Query about reducing wetted area

Discussion in 'Boat Design' started by Mr Efficiency, Jun 7, 2013.

  1. DCockey
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    DCockey Senior Member

    Sounds like Mr Efficiency wants the water to separate at the chine. That's that usually occurs when the boat is planing.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Nothing to do with water separation at the chine, I cited two separate examples of planing craft that have immersed vertical surfaces that aren't contributing to lift, they just add to the wetted area, and therefore increase drag, they were a sea-sled, where those surfaces are on the outside of the hull, and the "split" box tunnel hull where they are on the inside. The idea was to have those surfaces out of parallel with the direction of travel, so they tapered slightly, and hopefully no longer touching the water rushing by.
     
  3. philSweet
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    philSweet Senior Member

    I think you're trying to work with the dirty end of the stick. If you are talking about a modest planing speed or high semidisplacement speed then there is a good bit of dynamic lift pressure along the centerline of the hulls and this tapers off towards the chines because the flow is getting diverted towards the chines by the pressure. So there is outward momentum, and usually this is adequate to clear the topsides on a hard chined boat, or you should consider switching to a soft chine if the attachment is obstinate. If the boat is heavy and the chine imersion is fairly deep aft or the hulls are long and skinny, the outward flow may get ridden overtop of by the water alongside. Then you need to curve the quarters (in plan) with a radius that prevents the water from wiping the sides. This usually has more bad consequences than a bit of wetted surface, but for low planing speed it can work. Aft tumblehome can help as well. Often, you are talking about a smidgen's difference.

    Purely due to luck, when I converted my sailing skiff to power, it happened that the sides were dry at cruising speed of 16 knots, with a 5" wall of water about 2" off the side for the aft three feet. I suspect I would have been better off with more *** and a flatter trim, though. Attached flow increases the pressure on the hull bottom a bit and might help more than it hurts on a planing cat. If you are running flat, there isn't any sidehull worth mentioning. If you are not running flat, that probably needs to get looked at first. If you want to run fast, any wetted surface needs to develop dynamic pressure and contribute to the pressure on the bottom. I think this is why the tunnel design is so important on a tunnel hull. A foamy tunnel will wet the sides but not produce dynamic pressure or influence the pressure on the bottom. I think sleds should have a slight outboard deadrise to account for the pressure difference in the tunnel. This way the bottom dynamic pressure peak is along the center of the hull. Just an idea, I don't have any data on that.
     
  4. tunnels

    tunnels Previous Member

    Angle them slightly away from the water flow :confused:
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    My query was inspired by looking at a tri-hull planing boat the other day, the outer hulls were very nearly as deep as the main, central one, and the two tunnels extended right aft. The sides were flat, plumb panels, except for a bit of a lip to discourage water from lapping too far up the sides. My thinking was the bottom of the thing would have enough surface area to suffer from excessive drag, although a lot of the water would be heaviliy aerated inside those tunnels, without the added area of those external sides dragging through the water, parallel as they are to the centreline. I was thinking giving them a slightly negative angle lengthwise might have water breaking away from them, but it is just an idea.
     
  6. DCockey
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    DCockey Senior Member

    Water simply does not behave like you hope it will. The water will continue to flow along the sides even if "those surfaces [are] out of parallel with the direction of travel, so they tapered slightly" until the boat is traveling fast enough that it is "planing" and is supported by dynamic lift. Go out on a sailboat or other boat with the maximum beam around mid-ship and look over the side at the water flowing past the aft quarters where the sides taper in.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    I am talking about planing speeds of course, when the penalty of too much wetted area really kicks in.
     
  8. tunnels

    tunnels Previous Member

    Planning speed !! so what kind of speed is that ??
    10 mph 30 mph 50 mph 70 mph what ??
    where and what speed does skin friction against the waters surface become a problem ??
    does it start all of a sudden or is it a gradual thing ??
    have you ever water skied ?? fastest I ever been was 60 mph !! in a straight line :eek:
     
  9. DCockey
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    DCockey Senior Member

    There is a recent thread with extensive discussions of these questions. http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/definition-planing-45248.html Tunnels participated in the thread.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Skin friction is omnipresent of course, but for these purposes, 20-30 knots.
     
  11. daiquiri
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    daiquiri Engineering and Design

    Mr E,

    I think that you are giving too much consideration to the part of the drag coming from water wetting the sides of the hull. Yes, it will produce drag, but it has to be compared to other sources of drag in order to get the big picture of forces acting on the hull.

    The first thing you have to consider is that the wetted surface of the hull sides is normally just a fraction of the wetted area of the hull bottom. Hence, the resulting drag (when existent at all) will also be a minor fraction of the total friction drag.

    Next, consider a boat with vertical sides parallel to the water flow, in different speed regimes. What you have is:
    1. Low displacement speeds. The major part of the drag comes from the friction. So, if you have a boat with chines and if chines are wet (they normally are), you can do nothing about friction drag coming from the chines. Hence, the problem is not avoidable. Live with it or change to hull to a more appropriate type for that speed.
    2. High displacement speeds. The major part of the drag is due to waves, and the friction plays a minor role. The chines might be wet (which they normally are, again) or dry - it won't change things too much, because wave drag commands.
    3. Low planing speeds. The friction drag increases, but now you also have a drag induced by the dynamic lift. And it is pretty high, because at low planing speeds the trim angle is high. So once again the friction due to wetted side walls is a minor part of the overall drag. It wouldn't change things in any appreciable way if you managed to eliminate it - so not an issue in this case too.
    4. High planing speeds. The major part of vertical force acting on the hull is the hydrodynamic lift. The wave drag is low, the viscous drag is predominant. In this case it becomes convenient to run on dry chines, and that's what is usually done with the help of lifting strakes. If the boat is not running on dry chines, the hull walls will be wetted by a mixture of water and air, which has a much lower friction coefficient than pure water. So again, a scarcely important (when not nonexistent) problem
    5. Very high planing speeds. Chines are dry, the hull is partially sustained by aerodynamic lift. No need to worry about friction from the vertical sides, because they are not wetted.

    Cheers
     
  12. richard gray
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    richard gray Junior Member

    you could check out my photos of a hull you described at members site Rick Gray or at design site
     
  13. tomas
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    tomas Senior Member

    daiquiri, that's a succinct and helpful summary.
    Thanks.
     

  14. tunnels

    tunnels Previous Member

    A question !! what is the prurpose of the tunnel ??

    My question is this,
    what happens if those panels are not parallel, but are widest apart at the bow, and nearest at the stern, if only by a few inches ? At what speeds and negative angles will water no longer contact those slab sides, and thereby free it up from unwanted drag ?
    Might look odd, but who cares

    This statement is part way there to solving the tunnels function and water friction problems . The design of the inside of all of the tunnel , sides and top is a trick area and there's other things that come into play as well!, the tunnel roof, the actual shape if the tunnel ,and its only necessary to taper a part of the tunnel sides not all and the placement of a water deflector rail is really important along with a few other things as well !!,
    BUT remember everything is done with one purpose in mind to keep water away from the hull and reduce water friction .:idea:
    I spent a lot of my time working on tunnel boats all sizes and the designer and the company were what I consider to be way ahead of any tunnel designs I seen anywhere since then !!
    Its the first place I make for at boat shows or tunnel boats out of the water !!. :D:p

    Now think about this and get a pencil and paper and draw exactly what's been written (widest apart at the bow, and nearest at the stern) wouldn't it make better sense to be opposite ?? what do you think ??
     
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