catamaran added wave resistance

Discussion in 'Multihulls' started by ABD, Feb 18, 2018.

  1. ABD
    Joined: Feb 2018
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    Location: Argentina

    ABD Junior Member

    Hello everyone this is my first post on this forum so i apologized in advance for my english but it is not my first language

    I am in the process of changing the main engine for a 45 ft catamaran and i was able to obtain the hull resistance through the slender body method in maxsurf, the mast and rig wind resistance and the above water body resistance due to wind using a similar method than the one explained in "principles of yacht design". My problem is estimating the added wave resistance due to bad weather. I found methods in some books but they are for monohulls but that doesn't take in mind the fact that if the wave is big enough to reach the bridge it will add frictional resistance to the boat, and when i worked in big ships like tankers and container ships at university we used to add a 10% to account for wearing of the engine over time and a 15% to account for bad weather but maybe that is overestimating it.

    Any thoughts???

    P/D: The controlable pitch propeller of the boat will remain the same
     
  2. Richard Woods
    Joined: Jun 2006
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    Richard Woods Woods Designs

    Your English is good!

    I assume you are talking about a sailing catamaran as you mention the "mast and rig" Why are you changing the engine? To increase speed? What speed do you want? what speed do you do now?

    As a practical illustration of the speed loss into waves the 34ft catamaran I had some years ago had a 9.9hp outboard as its main engine. It would do 6 knots in calm water but 2 knots into 30 knots of wind and a steep sea. So sea state has a major influence on low powered catamarans

    Richard Woods of Woods Designs
    www.sailingcatamarans.com
     
  3. TANSL
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    TANSL Senior Member

    Does the boat really have a controllable pitch propeller?
     
  4. ABD
    Joined: Feb 2018
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    ABD Junior Member

    Richard:
    Thanks for your answer. Yes it is a sailing catamaran. The idea is to study the posibility of installing electric engines. Currently the boat has 2 40HP combustion engines and a cruising speed of 7 Knots, but i am seeing if smaller engines could do the job
     
  5. ABD
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    ABD Junior Member

    TANSL: Thanks for your response, you re right it is a fix pitch propeller, what i was tryng to say it is that the propeller diameter its set
     
  6. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    In the absence of hard data on your hull form and overall design, these values are a very good "average" estimate for added resistance.
    I use these values too, as a quick rough and ready approach.
     
  7. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    First of all, since it is an existing vessel: check the performance figures for the present state and validate versus engine, transmission and propeller data.
     
  8. ABD
    Joined: Feb 2018
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    ABD Junior Member

    Yes, but because we don`t have the original resistance curves and analysis I was aiming at doing the calculation again to see at what results i arrived and use the cunrrent propeller and engine data as a way to validate my results. I am not sure if i am explaining myself ok
     

  9. Richard Woods
    Joined: Jun 2006
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    Richard Woods Woods Designs

    off topic, but I have used controllable pitch props on a sailing catamaran. The logic is good. Motorsailing with one engine needs a different prop to powering with both into a head sea. And different again for high speed (that boat could do 15 knots under power). But later the owner changed the props to Max props as they were too complicated and unreliable

    yes your explanation for needing the resistance curves makes sense.

    But I doubt if electric motors will work well for long when you need them to

    What boat do you have?

    Richard Woods
     
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