Pocket Tunnel size? Help!

Discussion in 'Boat Design' started by agturbay, Nov 10, 2010.

  1. agturbay
    Joined: May 2010
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: argentina

    agturbay Junior Member

    Hello, I am from Argentina, and need help for some body who has experience in pocket tunnel desing.I have a 15 feet fishing boat, and i dont know if this measures are corret for mi boat tunnel: 3.3feet long ,1 feet 2 inch wide, and 6inch high.
    The shape of the tunnel is what I do not know which is better. the semicircular, rectangular or what g3 boats have.
    My outboard motor is a Yamaha 40hp 2 stroke 2010.
    Im making a jack plate .
    Any help will be apreciated.
    Many thanks!
    Gustavo.
     
  2. gonzo
    Joined: Aug 2002
    Posts: 15,774
    Likes: 1,201, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    It depends on the overall design. There is no fast answer.
     
  3. agturbay
    Joined: May 2010
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: argentina

    agturbay Junior Member

    Gonzo,ok I do understand, but some one who has similar boat with pocket tunnel and can run the outboard on plane 6 " high.The boat is a "soft v" hull 15 feet long, 5feet wide (fiber glass)I trying to modify the hull to run in shallow water(rocky bottom river).
    The transom is lifted 6 inch high, and my doubt is the shape of the pocket tunnel, because I see several diferent shapes (semicircular ,rectangular,trapezoidal)Which one provides more water flow to aviod cavitation or prop ventilation?
    Thanks for reply!
     
  4. gonzo
    Joined: Aug 2002
    Posts: 15,774
    Likes: 1,201, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    I don't think anyone can give you good advice unless they know what the design is. You want to take floatation off the stern and add more weight real far aft. That will change the center of gravity and the center of floatation locations. Depending on the displacement and other variables, the top of the tunnel may be dry or not. That alone makes a huge difference.
     
  5. frank smith
    Joined: Oct 2009
    Posts: 980
    Likes: 14, Points: 28, Legacy Rep: 185
    Location: usa

    frank smith Senior Member

  6. baeckmo
    Joined: Jun 2009
    Posts: 1,340
    Likes: 316, Points: 83, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    For a good shape of a propeller tunnel, see "DIY tunnel drive, post #46, attached zip-files". Note that the transverse section is not critical, since there is very little rotation in front of the propeller; a smooth longitudinal curvature is what matters in addition to the correct transverse areas! Also observe that the section aft of the prop cannot have a horizontal roof since that causes a reversing flow that will aerate the propeller from behind!!

    As noted by Gonzo, you must understand what happens to your hull when you change the buoyancy. The tunnel shape given above, is the shortest possible (=least loss of volume) for good inflow to the propeller.
     
  7. frank smith
    Joined: Oct 2009
    Posts: 980
    Likes: 14, Points: 28, Legacy Rep: 185
    Location: usa

    frank smith Senior Member

    Check the motor . I doubt that he is talking about a tunnel drive .
    More likely a motor well.
     
  8. baeckmo
    Joined: Jun 2009
    Posts: 1,340
    Likes: 316, Points: 83, Legacy Rep: 1165
    Location: Sweden

    baeckmo Hydrodynamics

    Mmmm... but he is describing something 3.3 ft long, 1'2" wide and 6" high. Seems like a tunnel to me, the inlet part of the tunnel I referred to is still relevant with an outboard.
     
  9. gonzo
    Joined: Aug 2002
    Posts: 15,774
    Likes: 1,201, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    With the cost and time of all that, it may be better to install a jetdrive. It can be an inboard or a replacement lower unit.
     
  10. agturbay
    Joined: May 2010
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: argentina

    agturbay Junior Member

    Thanks for your reply!here I uploaded four photos, two of them are from my boat(the orange and white one)the transom is painetd to represent how is now, and the black lines is the tunnel that i want to cut.The other photos are from diferent shapes.And finally is just draw two sides view of tunnels.Which one work better?Both of them,with 6" tall, will remain under water static level.
    Dont you see the alumacraft with tunnel, or G3 pocket tunnel boat ?
    Im trying to do some thing like that.
     

    Attached Files:

  11. gonzo
    Joined: Aug 2002
    Posts: 15,774
    Likes: 1,201, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Both boats you are showing, have sponsons that provide extra floatation aft. They are part of the design. You want to use half of the system.
     
  12. Bruce46
    Joined: Jul 2006
    Posts: 81
    Likes: 3, Points: 0, Legacy Rep: 49
    Location: Stuart, Fla.

    Bruce46 Junior Member

    Leave the hull alone. The chances are very great that trying to modify the hull will result in unhappy results. To get what you are looking for ie, shallow water performance an engine bracket like, http://www.dillon-racing.com/jackplate/index.htm this will raise your engine without doing anything to the boat.
    Depending upon how much you raise the engine you inital acceleration will wb slower, however, with the right porp your top speed will be higher. This is similar to the brackets used on high end bass boats. A word of caution, MAKE SURE THE CENTERLINE OF THE PROPELLER IS BELOW THE BOTTOM OF THE BOAT! Failure to do so could lead to a burnt out engine.
     

  13. agturbay
    Joined: May 2010
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: argentina

    agturbay Junior Member

    Gonzo, my idea is, if is necesary add sponsons to mantain original flotation level.
    Bruce46, and I will build a jack plate too.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.