24ft power cat design

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

  1. ecojet
    Joined: Jan 2017
    Posts: 33
    Likes: 1, Points: 8, Legacy Rep: 17
    Location: NSW Australia

    ecojet Junior Member

    It's only in the design stage at the moment and I will make sure I have the towing guidelines in writing before I decide to make it wider than 2.5m.

    Also the build plate is another issue, I'll have to find a competent person to sign off on the build which I will need to get sorted before the build starts.
     
  2. ecojet
    Joined: Jan 2017
    Posts: 33
    Likes: 1, Points: 8, Legacy Rep: 17
    Location: NSW Australia

    ecojet Junior Member

  3. HJS
    Joined: Nov 2008
    Posts: 341
    Likes: 49, Points: 28, Legacy Rep: 288
    Location: 59 45 51 N 019 02 15 E

    HJS Member

    24 feet and 16 to 22 knots is not a displacement boat. The selected speed is clearly a planing hull and must be designed for planing and nothing else. The tunnelsides must be flat to get low resistance. The planing bottom surface size must be adapted to the boat's total weight. Displacement speed is 5 knots in this example.

    JS
     

    Attached Files:

  4. ecojet
    Joined: Jan 2017
    Posts: 33
    Likes: 1, Points: 8, Legacy Rep: 17
    Location: NSW Australia

    ecojet Junior Member

    Maybe displacement isn't the correct term so lets call it an Efficient 24ft power cat design.

    Your example hull design is a mono hull with a tunnel through it which is a standard shark cat design that needs big hp to make it work, not something I'm interested in.

    The Leech 825 is one example and also the Supercat Sliver 29.
    http://www.supercat.co.za/sliver_29.htm
    http://www.leechboats.com/L825.cfm
     
  5. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,026
    Likes: 360, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Not a "shark cat" type of hull really, they have symmetrical demi-hulls, the boat illustrated is an asymmetric type. But of course, both are planing hulls. No reason why a 24' cat can't be made in a displacement format, but given that none are around, on an 8' beam, it clearly is a thing that involves a lot of compromises.
     
  6. sailhand
    Joined: Jan 2017
    Posts: 97
    Likes: 19, Points: 8, Legacy Rep: 29
    Location: australia

    sailhand Junior Member

    hi ecojet I think the leech cat looks like a great design. The performance with small outboards looks pretty good for the size of the boat. I think the low powered powercat is a bit of a new frontier in boat design and there are some great new designs beginning to appear. Richard Woods has been working on designs in this area over recent years with some great results. In the past it has been mainly high powered planing designs which are expensive to put together, maintain and run compared to the lower powered "displaning", as peter brady would say, cats that have appeared in recent years. I have designed and built several small catamaran tenders with this type of hullform and found them to be very successful. like all boats there are compromises to be made however I think these are less restrictive than the typical powercat compromises. I have recently posted some pictures of my 3.5 metre catamaran tender on this site running at around ten knots with a three horsepower yamaha outboard. the efficiencies of these hulltypes needs a lot more development effort to continue on the back of some great work already done by some of the more prominent multihull designers but I think your on the right track with this type of vessel.
     
  7. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,026
    Likes: 360, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    The biggest problems with an 8 foot beam are hulls causing wave interference at a speed it may be desired to travel at, and the need to have a high enough clearance for there to be little or no wave impact under the connecting span, creating a very high cabin structure eyesore. What clearance would be desirable on a 24' displacement cat ? Probably 15" at minimum, preferably 18" lightship. You finish up with a boat that is higher off the water, than it is wide.
     
  8. ecojet
    Joined: Jan 2017
    Posts: 33
    Likes: 1, Points: 8, Legacy Rep: 17
    Location: NSW Australia

    ecojet Junior Member

    At the moment it has a clearance of 500mm and yes the biggest issue now is the boats overall beam, 2.5m maybe just not wide enough, at 2.9 maybe ok.

    Hi Sailhand,

    I like the tender, well done.
    I found it interesting that the tender seems to hit a wall at 15 knots, would that be mainly due to hull length and maybe also due to hull spacing?
     
  9. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,026
    Likes: 360, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Do you know what the clearance on that Leech boat is, ecojet ? Appears not to be that much, but probably adequate for most purposes. But if the boat gets loaded up a little, and the weather goes bad, marginal clearance can become an issue.
     
  10. ecojet
    Joined: Jan 2017
    Posts: 33
    Likes: 1, Points: 8, Legacy Rep: 17
    Location: NSW Australia

    ecojet Junior Member

    Not sure but looks less than 500mm, I may lower mine to 450 or 400 but no lower than that as most of the time it will be used in sheltered waters but I still want it to have decent performance on a short coastal run.
     
  11. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,026
    Likes: 360, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Low clearance can be compensated by slowing down in the rough, I have done a lot of fast trolling (5-6 knots) in planing cats, in lumpy conditions, and tunnel slap isn't much to worry about, even though clearance is next to nothing. But a narrow displacement cat will likely be in the worst of interference drag at that speed. If being able to maintain fast cruise is the priority, you need clearance.
     
  12. sailhand
    Joined: Jan 2017
    Posts: 97
    Likes: 19, Points: 8, Legacy Rep: 29
    Location: australia

    sailhand Junior Member

    thanks ecojet , the reason the dinghy hits a wall at 15 odd knots is that it has rocker and for the hull to go any faster you would need to break the "suction"( if you could think of it in those terms or a low pressure area if you prefer ) caused by the hull shape as it moves through the water. perhaps a really simple example of creating this drag/weight would be to hold a glass under the water in your kitchen sink. making sure there is no air in the glass turn it upside down whilst it is in the water and slowly lift it up leaving the lip of the glass 10mm under the water. the glass should be full of water which is predominantly above the level of the rest of the water in the sink. you have just created a void that cannot vent and you can imagine dragging this water behind you. the faster you try and go the larger the volume of water you drag with you, this will cause the boat to squat more the faster you try and push it. the design of the hull will dictate the optimum speed. an example of different speeds on the behaviour of water and its effects at different speeds can sort of be seen in the wake immedietely behind your tinny at different speeds. at a slow troll speed you will see the water tumbling around behind the dinghy. this water is effectively being dragged with you as you move along. at a certain speed as you accelerate the water will be seen to be running off the keel of the hull and if you lean over you will be able to see the entire transom at that speed, which you couldnt see at trolling speed. if you look behind the dinghy now you will see the water converge and culminate in a bit of a rooster tail. as you speed up the rooster tail will move further behind the boat changing the shape of the "hole" in the water behind the boat. if you think about this interaction between hull and water you will realise that water interacts differently at different speeds and you must design your hull shape to suit its interaction with the water at a particular speed to achieve maximum efficiency. you can alleviate this maximum speed/increased drag conundrum by making straight sided planing hulls and riding "over" the water to some extent rather than pushing through it. however that would require a lot more horsepower/fuel/expense/weight etc etc. that is the reason for putting a slight rocker in the hull with a flat run aft, it will achieve the hulls optimum speed with very little power compared to a planing hull but it will not go much faster than its design speed without tons of power/fuel being thrown at it. like all boats it is a compromise and I personally would rather travel at consistent steady speeds in most conditions comfortably than flat out on calm days and getting pounded whilst staring skyward and watching the fuel gauge go down on rougher days. in the late 80s I built a cat dinghy about 15 foot long with a series of vented steps in a rockered hull to alleviate this problem and it actually worked quite well, maybe I should revisit that concept now that I know but a teensy bit more. and for all you naval architects out there this is a very simplistic and probably technically incorrect way of explaining this, I am sure you guys can do better. hope this helps you with the complexity of hull design a little bit. if you are heading down gosford way at all pm me and we can catch up. I have my boat and dinghy here for the next few weeks and you are welcome to check it out and go for a run. it basically embodies the simple concepts I have ineptly tried to explain here but may help give you some ideas for your design. I have actually designed a hull at 7.6 metres to run with a pair of twentys and I would expect a 14 knot cruise, 20 knot max, thats plenty fast enough for me!!
     
  13. ecojet
    Joined: Jan 2017
    Posts: 33
    Likes: 1, Points: 8, Legacy Rep: 17
    Location: NSW Australia

    ecojet Junior Member

    14 cruise with a max around 20 would be fine with me.

    I'm going to add a foot to the hulls at the rear and make it 25ft, the hulls are flat all the way, maybe a slight rocker to the front, bwl will most likely end up being 600mm just have to decide what the overall beam is going to be, trailered at 2.5m or 2.9m, or 3.8m and leave it on a mooring.
     
  14. Richard Woods
    Joined: Jun 2006
    Posts: 2,197
    Likes: 149, Points: 63, Legacy Rep: 1244
    Location: UK, USA and Canada

    Richard Woods Woods Designs

    Thank you for mentioning my Skoota powercats on this thread. I now have a Skoota 20, 24, 28, 30, 32, 36 plus the Jazz 30. The Skoota 32 is now also available as a demountable version, and the Skoota 24 with an aft cockpit as well as a centre cockpit. My Chat 18 is more of a motor sailer than power boat, but can be trailed without folding.

    You should have no problems running in mid teens with two 20hp outboards. We do over 16knots day cruising on our Skoota 28 and 12 knots loaded with twin 20hp

    I did seriously consider a catamaran dinghy for our Skoota, but the weight put me off, its hard to make a catamaran weigh the same as our "built in two days" 10ft Duo dinghy - 2 sheets 4mm ply - yet carries both of us and our shopping. See here (especially the last few seconds)

    https://www.youtube.com/watch?v=_eDo8Z9DP04&list=PLT7PbPvOm8lxpmhDyf2v3nkY7HAbvdWnU&index=17

    (Polite comment - paragraphs would help to read posts more easily!!)

    Richard Woods, currently cruising and living on board his Skoota 28 in the Bahamas

    www.sailingcatamarans.com
     

  15. sailhand
    Joined: Jan 2017
    Posts: 97
    Likes: 19, Points: 8, Legacy Rep: 29
    Location: australia

    sailhand Junior Member

    Ecojet you may want to check out the jarcat motor cruiser hulls they do around the 18 knot mark with a single 20hp outboard. They are lightly built ply hulls and the motor cruiser version was supposedly just the jarcat 6 hull stations separated a bit more to give 7 metres of hull. There are a few that have been way overloaded with massive cabins etc and the owners seem happy. The aesthetics of the topsides are definitely not to my taste and are somewhat dated but making higher hulls with the cabin sides staying the same and plumbing the bows would improve it greatly in my opinion. My 7.6 cat "should" be building in gosford starting soon it is 25 mm foam without too many stringers frames etc. I have done this to simplify the build and fairing and improve live ability and maintenance, easier to keep clean repaint etc no little nooks and crannies just a nice clean smooth interior. I don't think it increases the expense at all really if you consider build time for level of finish etc, I also think you end up with a higher value/better resale boat. The cost of foam is significantly lower nowadays and you save on resin as poly and vinylester are a lot cheaper than epoxy. I also have no choice as i am allergic to epoxy after playing with it for too many years. The entire boat will be cnc cut in bare foam and the internal edges rebated for taping. I have a helicopter for fairing so I can do most of my fairing on the table just the tape lines to fair in situ. As for the beam on your boat i think it is far better to have a 3.4 metre beam on a boat this size. It is much safer and roomier. In australia you can transport up to 3.5 metres without an escort and most crane trucks have standing approvals to transport loads to 3.5 with flags and signs etc. It is far far cheaper to put your boat in and out with a truck than it is to either build/maintain a large trailer or pay for a mooring maintain it and also pay for the extra maintenance/damage/theft incurred leaving your boat unattended on a mooring. To transport your boat a couple of miles to the water is less than a hundred dollars and you can bring it back home and put it back in the shed/backyard when your finished. You don't want to do this every day however I plan on doing trips of several weeks duration and then pulling the boat back out a couple of times a year so the economics of this method are far better than anything else and I don't have to worry about the boat left unattended so much. If you add up the build cost alone of the trailer and the maintenance rego etc you will be surprised at the expense. Then a few years down the track you have to build another one, been there done that twice never again it's too expensive.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.