Choosing engine size for my boat

Discussion in 'Boatbuilding' started by Rover220123, Oct 25, 2017.

  1. Rover220123
    Joined: Oct 2017
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    Location: Thailand

    Rover220123 Junior Member

    Hi all,

    I'm a newbie here and hoping someone can shed some light for me. I'm trying to find out the most efficient engine size for a boat i'm having made. I found info on the "recommended maximum power calculation," but is there such thing as a "minimum power calculation"? I want to find an adequate size, but wanna make sure i'm not underpowered.

    The boat is a 13' monohull fiberglass.

    Thank you.
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    More details would help. What is the weight, transom deadrise angle etc, a picture would not hurt either.
     
  3. Rover220123
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    Rover220123 Junior Member

    I don't have the weight yet, the boat is still being crafted. The length is 390cm, transom width 130cm, deadrise is est.9-10degree.

    I've included photos of the base mold, and the making of the top mold shape.

    Thank you
     

    Attached Files:

  4. DCockey
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    DCockey Senior Member

    How fast to you want/need to go?
     
  5. Rover220123
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    Rover220123 Junior Member

    Well, that's the thing. I don't know what the low-high speed should be for this boat Because i've never ridden a boat this size before. I assume that "speed" is relative to the "size" of a boat on how fast it would feel. Similarly that a smaller car would feel like you're going faster compared to when ypu're driving a larger, heavier car.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    30 HP should be more than enough, based on what you have shown, especially the modest deadrise,
     
  7. Rover220123
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    Rover220123 Junior Member

    Thank you :) that's also what i was wild-guessing.

    Forgot to mention, the boat will hold 4 seats. 30hp still enough, right?
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    What does the designer recommend? Have you applied the standard LWL/transom beam formula? Really, 4 adults in a 13' boat? I hope you're all quite intimate with each other, as you soon will be on a short boat.
     
  9. Rover220123
    Joined: Oct 2017
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    Location: Thailand

    Rover220123 Junior Member

    Hi PAR,
    I wouldn't call him a "designer." He does fiberglass works and he makes small boats. So, i'm seeking a more informed information. Can you tell me more about the LWL/transom beam formula?

    Btw, the boat is meant as a small, fun boat, not a day cruiser. It'll mainly be for 2 person, but there is a bit more space in the back so if i want to squeeze 2 more people in (if needed be), i want to have enough power to not have to boat be dragging.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    It will be packed like a sardine tin with 4 aboard, maybe OK for Thais of smaller stature, but 30hp seems enough. Depends how hefty the people are. But it will develop a mighty nose up attitude taking off with 4 on board, all in the rear half of the boat, and more so if you plonk a heavier engine on it. It is a small boat.
     
  11. Rover220123
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    Rover220123 Junior Member

    Thank you very much for the tip. Would a 25hp be too low on power?
     
  12. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    If the deadrise is as low as you say, 25 hp should suffice, most 25's are just detuned 30's, and will have similar grunt taking off, but not rev out as far. Unless the boat is heavy for its size, you should be OK.
     

  13. OzFred
    Joined: Nov 2015
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    OzFred Senior Member

    Make sure you are considering the right criterion. A 30hp two stroke Mercury is 51kg, the next smallest two stroke is 15hp and weighs 41kg, so for a 50% power loss you only get a 20% weight saving of 10kg.

    In comparison, a 25hp four stroke Mercury is 76kg, so more than 50% heavier.

    You may get different numbers for different manufacturers, but they're likely similar.
     
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