20ft Nemesis Cat

Discussion in 'Boat Design' started by Alexander Peter Bromley, Feb 24, 2021.

  1. Alexander Peter Bromley
    Joined: Apr 2020
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    Location: South Africa

    Alexander Peter Bromley Junior Member

    Hi Guys,

    I am beginning the build of a 20ft planning powercat, which is my design, boat hull designed in Maxsurf and finished in Rhino3D. This has been a dream of mine for a very long time and it is all coming together now, it is quite scary, stressful and exciting all at the same time.
    Boats application will be recreational fishing off the Cape waters, South Africa.

    LOA: 6.69m
    BOA: 2.4

    I will post some renders and pics if anyone is interested in following the build.
     

    Attached Files:

  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Interesting ! I can't quite see the detail of the demi-hulls, but they seem quite "fat", what is the width of them from chine-to-chine ?
     
  3. Alexander Peter Bromley
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    Location: South Africa

    Alexander Peter Bromley Junior Member

    Hi Mr Efficiency, yes I have kept them fairly fat to keep the tunnel clearance for the designed displacement if I reduce demi-hull chine beam I will end up with my tunnel sitting in the water at rest. It is a bit of a design constraint on small cat of this size.
    It will make the boat a little less "efficient" and I'm predicting it will throw quite a lot of spray, but I am looking for soft ride qualities over "efficiency". This might make the boat a little less conventional but I guess you need to leave the realms of convention to find something different that hopefully performs well haha.

    Plug design attached.
     

    Attached Files:

  4. Mr Efficiency
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    Mr Efficiency Senior Member

    If it isn't a "secret", what is that measurement, I can't get an accurate idea from the pics ? I would say 2' should be about the limit for best results.
     
  5. Alexander Peter Bromley
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    Location: South Africa

    Alexander Peter Bromley Junior Member

    Yeah it's a tough one because like I say if I bring my chine beam narrower it will sit with tunnel in the water, I don't want this. My demi-hull chine beam is 2.5ft, the benefits are a bit more planning surface for the demi's to run on, this raises my tunnel further out the water on the plane and makes the boat "sure-footed" on the plane which is my theory behind it.

    The knock off is lost slender hull efficiency, but I don't think it makes a huge difference because with the bigger planning area it should mean less wetted area on the plane where the slender hull may have more wetted area on the plane?

    What are the major pitfalls of the wider demi-hull?
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    I notice a few of the power cats over there in SA have "fat" demi-hulls, sponsons, call them what you prefer, but the most successful designs in Australia do not, and they are well-proven, Sharkcat and Kevlacat. But they also don't have deeply veed hulls that end in a "sharp" apex, they employ planks and fairly wide chine flats. There is a whole range of reasons they have stuck to that formula, I am suspicious of fat-hulled power cats because they amplify the "barrel roll" dynamic instability of planing cats, that is worsened by light weight, wide hulls, low deadrise, and high speed. Combine a few of those together, and in certain wave conditions, they can go over, surprisingly easily. Not a problem with slender hulls, and heavy boats. They can't go fast enough to get into that trouble, largely ! You actually worsen that tendency by having a high centre of gravity underway, with hulls that lift the boat higher, and the high tunnel height makes it even higher. I can tell you the most successful design here smaller than your boat, the Sharkcat 560, had hull width of 22". I've done many thousand of miles in them under all conditions, never felt unsafe for a second.
     
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  7. Alexander Peter Bromley
    Joined: Apr 2020
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    Location: South Africa

    Alexander Peter Bromley Junior Member

    You bring bring up some great points and I agree with all of them and we were actually discussing this on an SA boatfishing forum.

    The funny thing was one of our worst performing boats over here was based on a Sharkcat, take nothing away from Sharkcat I think the South African designer just made a few mistakes and it didn't turn out to be a great boat and broached quite a bit. The boat somehow lacked what the legacy of Sharkcat is riding on.

    All in all its hard to make one outright rule and say this is the only thing that works. The attempts of my design are a soft riding stable fishing platform. Soft riding has often come down to tunnel clearance on the plane, once wave heights exceed the tunnel height which is obviously going to happen but the bigger the wave that the tunnel is still clearing the better. The waters around the Cape here are consistently rough enough to very quickly expose any design flaws, the other thing is boats do not travel at excessive speeds compared for instance to America, this boats optimum cruise will be around the 18-22kts range any faster and the weather is giving you a gift on that day.

    "Hi Freddus, you make some very valid points. As you would know the broaching comes from a few different variables, the Ace cat 555 for instance has a few factors conducive to broaching. Weight and weight distribution with a heavy fwd console, small planning areas with low deadrise, narrow chines and a flat section on the transom, I don't think did it any favors. The difference is, I have made my hull deep vee (in excess of 25deg), all the way through to the transom, where the sharper the angle the more surfboard "fin-like" the stern will behave giving it more hold in a forward direction. Whereas the flat surface can slide out in theory. I have also created a much larger planning surface on my hull to provide more lift.

    The Tunnel thing is another valid point, I believe not so much in tunnel shape compared to tunnel height. A key factor is aerating the water being fed into the tunnel to make it more compressible like earlier discussed. A few sharp edges in the right areas I believe can do this. According to my current weight study my boat fully loaded, on a level trim sits with the tunnel still clear of the water. Full fuel tanks, 4 crew and 200kg of fish on board.


    We will also take the plug for a test run and refine any aspects that need to be changed. I am extremely excited going forward."

    Below is the Ace CAT 555 closely similar to the Shark Cat concept but i think the designer went wrong somewhere.

    [​IMG]

    [​IMG]





    Meanwhile this is one of the most popular, the Yeld Cat. Very good handling boat and yet the demi-hulls are fairly "fat".

    [​IMG]

    [​IMG]

    Another popular good riding boat over here is the Tom Cat...

    [​IMG]

    [​IMG]
     
    Last edited: Feb 24, 2021
  8. Alexander Peter Bromley
    Joined: Apr 2020
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    Location: South Africa

    Alexander Peter Bromley Junior Member

    A Sea Cat 565, I believe they are actually exporting a few over to Aus.

    [​IMG]

    This is a transom view of my boat.

    upload_2021-2-24_14-29-45.png
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I wasn't wrong about the "fat" sponsons over there. I think a "culture" develops where boats start looking similar to one another. I posted the actual dimensions of the Sharkcat 560 transom on this website, I will see if I can find it.
     
  10. Alexander Peter Bromley
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    Alexander Peter Bromley Junior Member

    Hahahaha!
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    This is the rough sketch, but the dimensions are pretty accurate. There were centre console versions sold, not a great many.
    Sharkcat 560.jpg
     
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  12. fallguy
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    fallguy Senior Member

    The only notable thing I see is the bow distance to dwl on the Ace looks lower than the others. It could be an illusion. Perhaps forward tunnel size is important..

    If not, the boat could come down into a wave and dig in, but it would need to be quantified. And I could be wrong.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    They did appear "low to the water" those old 18 footers, I think it was a blessing, lowered the centre of gravity, took a few waves over the front on bars, but no dramas.
     
  14. fallguy
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    fallguy Senior Member

    It took me a bit to realize the seacat pic is actually on the shore up above dwl. I sat here in a morning delirium perplexed at how she was so high at rest!!!!???
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    It is amazing how popular small power cats are in South Africa. I'm not sure why, they actually don't lend themselves to beach launching as well as monohulls, sitting higher on the trailer, and I do recall reading that beach launches are common over there.
     
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