (limited runway length).
    By jacking up, the boat is also trying to get takeoff (planing) at the lowest possible speed, by increasing AoA at the expense of greater drag.
    The theoretical case of the low-drag, easy-planing boat would be like an airplane that had unlimited runway length, and wanted to take off with minimum horsepower, instead of shortest takeoff distance. In that case, the wings of the plane might be trimmed to a lower AoA (lower Cl, lower induced drag), and the plane would get to a higher, more efficient speed before lifting off.
    For the boat, this might mean reducing the 'jacking up'. The obvious way to do this would be to have a wide, flat planing-type surface Far Behind the center of gravity of the boat, and no similar surfaces up front. Sure enough, these new hull designs like the 29er have narrow, steep-sided bows that don't widen out until fairly far back, but are very wide and flat at the stern.
    Does this make any sense?

    Matt
     
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