High speed foil design

Discussion in 'Hydrodynamics and Aerodynamics' started by Ollie60, Aug 20, 2013.

  1. Ollie60
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    Ollie60 New Member


    Currently I have a project where I need to position a camera underwater to view the behaviour of water below the anti-cavitation plate on a motor. The problem is that I am unsure what foil profile to use since the boat will be moving at up to 60kts. I was originally considering a NACA 6 series profile, but then I realised a super cavitating profile might be more suitable.

    Does anyone have insight into this?
  2. philSweet
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    philSweet Senior Member

  3. Ollie60
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    Ollie60 New Member

    That's a possibility, but I would really like to see the dynamic response from standstill to full speed so that I can easily identify the problem
  4. DCockey
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    DCockey Senior Member

    Foil section for a strut to hold the camera in position or something else? How large and what shape is the camera?
  5. tunnels

    tunnels Previous Member

  6. tunnels

    tunnels Previous Member

    have a look at Mercury racing and look at there racing gear cases on sterndrives and outboards as a mater of interest .
    Also add on nose cones for out boards and sterndrives !!
    The plate is above and behind the prop !! so its what's in front that's the important bit , !!:):D
  7. tunnels

    tunnels Previous Member

    view the behaviour of water below the anti-cavitation plate on a motor at up to 60kts.

    The anti-cavitation plate at that speed is possibly no where near the water !!just the gear case and half of the propeller should be in the water so the plate has no use !! you need to study high performance and racing gear to get a better idea of what happens at the rear end of boats . the less that's in the water the less drag and the faster you go !!get the drift of what speed on water is all about

    http://www.performanceboatsmag.com/...&utm_campaign=May 2013 Issue&utm_medium=email

    Here's a site dedicated to performance boats !! down load and have a look at what's on the front cover !!:)
  8. daiquiri
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    daiquiri Engineering and Design

    Very true.
  9. Ollie60
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    Ollie60 New Member

    @DCokey, the foil section for a strut is exactly what I am pondering over. The camera is 25 x 25mm and 20mm thick. I have started looking at the NACA 66 series foils. Will XFoil give an accurate representation of the hydrodynamic pressure distribution? Otherwise the use of a super cavitating section still is an option.

    @tunnels, thanks for the interest. I am not really interested in the behaviour of the anti-cavitation plate, but more the possibility of cavitation or ventilation occuring due to the shape of the gear case.
  10. tunnels

    tunnels Previous Member

    what you are doing is trying to invent the wheel again !!I know it interesting BUT !!!
    what you could be doing is using a smoke trail in a wind tunnels at slow speeds Or simply find a naval site with all kinds under water testing and data thats already done !!
    OR JUST USE tell tails of fine silk thread set at all different heights and movable back and forward and a ordinary fan with variable speeds !! I have used this method before doing air flows over shapes of fairings and and cowlings and you don't have to have sophisticated equipment to do it !! :D:p
  11. tunnels

    tunnels Previous Member

    Have you ever really had a close look at the gear case of an outboard ?? its really quite interesting how its been arrived at that particular shape . they have changed over the years but not as much as you might think and the standard case shape is quite efficient up to quite a high speeds before it needs reshaping !!
    Racing gear cases are not all the same either and vary quite a bit in shape !!some even have a slight bend but like a banana and there's a very good reason for that !! and its not till you sit with pencil and paper that it becomes clear what its all about !! there's some very clever people out there !! .these changes have happened over a log time what we have today !! :D:p
  12. johneck
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    johneck Senior Member

    A NACA 66 shape should work well at zero AOA. You can probably truncate the section at 80% chord since I don't think it matters if the TE cavitates. But any slight surface imperfections will begin to cavitate at those speeds.
  13. tunnels

    tunnels Previous Member

    Then why didn't you say that's was what you were interested in !! say what you mean !!
    You really are going over well trodden ground ! people been experimenting with this for ever and a day . do you think the outboard manufactures just spun a board and threw a dart and that was there answer ??
    they have researched and found the shapes that are best suited to there particular outboards , as I said the shapes have changed over the years and yes you can make changes by fitting nose cones for hi speed outboards ! first its a gear case and totally necessary or you don't go any where
    second it has to have shape ! and that shape has been selected from the company doing a lot of research .
    thirdly they need to be computable with each other !!

    when you start adding to the leading edge of a leg you better be prepared for the other changes you haven't even thought about probably !!
    steering for one and how it will effect with having now added a long leading edge and it passes the point of pivot of the motor and in a worst case scenario at speed takes control to the point you cant hang onto and you wont be able to hold the wheel !! your history !!
    mmmm there's more here than meets the eye !! :confused:
    first I would study what has already has been done and what's been achieved and how it all works !
    learn from others that have been and done .
    there's very little that hasn't been tried by many people looking for that edge over the guy in the next lane on race day !!:idea:
    wheels are always round and you cant change that very much :D:p!!
  14. tspeer
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    tspeer Senior Member

    What direction is the camera viewing the plate? Is it ahead of the motor, looking aft, to the side of the motor looking to the side, or behind the motor, looking mostly forward?

    If it's looking aft, you have a problem at 60 kt, because if the flow is cavitated behind the strut, as it would be with a supercavitating section, all you'll see is white foam. If the flow is not cavitating, you have to be looking past the trailing edge of the strut, with a lot of optical distortion.

    At 60 kt, a subcavitating section will have to be quite thin, with a thickness less than 5% of the chord, as indicated by this diagram. For your 25mm camera (looking sideways), you're going to need a strut that is at least 30 mm thick (allowing 2.5 mm for the thickness of the skin/glass) and 600 mm - 800 mm wide. This could create quite a lot of lift at 60 kt, so the mounting is going to have to be robust and accurately aligned. It might make sense to allow the strut to be free to trail, even though that will alter the sight line.

    A NACA 6-series section would be a good choice, with the roof-top region carried quite far back. Say a NACA 67-0005. I doubt you'll find coordinates for such a beast, so it might be better to use XFOIL to design one specifically for your requirements. The thickness distribution from an Eppler E817, suitably thinned down, may also be a good starting point.

    I agree with the posts about a lipstick fiberoptic camera being a good choice. It might be possible to mount it inside the motor's strut instead of having a separate strut. A periscope is another option, with a small mirror down in the water and the camera up above the waterline. A vertically oriented fiber optic lipstick looking at a mirror might be the thinnest arrangement, and thus the smallest strut.

  15. tunnels

    tunnels Previous Member

    Still trying to reinvent the wheel ??

    the one really important thing that's being completely overlooked and not one person has mentioned so far is if you mount anything regardless of size or shape or position you will changing the water flow pattern into and around the gear case , therefore you will never be getting a true and accurate view of what is really happening .
    fibre optics is the smallest and best option and could be mounted on the plate above and in front of the prop or where ever . its small unobtrusive and will not interior with any water flows any where .
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