24ft power cat design

Discussion in 'Boat Design' started by ecojet, Jan 20, 2017.

  1. ecojet
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    ecojet Junior Member

    I am in the process of designing a 24ft displacement power cat which will be powered by twin 25hp or 30hp outboards, looking for a cruise speed of around 16 knots, top speed at 20knots.

    I want the boat to be a weekender but also used for lengths up to a week.

    It will be mainly used in sheltered waters with the occasional run along the coast in good weather.

    The issue at the moment is the beam of the boat, do I keep it simple at 2.5m or possibly 2.9m still to confirm towing widths in NSW but 2.9m is doable with just warning signs in Qld. or make it demountable at a 3.8m beam.

    What effect will beam width have on performance and fuel efficiency?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    There is a resistance penalty from interference effects of closely spaced hulls, related to speed. The higher the cruise speed, the less it matters. If you go wider, though, you have a heavier structure, which imposes greater resistance of itself, so there is a trade-off there. You should be able to find resources in the search function here, pertinent to the separation . To achieve your cruise speed goal, you will need to watch weight carefully. Also, a wide boat may have a less comfortable motion in waves.
     
  3. ecojet
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    ecojet Junior Member

    Thanks for the reply

    The cat will be ply/epoxy construction and I will aim to keep it as light as possible.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Richard Woods, who posts here has his Skoota cats, which are pretty beamy. He may have some advice for you. What waterline beam do you have in mind for the individual hulls ? You need them pretty slender to work well as a displacement cat.
     
  5. ecojet
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    ecojet Junior Member

    At the moment it's a 500mm waterline beam.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    That should give you a length/beam ratio of 13 to 1 at least, which is sufficient.
     
  7. ecojet
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    ecojet Junior Member

    Great to see, I originally had them narrower but being a power cat I think it needs a bit of width to help stop it squatting.
    I like Richard woods designs but want the outboards on the hulls so a folding cat won't work.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    500mm is quite slender, you could not go less, imo, without penalties. I'd include in that power-head security of your outboards, depending on what the stern looks like and your engine mounting plans. You have I suppose to consider the tunnel clearance carefully, or slamming could be a problem. With an 8' beam, you might finish up with a high boxy look if you opt for more clearance. And if you load the boat up a little, the clearance will drop quickly on slender hulls. I think I'd settle on 12 to 1 L/B ratio as enough.
     
  9. guzzis3
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    guzzis3 Senior Member

    The 2.9 beam is australia wide. I've just had a look in the NSW website and man is it a dog breakfast. I can not find a simple document explaining it. Go figure something queensland transport does better...

    There have been quite a few powercat hulls on gumtree for pretty much materials cost. I don't know if any of those appeal. At about 24 ft you should be ok at 2.9. You can probably go to 10:1 on your waterline but make them good and fat at the back to support those big outboards. And keep your weight low.

    You'll get wave drag between the hulls but you just have to power through it. 16 - 20 knots you'd probably need 5 meters of beam not to get any drag from that, but it shouldn't hit your fuel economy too hard at 2.9. You can calculate the drag at different speeds and then calculate the power requirement if you care, but I don't have the equations to hand. I'm too lzy t do that stuff anymore 'd just go look at power cat manufacturers websites and see what they claim in fuel consumption at different speeds than look at the proportions of their boats.

    Here are a few abandoned projects. Not suggesting you buy any of them but might be of interest:

    http://www.gumtree.com.au/s-ad/toor...kis/fiber-glass-project-unfinished/1131950187

    http://www.gumtree.com.au/s-ad/bundall/motorboats-powerboats/project-noosa-cat/1136092322

    The noosa cat is 26' so not too far off ?
     
  10. ecojet
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    ecojet Junior Member

    I'm looking at 25 or 30hp outboards so not too heavy.

    Trying to find the towing width info online is nuts, their website needs a good clean up and good to hear it is Australia wide, thanks.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Haha, was that one at Toorbul a spaceship in a past life. No idea what the "wings" are about. The so-called Noosa Cat isn't a Noosa Cat that I'm aware of, but whatever it is, it is a planing cat, and will be going nowhere fast with 25hp motors. Whether it is feasible to have 8' beam in the type of boat envisaged in the OP, is somewhat questionable, when you realise there are none (that I know of) that you could buy. If you could power through the sticky zone with the interference drag, you might have a boat that looked like a small block of flats with a full head room cabin perched on a deck raised sufficiently to avoid the worst of tunnel slap. Unlike a planing cat, you don't get elevation with speed. Maybe the 10 foot beam is the way to go. But a trailing trial. You see less overwidth boats on the road than years ago.

    I just had another look at the Toorbul "boat". Extraordinary ! I had to stop looking, it might bring on a nightmare.
     
  12. ecojet
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    ecojet Junior Member

  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Nice boat, not sure about those outboards though. 2000 kg is quite light for a 26x10 boat, it is the type of thing where weight is critical though. I think there was a guy from NZ on this site that was tinkering with the idea of getting one to cross over to the island at the bottom of NZ, I seem to recall he had consulted with Leech about its suitability.
     
  14. guzzis3
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    guzzis3 Senior Member

    I wasn't recommending either of those hulls, just pointing out they are out there and for about the cost of materials, or less.

    I am 99.9% certain the oversize is australia wide, but you'd be mad to proceed building a boat that will cost 1000 hours and $1000s without getting something in writing from NSW.

    As I have said elsewhere I do not understand the 3.5 rules. I contacted queensland transport and was told you can't tow 3.5 with a light vehicle, that you need a heavy vehicle license and suitable vehicle. Maybe I was misinformed, maybe I did not word my inquiry correctly, they referred me to the oversize rules and said pretty clearly that the wide load can not be done with a light vehicle.

    Always remember the government is not here to help...
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Sadly, that seems to have become the case. You are kept waiting on the line for ages, and then have to tell them what the laws say.
     
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