engine choices

Discussion in 'Propulsion' started by wudenbote, Oct 25, 2008.

  1. wudenbote
    Joined: Aug 2008
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    wudenbote Junior Member

    Hello everyone! I am at that point in designing/building my boat that I have to think about framing for the engine. Since I first joined this forum and stated what I was building, I have made some changes and the boat is somewhat different than originally mentioned. The boat is 22 foot and of a flat bottom displacement type hull. It is 1/2 inch Hydrotek over white oak frames. This is not a boat that will plane and I am not interested in speed but rather, sturdy and stable. Originally I was going to build it with an outboard motor mounted in an inboard engine well. However, I am now thinking that I might prefer a small diesel engine of 2 to 4 cylinders and somewhere between 15 and 75 HP. I would very much like suggestions on what to look at. I would like it to run through a V-drive unit and be connected with a simple, mechanical shift which has only forward, reverse and neutral. Because I am pretty much dumb as to how to make choices to end up with the proper rpm's at the prop (including size of prop and pitch) I will need LOTS of help! I am not interested in a lot of speed. 15 knots would be plenty! Also, who makes, or where should I look, for V-drive units? In advance and as always...thank you for your suggestions. They are much appreciated!
  2. Fanie
    Joined: Oct 2007
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    Fanie Fanie

    For ease of installation, steering and power to weight, the outboard is the best. Inboard diesels are more economic, but everything is not always as simple as they seem, they just look that way. Good choice though, I like the idea of being able to hook a decent alternator up on an inboard. The small outboard's charging is a bit on the poor side.

    Also look for pictures of the thing you are interested in, that would give you a good idea of what to expect.
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    A 22' displacement type hull isn't going to do 15 knots. Assuming about one ton and a 20' LWL you'd be hard pressed to get typical a displacement hull over 7 knots, frankly just over 6 is about right. To reach these speeds you'll need a boat shape that can plane. A photo or drawing of the hull, would make understanding your hull form and the possibility of planning much easier.
  4. CDK
    Joined: Aug 2007
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    CDK retired engineer

    The small diesel is not a bad idea at all. FastFred can point you in the right direction.
    The 15 knots on the other hand are totally impossible within the given power boundaries (15-75 hp) and even above these, so aim for a really small engine, as using a large one won't change the hull speed noticeably.

    V-drive units for small engines do not exist, the makers all had at least a 350cu V-8 in mind. But you don't need one: with 15-20 hp you can use a double V-belt or a 1" timing belt and a small marine gearbox (Technomarine, now part of TwinDisc).
  5. StianM
    Joined: May 2006
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    StianM Senior Member

    I think I would prefer a 4-cyl over a 2-cyl since I would expect this to cause less vibration and smother sound.

    What are you going to use the engine for? is it only propulsion or do you also want to generate electricity and/or run hydraulic pumps?

    I know a local company just outside my window that sell nanni diesels complete with gear(10-320hk) and they can also deliver CP, Fixed and adjustable propellers.
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Wudenbote, without an idea of what your boat is (shaped), then engine suggestions are purely guesses and not especially valuable. Post some drawings or photos.

  7. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    Hi wudenbote,

    Drawings, photos or anything else would be of great help to us.

    A 22' displacement hull would go about six knots. Even for a dedicated planing hull, the 7-15 kt range is virtually unuseable; cruising in this intermediate speed range requires a hull designed specifically for this purpose.

    I have no idea how much your boat is expected to weigh. If it's around 2-3 tonnes, you'd probably be able to do 6 knots with less than 10 hp in calm water. 20-25 would be ideal; you wouldn't get any more top speed with this, but you'd be able to keep your speed up in a headwind. But it's really hard to make a meaningful guess knowing so little about the boat.

    As to your powering concerns, if you have $20 to spare to buy Dave Gerr's book "Propeller Handbook", you will find it a very worthwhile investment. All the formulae and data tables you need for the propeller calculation are in there.
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