How much power

Discussion in 'Boat Design' started by standman, Apr 3, 2019.

  1. standman
    Joined: Mar 2015
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    standman Junior Member

    Hi Guys,
    I want to build a stabilized mono hull (Trimaran). 5,000 KGs Displacement weight. Main Hull beam at water line is 3ft and waterline length of 48'. It has a fine entry and exit angle. E-glass- and some carbon fiber on PVC core foam. I want to achieve a cruising speed of 20 knots. How much power ( HP or KW) would i need? TIA
     

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  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, I can't see it needing much less than around 350/400 HP. What were you thinking ?
     
  3. standman
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    standman Junior Member

    Thanks Mr. E
    Building the hull slim 16:1 ratio, and with light materials, I was hoping to get away with much less.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    You might get away with a lot less power, but 20 knots as a cruise speed makes it difficult, and it is 5000 kg. You can pretty well put a minimum power to have any boat cruise at 20 knots at 5000 kg, and I would reckon you would get little or no change out of 300 hp. Bringing your cruise speed down, is about the only way to change that.
     
  5. HJS
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    HJS Member

    Based on the numbers you entered, about 150 horsepower is needed. But it requires a much more elaborate form than the one you show. The relationship length - speed is a bit unfortunate selected, It can be difficult to shape the boat so that the wave formation becomes minimal. The interaction between the hulls requires great attention in the design.

    JS
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    When you say 150hp needed, I take it you mean 150hp to reach 20 knots, not cruise at 20 knots, with plenty in reserve, it makes no sense to run engines flat out. If a nominal 150 hp engine will have this cruising at 20 knots and 5000 kg, I want one !
     
  7. standman
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    standman Junior Member

    Thanks for the input HJS. 150HP was what I planned on. I made the bottom rounded for maximum displacement. Made entry and exit very slim. What would you suggest I change? What elaborate form are you referring to?
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    What kind of engine(s) are you thinking for it ?
     
  9. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    I'd also would love to know how to best calculate power needed. From what little I've learned so far is that hydrodynamic is hard and you basically need to spend money and lots of work to get an accurate answer.

    You can use hullcalc.xls to get a rough idea, but no idea how accurate. It says 62kW propulsive power at 20 knots for a monohull with your length, or 56kW for a catamaran. Not sure what the small stabilizing hulls would do. With propeller losses you can easily get to 150hp brake power.

    Delftship has a resistance curve tool based on something something. And it can export to michlet.

    Michlet / flotilla seems to have the most elaborate calculation that is fast and relatively easy to use.

    The best way would be CFD simulations but those are expensive software or you need quite a bit of time to learn how to use the complex open source software OpenFOAM.

    Or hire an engineer to do it.

    Or tank testing with a model.

    Then the next problem is how to select the propeller and engine and rpm for your boat to deliver that power!
     
  10. standman
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    standman Junior Member

    @ Mr. E. Inboard Diesel
     
    Last edited: Apr 4, 2019
  11. standman
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    standman Junior Member

    Thanks Dejay, that would be nice if indeed that were true (62kw). The small stabilizers is just grazing the surface. I do have a 200hp outboard (zuki) that I can use to see how fast she can go then install the inboard diesel (bigger or smaller). But weight distribution will also be an issue since inboard near center while outboard at stern. Ballasts i suppose.
     
  12. HJS
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    HJS Member

     
  13. standman
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    standman Junior Member

    First pic (Grey) is very similar to my design with the exception of rounded hull as suppose to a more boxy shape. I suppose the flat surfaces can help in the dynamic lift. The hulls you attached has been built ( exsisting results)? if not, CFS confirmed performance?
     
  14. HJS
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    HJS Member



    One of my designs as the basis for other trimarans.

    JS
     
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  15. standman
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    standman Junior Member

    VERY NICE JS!! what's the power and speed? beam length ratio?
     
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