Sterndrive on semi-displacement hull ?

Discussion in 'Sterndrives' started by Vronsky, Apr 30, 2015.

  1. Vronsky
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    Vronsky Junior Member

    I noticed this 7m sportsfisher boat with a semi-displacement hull powered by a Volvo diesel sterndrive. The advantages of the sterndrive in the design are obvious, but as a propulsion solution I believed sterndrives are best for planing hulls only.

    What's your view ?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Do you have a view of the underwater part of the hull, are you certain it is "semi-displacement" ?
     
  3. Vronsky
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    Vronsky Junior Member

    Hope this ons will do >

    [​IMG]

    Maybe it's a semi-planing hull iso semi-displacement (cann't tell the diff :( )

    THANKS,
    V.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    It looks like a planing hull, low deadrise, but the picture in not that definitive. Is there any publicity for it that describes it as semi-displacement ? And yes, semi-planing Vs semi-displacement, doesn't really denote much.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    This is not totally indicative either, but it looks like a planing hull to me.
     

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  6. Vronsky
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    Vronsky Junior Member

    Bow:

    [​IMG]



    Stern, sort of:

    [​IMG]
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Definitely a planing hull.
     
  8. Vronsky
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    Vronsky Junior Member

    Thanks, Mr Efficiency, any comments on my original question, i.e. whether sterndrives are (not) suitable for non-planing hulls ??

    THANKS,
    V.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Some sterndrive legs have gearing and propellor diameter options that would be suitable for semi-displacement speeds, particularly in this size range. Some would not be. Typically semi-displacement boats are quite flat in the bottom aft, and to improve handling, have sizeable skegs fitted. In a sterndrive set-up, that would need to be tapered well forward of the transom, to avoid interference with the sterndrive, so perhaps losing some of the effectiveness of the skeg. The boat you are speaking about can probably cruise at lower speeds than a deeper veed boat, and better placed to accomodate a diesel for that reason. Being reasonably beamy won't hurt that either.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Considering the boat pictured has a 140 Suzuki, and some say that is not a particularly strong engine, it appears the boat is not power-hungry, or excessively heavy, or both. Which you would expect anyhow, with the modest deadrise. I would call it a slowish planing boat, if that is not a contradiction, some planing boats just won't function until they hit at least 20 knots, this one should settle for slower than that.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Depending upon your point of view, it si neither or both. Since it matters not what you call it, it is about what it is actually doing. You can call it a sluggish monohull, or a garden shed, it doesn't affect the hydrodynamics and what the hull can do.

    If you look at the spec's it tells all, rather than just guess as others prefer to do as that is easier.

    Length = 6.83m...one assumes LOA thus the Lwl is circa 6.0m

    Displacement (assuming correct - we don't know as it is a sales pitch) is 2 tonne.

    Speed max = 29 knots, cruising 19 knots.

    So from this very simple data we can establish what it does actually do. Despite whatever name you wish to call it.

    Firstly the length-displacement ratio is low it is just 4.7. Which suggests this hull has a high hump resistance and shall trim excessively when attempting to get up onto the plan and shall remain at high trims, unless the boat has trim tabs or interceptor.

    So the speed 29 knots, on this hull length suggests a Froude number of 1.9 which is high It is well into the planing region. A Fn over 0.7 one can consider to be planing. Thus, not semi of any kind.

    At cruising speed Fn = 1.3 which is still again, technically planing. Despite the large hump resistance.

    So lets look at the power it has. Stated 160 Hp which is a hp/ton of 80. Again for its length this also suggest it is planing. As the Talyor quotient is 6.5...again it is high.

    It is a planing hull...just not a very good one.

    But i'm sure others shall like to call it something else as they always like to to show they can press buttons on a keyboard rather than answer a simple question.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    No mention of the key indicator, the transom deadrise angle, or of the fact it appears to be a warped plane bottom, both of which are important in determining speeds at which it will be effective.
     
  13. WindRaf
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    WindRaf Senior Member

    Yes, Mr. Efficiency is right: it is a boat planing hull.
    Some manufacturers, to expand the sales, put on boats planing hull engines with less power, but it is not really a semi-displacement hull.
    The real modern semi displacement hull is only just beginning to be required.
    A true modern semi-displacement hull of this size can make 6 knots with 6 hp, and 18 -19 knots with 60 hp.
     
  14. WindRaf
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    WindRaf Senior Member

    ..and yes, sterndrive can be a good solution for a modern semi-displacement hull
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    From what I have seen, and I have seen a few attempts, turbo-diesel sterndrives in planing boats this length, don't quite reach the mark. Especially if they are constrained by a trailable beam of 2.5 metres, which this one isn't, though. The power-to-weight ratio still lags, and especially now where outboards have had a quantum leap in fuel efficiency, the reasons for going to diesel are not as strong as they might have once appeared. And there is a considerable difference in capital cost, as well. But a boat this size with a generous waterline beam, and not much deadrise to cope with, can work well enough with the diesel. Just. But I haven't seen one with 2.5 metre beam, under 7.2 metres long, I'd want. They were slugs.
     
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