Displacement powercat single 40hp

Discussion in 'Boat Design' started by Saqa, Oct 8, 2014.

  1. Saqa
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    Saqa Senior Member

    Guys
    As you might be well aware, I only have a fledgeling grasp of concepts for such boats. Will realy appreciate some discussion and pointers towards learning more about this for my project aspirations

    Project goal is a displacement powercat about 20-30' long. Sealed hulls, full deck with walk around fishing access, centre cabin/cuddy to sleep 2.5 in off weather. BBQ/wok burner combo gas cooking. Shower and toilet

    At max there will be 5 adults onboard on a day fishing trip or 2.5 adults cruising the inshore islands

    Build will be either glass on ply or alloy or a combination of both

    This is the king of small outboards, 40XWTL Yamaha. There is also an Enduro version. Power start, tilt and trim, remote controls. Can run all motor operations from the helm

    I am keen on running a single 40XWTL as the project motor. Firstly, is this a reasonable power? If enough attention is given to the design, what kind of speeds can be had? Is 25 knots achievable in a cat with this motor?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Richard Woods Skoota boats should give you an idea of what is feasible. Running a single engine on a cat is problematical, and even if you solved that problem, your speed target is very ambitious with a 40hp motor.
     
  3. Nick_Sinev
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    Nick_Sinev Junior Member

  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I'd say 15 knots cruise could be feasible for the 40hp displacement cat, but engine placement is a headache.
     
  5. Saqa
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    Saqa Senior Member

    That is what I came up with when doing internet research but in another thread here it was apparent that powercats like the Skoota 20 that Mr E mentions are claiming 15 knots with a 20' boat and 25hp single motor

    What I am trying to get my head around is how the additional 10' of length and additional 50hp, long narrow and slippery hulls and a lot less cockpit superstructure help? Does it help?

    One of my goals is to keep the overall weight down to a minimum by using appropriate materials and build techniques
     
  6. Saqa
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    Saqa Senior Member

    Yes mate, inspired by the Skootas

    25 knots is the speed to aspire to, I dont know if it can be had but a good attempt at getting it and 15 knot or more result is good. I am willing to try
     
  7. Nick_Sinev
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    Nick_Sinev Junior Member

    I think it is not a displacement mode.
     
  8. Saqa
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    Saqa Senior Member

    You are correct, listed as semi displacement. Makes me wonder about the potential of this mode in a longer hull and the 15hp extra
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Slender cat hulls don't generate the transverse wave amplitude that creates the resistance hump commonly encountered in monohulls at "hull speed", but running at speeds around 25 knots or more the efficiency falls away rapidly for small non-planing cats. Plus the hobby-horsing will be an ordeal !
     
  10. Nick_Sinev
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    Nick_Sinev Junior Member

    Yes, you are right.

    Right now I'm trying to find graphs made by Albert Nazarov "hull resistance vs speed for different catamaran hull shapes".

    As far as I remember.
    1) The difference in resistance was really small. So, you are correct.
    2) On the other hand, this difference exists and can be clearly seen on graphs.
    So, it is reasonable to design a displacement hull with a canoe stern and a semi-displacement cat with transom stern. Despite the fact the hulls are really narrow. So, you are not absolutely correct.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    The canoe stern has probably got more potential for excessive pitching than a transom stern. The canoe stern works better at slow speeds than a submerged transom, though.
     
  12. Nick_Sinev
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    Nick_Sinev Junior Member

    Graph by Albert Nazarov

    [​IMG]


    Sorry, I still can not find the reference to his article.
    Albert is a member of this forum, so, in case of emergency, it is possible to ask Albert directly.
     
  13. Saqa
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    Saqa Senior Member

    A pod mounted single motor seems to be the go if wanting to run a single motor on a cat. Does anyone know if this pod were to be designed like a planing pad, would it help top end by lifting the boat and reducing draft overall but using power trim to keep the boat flat and all of its overall length in the water?

    I know semi planing appears to be built into the main hulls on existing designs. My thots with above is with the intention of hopefully making a smarter use of the need to run a pod for single motor

    Due to lack of knowledge I could be wrong and a planing pad pod could end up negate any advantage of lifting by introducing its own drag
     
  14. groper
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    groper Senior Member

    You can build a cat like you have described. Many sailing catamarans are built in this fashion, using a central pod to mount a single outboard engine. Like everything in boat design, there is a raft of compromises you will have to decide between. Some decisions will affect speed, others will affect practical considerations such as manuverabilty in strong winds and tides, and even more subtle issue such as wave slap on the pod which can get extremely annoying depending on the severity. Other issues include propeller efficiency and cavitation. There is a careful balancing required so as to acheive a result which is desirable for your purpose and usage and that you will enjoy living with day to day.

    Personally, I wouldn't do it- a single motor that is. The price difference between 2x 20hp and 1x 40hp seems negligible, so I'd rather not make so many compromises. Having owned and lived on a 43ft cat with a single outboard, I can tell you that maneuvering it in a tight anchorage in windy conditions is extremely difficult. Getting in and out of marinas is highly stressful. A long cat doesn't spin on its transom like a monohull does, the turning circle is huge and you don't get the prop blast over the rudders to help. Turing the engine from a more forward location, doesn't give as much steering authority. If you must build a pod and go down this route, make it the deepest v possible, not flatter. There is a thread about it on here where RW recommends the same in order to avoid the wave slamming.

    25kts is extremely unlikely unless you envisage a boat with nothing to it, like a beach cat with no rig type of thing. Richard woods skoota does max speeds in the high teens with 2x 20hp which will most likely give better thrust from 2 props slung under the transoms instead of 1 highly ventilated prop up near the surface. I would expect cruise speeds in the low teens, and you'd do well to get it upto 15kts at full throttle. This is all highly dependant on how light you can built it. Displacement to length ratio is the most important design ratio when looking at a hulls resistance.

    I showed you the camera boat of the last americas cup in your other thread. How about something like that with twin 15-20hp ? You would get best chance of speed and a small amount of space for basic accommodation, quick and easy to build...
     

  15. Saqa
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    Saqa Senior Member

    Groper, thanks for all the info. I really do favor two smaller outboards for all the advantages they offer over a single motor

    The single 40 is a convenience compromise choice. It is the smallest that I am aware off that has all motor functions available at helm. If there was a smaller 2 stroke that had electric start, power trim/tilt and stock forward controls then they would be my first choice. Do you know of any smaller engines that I should look at?
     
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