Pocket tunnel design HELP

Discussion in 'Boat Design' started by bahamabay, Aug 13, 2007.

  1. bahamabay
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    bahamabay Junior Member

    We are working on a new boat model but are having problems with the pocket design. This is a flat bottom boat design and because it doesn't ride deep enough in the water we cannot get water in the pocket. This is not a full tunnel. We want to stay away from a full tunnel. A full tunnel takes away from the draft which we cannot afford. The pocket now is 22in. deep and about 20in. wide and 6in tall. The boat is 21ft. Can anybody help.
     
  2. bahamabay
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    bahamabay Junior Member

    We use tunnels to pull water up in the hull so you can jack the motor up higher to run in shallow water.
     
  3. messabout
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    messabout Senior Member

    Search for Wm. and John Atkin designs. They have produced some very workable boats with the feature you describe. See also a boat called Rescue Minor built more or less to Atkin design by the late Robb White. Rescue Minor has demonstrated the ability to run 12 to 15 mph in six inches of water. Not a fiction. I have seen the boat do so with my own eyes. It is an inboard of about 20+ feet LOA powered by a little Kubota tractor engine. RM is not a high speed boat so it may not suit your needs. I presume that you are talking about a "flats" boat with a large outboard and capable of terrifying speeds when there is sufficient water. The Atkins/White concept may be worth exploration even so.
     
  4. eponodyne
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    eponodyne Senior Member

    Have you considered shaping about the aft third of the hull into a great big NACA duct? That one's out of Tony Bingelis' book The Sportplane Builder, but the formula for layout can be scaled up. I'd consider it as the ducts are purposely designed to move fluids around with very low drag.
     
  5. dick stave
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    dick stave Senior Member

    There is some information on the Bateau.com website. The phantom 16, and the extreme flats 20 have pocket tunnels. Chris Ostlind also designed a pocket tunnel I saw on Duckworks (Blackwater 16) which is a scaled down version of Capt. Henderson's flats boat. You might want to consider sponsons to maintain minimum draft. They really work well on my FLAT BOTTOM jet boat.
     
  6. dick stave
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    dick stave Senior Member

    Here they are.
     

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  7. bahamabay
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    bahamabay Junior Member

    The problem is that this is a wide flat bottom design with a outboard that only runs in 2-3 inches of water so I am trying to pull the water up into the boat so I can raise the motor up below the bottom of the boat or close to it. I am trying to do this with a pocket that is 22 inches deep. I can put a full tunnel in and get all the water we need but then loose the draft I have to keep.
     
  8. kengrome
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    kengrome Senior Member

    How much draft do you actually need to keep? Do you really need it to draft only 2-3 inches?
     
  9. Pericles
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    Pericles Senior Member

  10. bahamabay
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    bahamabay Junior Member

    I want the boat to draft less than 6inches and run in 3-4 inches.
     
  11. kengrome
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    kengrome Senior Member

    This would require a boat that lifts while running at speed, and it also leaves out the Seabright skiff hull design -- along with its well-known seaworthiness in offshore conditions -- so I think the flatscat suggestion is more along the lines of what you need.
     
  12. Pericles
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    Pericles Senior Member

    Bahamabay,

    Did you look at the specs?

    21' FlatsĀ®Cat Beam 7'6"

    Bottom 7' Weight 1350lbs

    Gunnel Height 20" Transom Height
    27"

    Planing Distance 10' Take off depth 6"

    Minimum running depth 3 1/2"

    Bow rise on take off 5"

    Boat Draft at Rest 4 1/2"

    Boat Draft at Speeds 0-1/2"

    Maximum HP 150

    Recommended HP 90-130

    Speeds 115 HP = 43.MPH 130 HP = 48 MPH

    Just what you asked for.

    Pericles
     
  13. Capt. Ken Owens
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    Capt. Ken Owens New Member

    The size of your hull needs a longer tunnel. I am a consultant with the Bateau design called XF 20. I came up with the design and notified Jacques Mertens owner and designer of the company.We put our heads together and came up with the XF20. Jacques did all the engineering and design work, The tunnel needs to be at least 36 to 40" long. 20" wide will work. From the bow forward area go from ) back 12" to 5" high,then rake back to the transom area finishing at 4" high. Install a 3" radius on each inside of the tunnel. This allows water to enter the tunnel and compress and forces it out the stern. Very simple. From the top of the tunnel to the top of transom you want 20". This allows the jack plate to come into play by raising the engine so prop runs in tunnel backwash.The Hendersen design is a proven hull design. I know Capt. Hendersen personally and have seen his hulls at work. We used his hulls as an inspiration and came up with our own design.Go to bateau.com and click on XF20 in the power boat section and see pictures.
     
  14. dick stave
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    dick stave Senior Member

    Pocket Tunnel

    The pocket tunnel design on the Henderson skiff is interesting. I would be interested to hear from someone who is running one of these skiffs. I have done some experiments with jack plates on flat bottoms boats. The rule of thumb as explained to me is 1/2" of lift to 1" of setback to a maximum of 10" setback ( break even point ). I did not use low water pickups or expensive double cupped props and achieved some pretty skinny water results with no cooling or cavitation issues. I have also read that these tunnels are also sometimes vented at the top of the entrance with an elbow and tubing thru a ball valve and then discharged over the transom to alleviate priming problems. The way it was described is that you had released an anchor when the valve was opened. Good luck.
     

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  15. kengrome
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    kengrome Senior Member

    Hi Captain Ken Owens,

    I've designed an extreme flats boat that's even more "extreme flats" than most. It requires a simple surface piercing propeller drive, and the boat (including propeller) drafts less than 4 inches at 1000 pounds displacement. I'm wondering if you'd take a look at it and give me your opinion, assuming you're interested. Other's opinions are welcome too of course!


    [​IMG]


    Note that the top of the tunnel will be vented to allow compressed air to escape rather than be forced into the water in the tunnel. I expect this will keep the tunnel water more 'solid' with less entrained air.

    You will probably also note that the boat is designed to use an inboard engine installed in the center console (not shown, nor is the rudder and other non-hull features). The boat is supposed to be like a 'scooter' with a big, wide, flat casting deck forward, and the rest of the boat with a flat deck 6 inches lower.

    I like the inboard diesel for fuel economy as well as the fact that the stern is completely free from equipment, so you can walk around the aft end of the boat and fish from there with no obstructions to get in your way. I also plan a "trap door" over the prop to clear it without getting wet.
     
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