Displacement catamarans and single hull power.

Discussion in 'Boat Design' started by Typhoon, Apr 5, 2010.

  1. Typhoon
    Joined: Mar 2009
    Posts: 125
    Likes: 8, Points: 0, Legacy Rep: 150
    Location: Australia

    Typhoon Senior Member

    The other thread http://www.boatdesign.net/forums/bo...powercat-run-one-engine-32180.html#post355760
    on a single engine in one hull got me thinking.
    How much yaw due to drag would there be on one catamaran hull, say a 30-35ft cat, with medium displacement (say hulls 1m wide at waterline, 600mm deep) and how much rudder would be required to overcome the drag at say a cruising speed of 10 knots, with a sprint speed of 15 knots?
    I've had experience in a heavy displacement sailing cat that would do 8-9 knots flat out, but was wondering if it would be feasable at higher speeds?
    Was thinking otu loud that a single bow thruster would work for manouvering, but what about a low speed only propulsion device in the other hull? Why not main engine and shaft etc in one hull, and mount a small thruster mounted fore and aft in the rear of the other hull, purely for manouvreability/low speeds? You could probably get fancy and make it's aperture very low drag in some way, for high speed work. Could even run a small electric motor and folding prop, again, sized purely for low speed manouvering.
    The advantage would be huge cost savings due to a single engine installation/lower running costs and much more useable interior space in the hulls(always at a premium in smaller cats). The thruster may cancel out the low cost/low drag, but single engine cats do work, my question is, at how high a speed can they be made to work before drag/negatives outweigh advantages?

    Thoughts?

    Regards, Andrew.
     
  2. masalai
    Joined: Oct 2007
    Posts: 6,823
    Likes: 121, Points: 0, Legacy Rep: 1882
    Location: cruising, Australia

    masalai masalai

    Hi Typhoon,

    Have a look at the oram designs - I know they sail (motor) straight in one engine, the Goss fastbacks mostly had one engine, and I think a few Snell Easy's were fitted with a single engine...

    Bob Oram suggested, at one stage that I fit a single saildrive when electric engines seemed not the way to go yet... Several sailing cats operate quite happily on a single engine in a pod midships - I understand the trick is NOT to steer the engine but by sail rudders only...

    Mine is a tad under 40ft and with a 21ft beam, the Fusion Cats, from up Airlie Beach way, both power and sail are 42ft by 24ft so just ease in to the lock marina in Darwin - which has a 25ft wide opening...

    In Australia the beam is determined by marina berth space and price/availability.... Engineering is no special problem... look at the ferries serving Bass Straight and the inside/semi-sheltered waters of the Great Barrier Reef...
     
  3. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 110, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    The engine in a pod with a fold up leg might solve manuvering , space and noise problems?

    Why give up good hull space for a chunk of iron?

    FF
     
  4. masalai
    Joined: Oct 2007
    Posts: 6,823
    Likes: 121, Points: 0, Legacy Rep: 1882
    Location: cruising, Australia

    masalai masalai

    I came close to a pair of outboards in nacelles added to the inside of the hulls but no high thrust options available in 4 stroke long shaft so reverted to saildrives (NOT steered).... I feel it is a personal matter of preference and what fits ones need and desires in the build... 6 of one, half a dozen or 0110 in 4bit binary...
     
  5. FAST FRED
    Joined: Oct 2002
    Posts: 4,519
    Likes: 110, Points: 63, Legacy Rep: 1009
    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Remember most cats will require more power /fuel at lower speeds than a lead sled.

    Going slow the propulsion cost is surface area , not wave making.

    FF
     
  6. masalai
    Joined: Oct 2007
    Posts: 6,823
    Likes: 121, Points: 0, Legacy Rep: 1882
    Location: cruising, Australia

    masalai masalai

    Typhoon, There are HEAPS of power and sail cats that would meet your purpose - second hand... Have a look around http://www.yachthub.com.au/ - click on <boats for sale / used boats> there are 62 0n the 31ft - 35ft category of sailing cats , then in the 36ft to 40ft range a further 86 sail-cats then further down in the multihulls / power boat section a further 286 boats in the over 30ft category, and you may find others in the charter and commercial vessels listings...

    You WILL NOT cruise at that speed with barge hulls like you suggest... I have about HALF the HULL beam you suggest, and may achieve those speeds with twin 21hp nanni saildrives, (at a list price of around Au$32,000 the pair), if I am lucky and I weigh in at 4 to 5 tonnes, which is a bit more than a maritimo racing boat with twin 500hp and surface drives which does more than 100mph (both boats are built using similar materials)... I may burn about 1.3L per hour, Maritime does that in a few seconds
     
  7. u4ea32
    Joined: Nov 2005
    Posts: 416
    Likes: 14, Points: 0, Legacy Rep: 192
    Location: Los Angeles

    u4ea32 Senior Member

    I tried a 27 foot x 10 foot beam power cat with twin I/O power. It cruised easily on twin engines at anything from 15 to 30 knots. I then stopped, turned off one engine, and tilted that drive up. The boat could only go 6 or 7 knots on the single engine.

    Not much different from my boat that can go 70 on both engines, but only 8 knots on one.
     
  8. Typhoon
    Joined: Mar 2009
    Posts: 125
    Likes: 8, Points: 0, Legacy Rep: 150
    Location: Australia

    Typhoon Senior Member

    I'm not looking for a boat (well, not just now, but in the future I am considering this path). I was musing about single engine power and perhaps thoughts on optimising it/how much yaw you could expect at higher displacement speeds and if that drag would have a seriously negative effect at those higher speeds.
    I think a well refined displacement power cat with fairly minimal power would make an excellent, efficient cruiser/semi live aboard vessel. If it could be done with a single engine at the speeds I stated, it would be super economical to boot, not just in running, but initial construction.
    Central engines and pods have serious issues in rough water, and if you are single engine power as your only propulsion, you don't want that......
    I also posted on the other thread I linked, my first hand experience with a Eureka 32 with a 17hp Volvo diesel and shaft drive in one hull, that would do 8 odd knots flat out with that power only, with hulls of approximately the dimensions I gave. And that was also dragging a mast through the air.
    It was a very heavily built and outfitted boat too.
    By the way, that boat would happily run up to 12-15kts down a small wave, so hull speed was not an issue, engine power was.
    You are right that there are plenty of cruising cats in this size range, but they're all sail and you pay a hefty premium for all that junk above the deck, especially when you don't want it! :)
    Plus, they've all got heavily compromised engine installations (if you want maximum power performance).

    Regards, Andrew.
     
  9. masalai
    Joined: Oct 2007
    Posts: 6,823
    Likes: 121, Points: 0, Legacy Rep: 1882
    Location: cruising, Australia

    masalai masalai

    Typhoon,
    Not really, in my view, as most power-cats are designed for high speed and feel decidedly different at low (displacement) speeds - Your thinking may work for a sail cat, with a slightly modified bottom to reduce "stern squat" and provide protection for the prop which can then be installed on the end of a horizontal shaft...

    The C10 in my gallery had that capability and maxed out at 16knots by gps in sheltered waters to cruise nicely using twin 50 hp Kobota marine engines was very economical at 8 to 10 knots... Twins gave very good manoeuvrability by rudder of engine control... Designed by Robin Chamberlin... At present could be on the market? based in Hervey Bay... Her hulls are about 700mm w/l beam... At the time I was looking for a 40ft loa boat and after a bit more research opted for the Oram design which comes with a modified hull below the w/l for use as a power-cat... Mine is the "half and half", as a motor-sailor with twin 21hp Kubota/Nanni saildrives and 70 sq metres of sail.... Build details are here http://boats-n-stuff.com.au/forum/index.php/topic,29.0.html and http://boats-n-stuff.com.au/forum/index.php/topic,2.0.html - - - You may need to register to see the images...
     
  10. playmaker
    Joined: Jan 2010
    Posts: 6
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Milano

    playmaker Junior Member

    Hi,
    I am working about catamaran and its hydrodinamic characteristics, so have some problems to determine its main dimensions. How to decide main dimensions hydrodynamic aspect...?
     
  11. playmaker
    Joined: Jan 2010
    Posts: 6
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Milano

    playmaker Junior Member

    I aim at least 25 knot and 15 m length...
    grazie..
     

  12. masalai
    Joined: Oct 2007
    Posts: 6,823
    Likes: 121, Points: 0, Legacy Rep: 1882
    Location: cruising, Australia

    masalai masalai

    Playmaker, - - - WHY? - - - where are you cruising, - - - what do you intend to do, - - - How will you pay for the fuel?

    Near 100 gallons per hour AT LEAST, for a stripped out short passage "speed boat" of that size 15m = 50ft as a power-cat? (20 tonnes as a "gin-palace") and I could be vastly underestimating things there as I cannot be bothered to do the calculations on such thoughtlessness...

    If you have the money then go see a good Naval Architect and put down some 5million Euro for a good one... (What are you doing here?) - If you do not have the money - time to do a quick re-think...
     
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.