Saildrive or shaft with electric motor

Discussion in 'Propulsion' started by xellz, Feb 10, 2018.

  1. xellz
    Joined: Jul 2017
    Posts: 74
    Likes: 3, Points: 8
    Location: Japan

    xellz Junior Member

    Currently planning to start building 30ft catamaran from plywood around July. Now i'm not sure i can handle to go with electric right away, but i do want to convert at some point. So to make things easier, i want to prepare as much as i can for conversion during build process.

    I thought that shaft is the only way to go and didn't seriously consider other options. But after reading about saildrives in more detail it might be a good choice after all. But i would like to confirm, since there are quite a bit of contradiction in sources i got over.

    Efficiency, this probably most important factor for me. In few sources in pros for saildrive was it's higher efficiency, horizontal thrust vs at angle in shaft. But by how much, i could find only rough guessing.

    Most of negative reputation saildrives got from early versions and lack of maintenance? Like not sufficient area on zinc anodes or not replacing in time, not checking coating etc. I.e. properly maintained saildrive isn't less reliable than shaft? Boat will be pulled out of water at least few times per year to protect from typhoons and maintenance.

    Less vibration and more easy installation. Is this true?

    My goal is to achieve most quiet and least noticeable propulsion which will be main selling point. And of course i would enjoy it myself quite a bit. It takes a while for my ears to return back to normal after full day out on usual fishing boat.
     
  2. gonzo
    Joined: Aug 2002
    Posts: 12,522
    Likes: 212, Points: 63, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    You must have a boat with an older, really loud engine. Modern outboards have low noise levels. Further, if you need to run a generator to charge the batteries, the noise will still exist. I agree that it can be intermittent though. Regardless of whether it is an inboard, outboard, saildrive, etc. any can be powered with an electric motor. All you need is to match the power and RPMs to it.
     
  3. xellz
    Joined: Jul 2017
    Posts: 74
    Likes: 3, Points: 8
    Location: Japan

    xellz Junior Member

    All boats here are fairly old, most of them are well over 30yr and have straight shaft with yanmar diesel. At cruising speeds can't really have a conversation when staying behind pilot house unless speak really loud and near. Just to give an idea. In fact i was offered to buy one of not used boats that is stored on island. For years no one touched it, hull in not so good shape, engine quite decent and with engine overhaul will cost in the range of 10.000usd, this is for 26ft FRP hull, 25hp yanmar inboard. Less than material cost for catamaran, but i would have to give up charter fishing or sightseeing tours, fishing during most of the winter season. Outboards are quite rare to see in open ocean, in bays small pleasure crafts or skiffs for fish/shell farms, various trailable boats. I'm also a bit uneasy to use outboards here, i.e. worry that it can be hit by too high wave when drift fishing. Probably i'm worrying too much, but i would feel more relaxed having engine inside hull than hanging from stern.

    Back to the question, if i can convert to electric i definitely don't want outboards, even if they can be made completely water tight. First of all, they take quite good space on the boat, aesthetics can be important too. Price difference between shaft/saildrive or even outboard is negligible compared to total cost. Distances aren't big here, normal day operation will be mostly at displacement speeds, drifting or slow trolling. Sightseeing tour around island is also at low speed, roughly 18km in 2-2.5h. I.e. it's possible to include large enough battery bank for one day, generator for backup or long distance cruising at higher speeds. NMC batteries with over 150wh/kg and decent cycle life in various setups starting to appear on local market. If saildrive is noticeably more efficient, it can give me some extra room for battery bank. How i understand, in addition to horizontal thrust, it's easier to use larger prop on saildrive than with shaft. For example Bellmarine has ready 15kW units with yanmar saildrive leg with top 750rpm. But i have hard time figuring out how much it can affect efficiency and if it's worth the trouble.
     
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