Ganging Outboards - Does it work?

Discussion in 'Propulsion' started by CatBuilder, Jan 14, 2012.

  1. CatBuilder

    CatBuilder Previous Member

    Richard, at $6000 a piece for those legs, they have become unaffordable. I would need a pair of them because I need two props for moving a boat this size around in tight marinas in New England. Sillette legs and such are out due to exorbitant costs.

    DCockey: Yes! We have arrived at the exact same conclusion. I agree 100% with your last couple posts and I was going to go with the electric motors, until I started looking at weight and finding a way to retract the props.

    Do you (or does anyone) have any suggestions on retracting the props using a pair of electric motors? I'm open to any configuration that works. In fact, let's give this a formal statement of requirements:

    1) Large, massive generator will be installed to run heating and air conditioning loads via reverse cycle heat pump. Generator will be standard inboard diesel type.

    2) Must have two props, each capable of forward and reverse

    3) Weight must not be excessive

    4) Props must come clear of the water when sailing

    A "nice to have" would be having the generator run the drives somehow.

    Anyone, please feel free to drop ideas in on how to meet these requirements. Please, nobody make fun of any outlandish ideas. I think an open discussion would be best. Discuss the merits, but don't give anyone a hard time for an unconventional response. I'd hope to see some brainstorming here.

    I have been unable to figure this stuff out. Looking at outboards for simplicity, but I don't want to build boxes under my bridgedeck. Looked at mounting them in the stern too, but wish for a more elegant solution.

    Anyone?
     
  2. Mr Efficiency
    Joined: Oct 2010
    Posts: 9,059
    Likes: 639, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Might be some merit in outboards with jacking plates giving more scope to where they sit both when running, and when clear of the water.
     
  3. WestVanHan
    Joined: Aug 2009
    Posts: 1,374
    Likes: 56, Points: 0, Legacy Rep: 746
    Location: Vancouver

    WestVanHan Not a Senior Member

    About 10 years ago when I was in my mid 20's I got it into my head to have a large sail cat..80' or so..and live the vagabond life in being a sea bound trader-taking stuff to far off Pacific islands rarely serviced etc. I drew it all up,room for provisions etc and I came up with a drive.

    For each hull, a shaft would exit as high as possible off the water,on the inside of the hull-near the stern of course.
    I designed an aluminum chain case,at the top would be a bevel gear to change direction from the shaft.
    Using hyvo for the drive from the case down to the prop..no bevel needed at the bottom.
    As the prop would push,the top case and mount would have an interior support and stop,and would be lifted via a hydraulic ram pushing on alever inside the boat.

    In reversing,the ram would stop the leg from pulling up.
    Could be retarcted up into channels under the bridge deck and be almost invisible.

    All heavy bearings,o-rings and seals of course....could be serviced via dinghy or beach.


    Decided that control pitch props would be a simpler way and be feathered so as to give almost no drag.

    I guess if you mounted AC drive motors on the outside,you could do away with some of the stuff
     
  4. whitepointer23

    whitepointer23 Previous Member

    i knew of 4 of those yanmar outboards pushing weed harvesters in wa and they had no end of trouble with them. the local boat place had a maintenance contract and he hired a diesel mechanic who stayed on the boats while they were working just to keep them going.
     
  5. Mr Efficiency
    Joined: Oct 2010
    Posts: 9,059
    Likes: 639, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Maybe goes towards explaining their extinction.
     
  6. CatBuilder

    CatBuilder Previous Member

    I'm reading the replies and considering quietly for the time being. The diesel outboards are out. I can't buy something that has no parts available (in the States).

    As to jacking plates, could you please elaborate on where to locate the jacking plates so that the outboards don't get wet when doing 20 knots under sail, but will still work well when motoring into steep, 9 meter seas?

    Also, as to the electric drives outside, those need to be kept 100% dry at 20 knots too. How do I mount them?

    The main problem here isn't so much what to choose - I can choose electric motors (both inboard and outboard), regular outboards, or regular inboards (though they are too heavy with the generator).

    It's really a question of, "How do I mount any of those 4 choices, given that the boat will do 20 knots under sail and have tons of spray shooting down the bridgedeck tunnel, also keeping in mind they need to retract out of the water?"

    For a look at what happens under a bridgedeck of a cat in motion, please see the following YouTube

    http://www.youtube.com/watch?v=Lp0Z1SzPJw4


    At 0:31, notice what it is like to motor (or sail) into a steep chop. Look at the aft part of the boat, right under the bridgedeck.

    At 1:53 see 20+ knots of spray down the bridgedeck tunnel. Imagine outboards or an electric motor head in there and remember this is salt water.

    These things need to be overcome somehow... but how?
     
  7. Mr Efficiency
    Joined: Oct 2010
    Posts: 9,059
    Likes: 639, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Motoring into 9 metre seas ? Depends on the wavelength. :p
     
  8. CatBuilder

    CatBuilder Previous Member

    He he he... yes, I meant 3 meter sea, but was thinking about the conversion (to 9ft) while typing. Picture a steep, 3 meter sea.
     
  9. hoytedow
    Joined: Sep 2009
    Posts: 5,669
    Likes: 285, Points: 93, Legacy Rep: 2489
    Location: North of Cuba

    hoytedow Wood Butcher

    Measure twice, cut once! :p
     
  10. Mr Efficiency
    Joined: Oct 2010
    Posts: 9,059
    Likes: 639, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Clearly if the transom of your sponson is not an immersed one, which I assume is the case, heading into steep 3 metre seas it will clearly be well out of the water at times, making life difficult for any outboard hung on that transom, as the prop loses traction due to aeration. Hopeless. Dropping it lower will lead to excessive drag. I don't think there is a satisfactory solution with an outboard unless you are happy to tootle along slowly with an engine that has an extended leg to get down deep, and the extra drag won't be too much of a problem going slowly.
     
  11. WestVanHan
    Joined: Aug 2009
    Posts: 1,374
    Likes: 56, Points: 0, Legacy Rep: 746
    Location: Vancouver

    WestVanHan Not a Senior Member

    Gunboat!!

    And IIRC they use saildrives......
     
  12. waikikin
    Joined: Jan 2006
    Posts: 2,352
    Likes: 124, Points: 73, Legacy Rep: 871
    Location: Australia

    waikikin Senior Member

    Some great footage of the orange cat, very nice. On Beach Marine cats, outboards were usually located to a "nacelle", I had a 10 meter Beach Marine Performance Cruiser where the power head was housed inside the nacelle to a mounting "transom" , a slot allowed the outboard to pivot up as usual using a remote pull to unlock & a simple 2:1 purchase through a nylon dead eye(attached to cav plate) to assist the manual lift. This cat used a 9.9 4/ yamaha, I thought about fitting rubber "lips" to the slot but never bothered, heres a link to a beach marine cat discussed on the site but wwith out a slotted Nacelle.http://yachthub.com/list/yachts-for-sale/used/sail-catamarans/crowther-10/103316
     
    1 person likes this.
  13. Mr Efficiency
    Joined: Oct 2010
    Posts: 9,059
    Likes: 639, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    The idea has crossed my mind that a cat with a central pod to hide the outboard behind might go OK if it is near the point of pitch, i.e. up the tunnel aways from bow or stern. But you'd still require some immersion to have your prop in the water, and it wouldn't be steering the boat in there, you'd need rudders. Not applicable to fast planing boats though, they get airborne too much.
     
  14. Richard Woods
    Joined: Jun 2006
    Posts: 2,207
    Likes: 162, Points: 63, Legacy Rep: 1244
    Location: Back full time in the UK

    Richard Woods Woods Designs

    You are right, I wouldn't recommend the Sillette legs, I just posted the link for reference to others who are trying to help you out, in case they didn't know about them.

    Realistically, for what you want to do I think you have to use conventional props, even if they are hydraulically or electrically powered. And although I think electric power makes more sense in theory, (it's lighter, quieter, gives instant torque), in practise it seems people are still trying to get the bugs out of the design details.

    People have tried "bomb bay" doors, but if you do that you have to ensure the doors are watertight or you lose the buoyancy of the engine space. And probably lose interior space as well. (Been there done that)

    You're right that a more centrally mounted engine will not aerate when pitching, but usually it gets in the way of the accommodation (been there, done that)

    I sailed across the Atlantic to Panama about 5 years ago in a sistership to the one in Waikikin's link. The single 9.9 worked well in the central nacelle, but I have found that twin nacelles don't work so good (again, been there done that)

    I do hope you get a good workable solution as then we will all copy it, because right now I don't think there is a perfect solution for large daggerboarded catamarans

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     

  15. Mr Efficiency
    Joined: Oct 2010
    Posts: 9,059
    Likes: 639, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    I can see some merit in the idea of an outboard with a leg extension mounted on a jacking plate that will give running depth as well as the ability to withdraw well clear of the water, but you still have the extra appendage drag compared to normal transom mounting, though at more sedate speeds that wouldn't be prohibitive. It depends on the shape of the outboard leg, the ones with the insert piece would be the go, rather than the one-piece type with a 'blunt' leading edge above the cav plate.
     
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.