How to design an outboard on an Atkin Seabright Skiff???

Discussion in 'Boat Design' started by SalmonMan, Mar 30, 2007.

  1. SalmonMan
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    SalmonMan Junior Member

    I want to build an Atkin designed Seabright skiff with a V-bottom and a stern tunnel like the Shoals Runner (http://www.boat-links.com/Atkinco/Utilities/ShoalsRunner.html). There are also a few other similar Atkin designs like the Everhope, Heron and Rescue Minor. I would like some feedback on the idea of how to use an outboard instead of an inboard. I realize that with this type of design that the box keel directs water into the inverted-vee stern tunnel and that the inboard prop also pulls that water into the tunnel.

    I have several ideas but am unsure as to how they will work.

    1. Locate the outboard prop in the same location as the inboard prop using an outboard motor well. This will remove some of the stern tunnel’s surface area and will probably change the dynamics of the water movement into the tunnel.

    2. Locate the outboard on the stern with a hydraulic jack plate and experiment with adjusting the outboard with the jack plate to find the best location of the prop. The inverted-vee stern tunnel is similar to the stern of the Hickman Sea Sled and that design had the propeller located aft of the stern.

    3. Extend the box keel all the way to the transom and locate the outboard on transom as in #2 above. At this point I’m changing the entire hull design and I’m not sure that I want to experiment that much.

    I am open to other ideas on how to get an outboard to work on this type of hull design.

    Also are there any other Seabright box-keel designs that are available with outboard power. My searching has not come up with any.

    Thank you in advance for your replies!
     
  2. charmc
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    charmc Senior Member

    I suspect that either #1 or 2 can be made to work. If you print out the order form for the plans, you'll see contact info for the designer. Why not ask him/her the question? I doubt it's the first time it's come up.

    Cheers,

    Charlie
     
  3. tom28571
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    tom28571 Senior Member

    I am dubious of all three solutions. Dave Gerr says that the tunnel is tricky to design and must be tailored to fit the specific boat. My 30 second solution is to fix a sealing plate at the ventilation plate that matches the tunnel surface. When not in use, jack the motor vertically until the prop is above the waterline. This would leave a big hole in the top of the tunnel but that is well above the waterline.
     
  4. duluthboats
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    duluthboats Senior Dreamer

    A few years back there was a traditionally built Sea Bright skiff in the launchings section of WoodenBoat. It had a 25hp outboard hung from the transom. That is much different than your idea; I doubt putting an outboard in a tunnel stern would be worth the hassle.

    Gary
     
  5. sal's Dad
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    sal's Dad Atkin/Bolger fan

    None of your three solutions. Tom's idea is close, but I would permanenetly seal (weld) the venitilation flush with the hull botton (tunnel top), and the skeg welded to the shoe, with the prop precisely in the same position as designed. I may still do this, if I can't get my act together on the inboard installation (all this transmission, shaft alignment, stuffing box, bearing, and custom prop stuff has me pretty frustrated right now! and I have a spare lower unit...)

    There was a bunch of discussion of this a while back on the AtkinBoats forum.
    Charm, you might notice the designer has been dead for 45 years...

    Where are you located, SalmonMan?

    Sal's Dad
     
  6. ted655
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    ted655 Senior Member

    :confused:
    So...., you like the design (which didn't load by the way), but not the propulsion system? If you have no need for the inboard tunnel then why wirk around or in spite of it? Is this a new build or are you converting an existing boat? Build a hull for the OB. What size OB anyway?
    Tunnels are for shallow operation. There are other ways to do that using an OB.
    1. Jet bottoms. 2 Surface drives. 3. Jack plates
    Remember that extended operation with a jack plate requires modifying the water pickup & delivery system and a water pressure guage.
    ==="When not in use, jack the motor vertically until the prop is above the waterline."==
    Any I've seen only lift 6"'. Depending on transom height, this is not enough travel range to clear the waterline AND still provide adequate settings for depth.. This means the hole for a motor well will still have to be big enough for tilt up.
    I personally always wanted to build an updated version of a "Dippy", incorporating a tunnel hull. But now we're back to an inboard engine. It's still the ultimate shallow running design.
     
  7. sal's Dad
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    sal's Dad Atkin/Bolger fan

    [​IMG]
    6" would be plenty to lift the prop and skeg clear. But again, I don't think it would be necessary or desirable. The total draft (like its more primitive sister, Rescue Minor) is only 6" or so, and the prop shaft is very near the waterline, at rest.

    It is really very challenging to find the components for a low-powered "high-speed" inboard installation. The designer writes: "The motor specified in the plans is a Universal Atomic Four, 22 hp at 3,000 rpm. The speed of Shoals Runner will be close to 17.5 mph. Do not install greater power than this."

    What is a "dippy"? sounds intriguing!
    Sal's Dad
     
  8. FAST FRED
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    FAST FRED Senior Member

    "My 30 second solution is to fix a sealing plate at the ventilation plate that matches the tunnel surface. When not in use, jack the motor vertically until the prop is above the waterline. This would leave a big hole in the top of the tunnel but that is well above the waterline."

    That's a reasonable solution, IF the plate is fair.

    We intend such a plate with diesel inboard.

    I have been on British narrow boats that transit rubbish filled canals , that have a similar system. A simple box with Watetight gaskets , the cover and fairing piece lift out together , and the prop can be cleared of line ., plastic etc.

    The cover is just above the WL,so no water enters the boat.

    With the Atkin it should be easier as the top of the tunnel, where the prop sits is above the WL at rest.Sure should make lobster pots easy to clear!

    Have never been an enthusiast of knife in the teeth and 55Fdeg water.

    FF
     
  9. ted655
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    ted655 Senior Member

    http://www.prodriveoutboards.com/
    There is also Go Devil, Gator Tail & Mud Buddy
    .
    Fast fred describes the essence of the Dippy. A inbosrd "box" with a removable top. The small inboard engine /trans is fixed but the shaft is CV jointed & extends down into the water, a prop guard similar to a 3 tine pitchfork is affixed just forward of the propeller. A seperate foldind rudder trails behind. When the shallow bottom or stump is passed over, the spring tempered tines lift the prop & shaft up, into the box. As F Fred says, there is a lever That can be pulled to manually lift the shaft for debri cleaning by removing the cover.. There is also a self locking latch that activates if the shaf up far enough.
    Google them. They were/are being used in lakes & rivers up in Canada. As with all things, there is a following of a few keeping them alive & repairing the originals.
     
  10. charmc
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    charmc Senior Member

    Sorry 'bout that. I was focused on the contact info, assumed there would be a designer at the other end.
     
  11. dick stave
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    dick stave Senior Member

  12. ted655
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    ted655 Senior Member

    :D
    I won't steal the thread but thanks. I had deleatrd all my bookmarks on them
    Imagine what we could build with todays technoligy.
    If I ever hit the lottery.....:)
     
  13. sal's Dad
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    sal's Dad Atkin/Bolger fan

    What do you think? This is the lower unit of a Yamaha F25. The line drawn on the box keel is the approximate design waterline.
     

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  14. dick stave
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    dick stave Senior Member

    You are a Trendsetter...
     

  15. SalmonMan
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    SalmonMan Junior Member

    Thank you to everyone for your reponses and info.

    Sal's Dad - What do I think? I think that it looks great! Is that your boat that you are building? I've seen a few other photos of it on another thread on this forum. What plans is it being built off of? Can you tell me more about it? Do you have a website or a gallery with more photos?

    Thanks!
     
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