Hull Shape Enquiry

Discussion in 'Boat Design' started by ancient kayaker, Nov 1, 2008.

  1. ancient kayaker
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    ancient kayaker aka Terry Haines

    What would be the optimum hullshape for a small boat that will spend most of its time being towed on rivers or lakes? It should handle speeds from below hull speed to full plane, and should not cause excessive drag at any speed. The towed weight would be about 250 lb.

    For the purpose of this discussion, the behaviour of the hull as an independent boat is irrelevant. Low drag under tow is the only design driver. This is somewhat of a theoretical study for the present but I do have a potential need.

    I am familiar with hull shapes like canoe/kayak, rowing, sailing dinghy and keelboat, each of which has good reason for the shape it has. I can see the design logic behind a long and skinny hull or a deep-vee entry with flat aft, and of course the classic keelboat, so lovely in or out of the water.

    A lot of boats are dogs to tow because of a tendency to jig from side to side. However, for this application, if jigging is a problem it can easily be prevented.

    For this application I don't see the need for for a sharp entry but I can foresee good lateral stability being needed so I wondered if a skimming dish hull somewhat like a coracle would work best all round.
     
  2. marshmat
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    marshmat Senior Member

    This is an interesting question, and one that probably deserves more attention than it generally gets.

    While carrying the dink on board is obviously ideal (and essential when offshore), lake sailors often seem to lack the space and time to dismantle the dinghy and lash it to the foredeck (or wherever).

    I've never actually towed a boat substantially smaller than my own, unless you count skiers. (Towing a vessel or object larger than my own, which I have done on occasion, is a very different matter.) But from what friend have told me about towing their dinghies, directional stability seems to be one of the big issues- the dink starts yawing wildly as the wind and waves get stronger, until the point where it is thrashing about and going sideways more than forwards. When you look at the waterplane and lateral plane of the typical 10' Zodiac, it's easy to see why- the weight (motor) is in the back, the lateral plane is tiny, and the waterplane is as wide as it is long. I wonder if a fuller-bodied, somewhat heavier dinghy might fare better?
     
  3. ancient kayaker
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    ancient kayaker aka Terry Haines

    I was not thinking about a motor, but the weight mentioned of 250 lb would include a load, which could be carried in whatever manner best suits the boat.

    I know what you mean about yawing; the only towing experience I have had so far with towing is with kayaks, and they just want to go anyplace except where you are trying to go to, even in calm water. For kayaks towing is best done with 2 boats pulling a 3rd I understand, but we were 1 on 1 that day.
     
  4. Fanie
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    Fanie Fanie

    You may want to take into consideration the wake of the towing boat, but I would think the towed boat should have a flat hull bottom, with little fins near the transom. The tow rope should be tied close to the water on the towed boat so the rope would tend to lift the bow up a bit when pulled, could make for easier planing.

    My 2 .za cents.
     
  5. ancient kayaker
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    ancient kayaker aka Terry Haines

    The towing arrangement can be such that the wake of the towboat does not influence the towed boat.
     
  6. PAR
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    PAR Yacht Designer/Builder

    At the beginning of the last century this issue was addressed with tenders and the results were the likes of The Lawley and the Lawton tenders. The Lawley appears to have some Whitehall ancestry in her, though a shorter, fatter representation because of it's diminutive size. It carried some reverse curve in her garboard from about midship aft and softer bilges in comparison to the Lawton. The Lawton had no reverse in her belly and carried beam farther forward in her waterlines.

    The Lawton had more dead wood exposed then the Lawley and her forefoot had slightly more "bite". Both row and tow well, plus can carry quite a load.

    The Lawton had flatter midship bilges then the Lawley, with a V entry and exit. The Lawley had a more U shaped entry, softer bilges as mentioned and a squat wine glass stern section.

    I suspect the Lawton preformed better at higher towing speeds then the Lawley, with the Lawley being less resistant at lower S/L ratios.

    Towing performance can be greatly affected by how she's towed and her trim. I've found a bridle works better then a painter to keep the dink under control.
     
  7. daiquiri
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    daiquiri Engineering and Design

    My few toughts on this interesting problem... actually a "design theme". :)
    Let's consider your requirements:
    1. the boat has to operate in lakes and rivers
    2. it will operate at speeds ranging from very low to planing.
    3. it has to minimize towing resistance.

    I would deduce the following:
    1. it will have to operate in shallow waters, so the blockage effects need to be taken into consideration. I had briefly addressed the Shallow Water Effect in this page (though it was a bit off-topic over there):
      http://www.boatdesign.net/forums/boat-design/dead-water-designers-24692.html
      It depends basicaly on "Depth Froude number" Fnh = V / sqrt(g H) and on H/T ratio, where V is boat's velocity, H is water depth and T is boat's draft. The towing drag starts to increase due to low water depth when (H/T) < 4.
    2. very low speeds require a minimum wet surface, while the planing mode require a high Cp plus low L/B ratio and a chine hull.
    3. the minimum towing resistance (under the constrains given above) is implicit in the above choices.

    In conclusion (kind of):
    1. Choose the minimum water depth expected and then make sure that your boat's draft is less than H/4. You need to use consistent units during the calculations (either all metric or all imperial).
    2. Minimum wet surface require a circular section shape, while the planing mode would idealy require a flat-bottomed chine hull. Hah, and now what? Maybe a compromise solution could be a mid-secton with flat vertical walls until just under water surface, followed by a chine and a large-radius circular or an elliptic bottom.
     
  8. TAH
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    TAH New Member

    to PAR of Ricelli Yacht Design;
    We wrote a few times about Floom. Tried to contact you again, but failure notice keeps coming up. Please give me a holler with a working address.
    Thanks. hillofbeans007@yahoo.com
     
  9. messabout
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    messabout Senior Member

    I note, with interest, that some dinghys tow remarkably better when stern to. That may provide some fodder for discussion.
     
  10. Stumble
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    Stumble Senior Member

    Not much comment on the hull shape, but I keep thinking that some windsurfer fins on the back of the boat and one in the bow should really help to keep it tracking strait. But then I once created a very complicated and unfortunatly never sucesfull tiller stearing rig for dingies, so my luck with them on tow is bad to worse :D.
     
  11. T. Lee.
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    T. Lee. New Member

    I made a boat design once that seems to tow well. Made of plywood with a single chine, it was long and skinny, like a canoe, with a long flat area on the bottom. By flat, I mean absolutely flat, in the shape of a parallelogram with the long axis along the centerline and the short axis at their widest at the chine. I made sure that the pressure of the water being pushed down under the boat was at least equal to that pushed sideways by the sides, so it did not develop a relative vacuum under the boat so that the hull did not squat at higher speeds (most canoes have the flat bottom, but at speed the pressure is higher on the sides than on the bottom, which causes the boat to squat). Unfortunately, the boat was stolen before I used it much, I tried it only in flat water, never had a chance to see its performance in rough conditions.

    T. Lee.
     
  12. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I don't think that there is much you can do to reduce the wave resistance because you are planning to operate over a very wide range of Froude numbers. (Towed barges can be quite efficient if the spacing between them is chosen judiciously to reduce the wave resistance).

    Ideally, at low speeds you want to keep the two boats about 1/2 wavelength apart.

    The wavelength (of pure transverse waves) is given by:

    Lambda = 2*pi*U^2/g

    where g is gravitational acceleration (in m/sec/sec), and U is the speed in (metres).

    At higher Froude numbers there is little wave cancellation possible because the wavelength is so long. It's then best to keep the boats as far apart as possible, but that is obviously going to create other problems.

    It's a very interesting problem. Sorry that I can't help much.

    Good luck!
    Leo.
     
  13. ancient kayaker
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    ancient kayaker aka Terry Haines

    Sounds like I need an approximate half-round or elliptic section with a broad beam midships to minimise wet surface for the given displacement, morphing to a near flat bottom with chines at the stern to encourage planing and with the stern cut back to a narrow transom. Perhaps a small skeg for directional control. Minimizing deadwood at the stem should help prevent diggin-in under tow. Starting to sound like a sailing pram.
     
  14. philSweet
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    philSweet Senior Member

    I'd like to know a bit more about the intended towing arrangement, but lacking that, here's my two cents.

    The boat ought to perform well on its own (not under tow) over the speed range intended for towing. There's nothing magically different when it is being towed, just a few extra things for the towing skipper to consider.

    If the towing vessel is several times heavier than the tow, the towed vessel will be sitting in a hollow between the wake. As speed increases , the tow may endup with its bow and stern immersed and you will not have a happy tow. No tow will be happy at every speed. When the wakes's wavelength is a bit more than the towed vessel's length, it's hard to find a stable place for the tow to sit on the wave. If you have a planned cruising speed, take a look at the wake at this speed. If your speed is seven knots and your thinking of a 13 foot skiff, you may have to add some kilp to the skiff to get it to sit in the wake.

    If the boat is being towed without crew or simmilar load, then the wetted surface will move around under the boat to a far greater extent when chop and wake hit it. This, and the hollow caused by the wake, causes the dancing. The only effective way to reduce the extent of the dancing is to control the bow of the tow. Bridles, tag lines, whatever works. In my oppinion, skegs do more harm than good. A boat with an effective skeg may travel in a staight line, but it might be surfing on your wake 30 degrees to port. In a strong cross wind it is easier on everything if the tow can drift at an angle. Don't add addition skeg to a design to improve it's towing.

    If you are thinking of a transom hung outboard, this is cause for concern because of the large change in the longitudinal center of gravity when the crew is removed. A cut in motor mount is very nice to have on vessels being towed. Nice to have if it's a 5 hp. Important if it's a 70 hp.
     

  15. ancient kayaker
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    ancient kayaker aka Terry Haines

    As mentioned earlier, I am not much concerned with the towing boat's wake for the purpose of this exercise. The wake does not always have to be a problem, two boats can tow one boat or a boat towed by a catamaran can be towed close in to take advantage of the wakefree zone between the hulls, at least in theory. Another option would be to fit the boat with foils or lash the rudder so it tows off-center, clear of the wake.

    Some examples are in order. When towing a kayak it is best to have two kayaks doing the towing, they can spread out and tow using vee arrangement akin to a bridle; this helps keep the towed boat on course. A kayak is notoriously difficult to tow in a straight line, on a single line it can zigzag wildly, and often lowering a skeg or rudder may simply change the frequency of the zigzag or cause it to hang way off-center as you noted.

    The boat will be towed with a load, and the balance of the load can be adjusted for optimum behaviour under tow; it will not have an outboard.

    What I want to do is disconnect the problem of the towed boat's behavior at a variety of speeds from the effect of the towing boat's wake or worries about the boat's behavior on its own. Somewhat more akin to its behavior in a testing tank, which might have been a better choice when I started the thread.

    It seems the guys reading this thread are practical sailors concerned with how to safely tow an existing boat, rather than how to design a boat specifically for towing. Nonetheless, all of your inputs are appreciated. Thank you.
     
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