I've never seen a stern-drive on a sailboat(mono or mulit-hull).

Discussion in 'Boat Design' started by Squidly-Diddly, May 17, 2010.

  1. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Is there a real good reason for that?

    I'm thinking it could have a few advantages on either mono or multi.

    1) it can be raised out of the water for less drag, while not being as much in the way as a outboard, when not used (and a pure sailing rudder(s) like a MacGreggor dropped either with or without prop dropped).

    2)the thrust can be vectored, so it could be better for docking, which is when a sailboat really needs its motor.

    3)the placement of the engine full aft COULD be desirable depending on overall design, and the kickup feature nice for shallow waters, logs, or beaching, while still using an large inboard motor.

    While I read there are some extra maintenance issues, stern-drive seems a well sorted out application, and should be fairly cheap and well supported. I'm sure there are hundreds of power boats getting junked for other reasons with perfectly good stern-drives.

    Any good reason these aren't used on any sailboats, even motor sailors?
  2. rasorinc
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    rasorinc Senior Member

    stern drives are usually connected to a V6 or V8 engine with high HP. put a 10 HP outboard on an extended transom bracket and you can tilt it up.
  3. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I'm talking about sailboats with fairly large inboard motors of course.

    I've also noticed there doesn't seem to be much range, if any, in the size and cap of stern-drives.

    I was expected to see something like every maker offering "50-90hp, 70-120hp, 100-170hp, 150-250hp..." and so on. Or at least a Small, Medium and Large!

    It only seems to go by year.

    I'm sure there is an almost infinite number of offerings for inboard apps.
  4. Gilbert
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    Gilbert Senior Member

    Stern drives or inboard/outboard units are more complex than either outboards or inboards and therefore are inherently less reliable.
    Their popularity is based on their weight in the stern not being a disadvantage in a planing hull and manufacturers get bulk discounts for large orders and they are quickly installed.
    They do have some virtues as mentioned, but sailboats just aren't normally a good fit for them.
    I think David Beach and a friend put one in a catboat once, long time ago.
  5. CatBuilder

    CatBuilder Previous Member

    The reason they are not used is that a sailboat will pitch. (bow and stern will act like the ends of a seesaw)

    When the stern comes up, the prop of a stern drive would pop out of the water.

    This is also why you always see guys with outboards on a sailboat looking for the "long shaft" models of the outboards. It's impossible to keep the stern of your sailboat in the water at sea, so you need to reach way down into the water with your prop to avoid having it breach the surface.

    Locating your props closer to the center of the boat (half way between bow and stern) would minimize the possibility of the props coming free of the water, however, you can't exactly mount a stern drive there.
  6. daiquiri
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    daiquiri Engineering and Design

    An added (and pretty big) weight concentrated at the transom, due to the engine/stern drive, translates into an increase of the moment of inertia around pitch and yaw axes. It means lower hull pitch and yaw frequencies, which is the opposite of what you want for good seakeeping.

    The hull shape would also be affected, because more volume would be necessary in the aft, to lift all that weight. While it is a good feature in case of fast powerboats, it would compromise the overall hull efficiency at displacement speeds for which generally sailboats are optimized.
  7. Joe Petrich
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    Joe Petrich Designer

  8. CatBuilder

    CatBuilder Previous Member

    Thanks a lot! (sarcasm)

    I'm trying to keep my catamaran under budget!! :D
  9. Paul B

    Paul B Previous Member

    I believe Bruce Nelson did a sailboat around 45 feet that had a stern drive. This was one of his first orders and the boat was called Fast Mast (how original!).

    It didn't seem to sail very well. I never saw it under power with the hammer down, so I can't say if it powered well either.

    It did have a big, boxy stern to give it enough volume to support the weight back there.

    I think it might still be sitting in a slip in Long Beach, 30 years after it was built.
  10. webbwash
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    webbwash Junior Member

    location of a stern drive and motor will give your weights program fits and you will be forever dragging around your transom. placing the engine centrally and definitely forward of the transom gets the weight down low and below the waterline, putting ballast where it belongs. the prop being deep below the hull will also reduce cavitation (see long shaft vs short shaft outboards). and then there is cost - for installation and then for repair a simple propshaft and engine is much cheaper than an outdrive anyday.
  11. marshmat
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    marshmat Senior Member

    A few thoughts on the matter:

    The main purpose of a sterndrive is to give an inboard engine the trim and thrust vectoring abilities of an outboard.

    In the process, you lose the outboard's ability to be lifted clear of the water. So you have a complex, expensive lump of mixed metals hanging off your transom.

    I'm a big fan of sterndrives for planing boats- from about 180 hp and up, they have a lot of advantages over outboards. But there aren't many displacement hull shapes that suit them. The weight distribution doesn't have to be an issue- you can always use a driveshaft between the engine and the drive- but the numerous other issues make them generally unsuitable for sailboats.
  12. Easy Rider
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    Easy Rider Senior Member

    I/Os are made for high speed craft w high speed propellers unsuited for low speed craft and heavy loads. My 2 cents.

    Easy Rider
  13. dskira

    dskira Previous Member

    It keep part of the transom underwater, a bad feature for sail boat.
    It was tryed in the 1970, good under power, a disaster under sail.(read above)

  14. Stumble
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    Stumble Senior Member

    I think the other issue is that most sailboats need relatively little power compared to a planning hull. Even large (60' or larger) sailboats normally have less than 100hp diesels in them. Outdrives are just to complicated to justify for such small engines, though you can take a look at sail drives, which have some of the same features.
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