About inclined underwater hull form

Discussion in 'Hydrodynamics and Aerodynamics' started by fredschmidt, Feb 18, 2012.

  1. fredschmidt
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    fredschmidt Naval Architect

  2. fredschmidt
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    fredschmidt Naval Architect

  3. daiquiri
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    daiquiri Engineering and Design

    If it goes well to windward, the merit goes almost certainly to this long and slim keel:

    RC boat.jpg

    It is simple to verify - make a boat with a same LWL, Sail area, displacement and and keel shape, but with a less (or none at all) foil-shaped waterplane area. then see how it performs and compare it to the ORCA RG 65. I can bet it will have the same windward perfomance.

    Cheers
     
  4. rwatson
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    rwatson Senior Member

    I have been told by an engineer, the foil shape at this scale has virtually no performance difference at all.
     
  5. daiquiri
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    daiquiri Engineering and Design

    The foil shape (which is an airfoil, the cross-section of a keel) and the keel planform shape (which is what you see when looking at the keel from abeam) are two very different things. The first one has a minor influence at this boat scale, the latter one is fundamental at any boat scale. The foil shape is not visible in the above pic, only the planform shape is - and that's what my comment was about.
     
    Last edited: Aug 13, 2012
  6. rwatson
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    rwatson Senior Member

    Thanks for the clarification on that. My comment was about the foil definitely, and that the actual performance difference of that size foil was infinitesimal.

    I often wonder how much actual effect the hull shape on performance for scale models is, and how much the psychological effect of the operator comes into play. In watching model boat racing, it appears that the 'speed' is very much less important than the operators skill. One twitch of a finger destroys the race.

    Your original suggestion of getting two similar model boats to actually do a side by side performance ( say a windward beat over 100 yards ) is not something you see many 'developers' do. They seem to prefer quote race results, which could be just be the relative skill of the operators in the tricky business of remote control.
     
  7. fredschmidt
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    fredschmidt Naval Architect

    Daikiri

    Thanks for suggestion.

    I think that your knowledge would be very welcome to the classes involved in the RC, since, contrary to what you think, they came to the conclusion that the foils with 6% are more efficient than the foil of conventional boats because the actual number of Reynolds be much smaller.
    In my case I'm using a helicopter blade, totally inadequate for RC keel 15 %, but it was what I had at the time.
    Maybe I, like Pasteur, has discovered, thanks to you, the penicillin.

    I am impressed with your wisdom.
     
  8. fredschmidt
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    fredschmidt Naval Architect

    Bob Wells sent to me the Seattle IOM Update Newsletter - september 2012 that I attached here.
    It contains thinkings from David Hollom about chine in sailboats.
     

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  9. fredschmidt
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    fredschmidt Naval Architect

    David Hollom considerations are extensive, I will comment only the beginning and before commenting on the rest would like to know what you think about it:



    1 - "Firstly, lets dispel the myth that reducing leeway allows a boat to point higher. As Lanchester
    pointed out in 1907, how high a sailing boat will point is dependant only on the combined hydrodynamic and aerodynamic lift/drag ratios. Of two boats sailing to windward at the same heel angle, one at three degrees leeway and the other at six degrees, if the boat with six degrees leeway has less hydrodynamic drag, provided the rig forces are the same, it will point higher, full stop."



    What are we talking about?

    We're talking about chine.

    We're not talking about leeway angle of different boats, we're talking about a lower POSSIBLE leeway in the same boat, with and without chine!

    For the same vessel, the resistance increases with increasing leeway?

    Drag is usually associated with foils - induced drag. We are confusing drag with resistance?

    A boat only wins a competition test in the tank if it has less total resistance than others. Total resistance.

    What is the influence of leeway angle, in the other resistances other than as the induced drag?

    Are we confusing "less hydrodynamic drag" with a lower total resistance?
    We need to understanding what the other speaks.

    I do not know if what David Hollom says is the phenomenon raised in this thread.

    I have a sneaking suspicion that it is not.





    2 - "We should also dispel the other popular sailing myth that if you produce more hydrodynamic lift you will point higher. As Tom Spear pointed out, for equilibrium reasons hydrodynamic lift (side force) must be equal and opposite to aerodynamic heeling force. Hydrodynamic lift is a reaction force. It is a consequence of the aerodynamic forces so you cannot increase it unless the aerodynamic force increases. If you power the sails up aerodynamic force will increase and to compensate hydrodynamic lift must also increase, but it is not something you can increase independently of the rig. As how much you can power the sails up depends, to a large extent, upon the stability of the boat, we reach the inevitable conclusion that hydrodynamic side
    force is, primarily, righting moment dependent.
    Looking at the example of two boats sailing to windward at different leeway angles, you could
    argue that the boat making the larger leeway angle, because its hull is proceeding through the water at a more sideways angle, would have a higher drag. It might or it might not but either way, to make a larger leeway angle (all other things being equal), its fin would have to be smaller which you might expect would reduce fin profile drag, but that would depend on where the resulting lift coefficient (CL) was situated on the lift/drag polar. It may, in fact, go up. The final drag answer is therefore not that simple but if the combined hydrodynamic drag were less it would, as already mentioned, point higher even though the leeway angle was greater."


    The problem is not point higher, the problem is produce a boat with EFETIVE less resistance to win a course.



    We can have a boat pointing higher but with less velocity to win a course than another boat.

    And it seems that the boats with chine do.
     
  10. Mikko Brummer
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    Mikko Brummer Senior Member

    There is a note about the chine effect in David Hollom's excellent writing:

    "...an American football or rugby ball, again not spinning, but presented to the flow at some angle to its long axis would produce lift because the attachment point would now be below its nose and the mean separation point around the centreline of what would be its tail. The flow from the attachment point to the separation point would now have to travel further on the top surface than on the bottom surface and there would be circulation and thus lift. (Because of friction which takes energy from the boundary layer flow, the flow never reaches the extreme tail but separates some distance before reaching it hence my term mean separation being a point midway between the detachment points on the top and bottom surfaces). However, the mean separation point would not be quite on the centreline of the tail but slightly above. Because of low pressure on the upper surface and high pressure on the lower surface, caused by circulation, the mean separation point would be dragged, to some extent, from around the centre of the tail onto the top surface and this would reduce the circulation and thus the lift. That is why airfoils have sharp trailing edges. The sharp edge helps define the point at which separation will hopefully occur thus helping prevent flow from the lower surface migrating to the (in aircraft terms) top surface and thereby reducing circulation and lift. Nevertheless, even with a sharp trailing edge, the top surface in particular does, to some extent, separate before the flow reaches the trailing edge, which does reduce lift, but to a greatly reduced extent compared to a rounded trailing edge.
    The yawed boat hull with a round bilge behaves in much the same way as the rugby ball. The point of separation will be some way round what would be the trailing edge on what would be the top surface of an aircraft wing with a consequent reduction in circulation and thus lift. Also, the area of separation will be large and the wake wide which will increase pressure drag. When heeled, a well designed chine gives a sharp trailing edge to the canoe body foil making it, for the reasons just discussed, into a much more efficient lifting device producing greater circulation and thus lift and also, less drag. Of course, when upright, the chine works in the same way as on a powerboat, allowing the water to leave the canoe body cleanly and thus reducing drag."
     
  11. daiquiri
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    daiquiri Engineering and Design

    I am (almost) sorry for being skeptic, but until I see some measured numbers for these claims, for me they will remain just a personal opinion.
     
  12. Mikko Brummer
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    Mikko Brummer Senior Member

    Just a personal opinion, yes, but the opinion of a really knowleagable person in the subject.

    I don't have any measured numbers but to me the best proof is the Star, which performs so much better in real life than in the VPP which ignores the chine.
     

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  13. fredschmidt
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    fredschmidt Naval Architect

    3 - “If we look at the hull and the fin and the rudder as separate devices that can each produce lift
    (side force) then you would be drawn to the conclusion that, as lift induced drag, for a given lift, varies inversely as the square of span, the long span of the keel makes it a more efficient lifting device and the very small span of the canoe body makes it a very inefficient lifting device. However, I would suggest that this is the wrong way of looking at the problem. You must look at the whole combination of hull, fin and rudder as a combined lifting device where each part has a SYMBIOTIC relationship with each of the others and where their combined effect is greater than the sum of their parts.”

    For me one of the best conclusions of David Hollom.

    To me it explains almost everything in question, perhaps even more than the words that Mikko emphasizes in the David Hollom explanation of the phenomenon, by circulation theory.

    It is very intuitive to see that the chine, ever, cut the water lines so that hull low pressure side is on the side from which the wind comes from, like the keel. That is, the hull and keel are in symbiosis, are not conflicting.
    A boat without chine the heeled waterline is a foil or symmetric. Many boats without chine have their heeled water lines as a foil, however, in conflicts with the keel, that is, the low pressure side where the wind will, and this decreases the efficiency of the keel. It's a conflicting relationship, as the case of Riptide at low angles.
    Changing the shape of the foil (changing the shape of the waterlines), putting the side of the low pressure where the wind comes, this relationship conflicting ends, as in the case of Riptide in greater angles of inclination.

    4 – Other interesting observations than those motivated by the circulation theory:

    “Minimum drag, for a given side force, was always achieved with some appreciable yaw angle which would suggest that canoe body lift is not as inefficient as some people would have us believe. Indeed it must be quite efficient to produce the results we observed.”

    “If one then accepts that the hull does produce lift reasonably efficiently it would behoove us to
    make it the best possible shape to produce lift.”

    Here this statement reminds me of the idea I had to make elevations on the hull to change the shape of the waterline foil. I think in future we may see more hulls not faired, but with shapes that produce optimized foils to improve the symbiosis hull / keel. The new problem is know the optimum form of the foil.

    CONTINUE...
     
  14. fredschmidt
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    fredschmidt Naval Architect

    I made a great little error, the correct name of the author is Dave Hollom, I pray he forgives me.
     

  15. fredschmidt
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    fredschmidt Naval Architect

    My conclusions about the thoughts and conclusions of Dave Hollom:

    Well, it seems that someone finally agrees with the thoughts of this old fool.
    But to me the truths are truths only a certain amount of time, due to the stage of human knowledge. I am very skeptical about the truths, which allows me to go where many settle with truths divulged, but not as truths. And it takes courage to do that.
    And the great truths or truths are not full, as the 3 laws of Newton, Einstein showed that they had strict validity in strict speeds.
    I would like to bring my experience to you in this forum.
    Old, unknown, a new country with no tradition in discoveries was kind of hard to keep my opinions here.
    I think the posture of humanity should be opening. If someone lift something, should be warned of possible difficulties in relation to current knowledge, but with love, not with indifference or contempt. It will be better for everyone.
    But the truth can not be true or be halfway to the truth.
    Let us follow the path.
     
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