Exploring another idea

Discussion in 'Boat Design' started by Southern Cross, Apr 30, 2013.

  1. Southern Cross
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    Southern Cross Senior Member

    Just an idea ...

    I start each day with a 2 - 3 mile swim in the ocean. Open water swimming gives you some insight into fluid dynamics. Rather than observing water move past an object in a tank, you are the object, immersed in water, propelling yourself through water. While swimming through the open ocean, the tactile feedback is incredible.

    It takes many thousands of hours to master the stroke technique to swim long distances. Body position, head position, legs and hip position, the catch, the pull, the recovery and strokes per minute must all be performed with great accuracy lest you tire too quickly. Body position is paramount to keeping frictional drag and form drag at a minimum. (See Sun Yang http://youtu.be/XvM3JYC--hM.

    The condition of the water is never the same. Temperature, current, wave height, visibility and the number of indigenous sea life including seals, dolphins and Great Whites varies dramatically from day to day. So, you learn to adapt to the conditions. For example, there is a way to surf in a following sea and another way to press your chest and head down when swimming into chop. There are ways to use your hands as foils and lengthening the body is as important as a long waterline on a boat.

    After many thousands of hours swimming it's easy to understand how swimming beneath the surface can be more efficient than swimming on the surface. This is always made more obvious to me when a school of dolphins glides under me. With barely a noticeable flick of their tail, they disappear into the depths. Efficiency underwater is why Phelps and many other world class swimmers began to extend the time spent underwater after each turn (working the walls).

    When I've observed dolphins and seals or Great Whites at sea, I've noticed it takes a great deal of energy for one to break the surface and swim on the surface. However, at the Long Beach Aquarium, for example, I've seen seals glide almost perpetually underwater with the slightest movement of their flippers, tightening their body into that perfect torpedo shape.

    Now to the point ...

    So far sailboat design seems to be about varying degrees of immersion, from heavy displacement hulls, to planing hulls to hulls on foils. One aspect of increasing speed depends on reducing contact with the water, or the amount of wetted surface because of the density of water. But isn't it generally true that water at the surface is disrupted more than water beneath the surface and plays a significant role in drag? I recall reading that crew on the new AC boats spoke about the dampening effect of foils, how the effect of wave motion is hardly felt at all. And the deeper you go, the less the effect of the surface waves have.

    I can imagine that research into optimal bulb shape is pretty advanced, given Naval Submarine design and torpedo design. I can imagine that keel bulb design is pretty advanced also. Could it also be true that it would take very little energy to move one of these bulbs beneath the surface?

    My question...

    An air tight bulb fully immersed will produce buoyancy equal to the weight of the liquid it displaces which is water at approximately 62lbs/cu ft. So, a bulb with 18 cu/ft will displace approximately 1100 lbs. 4 bulbs will displace 4400 lbs. Right so far?

    What if you placed one bulb fore and the other aft, one to starboard and the other to port connecting each with narrow cross beams and with additional structural support. The idea is that the total weight of the supports, rig and crew etc will equal the weight necessary to float the bulbs beneath the surface. Steering could be done from the aft bulb or from the outrigger bulbs. Maybe some aspect of foils could be added to the bulbs as seen on some foiling surfboards. The rig could be something akin to Sailrocket.

    Looking at the Weibel design http://www.wiebel-sailing.com/ whether it will ever work especially in true ocean conditions or not, it seems that dividing the hull up into smaller components with less surface area might be a move in the right direction. We'll see.

    I've attached a sketch. It's a starting point. My cigarette is lit and my blindfold is on. Fire away.
     

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  2. jonr
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    jonr Senior Member

    Looks like a boat I used to have - very fast but it couldn't handle rough water. It was an iceboat :).
     
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  3. Petros
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    Petros Senior Member

    you have overlooked something: four small pods of the same volume (displacement) will have much more surface area than one large one. So skin friction drag will be higher.

    Your idea is not without merit however. The whole idea with a multi-hull is to get more righting moment with less weight (wider stance, less displacement) so proportionately you can carry more sail area.

    IOW, the more sail area you can carry for the amount of hull in the water, the faster the boat will sail. So if submerged hulls can create displacement with less drag than a conventional design, you might have improved performance.

    Though in most boats the hull is also a handy place to get out of the weather and store supplies, not sure how that would work if fully submerged.
     
  4. Southern Cross
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    Southern Cross Senior Member

  5. John Perry
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    John Perry Senior Member

    It's a SWATH with sails, or if it has four hulls maybe a SWAQH. As such its certainly been discussed over plenty of yacht club bars but practical implementation has been discouraged by the lack of hydrostatic stability. However, combined with a non-heeling rig (like Sailrocket) (or a kite) together with a keel surface that is canted to produce a downforce to counteract aerodynamic lift (also like Sailrocket) it might just work, well maybe.

    As AdHoc will tell you, the advantage of a SWATH is superior seakeeping, it is not a particularly low drag hull form.

    For a really high performance sailboat you are probably better off using hydrodynamic lift from hydrofoils or aerodynamic lift from a tilted rig (or kite) rather than hydrostatic lift from any kind of hull shape, submerged or on the surface. So if you want to go fast maybe you need your pods to become thinner as speed increases so that they turn into efficient submerged hydrofoils - maybe make them from something flexible and let compressed air in and out to change the geometry.

    Any more mad ideas?
     
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  6. messabout
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    messabout Senior Member

    I just finished a book by a solo round the worlder whose boat was named Southern Cross. The skipper was a swimmer, female, and adventurer with more guts and determination than most of us. She will probably be located somewhere in So. Cal. Coincidence???

    Title of the book: By The Grace Of The Sea. Author: Pat Henry.

    As for the submerged quad pod idea............we'll have to contemplate a method for developing reserve stability.
     
  7. Southern Cross
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    Southern Cross Senior Member

    I did think of that but I was also thinking along the lines of swimming beneath the surface. Michael Phelps can swim further faster underwater than he can on the surface. Same Michael Phelps. Same displacement. Form drag is reduced so significantly beneath the water when tucked into a stramlined shape and also drag is reduced as a result from surface waves being eliminated. Storage etc can be housed on the platform above the water...
     
  8. Southern Cross
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    Southern Cross Senior Member

    I have always liked the idea of morphing shapes below the waterline. In my view it makes sense for a hull shape to adapt to the infinite number of sea conditions such as running rigging and sail configuration do. I started a thread on it and got crucified.

    I like your idea but I can just hear how others will think it too complicated of a system.

    The more water on the brain I get (from swimming, sailing etc) the more the mad ideas come!
     
  9. Southern Cross
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    Southern Cross Senior Member

    Just ordered. Thank you.
     
  10. Southern Cross
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    Southern Cross Senior Member

    Quote from Wikipedie:

    A Small Waterplane Area Twin Hull, better known by the acronym SWATH, is a twin-hull ship design that minimizes hull cross section area at the sea's surface. Minimizing the ship's volume near the surface area of the sea, where wave energy is located, maximizes a vessel's stability, even in high seas and at high speeds. The bulk of the displacement necessary to keep the ship afloat is located beneath the waves, where it is less affected by wave action. Wave excitation drops exponentially as depth increases (Deeply submerged submarines are normally not affected by wave action at all). Placing the majority of a ship's displacement under the waves is similar in concept to creating a ship that rides atop twin submarines.

    The twin-hull design provides a stable platform and large, broad decks. The main disadvantages of SWATH watercraft are that they are more expensive than conventional catamarans or mono-hulls, require a complex control system, have a deeper draft, and have higher maintenance requirements. The design of SWATH vessels is also considerably more complex due to the structural complexities inherent to the design. Additionally, SWATH vessels cannot operate in planing or semi-planing modes and as such gain no drag reduction when operating at speeds normally associated with such modes. This marginally limits SWATH speed when compared to equivalent catamarans.

    End Quote.

    I wonder if splitting this up in the way I have makes a significant difference?
     
  11. Southern Cross
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    Southern Cross Senior Member

    Water insect floating on air pockets.
     

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  12. Southern Cross
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    Southern Cross Senior Member

    http://www.sciencedirect.com/science/article/pii/S0029801808001765

    Abstract

    Traditionally autonomous underwater vehicles (AUVs) have been built with a torpedo-like shape. This common shaping is hydrodynamically suboptimal for those AUVs required to operate at snorkeling condition near the free surface. In this case, the wave resistance associated to the wavy deformation of the sea surface induced by the motion of the platform is an important component of the drag. This work has investigated the optimum hull shape of an underwater vehicle moving near the free surface. Specifically a first-order Rankine panel method has been implemented to compute the wave resistance on a body of revolution moving close to the free surface. A simulated annealing algorithm was then employed to search those set of parameters defining the hull shape that minimize the wave resistance. The optimization was constrained to keep constant the total volume of the vehicle. The total drag of scaled models of the torpedo-like and resulting optimum shapes was measured in the naval tank of the University of Trieste. Measurements showed a smaller resistance of the optimized shape in the range of the considered Froude numbers.
     

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  13. messabout
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    messabout Senior Member

    Observing the creatures of nature and attempting to apply the observed details to our own propulsion details is a long standing practice. It is sometimes a useful exercise, but sometimes completely misleading.

    One of our earlier serious inquisitors and experimenters was Manfred Curry, a German man of notable social and educational status. He wrote books. One of them was/is: Yacht Racing, The Aerodynamics of Sails and Racing Tactics. Originally published in English in 1928. Curry spent a lot of time and careful effort looking at things like eagle wings and water bugs. He attempted to relate the design of the eagle wing to the design of a sail and many other comparisons with animals in nature. He was very much involved in attempts to "streamline" moving objects, both in air and under water. He was a brilliant man but he was often wrong when attempting to relate bird wings to sails or dolphins to boats.
     
  14. Southern Cross
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    Southern Cross Senior Member

    Didn't Da Vinci predate him somewhat?
     

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  15. Ad Hoc
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    Ad Hoc Naval Architect

    If you want to be taken seriously, then you should find authoritative sources rather than Wiki. That’s just glossing the surface as is not always correct too.

    Sorry no. The clue is in the acronym. SWATH…the SWA part = small waterplane area. Not the volume the waterplane area!

    Not so. Like everything it depends upon the SOR and experience of the designer too.

    That’s a bit like saying an aircraft cannot perform underwater because the engines will flood.

    Sounds like the citation is written by someone with no understanding of SWATHs at all. :eek:
     
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