pocket tunnel will outboard bracket help

Discussion in 'Powerboats' started by fjlegend, Jun 28, 2010.

  1. fjlegend
    Joined: Apr 2008
    Posts: 12
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    Location: HotLanta

    fjlegend Junior Member

    I am still messing around with getting my pocket tunnel to perform on my 20' flatback proline (1975 model). I have upgraded to a 4 blade trophy prop and added a mantaray compression plate. Both of these items helped but it still could use some help.

    I run a 6" setback Bob's JP with a 140 Zuk 4 stroke. So now I am thinking of adding a 12 - 16 inch setback bracket with the JP.

    It seems that the bracket will give me the benefit of increased flotation and putting the engine in cleaner water.

    I need some theoretical and practical advise on this subject.

    Many thanks in advance,

  2. tunnels

    tunnels Previous Member

    This just what i found having a quick look at POCKET DRIVES
    Pocket drive, catamaran tunnel and some theory:

    There are different ways to design a flats boat with minimal draft. Some catamarans claim to have less draft than monohulls but that is simply not possible.

    Let's demonstrate by comparing the geometry of two simple hulls.
    The sketch below shows the waterplane area of a monohull compared to a typical catamaran hull, each simple rectangles but the comparison works just as well for more sophisticated shapes.

    You can see that the catamaran hull as less foot print than the monohull: at equal draft, the cat displaces only half the water of the mono hull

    and therefore, at equal boat weight will have two times more draft. Even if the tunnel is narrower, the cat hull will always need more draft but there is more: a cat structure is more complicated and the hull area is larger, therefore heavier.

    A cat type tunnel will always have more draft because it has less waterplane area and a heavier structure.

    That is for static draft but what happens when running?

    There again the monohull is superior.
    The water between the hulls of the cat is turbulent. All kind of steps and other contraptions installed between the hulls have been tried to reduce that problem but despite the claims of some cat manufacturers, the prop runs in aerated water and is not as efficient. The prop has to be lowered to run in "hard" water but this increases draft.

    This is where the tunnel is clearly superior. Not only does the water coming out of the tunnel rise higher than along the hull sides but it is compressed by the tunnel shape: the prop will cavitate much less than between the cat hulls.

    The correct name for our type of tunnel is pocket drive. The theory is well known: the forward part of the tunnel is higher than the exit and the aerated water is compressed before it reaches the prop. At the transom, the water makes a hump and that is the level of the cavitation plate at planing speeds. For the hole shot, the prop must be deeper but once the pocket drive is "primed" the engine can rise on a jack plate allowing the boat to run in 4 or 5" of water.
    (Some of the XF20 builders report running in as little as 2-1/2"!).

    A pocket drive is not perfect: there will always be a turbulence at the mouth of the tunnel. There is simply no way around it. At planing speeds, the water going through the pocket has to cover a longer distance than the water running under the straight parts of the bottom. This will create cavitation but we can reduce its consequences with the shape of the tunnel. The tunnel is not a plain box: it is tapered in profile and all corners have large radii fillets. It is heavily reinforced to withstand the pressure variations.
    The pioneer of this type of tunnel is Uffa Fox: he designed the first ones for the British Navy boats during W.W.II. Today, we know what the ideal shapes and proportions are and that is how the XF20 pocket drive was calculated.
    Don't forget that as you move your motor back, your center of gravity will shift aft and cause more draft and change how your boat rides. It would be best to find someone with a similar setup and see how it works. More setback is not always the answer. Also maybe a low water pickup will let you get the motor up higher.
  3. brianincc
    Joined: Apr 2010
    Posts: 1
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    Location: crescent city Fl.

    brianincc New Member

    I have built two pocket tunnel boats. The key to getting one to perform is to vent the tunnel. I picked up 4 mph and the boat ran cleaner and higher after venting. If you go to the bateau2 site and do a search for tunnel vent you will see what I mean. I think adding more setback to that hull will do more harm than good. Good luck
    1 person likes this.
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