Resistance prediction methods/series for catamarans

Discussion in 'Multihulls' started by Roni, Jul 8, 2020.

  1. Roni
    Joined: May 2020
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    Roni Junior Member

    Hi there,

    I am looking for resistance prediction methods/series for catamarans around 10 m lenght, 6 ton displacement and 10 knots of speed (Froude number around 0.52). Any recommendations?

    Thanks
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    You can try HERE as this excellent research may give you a good first indication.
     
  3. Alik
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    Alik Senior Member

    We use following methods:
    - Molland series - see link from AdHoc, for round bilge shapes
    - Muller-Graf series - for semi-planing catamarans with chine
    - Savitsky planing method with hulls interaction factor - for planing catamarans
    - For narrow and light hulls, there is 'slender hull' method, this is originally a graph published by Doubrovsky in his book. However use with care, as comparing with tank tests seems the method underpredicts the resistance.
     
  4. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Which is also available HERE.

    But with these parameters, the M-G series is generally outside your scope. But may offer you some guidance in addition to the Molland paper.
     
  5. Richard Woods
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    Richard Woods Woods Designs

    10 knots on a 10m powercat is only just over the hump speed. So it would be best to either aim for displacement speeds of under 8 knots or go for mid teens as a minimum speed.

    see here for an example of one of my home built designs in that size range



    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
    bajansailor likes this.
  6. Roni
    Joined: May 2020
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    Roni Junior Member

    Thanks a lot everyone, will have a fun weekend studying.

    Yes the froude number is 0.52. Maybe at around 0.4 (~8kt) would make more sense.
     
  7. Dolfiman
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    Dolfiman Senior Member

  8. Bureau Naval
    Joined: Oct 2021
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    Bureau Naval Junior Member

    Hi all. There is some time we arfe working with this series and we get som values of the Residuary resitance - weight ratio negatives for centains Volumetric Froudes. Does anybody knows if perhaps, there is some printing error in the coefficients values. Any help, most welcomed. Regards
     
  9. guzzis3
    Joined: Nov 2009
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    guzzis3 Senior Member

    Hi.

    I have found this calculator gives not unreasonable results for the power and cat specs I am interested in, but my interest is small motors on sailing cats.

    Free to use Propeller Sizing Calculators https://www.vicprop.com/free-propeller-sizing-calculators

    It predicts about 5 knots from 4hp on a 32' cat weighing 5300 lbs. Interestingly it predicts only 5,5 for a 6hp. When I input beam I double the hull beam to account for both.

    I would be very interested if anyone knows of a better calculator for speed predictions for displacement situations.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    I've used them in the past and not come across this...

    Can you post your numbers/data?
     
  11. Russell Brown
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    Russell Brown Senior Member

    That is a really fine power cat! Congrats Richard.
     
  12. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    Can you test mine, SA-VPP Power Catamaran 1.1, based on Molland and al. results for Froude 0,4 to 1,0, and a connection with Holltrop for lower Froude < 0,4. It is a spreasheet application working with Open Office or Libre Office.
     

    Attached Files:

  13. Bureau Naval
    Joined: Oct 2021
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    Bureau Naval Junior Member

    Here in my Excel spreadsheet, you can see there are values for some Froude that they are negatives....????
     

    Attached Files:

  14. Bureau Naval
    Joined: Oct 2021
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    Bureau Naval Junior Member

    Thanks Dolfiman, I will check out with my data. We worked with Molland with good results for slow and medium speed. We are working with some high speed model with chine for easy and amateur construction...
     

  15. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    Ok, fine, just to mention a typo mistake in my User Guide, it is :
    Cms : midship section coefficient (section / Tc Bw) , and not of course .... (section / Lw Bw).
    (This coeff Cms is used in the Holtrop / coeff c5 formula)
     
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