Tunnel Design help for Flats Skiff

Discussion in 'Boat Design' started by DanAyo, May 11, 2003.

  1. DanAyo
    Joined: May 2003
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    DanAyo Junior Member

    I am planning to build a new skiff. This will be the third skiff I have built. It will will a flat bottomed aluminum skiff with a 20' by 4' bottom with a 40HP outboard. It will draft around 4".

    I'm looking for ideas and help with a small tunnel design. I'd like to elevate the motor approximately 4 or 5". I'm concerned about losing too much top speed and handling characteristics.

    Looking for advice, opinions, books, web sites, etc..

    Any help appreciated.
    Dan
     
  2. badges65
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    badges65 Junior Member

    Hi,
    you could look at a variation of the IVB for your needs!!
    don
     
  3. ted655
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    ted655 Senior Member

    Tunnel

    I'm cursed with a slow memory. Just 2 weeks ago I ran across a homemade ply design that featured a page of explanation on why a flat bottom is perferable to a cat tunnel. The way they handled the tunnel made sence. I remember the hull was built, leaving a hole aft. The tunnel was built and added last. I will remember where in a few days and post the site here if you are interested. You may already have this information.
     
  4. Doug Carlson
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    Doug Carlson Senior Member

  5. ted655
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    ted655 Senior Member

  6. Bill the Cat
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    Bill the Cat Junior Member

  7. DanAyo
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    DanAyo Junior Member

    Thanks for the post. The Bateau Tunnel Hull is close to want I'm thinking of building.

    Do you know of any other resources (links, etc.) where I can get more info on Tunnel Drives or Pocket Drives?

    Technically, I'm interested in a Pocket Drive. It's basic measurements would be approxiamtely 14" x 32" and about 4 or 5 " high. By elevating the motor (40HP) I could get the prop out of the oyster shells. I'm also hoping to un out of shallower water with soft bottoms better.

    My major concern is reduction of top speed.

    I plan to build the 20' x 5' skiff myself out of aluminum.

    Thanks for the help,
    Danny Ayo
    Houma, La.
     
  8. DanAyo
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    DanAyo Junior Member

    tunnel design images

    By-the-way: I described the new skiff as 20' x 5' aluminum. The 5' indicates the bottom of the hull and not the beam. the beam will probably be 6 1/2 or 7'.

    [​IMG]
    [​IMG]

    The images above show some of my present thoughts concerning tunnel/pocket drives.

    NOTE: The approximate dimensions of the Pocket illustrated is 32" long X 14" wide X 5" high. These are also subject to change.

    First, consider A & D. This is used by some of the locals. I'm told that Hell's Bay Boat company is using it also. In example A notice the red line and compare it to G. The light gray area in G indicates a flat plate that is welded flush with the bottom of the skiff. Not only does the red line indicate a steady rise from entry to exit, but at the entry side you will see that a flat plate is welded in the "nose" of the pocket. Now notice the green line. It indicates a small pipe that is supposed to vent the air pocket created by the welded plate. this venting is said to eliminate any suction or trapped air that might be caught in the top of the pocket.

    My thoughts on B. If a pocket was constructed so that the water never hit the top it would self vent and cause no extra drag. It might work similar to a "step" in the bottom of most bass boats.

    The concept in C came from http://www.bateau.com/plans/power/XF20_study.htm
    I really like this concept, but it seems like it would add considerable drag and the water is compressed in the pocket. According to the plans it would also have the configuration of E.

    What about F? I could have a peice of 3/16 or 1/4" aluminum rolled into a pipe shape and then cut on a diagonal to make an oval shaped pocket.

    I'm obviously no engineer, but I am interested in learning. If any thing stated above in unclear, please let me know so I can try to communicate this better.


    NOTE: D,E, & F are illustrated as seen from the back of the transom. G is illustrated as seen form the bottom of the skiff looking up.

    The goal of a tunnel (pocket) is to elevate the motor so as to raise the prop in shallow water with soft muddy bottoms that are covered with oysters. I need to get up on step from a dead stop. I am very concerned about losing top end speed. My skiffs are under powered for shallow drafting.

    Thanks to everyone that took the time to read.
    Any thoughts, opinions, or help greatly appreciated,

    Danny Ayo
     
  9. Peter_T
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    Peter_T Junior Member

    http://www.glen-l.com/designs/outboard/consoleskiff.html
    http://www.unclejohns.com/skiff/builders.htm
    http://www.responsemarine.com/slides.htm

    The above links will be good show case on some skiff boats. Even wood structure can be replaced with aluminum works. Some there are similarities.

    Your quotation on 4" draft, is it a dynamic lifted draft. If not, then the boat has to be very light. Get a quick check on the intended vessel weight and the available displacement. Some of these links will guide you through to the proportions.

    For such a shallow draft boat, is the tunnel intended to get water to the propellers. In most cases, you can let water run to the outboard motor propeller with a aft hull tunnel. Giving a fore tunnel will let water into the bottom and continuing the tunnel will let the water stream flow to the stern. The end result, the boat became a twin hull with full waterplane, since it is likely there is no air gap. The tunnel you wanted will take away the floatation you needed.

    Sometimes, putting a feature will have "pros" and "cons". You only put features that is best for the intended speed and must provide adequate floatation support (displacement).

    Some of our comments were stated, because we did not see pictures of your existing boats and those in speed trials. If you post these existing boat pic. files. Then, we can follow your thoughts in line. Most times we are guessing on the limited inf. from the questions.

    Peter
     
  10. DanAyo
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    DanAyo Junior Member

    Thanks for the reply Peter.

    My present skiff is 18' with a 4'
    bottom. It is made with Aluminum and I estimate it's weight at 450. I run small O/B that weighs 115#. I'd estimate the gross weight at 1000#.

    The new skiff will be 2' longer and 1' wider so I don't expect the hull to float any lower with the pocket even considering I plan to use a motor that will weight 200#.

    The goal of the Pocket is to raise the motor higher, especially at dead stop and trying to get on step. I am concerned about losing top speed.

    The illustrations below are not very technical, but I hope they are understandable.

    Along with any other opinions, suggestions, or comments I'd like to know the following:
    1. pocketdesign, size, and dimensions
    2. pocket performance (Pro's & Con's)
    3. If I run a hydraulic lift plate and raise the motor at slow speeds or steady acceleration from dead stop with No Pocket will the motor get water and the prop perform well enough to get me out of shallow water? I realize I would have to lower the motor once the hull go up on step.

    4. This question is on another suject. Would tucking in the width of the bottom at the rear of the boat increase performance (speed)? See bottom image in illustration.
    [​IMG]

    I know that I have asked a lot of question and I don't expect readers to answer them all. I do greatly appreciate any help rec'd.


    Thank you,
    Danny Ayo
     
  11. Peter_T
    Joined: May 2003
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    Location: Gulf Coast

    Peter_T Junior Member

    "Bill the Cat" shallow draft boat appear OK for the hull form and ease in construction for running swamp and really shallow water region.

    To get water to the propeller, you will either lower the propeller circle wholely below the keel or wing bottom (for twin outboards). For partial lowering of the propeller with the hub center to line up with the hull surface, then make a concave conic appendage to direct water flow to the propellers.

    Danny's design outline is satisfactory. Ride comfort is in trouble since the real waterplane may rise and fall below the chine for a flat bottom hull. Recommend to give some rise of floor. This will increase immersion draft, but is more steady in running over rough water. The center profile contour should run well below the chine. The chine should run up to intersect the waterline.

    Your question on tucking the stern side shell in. What you show is not enough to make a difference, if you want the water to flow with the waterline. Water will flow in bottock pattern aft in prismatic hull section. Speed boat with constant aft section is more advantageous, it even beat hull with the stern bottom faired to the waterline or just below the waterline.

    Your short tunnel need to widen and lengthened with sloping boundaries. Since water particles flow in a turbulant state.

    Note the best result is to set the outboard propeller below the hull surface. Water delivery to the propeller will not be restricted.
    Perhaps, put in a small tunnel to help in condition running back in shallow water with the outboard stem elevated.
     
  12. lagc

    lagc Guest

    tunnel design help for flat skiffs

    lifting the prop could bring in the 'prop rider effect' used in very fast boats. Get the right prop for that job
     
  13. more thoughts.
    what about a hamilton jet? In fact make a suitble cutout in the back of your skiff to lock in a jetski,to drive the skiff.
     

  14. Guest

    Guest Guest

    Tunnel Hull Design

    Go to www.aeromarineresearch.com Here you will find any nformation you need on tunnel design. Go on the forum or e-mail Jimboat and he can answer any and all questions . he is one heck of a nice guy.
     
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