20ft Nemesis Cat

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

  1. fallguy
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    fallguy Senior Member

    I am mostly lurking here. I think you'll appreciate my thought, though.

    If it were true wider sponsons contribute to more stability, then that would flow to a jon boat providing more stability. But we know that is not true. Wide and flat boats are notorious for capsizing in rough seas.

    The opposing argument would then be that narrow and narrower hulls are more stable. Of course, this lends itself to not planing at some point. Or a boat that rocks a lot adrift, etc. Or one that planes horribly, etc. But a sharp hull is very hard to roll over unless top heavy.

    I am not smart enough to delve into the details, but I do believe you are generally incorrect here.

    It would be interesting to see if anyone has ever made a cat that does not heel 'the wrong' way in turns. Also, if chine flats can help or if the asym hulls of the hammercat offer any resistance to capsize.

    I will return to my corner. The thread is very fun. Thanks.
     
  2. Alexander Peter Bromley
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    Alexander Peter Bromley Junior Member

    Hi Fallguy, I love a good boating discussion and love hearing peoples understandings and own experiences they have had on all things that float.
    Thanks for the reply, I think the flat vs deep vee is an interesting topic. If it was a mono I do believe if the CG on a flat bottom hull is in the correct place on the plane and riding in completely flat water compared to a deep vee riding in flat water the flat bottom will be more stable on the plane and less susceptible to lean if weight is shifted tranversely off the centerline, it will also be producing a lot more dynamic lift. Without any keel strakes or fins the flat bottom will tend to slide out when cornering. The flat bottom will have a steep initial stability curve to the point of capsize where the deep vee will have a much more gradual curve and be more susceptible to rolling but having a much wider range of heel angle before capsize occurs.

    I have been on deep vee's that have had too small a chine flats and have been known to broach quite notoriously, their problem was not having a large enough chine flat to create the desired dynamic lift and in turn stability on the plane so they were incredibly weight sensitive you have to place each crew member carefully to get the boat on an even keel on and off the plane. They wouldn't capsize and they would always self right after the broach but at the same time it was extremely terrifying.

    Basically what im trying to get at is that wider or flatter will have more dynamic lift than a narrow vee shape in flat water, this in turn should have more dynamic stability in flat water than a vee shape hull. That is assuming the CG's are both where they should be for each respective hull.

    Please I am no scientist and this may all be completely incorrect. this is just my theory haha.
     
  3. Alexander Peter Bromley
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    Alexander Peter Bromley Junior Member

    So here is a pic of my design, I have gone with deep vee all the way through to the transom but the sponsons are quite "fat" as Mr E. put it haha. We will run the plug before starting the mould.

    upload_2021-2-25_21-21-28.png

    upload_2021-2-25_21-21-48.png
     
  4. Alexander Peter Bromley
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    Alexander Peter Bromley Junior Member

  5. Mr Efficiency
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    Mr Efficiency Senior Member

    I have certainly learnt here ( I think) why the SA cats are the way they are, largely the surf launch requirement. That keeps them small, and with shallow draft so they don't bottom out so easily. I first became aware of the popularity of these boats in SA decades ago, and recall them being known then as "ski boats", not at all what that term means elsewhere ! It is interesting the way certain designs get adopted in certain places, and people follow the trend, despite one wondering whether it really is the best boat for local conditions, or that people are "dedicated followers of fashion". In life in general, they definitely are. But it is possible to see why they might have evolved in the direction they did.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    APB, what is the deadrise angle of your hulls, and the width of the flat at the chine ? Are you having outboard pods, or just transom hung ?
     
  7. Alexander Peter Bromley
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    Alexander Peter Bromley Junior Member

    Correct! We ended up calling your trailer boat equivalent "Skiboats", don't ask me where that came from haha. We also call Utes, bakkies...

    We do have to get you for a days fishing on a "Fat cat", and you guys can take me out on a Sharkcat or Kevlacat and we can really get a feel for the differences.

    I worked at a factory here in Cape Town, they build a boat called a Magnum 32. Here is a transom view of her, there is a video of this exact boat beach launching will try find it. The video shows exactly what you say, these boats are built for a shallow draft in the surf. They ride really well, we fish up to 80Nm offshore off them in all sorts of weather.

    upload_2021-2-26_7-16-12.png
     
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  8. Alexander Peter Bromley
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    Alexander Peter Bromley Junior Member

    Here is the vid of her beach launching. This is on the limits of the size you can beach launch.

     
  9. fallguy
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    fallguy Senior Member

    Does it take more power to get the deep vee on plane vs the flat bottom? A lot?
     
  10. Alexander Peter Bromley
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    Alexander Peter Bromley Junior Member

    You need more power to get a deep vee on the plane.
     
  11. Alexander Peter Bromley
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    Alexander Peter Bromley Junior Member

    Another theory I have on sponson width is comparing two boats of the same beam the one with wider sponsons will behave more like a mono whereas the narrower sponsons boat will be more "cat-ish" ie leans outwards in turns and so on... What Bruce Harris cleverly did to counter the lost volume up in the bow was to add bow flare on his Sharkcat, because his sponsons were narrow they needed some sort of lift for running with the sea so the bow wouldn't bury itself.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    The more I see of these SA cats, the more obvious the beach launch is a big design input factor. It accounts for a lot, I would say.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    A couple of videos that may interest:

     
  14. Alexander Peter Bromley
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    Alexander Peter Bromley Junior Member

    A noticeable design trend you guys have over in Aus seems most of the cats are running with outmounts or pods as you call them? I understand why you might add them but why would a designer go and design a boat with outmounts in mind from the start? Extend the hull if necessary? Surely if you designing a boat you design it with the required volume for the application of the hull. Outmounts are a remedy to insufficient displacement volume the sized engine weight...
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't think they work too well on smaller boats (pods) if they are heavy motors, as attested by the common use of bolt-on "hydrofoils" to the outboards. One thing you haven't got, on cats, is the trim tabs that can compensate to some degree for heavy motors perched out there, and with a cat, the effective bottom width is rather less anyway, I much prefer transom hung engines, but of course loss of internal space pushes builders toward pods. I notice that bloke who went through 30 sets of engines on his 18 footer ( video posted earlier), selected a pod that was a continuation of the bottom, I think a wise choice, there was an attempt to fit elevated pods to that hull, at one stage, with light two stroke 70's, that worked OK, but it is not using the high cruise speed potential of the hull.
     
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