New Hybrid: Small diesel/ Big OB in 33' S/D Hull.

Discussion in 'Boat Design' started by rustybarge, Oct 28, 2013.

  1. rustybarge
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    rustybarge Cheetah 25' Powercat.

    At this stage in the discussion I feel that the concept has some merit. I would also like to draw your attention to hybrid cars with two different engines, electric and petrol, that have now become common place. In my opinion electric drive has no place on a boat because if the high hp requirments, and the terror of 400 volts DC with all that water sloshing around on the decks and in the bilges.

    Summary:
    A seaworthy 33' S/d hull, Nelson design ' gold standard' type hull form as used by lifeboats, customs, pilot boats, patrol boats....( 3 different hull manufactures available).
    8-10 tons displacement.
    About 12' beam for fuel economy.

    Target fuel consumtion: 4.5ltrs/ hr (1 uk gal) to give 7mpg at 7 kts.

    Weight saving of about 1/2 ton by using 75hp diesel as main engine.
    massive range with an extra 1/2 ton of fuel, and cheap fuel bunkering in low cost ports.

    Lightweight big engine for 15kts cruise, petrol outboards or inboard ?
    Steering with rudders or outdrives?
    we also have to examine drive sytems.


    These are interesting outdrives that tilt completely out of the water like an outboard, but limited to 60hp. When in the down position the gearbox stays clear of the water.
    http://www.sillette.co.uk/documents/sonic_000.pdf

    Enfield do a larger version that will take 75hp.........http://www.bobknowles.demon.co.uk/prod01.htm

    Less trouble than a Merc. outdrive, no bellows to replace?

    these bolt on surface drives at £3000 look fantastic, can't get simpler than this:
    surface drives page 16 :http://www.lancingmarine.com/databook/files/index.html
    [​IMG]
    Any suggestions?
     
  2. eyschulman
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    eyschulman Senior Member

    Here is an example of a boat designed to get the type of fuel economy you target. Note one single diesel 220 hp motor does it all in the proper boat design. This is not a pie in the sky design it is successfully built and marketed in Northwest USA by a knowledgeable engineer and builder and has won awards for its design and performance. Once again for effect it's not the motor it's the boat. This boat will run all day at 7K and perk up to 20K and management of the motor doing this is also not pie in the sky it is well known and proofed. I believe your idea of a little diesel and big gas OB is the wrong way to go. Better is an efficient boat with an efficient motor that can handle both speed ranges and the Aspen 32 is just one example of that kind of design and engineering. Your old school ideas about ruining a diesel by running at 7 K is out of step with what is becoming common practice with big modern diesels. I am sorry that I have to be negative to an idea that obviously excites you.
     

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  3. rustybarge
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    rustybarge Cheetah 25' Powercat.

    Now that is impressive!

    In my design brief I had excluded Cats because in Europe marina's charge 50-100% extra for a multihull!!! (can you believe it, totally ridiculous)

    Just guessing that if a 33' lightweight 4 ton Cat burns 1/gal at 6kts, an 8 ton mono will burn double that at 7-8kts?

    The standard Nelson 33' has about 300 gal of fuel.
    The hybrid will have another 1/2ton available= 130 gal.....giving a total range at 7 kts of 450 mn.
     
  4. groper
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    groper Senior Member

    I was reading the first page of this thread and from what you were trying to acheive i immediately thought - this guy needs a displacement catamaran. The high length to beam ratio hulls dont suffer a significant hump speed barrier and so requires very modest power right upto 15kts easily. They have a snappy roll motion in beam seas, but they dont roll as much in anglar fashion.

    My cat, when its finished, should burn about 4gal/hr @ 15kts - its 4 ton displacement @ 10.6m LWL.

    If you want efficiency, its dead simple, - get the weight out of it.
     
  5. eyschulman
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    eyschulman Senior Member

    Groper is on the right track. If you want efficiency look to the craft not the motors. Modern motors and drive trains are getting what they can out of a given gallon of fuel. Unlikely to gain any big moves to efficiency there one or two motors not so important since it takes a certain amount of power to move the boat at any given speed. The biggest pay off is in hull design and building methods particularly those that lower weight and resistance in the water and above. If you want economy at low and higher speeds you will have to go light either multi- hull-long waterline-or get the boat out of the water with lifting fins. A standard relatively heavy recreational motor boat is unlikely to fill your fuel needs no mater what combination of motors.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Unfortunately for cats, the need for twin motors running simultaneously cuts into their efficiency somewhat, could be as high as 30 % in some cases. Getting them to operate satisfactorily with a centrally mounted single engine is a task.
     
  7. eyschulman
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    eyschulman Senior Member

    Please note the Aspin 32 has one motor and it operates just fine that's why it wins awards. Also two motors burning enough fuel to get the needed hp are not that far behind one. The loss is mainly due to friction of the drive and the extra weight and gear in the water. The reason for the single motor in the aspin is probably related to coast weight and the asymmetric hulls. only one hull is big enough to hold the motor.
     
  8. rustybarge
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    rustybarge Cheetah 25' Powercat.

    The Aspin Cat is really a modern take on the Polynesian outrigger canoe, one large hull and one smaller secondary hull to add stability, but not too much drag.

    Of course a Tri would extend the concept one stage further.
     
  9. rustybarge
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    rustybarge Cheetah 25' Powercat.

    What type of Cat are you building?
     
  10. rustybarge
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    rustybarge Cheetah 25' Powercat.

    My Cheetah 25' Cat is a narrow beam design (8') , so it tracks straight on just one outboard. Although the hull was designed for twin 60's, lots of fishermen use a single 90hp to drive the hull, saving over 100kg in weight on the transom.

    Not sure how one motor on a wide beam 10mtrs by 5mtrs beam would work mounted on one of the hulls, would it crab along sideways? Apparently an OB in the central position between the hulls picks up turbulence from the hulls, decreasing performance at 15kts and above.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Even a full planing hull will be more efficient mpg-wise pottering along slowly off-plane, than at any planing speed. You have to decide what your operating cruise speed is, and fit an engine to suit. Buggering around with one engine for slow, and another for fast is just so much bother for very little actual advantage.
     
  12. eyschulman
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    eyschulman Senior Member

    What about a tri with folding swing in alongside outriggers as on the Dragonfly sail tris but in a motor mode with a single motor. The tri motor mode is not new and has been well developed including the Yanmar OB diesel tri that did a circum-nav. This hull form would allow for light weight slow and high speed economy. Accommodations are sacrificed to some degree in all high performance multis they must be very light to get the most bang performance wise. They really do preform in the mid 1980s I owned a dragonfly type sailing tri capable of over 20K under sail and a 10 hp ob close to 10K. I would think 20-30 hp would match the sailing speeds.
     
  13. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    This idea really appeals to me.
    A slim mono might be easy to push through the water, but the norrower the beam the more susceptible to rolling. I once rented a narrow boat on the canal, it was about 50' long and 8' beam, and suprisingly enough the accommodation was fantastic. all the seating was fitted along the sides parallel with the hull, and sleeping cabins were formed by closing doors across the full width, and converting the couches into beds.

    So in a Tri a long slim hull would work very well.

    On the basis that you can't have too much power, a high thrust Yamaha 40hp would be more than adequate, if you think 20-30hp would be able to push the hull.

    What are the advantages, and disadvantages of Tri's?
    Do you have any links to interesting Tri's about 40' or smaller?
     
  14. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    I think the biggest hitch is going to be the drives.
    The easiest option would be twin Merc outdrives, as you would get perfect balance, the 75 hp diesel weighs nearly the same as a petrol inboard, but a maintenence nightmare.

    Saildrive 75hp diesel with folding prop needs a massive hole cut in the hull, but could work well with a large outboard hung on the transom.

    A big petrol inboard shaftdrive as the main engine with the 75hp diesel driving the same shaft would work, but the small engine will be running constantly so I would worry about lubrication and side loading on the main engine gearbox output shaft.

    I think you may have a point about the complexity of the mechanical setup.....
     

  15. eyschulman
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    eyschulman Senior Member

    See if you can get the latest Kurt Hughs design book. While most are for sailing some are motor. This will give you an idea of interior room and construction. The swing folding arrangement for outer hulls is accessible on the Danish Dragonfly boats. A motor tri would not need large volume outer hulls several large motor tris with fixed outer hull have been built. Fixed is not so dock friendly. Hughes is in Seattle Wa USA look up on net.
     
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