Notched transoms - info needed

Discussion in 'Boat Design' started by Olav, Sep 14, 2011.

  1. Olav
    Joined: Dec 2003
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    Olav naval architect

    During the design of a small (5 m) fast rescue boat for a local boatbuilder I came across the interesting concept of incorporating a notch into the lower parts of the transom (see attached picture for clarification).

    From what I understand this has the advantage of being able to raise the outboard engine a bit - which would be a highly welcomed feature on the rescue boat since draught is critical.

    However, I only managed to find some rather unscientific statements on several boat forums but nothing with some more in-depth about hydrodynamics, design practice or other hints on how to determine the size (length and depth) of such a step/notch and how it affects lift, running trim, and resistance of the hull.

    Is anybody aware of any papers dealing with the subject or has other sources of information?

    Any help is greatly appreciated! Thanks in advance!

    Attached Files:

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  2. Petros
    Joined: Oct 2007
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    Petros Senior Member

    I do not know a lot about power boat planing hull design but it seems to me that notch would increase the drag at all speeds except the fastest speed where the hull is fully on plane.
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  3. tunnels

    tunnels Previous Member

    The disadvantages far outweigh the advantages !! If you are a designer what books have you been reading this have been round for years and years . Look for race boats if you want to find info You need to update you book list ,this is 2011 !!!
    Having been involved with rescue boats and also race boat i would simply say forget it It is designed for continuous high speed boats and if not done properly can give more problems than you will ever want in a rescue boat !!,like dragging the back of the boat down , without lots a hp it will make a hull slow to get out of a hole .
    it need air to work . when the water is tumbing against the transom its not working and sucks the back of the boat down !.
    Last thing you need in a boat thats in rough water and big swells !!
    . It was all the rage a few years back and lots a small power boats had them but have gone for other things now and filled them in . Understand what you are looking at and how it works ! find the advantages and the dis-advantages . With all good ideas there always a tra:confused:de off some where .
  4. FMS
    Joined: Jul 2011
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    Location: united states

    FMS Senior Member

    What books do you recommend tunnels?
  5. tunnels

    tunnels Previous Member

    Trouble is most of These types of things are not in books !! I always carry a camera and take pictures!, i have thousands from over my life time . I worked for a race boat manufacturer and thats a place to learn from people in the know .
    I worked for a surf life saving manufatcturer in nz and developed ridged bottomed surf boats using race boat technology , TThese boats could turn in there own length at 35 mph with the crew lashed in so they wouldnt get thrown out with the G force when the motor was locked full round . These boats could also punch out the top of a curling breaking wave which is almost totally unheard of anywhere ,soft bottoms usually bend in Half or flip over on there back upside down .
    Why would you want to punch through a wave ? to get to a drowning person as quickly as is possible !!!every second counts !!. also in compatition you can be out to the marker , round it and on your way back before the other boats are even 1/4 the way out!! .
    The boat you are designing inboard ? outboards ? jet ? Strakes are what you need to be getting your head around to get lift and get up and go ! Forget notched transom's If you have outboards with 25 inch shaft a stepped back transom would be heaps better to look at .
    Tricks from racing is another thing that really works .
    A mono that turns at full speed in its own length !! How was it done ??. LOOK very crefully at Jet sprint boats how are they are able to turn at such hi speeds?? understand how its done !!ask ! Take pictures and study them . This is not in any manual of written its learned and handed on from one person to the next . Trick stuff !!!
  6. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    The notch may or may not allow the lower unit to be raised. If that is so, then the amount it can be raised is so small that it would be inconsequential for reducing draft.

    I have been led to believe that the notch has a simple, practical, function. With the outboard mounted close to the transom, there are occassions when water will climb the leading edge of the lower unit housing, creating an unwelcome geyser of water that can and does crawl over the transom into the boat. The notch is said to diminish this problem somewhat. I will be intersted to learn the more technical reason for the notch. That feature is quite common on many small, fast boats.
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  7. IMP-ish
    Joined: Jan 2011
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    Location: united states of america

    IMP-ish powerboater

    When did the Fountain notched transom come in? Maybe a comparison is possible there by looking back at the first year it came in.

    I know the 38SC got a notch in 95.

  8. orb353
    Joined: Feb 2010
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    Location: GA

    orb353 Junior Member

    Notches, tunnels, or pockets, whatever you want to call them, are used to reduce the draft of a hull. These are usually incorporated into planning hulls. The basic principle is to allow the water under the hull to raise up into the tunnel, and therefore exit the tunnel at the transom at an elevated height, in comparison to the height of the water coming out from under the rest of the hull at planning speed. If you notice the height of the water coming out from under a planning hull is lower than the level of the water surrounding it, because of the weight of the hull plowing through the water. It takes several feet behind the boat for the water come back up and return to the normal height. Thus the further the prop is from the transom, the higher it can run. Thus , jackplates and porta brackets for outboards. I think the basic idea of a tunnel is to return the water to sea level closer to the transom, so the prop can be run higher, while still being submerged. I am not sure at what speed this starts to work.

    Look into the little rescue minor hull.
    Last edited: Sep 28, 2011
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