Most efficient form of propulsion

Discussion in 'Propulsion' started by jmac, Mar 19, 2006.

  1. jmac
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    jmac Junior Member

    I hope all of these answers are scientific.;)

    I'm building a 38ft express cruiser in aluminium. 18,000lb displacement. 11'10" Beam, LOL 32'8". 2'6" Draft. It is a slower speed planer (24knot top)

    Power will be 310 Shaft HP.

    There are so many ways to power this boat. What is the best type of drive system to use, (maintenance to come 2nd) and also include how you would add the get home engine.

    Please rate them the following
    1) Reliability
    2) Efficiency
    3) Safety (ie damaged props ect.)

    Also I would like to here about surface drives if this make sense.

    Thanks.
     
  2. Corpus Skipper
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    Corpus Skipper Hopeless Boataholic

    The get home engine kinda limits your options to conventional inboard. Otherwise, as far as the safest, most efficient, and reliable system, in my humble opinion, is diesel stern drive (inboard outboard), twin engines for the get home capability. Safety comes from diesel fuel instead of gasoline, the drives will kick up if you strike a submerged object, and the skegs protect the props. Stern drives deliver higher top speeds for a given power input versus straight inboards due to less drag, though a single screw inboard may negate this advantage. On that note, a single screw inboard is very efficient, especially for the lower speeds you are looking at. A full keel before the prop protects both prop and rudder from grounding/ debris, and is more corrosion resistant than the stern drives (where are those composite drives, Volvo?:rolleyes: ), especially if the boat is to be wet slipped in salt water. If not, I'd go for the I/Os any day. With a single inboard, you can couple a "get home" engine via pulleys and belts, or chain, or whatever to the main shaft. I believe surface drives are not an option for your low speed boat, but I may be wrong.
     
  3. jmac
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    jmac Junior Member

    Corpus

    You mention a single screw inboard being good. Is that the best? What if I had that plus a wing engine with some type of surface drive that could come out of the water when cruising, and the wing engine could be used for 1) trolling 2) generator 3) get home. Efficiency wouldn't matter too much for trolling and get home engine.
    The other issue I think with single screw is manuverability. I am willing to put in bowthrusters though.
    Like I said I am open to all types off configurations. It just has to make sense and not be too radical. My budget is pretty good to do it right.
     
  4. Willallison
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    Willallison Senior Member

    Corpus is spot on - strendrive(s) - whether single or twin - are definitely the most efficient choice. You only have to read a few comparison tests to see that sterndrives will be about 15% more efficient than shafts.
    There is, however, always a bit more to the decision than this. At around 9 tons, you probably won't find a big enough sterndrive to be able to fit a single engine, so as Corpus says, you may not get the full advantage as you would have to fit twins.
    There are other issues to consider too. Maintenance on sterndrives tends to be a bit higher than with shafts. Even more so if you have two motors vs one.
    Shoal draft is a big plus for the sterndrive. Even a skeg protected shaft is no match for a sterndrive's ability to sneak around in shallow water.
    You are more restricted with the size prop you can swing. Probably not too much of an issue with a sports cruiser, but for some things it can be.

    You can forget surface drives - they only come into their own at around 40 knots. Waterjets are similar, only being a sensible option (from an efficiency point of view) at over about 30 knots
    Your only other real choice is outboards. Some of the new bigger 4-strokes and direct injected 2-strokes off some real competition to the inboard alternatives...

    As far as your get home goes, it depends a bit on your field of cruising. If you're not planning on being miles off shore, then an outboard with a suitable low pitch prop is worth considering - it can be lifted out of the water when not in use and is by far the simplest and cheapest alternative.
     
  5. jmac
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    jmac Junior Member

  6. Willallison
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    Willallison Senior Member

    I am aware of the Konrad drive - though I don't think there's much point in using one on an engine any smaller than around 400hp - as you say, others make sterndrives capable of handling up to around 350 hp.
    The problem with the Konrad drive - and bear in mind that I've never had any personal experience with them - is that they are relatively scarce. As such parts, warranty, repairs etc MAY be an issue. There have also been comments from others on these boards in the past that suggest that they may be quite expensive to maintain. Again, I've never had any personal experience with them - indeed I've only laid eyes on one once at the Sydney boat show.
    You could think about Volvo's IPS or CMD's new Zeus (an interesting derivative of the same basic idea) but then your back to having to use twins and you can forget about going anywhere wher you might so much as nudge the bottom. They are also bloody expensive!

    As far as your outboard goes, I'd think that any high-thrust motor of the righ power would be ok. The biggest problem you face is ensureing that the prop remains sufficiently submerged to avoid cavitating (sorry - ventilating;) )
     
  7. jmac
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    jmac Junior Member

    How does this sound. Will it work?
    Cummings QSB 5.9 305 Wt. 1350lbs, ZF Drive 315lbs, Drive Shaft, SternDrive.
    Corpus Skipper suggests I could hook up pulley ect to the generaator to make get home engine. (Anyone have a sample dwg).
    I have been told that this engine is 2 speed and I could run it at 1000rpm and use the trolling valve and run all day like that.
     
  8. Willallison
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    Willallison Senior Member

    you mean run thru a 2 speed gearbox, then a jackshaft, to a sterndrive?
    yes it would work - sort of. Sterndrives have the gearbox in their 'foot'. Sometimes you see them with the gears disconnected - basically acting as a direct drive.
    It would be much cheaper to go for a 2 speed prop though. These change pitch at certain rev / loading to give you the same effect.
     
  9. jmac
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    jmac Junior Member

    What would be cheaper? and what is a two speed prop. (could you provice a link)
     
  10. jmac
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    jmac Junior Member

    Willallison
    The Konrad Drive has this type of connection. http://www.konradmarine.com/products/520_installation.pdf
    Does Bravo ect use this type. I was hoping to put in a short intermediate shaft inbetween the drive and transmission and somehow connecting the generator engine to turn the shaft. Anyone done this before and has a sample dwg.
    Thanks
     
  11. Willallison
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    Willallison Senior Member

    A 2 speed prop is one that automatically changes pitch once a certain speed / blade loading is reached. As I said, it effectively gives you the same as a 2-speed transmission - just at a vastly lower price

    you can read a bit more about them here -
    http://www.land-and-sea.com/articles/marine/trailer-boats/trailer-boat_prop_article.htm

    Almost all sterndrives can be configured to run a jackshaft. I don't have any drawings to show the get-home arrangement that you describe - you could try googling... - but I can tell you that it wouldn't be cheap or simple...
     
  12. 67-LS1
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    67-LS1 Junior Member

    Prop shaft angle

    Very interesting topic.

    Does the efficiency advantage of the sterndrive have to do with the prop shaft angle? Is this also an advantage of the Volvo IPS?

    Assuming a 35-40' twin engine planning hull, what is the typical shaft angle of an inboard engine? Does it change much between a straight shaft and a V-drive?

    Has any one ever seen a counter-rotating propeller system for inboards? Would it even work with the shaft angles of an inboard?

    Thanks,
    Dennis
     
  13. Willallison
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    Willallison Senior Member

    The bulk of the efficiency gain with a sterndrive is from a reduction in drag - from the struts, shaft and rudder. There is some gain as a result of having a horizontal thrust too. Though as some of the thrust from angled shafts are directed downwards, this has the effect of lifting the stern of a boat, which can be used to advantage.
    There are counter-rotating systems for shafts - I think I recall seeing pics of them somewhere in the forum...
     

  14. Steve H
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    Steve H Senior Member

    Don't throw surface drives out the window. There are many being used commercially in applications very similiar to yours. As far as the "only come unto their own over 40 knots" is pure bull. Spoken by someone who has no personal experiance with them.

    Take a look at

    www.pacificsurfacedrivesinc.com/kuroshio.htm. Doesn't look much like a go-fast to me!

    Give him a call. He does mostly Arneson conversions for work boats.

    Steve
     
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