What’s formula for estimating catamaran speed? Speed from length better than from design?

Discussion in 'Multihulls' started by JunkRat, Oct 31, 2020.

  1. JunkRat
    Joined: Dec 2018
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    JunkRat Junior Member

    Please respond with formulas and science, not opinions about what boat I should buy. Any boats mentioned are just hypothetical for example purposes.

    Given basic stats and DLR, SA/D is there a formula to estimate the speed of a Catamaran on a beam reach in 20 knots of wind? This would allow me to compare widely different designs across different lengths.

    I’m trying to understand all the tradeoffs. DLR and SA/D gives you an idea of how performant the boat is compared to other boats of the same length.

    But I would like to compare across lengths and design types.

    A 60 foot outremer has the living space of a 40-45 foot Lagoon. And costs a more.

    Does a 52 foot Lagoon perform as well as a 45 foot Outremer?

    Seems quite possible a heavier but longer boat may deliver same speed as a shorter, less comfortable but more performance oriented boat, at the same cost.

    Also, is there a way to factor in the length/width ratio of the catamarans hulls? This seems yo be a factor but isn’t included in any formulas I’ve been using.
  2. BlueBell
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    BlueBell Ahhhhh...

    Yes, but you know that already.
    Long and skinny.
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    when resistance = driving force, you get the speed figure. Obviously there is a matrix of factors, such as weight, length to beam ratio, wetted area, sail area etc, to consider.
  4. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    The simple way of doing a speed comparison is to use such formula's as Kelsall speed ratio, Bruce number, W performance factor by Bommer (?). They all work on the same basic principles length, weight and sail area which are the main speed controlling issues. If you want real refinement the OMR rule is available at the following site OMR Preamble http://www.mycq.org.au/omr/omr-preamble

    This rule is used to rate multihulls in racing and gives a rating based on the true weight of the boat when sailing, the rig type and efficiency of the rig and the true sailing length of the multi when sailing. The rule is reasonably accurate as to the relative performance of a multihull compared to others. Beyond this Velocity Prediction Programs are the ultimate but are hard to use and expensive. Another approach is download the free Hullform version P9 from the following web site which will lead you to the Hullform software: https://www.storerboatplans.com/boat-design/hullform-9p-now-working-with-windows-10-download-links/ which has a section on hull drag in it after you input a close approximation of a hull shape. The hullform program demonstrates the effect of length to beam and EG weight on the drag of a hull. Have fun, there is a bit of work ahead if you want accurate answers.
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  5. gonzo
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    gonzo Senior Member

    The wave interference between hulls is an important factor on resistance too. Using a single hull and multiplying by two will give lower resistance values.
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  6. fallguy
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    fallguy Senior Member

  7. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    I have adressed the residuary drag of a sailing catamarans in a previous thread, with formulations and process valid for designs with Froude from 0,4 to 0,9-0,1 (the usual range for cruising catamarans), here attached with complete numerical examples from page 25 .
    It is not an answer to your question, just an insight on the residuary drag issue involving two hulls which are, in sailing conditions, with different Lw, Bw, Displacement, Froude, ... + the wave interaction drag between them.
    For the speed prediction itself, you need also to involve the friction drag component (>> to know the wetted surface and its variation with each hull displacement), the aero drag, the righting moment with heel, the sails thrust and side forces, ... in brief without a VPP integrating all that stuff, it is hasardous to discriminate two designs. It is my intention to do one day such VPP for the sailing cata issue within a spreadsheet application, but that takes time ...
    Overall formulas based on ratios can be helpful to have order of magnitudes and trends, to start a new design for example, but they are not magic, a VPP tool with justificated data should be more reliable and accurate for a comparison of 2 designs.

    Attached Files:

    Doug Halsey and oldmulti like this.

  8. JunkRat
    Joined: Dec 2018
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    JunkRat Junior Member

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