Remove saildrive - gain?

Discussion in 'Hydrodynamics and Aerodynamics' started by andysailor, Apr 5, 2019.

  1. andysailor
    Joined: May 2017
    Posts: 65
    Likes: 2, Points: 8
    Location: Sweden

    andysailor Junior Member

    Have been thinking about to remove my saildrive and engine from the boat (46 ft sailing trimaran) and replace with outboard. At the moment engine and saildrive weighs about 230kg. Engine sits aft of the saildrive and very much aft....boat is pretty narrow and I guess this is a lot of weight for the boat...especially so far aft.

    Considering putting an outboard motor (around 50kg)...mounted on the side of the hull...maybe about 8 ft from the stern.

    How would you think this would affect the boat and it's sailing? I'm trying to loose as much weight as possible and will probably loose another 150kg when going to lithium batteries....and some other weight savings as well..
  2. messabout
    Joined: Jan 2006
    Posts: 3,127
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    Location: Lakeland Fl USA

    messabout Senior Member

    Andysailor; Your post has been here for a while without reply. That is unusual because our members are either legitimately helpful or honestly critical of your project.

    If you are working with a 46 foot tri, the difference between the weight of the 230kg saildrive and an outboard of some 50kg is not likely to make a significant difference for a boat of that size. I am all in favor of saving weight if doing so is a practical matter. Jettisoning something on the order of 400 pounds..... 230kg -50 kg x 2.20 (forgive me for thinking in terms of the antiquated Imperial system)....= a bunch of weight but only a small percentage of the total weight of a boat of the size that you describe.

    Hanging the outboard over the side is unconventional but it might be alright for your tri......depends on whether you use it for routine voyaging or merely for docking convenience.

    Tell us more and perhaps your post might attract more attention.
  3. andysailor
    Joined: May 2017
    Posts: 65
    Likes: 2, Points: 8
    Location: Sweden

    andysailor Junior Member

    Thank you for the reply.

    To describe the boat a little bit more. It is a performance cruiser. The total weight is around 7000 lbs. The beam of the main hull where the engine around 3-4 ft at waterline level. And the cc engine to the transom is about 3ft. The beam at the point furthest aft of the main hull is about 2 ft.

    What I wanna say is...there is not a lot of buoyancy where the engine sits which may result in very heavy stern?

    So why am I considering to taking out the engine and replacing it with on ob?
    -to improve sailing capabilities in light wind
    -with ob I can take the prop and leg out of the water (better sailing...but how much better?)
    -No stinky engine inside the boat...more quiet inside
    -Give us some more space inside.....but not really any space I would like to fill up due to the weight..hahah
  4. KeithO
    Joined: Jul 2019
    Posts: 306
    Likes: 57, Points: 28
    Location: Michigan

    KeithO Senior Member

  5. rnlock
    Joined: Aug 2016
    Posts: 72
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    Location: Massachusetts

    rnlock Junior Member

    Seems to me that saving 5 or 6 percent of the weight of the boat could be worthwhile, particularly if it's overweight now. You might have to move something heavy aft to maintain the trim of the boat. Unless the batteries are forward, since they'll be lighter after the change.
    Unless you want to go tearing around at high speeds, an outboard which can swing a big propeller slowly will be more efficient than one that has a smaller, faster spinning prop.
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