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Opinions for Underwater Exhaust Deflectors

Discussion in 'Technical Discussion' started by comship, Dec 8, 2007.

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  1. comship

    comship Senior Member

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    As the underwater exhaust deflectors in WET EXHAUST SYSTEMS are a very common practice nowadays, indeed only few are sure wether they work or not, I would really appreciate If somebody could submit any experience regarding these systems
  2. K1W1

    K1W1 Senior Member

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    Hi,

    Can you eplain what it is exactly that you refer to as a "deflector"
  3. Garry Hartshorn

    Garry Hartshorn Senior Member

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    I understand that you are refering to the venturi often fitted to the underwater exhaust. If correctly designed they reduce the back pressue on the exhaust and they do work, However a poorly designed one does little more than add a large amount of drag and turbulance which does nothing for the vessels performance.
  4. K1W1

    K1W1 Senior Member

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    Hi,

    Would you care to share with the rest of us what consists a "correctly" designed one Gary?
  5. comship

    comship Senior Member

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    the exhaust deflectors are laminated in wet systems and consist of a longitudinal device like "spray rail" , their length is six times the diameter of the under water exchaust. their purpose is to deflect the exhaust gases away from propellers.
  6. comship

    comship Senior Member

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    in the following photo you can see the STBD deflector of our Yacht which is currently in the Italian Shipyard

    Attached Files:

  7. Garry Hartshorn

    Garry Hartshorn Senior Member

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    I believe that the less turbulance that the front of the venturi creates the better, rather than a box with an abrupt front surface. I have attached a quick drawing of what I think is correct

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  8. comship

    comship Senior Member

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    Don't you see that these devices have nothing to do with the wedge and the turbulence which is created by them?
  9. Garry Hartshorn

    Garry Hartshorn Senior Member

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    Sorry I was messing with my drawing while you last posted and did not see your photo. NOW I AM CONFUSED.
  10. comship

    comship Senior Member

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    -Exhaust thrust slot.

    Just to put the things in order the device's you are referring as ventury, correct term is Exhaust Thrust Slot and invented by Cpt. George Vonwidmann (the wet exhaust system)
  11. Garry Hartshorn

    Garry Hartshorn Senior Member

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    I do not believe that I have ever seen such a deflector rail, was it original to the boat or were they dealing with a cavitation issue. In my experiance the exhaust is usually high enough on the bilge panel for the exhaust gases to be well above and out of the way of the propellors.
  12. K1W1

    K1W1 Senior Member

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    Hi,

    Gary, Here is the definition and a drawing of a venturi http://en.wikipedia.org/wiki/Venturi_effect

    Comship, I have never seen anything like your photo shows, I have been involved in the construction and operation of several high speed high horsepower boats and displacement motor boats.
  13. Garry Hartshorn

    Garry Hartshorn Senior Member

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    A venturi does not have to be in a pipe, the essence of a venturi is created through the flow of fluids over a correctly shaped surface to create a vacumme or a low pressure area on the trailing side. The wiki is correct but simplified.
  14. Garry Hartshorn

    Garry Hartshorn Senior Member

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    Just to illistrate my point I found a picture of a venturi bailer for sailing dinghies.

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  15. comship

    comship Senior Member

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    It is trent now in almost all Italian Shipyards to install them in wet exhaust systems. The exhausts gases as vacuumed to the sea (when yacht is underway) form a gas layer (covering the underwater keel of the Yacht above seawater – as the specific gravity of the gases is less that seawater special gravity) which is then sucked by the propellers. This phenomenon creates turbulence to the propellers area and thus cavitation and vibration. In order to eliminate this phenomenon designers are incorporating underwater exhaust deflectors. These deflectors, in the form of laminated curved spray-rails, begin from the under water wedge (the side of wedge which in near the keel) and run curved to the bow. The length of the exhaust deflectors is 6 X Diameter of the Slot, the Height is equals to the height of the wedge and turns to zero at the end point. The end point is located in the same line with the centre of the slot.

    As a consequence the exhaust gases are driven away from the propellers to the sides of the Yacht. These structures do no affect the drag or resistance of the Hull.
  16. comship

    comship Senior Member

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    K1W1 devices like these are absolutely necessesary also in case that somebody wants to install interceptors.

    In our Yacht even though we hadn't considered the deflectors value in conjuction with the Interceptors, indeed when the engineers of the interceptors company saw that the Yacht was featuring these devices they were very happy.
  17. K1W1

    K1W1 Senior Member

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    Hi,

    Comship, I am intrigued, is it possible to get a few more pics of this setup?
  18. comship

    comship Senior Member

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    Below is a rough made drawing showing the correct dimensions and layout in respect with the slot

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    • def.jpg
      def.jpg
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  19. comship

    comship Senior Member

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    of course the dimensions there refer to our Yacht.
  20. comship

    comship Senior Member

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    Below a photo taken during their lamination. You can see their layout in respect to the shaft tube and shaft bracket

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