Rudder Hydropower Generator & Solar Panels

Discussion in 'OnBoard Electronics & Controls' started by Bahama, Jul 9, 2010.

  1. latestarter
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    latestarter Senior Member

    I find your ideas useful in making me get back to first principles.

    I will try to be brief in reply to various points, I hope it does not come over as being curt.


    Stops only
    Apart from a sea anchor or when running before the wind, I can not think of a case where friction/drag is needed or helpful. When close hauled or reaching and with keels and rudders you are using lift.
    Very sensible, this is free energy, the only differences is the water downstream of the turbine will be moving slower and just upstream the water level is higher

    See http://www.seageneration.co.uk/ as an example
    The flaw in your idea is that you are ignoring the movement that has to take place.

    Assuming you step on the pavement and it moves down 5mm, you then need to step up 5mm to get on the next part of the pavement. It would feel like you were walking up a staircase with very small risers. Any energy at the turbine is being provided by the extra effort the walker has to put in, to effectively be walking up hill.




    Just to add to DaveJ’s explanation

    The parasitic drag is made up from the shape moving the water aside and skin friction due to the viscosity of the water flowing over the surface.
    Wetted area is the part of the hull, keel, rudder etc in contact with the water, the more wetted area there is, the greater the resistance. Ideally the cross section of a rowing shell is semi-circular as it would have the least possible wetted area for a given displacement ( unfortunately it would also have no stability)

    The object of a rudder is to create a side force at the stern, to turn the boat.

    Lift is at right angles to the rudder.

    Drag is parallel to the rudder.

    If the rudder is turned 10 degrees to port; this generates lift which moves the stern to starboard.

    As the rudder is 10 degrees to port, the drag force on the rudder mainly slows the boat down but also pulls the stern to port.

    The lift/drag ratio of an efficient rudder at 10 degrees can be in excess of 17.

    So which is more likely to move the stern the way we all experience, a large force pointing roughly in the correct direction (lift) or a small force slightly pointing in the wrong direction (drag). The more drag from the rudder the slower the boat turns.



    If you don’t believe me, I quote from 'All about Powerboats' by Roger Marshall (technical editor for Soundings magazine)
    “An efficient rudder operates just like an airplane’s wing. It is a lifting surface, and turning it increases lift on one side, forcing the boat into a turn.”

    Rudders would still work if the fluid in which they moved had no viscosity.

    You could visualize the drag on the rudder as being the equivalent of a rope tied to the back edge of the rudder and being pulled backwards in line with the rudder. In the above example, this will be tending to pull the the stern to port not starboard.
     
  2. Bahama
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    Bahama Junior Member

    Thanks for the kind email, I hope my reply comes across kind as well. I posted in inside yours with red color.
     
  3. latestarter
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    latestarter Senior Member

    Yes it does. There is no point turning these discussions into another Origami boats thread.:)

    In my previous post I said

    "Apart from a sea anchor or when running before the wind, I can not think of a case where friction/drag is needed or helpful. When close hauled or reaching and with keels and rudders you are using lift."

    You replied

    I was confused why you keep to a view of how a keel works which is wrong, then it struck me that there was one condition where you are totally correct. When the boat is not moving through the water.
    In that case, with no movement there is no lift.

    I used to race dinghies many years ago. Waiting for the start I would just let the sails flap and stop moving. The dinghy used to drift sideways as the centre board was using inefficient drag to resist the movement.
    Pulling the main sheet in, the dinghy started moving. Even moving slowly the much more efficient lift took over, and the drift seemed to disappear.

    It is similar to an aircraft flying through air creating lift.
    If it was not moving, there is no lift, it drops out of the sky, the wings would create drag upwards slowing the speed of descent.
     
  4. DaveJ
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    DaveJ Senior Member

    This brings me to question why they don't they have Centre boards/Keels that can change there profile to help resist the push the leeward (create lift to windward) due to wind forces. Imagine at wing style centre board that is like the BMX Oracle mailsail they used during the american cup race with the goal in a closed haul position as the boat speed builds up its pofile changes to resist the push to leeward.

    Maybe the drag created will drop the boat speed too much that a boat on normal track will still get to the top mark at the same time as a boat pinching with this style centre board.
     
  5. tspeer
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    tspeer Senior Member

    What you are talking about is a tab or flap on the centerboard/keel. Very commonly used. USA 17 had a tab on its central daggerboard, before they took out the daggerboard and went with foils in the amas. Even the 12 Meter boats had tabs on their keels.

    However, it doesn't do what you think it does. It doesn't produce any more lift than a symmetrical board, because the lift on the board has to exactly oppose the side load from the sails. Anything different would have the boat going in circles.

    Instead of producing more lift, it produces the same lift at a different leeway angle, as measured between the boat centerline and the course through the water. Deflecting the tab (or rotating a jibing symmetrical board) rotates the bow off the wind, while the boat continues to follow basically the same path through the water. This allows the hull to align itself with the direction of travel, and it also changes the angle of attack of the foretriangle - resulting in different sail trim.

    The tab can be used to reduce the profile drag of the board. It allows the low-drag region of the profile drag to be centered about the operating condition, instead of being centered about zero lift. It does not have a significant effect on the induced drag due to lift, however, which accounts for the majority of the drag of a board.
     
  6. latestarter
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    latestarter Senior Member

    I agree with Tom's analysis, my understanding is they are so effective that a lot of design rules ban them.
     
  7. DaveJ
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    DaveJ Senior Member


    I have never seen them or heard of them, i've heard of the self jibing centre board though. This justs disapoints me, if we have such a mechanism why don't today designers put them on the hulls of all boats that could benifit from them. Not everyone races, and at some point we have to beat to windward, anything that made it quicker crusier or racer would be better.
     
  8. Bahama
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    Bahama Junior Member

    Flexible Solar Panel Raft

    Flexible solar panels are now being used on boats--but has anyone mouted them onto a blow up raft that can be dragged behind?

    For smaller boats that are challenged for space, this seems like a nice option, and it would work out well for a life-raft as well I'd think.

    I would have 3 blow up pontoons and mount them in between the pontoons. This design would help to keep them from being overturned. Perhaps what I've describe is already out there, I've just not seen one.
     

  9. CDK
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    CDK retired engineer

    Flexible solar panels are very poor performers. They are not really flexible but built up from lots of small cells that get damaged over time, reducing the output even further. The ratio of both price and surface area vs. performance makes them virtually useless.
     
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