outboard power on larger cruising cats

Discussion in 'Multihulls' started by Steve W, Aug 14, 2016.

  1. UpOnStands
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    UpOnStands Senior Member

    got to respect the effort expended to get the frame constructed.
    seems as if your O/B is just inside the end of the bridge deck?
     
  2. raf pali
    Joined: Mar 2012
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    raf pali Junior Member

    Ya right, at the back of the O/B is the aft beam.
     
  3. UpOnStands
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    UpOnStands Senior Member

    at the moment. how much slack do you give the sled?
    under flat water conditions the transom should be, what, 50cm above the water surface but you let the sled surf so assume you give it some slack to fall into the troughs.
    and the sled has positive buoyancy or just dynamic skidding effect?
     
  4. Spiv
    Joined: Jun 2007
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    Spiv Ancient Mariner

    Phil,
    I am not sure what you refer as "braket".
    The nacelle bottoms were antifouled, but they actually were just out of the water when at rest.
    The prop was above the keel so we could beach the boat and reverse out, also no fear of hitting a reef with the prop....
    No part of the motor was in the water when at rest, underway they'd get wet because of waves action, I did consider closing the aft section with two sheets of rubber so that the motor leg could slide up and down between them, but never got to it.


    Prior to decide on nacelles, I talked to an owner with vertical sliders and consulted with my builder: the unanimous consense was that they are:
    • complicated to build strong enough
    • motors tend to get stuck
    • rattling
    • opening in the bottom of your boat is a real concern with water entry, drag etc.
    Also, if you place the motors aft, you will for ever cavitate in a headwind, just when you need your motors the most.
    And, lets not forget that if you motor in shallow waters, there is a good chance you will suck up dirty water and perhaps even damage your props.
     
  5. waikikin
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    waikikin Senior Member

    I also considered putting some rubber lips on the nacelle for my beach marine cat but never got to it.
    Also some survey authorities didn't like the outboards in the closed wells for fuel leak/vapour issues though I have seen one in survey that had the wells ventilated with slots that also made the form of a ladder in the side. That may have changed, not sure what they require now.

    Jeff
     
  6. Steve W
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    Steve W Senior Member

    Stefano, when you say the motors are mounted under the main b/head are you referring to the mast b/head? that's a long way forward but a lot of Malcom Tennants net and tube cats have their single motor in a nacelle up there. Another question I have for those running twins is do you typically run on both or just one motor when cruising under power? I know a lot of diesel powered cats typically run on one unless they need extra thrust or of course when maneuvering. May I ask why you switched from the Yamahas to the Johnsons?

    Steve.
     
  7. brian eiland
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    brian eiland Senior Member

    Got any photos/dwgs of those installations?
     
  8. Steve W
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    Steve W Senior Member

    UpOnStands, my only experience is with the Yamaha high thrust and it can be carried for transport either with the tiller handle down or with the tiller folded and the front side down which would put it at 180 degrees so I see no reason why it could not be tilted up that far on a bracket. I would think that all the 4 strokes would be the same but I would check with the dealer to be sure.

    Steve.
     
  9. brian eiland
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    brian eiland Senior Member

    Central pod/nacelle idea

    If I were looking to use my auxiliary engine in a strictly aux manner, rather than in a motor/sailing demand, I would seriously consider a single engine installation. This engine would be conveniently mounted in a boxed enclosure on the cockpit deck and would belt drive a steerable out-drive leg that would be incorporated into the rear portion of the central nacelle structure. (note: I am suggesting a central nacelle structure for several other reasons also)

    Maybe this rear nacelle might appear as on "Earthling's pod" (attached photo). This saves the cost and weight of the second engine, trans, shafting, prop, etc, and opens up the rears of the hulls for a nice master bath, or whatever.
    [​IMG]

    I say steerable as a result of my personal experiences with an installation I had on a Louisiane 37 cat. I had lines run from the tiller arms to the single outboard engine that would be engaged only during low speed maneuvering under power. that steerable engine combined with the rudder action provided extremely good control. I could almost 'parallel park' that vessel.:!:
     
  10. Steve W
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    Steve W Senior Member

    raf pali, I'm a bit surprised at all the issues you had as I have a Yamaha high thrust on an old Gemini cat which is mounted on what they call the bucket which is a fiberglass mount which pivots up and down with tackle. It works very well actually although it does present a couple of the issues you mention which are it can slam in waves but this could be alleviated by gluing some foam to the flat surface, shaping it to a vee shape and glassing it. The other issue is the sides buzz at certain rpm, again easily fixed. Other problems are that it there is no hold down for reverse and the tackle lines tend to get knotted up. All fairly minor to fix. But I was surprised that we get no cavitation despite everything I had been told and I have motored it in everything from flat calm to 25 -30 knot headwinds while motoring for 32 days straight while delivering it from Connecticut to western lake Superior in 2013. The only slamming we had was because I had screwed a couple of blocks of wood to stop the bucket from bouncing up when using reverse and I forgot to remove them when we got out in open water, once I realized this and removed them the bucket floated up and down and stopped pounding in smaller waves but still does in larger waves.

    Steve.
     
  11. rustybarge
    Joined: Oct 2013
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    rustybarge Cheetah 25' Powercat.

    This company sells liftable outdrives for cats that bolt onto the transom like an outboard or could be bolted on centrally; engine could be diesel or gas.....


    http://www.sillette.co.uk/index.html
     
  12. UpOnStands
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    UpOnStands Senior Member

    not a sled as such

    this is a rough idea for 25 HP single on centerline midships
     

    Attached Files:

  13. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

    Prout and Gemini used these quite often in the early days. If I remember correctly the only major complaint was the lack of a really positive lock down when going into reverse,...and with that big lever arm it could pop up drastically.

    The Gemini bracket, and the one I had on the Louisiane 37 were to 'brief' in their length (thus small swing radius), and this resulted in the outboards assuming a somewhat radical rake at different immersion heights.

    I ran some test on that Louisiane with the 9,9 Yamaha and discovered what lots of outboard race boat guys know:
    1) You can significantly cut down on the drag presented by the lower unit by fine tuning the immersion depth of the lower unit/prop. (that's why those racers all use 'jacking plates')
    2) 'Cupping' the props can be an effective way to gain another bit of pitch in the prop. I was able to gain another almost half a knot of speed.
     
  14. raf pali
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    raf pali Junior Member

    Steve, all the issues I mentioned are related to one another, slamming causes them all accept cavitation.
    The original centre pod on my cat was badly designed too, long, fat and low to the water line.
    Also is to be considered the environment in which the boat is sailing: long ocean swell causes little to no problem even in brisk winds but, as in my case within the Great Barrier Reef of East Queensland in Australia, tidal streams regularly run fast and the chop can and does develop into a steep, short and crossed mess where the boat gets pitched severally causing cavitation all too often. This is especially so while crossing sand bars at the mouth of rivers and within narrow passages between island and headlands.
    Cheers
    Raf
     

  15. raf pali
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    raf pali Junior Member

    The swinging frame, when under power, is hanged on bungy cords which keep the sled on the water surface..
    The sled has little buoyancy, is just a sheet of foam 25mm thick fibreglassed both sides and is free to move down as far as needed and dynamically skids up/down on waves The max down swing can be regulated by adjusting the Up/Down cord length.
     
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