Semi-displacement? (photo)

Discussion in 'Boat Design' started by Datacat, Sep 25, 2015.

  1. Datacat
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    Datacat Junior Member

    Will a hull like this, plane? The description of the vessel says it's 42 feet with 350 hp, and cruises at 15 knts. Is that getting close to the practical speed limiting factors of the hull, or could the boat still benefit significantly from an increase in horsepower?
     

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  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    It seems very unlikely that boat will plane off and I also doubt it'll do 15 knots on 350 HP, unless it's especially well shaped below the LWL and very light. She appears to be some sort of service vessel, possibly a small "hop" ferry, but without a look at her bottom or knowing what make, year and model she is, nothing more than guessing can be done. Her weight would need to be well below 20,000 pounds (9,090 kg) to be able to manage 15 knots (350 HP), which I find hard to believe in a 42' cruiser with a cabin 3/4's the on deck length.

    Simply put, if you make direct comparisons, you'll find boats like the Grand Banks 42, which with twin 220 HP diesels will manage 12 knots and displaces well over 32,000 pounds. Even the all carbon fiber Wider 42 (one ridiculous boat) is 22,000 pounds.

    Who told you it does 15 knots - a broker?
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

  4. PAR
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    PAR Yacht Designer/Builder

    It's listed at 61,000 pounds, so 350 HP will produce 10 MPH at best.
     
  5. Datacat
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    Datacat Junior Member

    Thanks. I thought it might be optimistic.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    The weight seems very high, I'd have guessed 10 tons lighter.
     
  7. Datacat
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    Datacat Junior Member

    http://commercial.apolloduck.ca/feature.phtml?id=437605

    It's been advertised on a few sites. This is the ad that I bookmarked. I'd like to determine what good options exist in the forty-foot neighborhood for an aluminum powerboat hull that favors superior sea-keeping as the first consideration, and the ability to plane for speed as a desirable second trait.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    That ad says the hull is steel ?
     
  9. Datacat
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    Datacat Junior Member

    So it does ...
    and yet the title of the ad is "1990 42' x 14' x 4' Aluminum Dive/Crew/Work/Patrol Boat".
     
  10. aussietrev
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    aussietrev Junior Member

    I wouldn't be touching it full stop unless they CAN tell you truly what it is and does. What else don't they know, but are telling you? Advertising has a lot to answer for.
     
  11. fredrosse
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    fredrosse USACE Steam

    Looks like they know the hull is at least made of metal? What kind of metal is the question.
    Cruising speed of 15, maybe they should list this al km per hour rather than knots?
     
  12. FAST FRED
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    FAST FRED Senior Member

    "Cruising speed of 15, maybe they should list this al km per hour rather than knots?"

    Also beware many in the US will list statute mile speeds 5Kn = 6Mph as the boats are used inland where lubber miles are on the ICW charts.
     

  13. Datacat
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    Datacat Junior Member

    That makes me laugh. I guess it's hard to learn anything reliable from internet ads. Thanks to all for your comments.
     
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