Multihull power requirements

Discussion in 'Multihulls' started by Brian Alsum, May 27, 2021.

  1. Alik
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    Alik Senior Member

    It is not a 'simple math'. It is a calculation procedure based on either graphs or regression procedure, and on de-composition of resistance components. Normally done in Excel spreadsheet or other software.
     
  2. Brian Alsum
    Joined: Nov 2018
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    Brian Alsum Junior Member

    Great... so lookup tables based on known hull geometry?
     
  3. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    So one thing I've learned from the pros on this forum is that the exact hull shape almost doesn't matter at all for resistance - as long as it's a fair and smooth shape overall. Displacement and length does. And wave interference. So you don't need a scan.

    So just model any kind of fair multihull shape then all you need is length, displacement, beam over all and beam of hull and throw that into prelimina.com. It is verified against existing series data and apparently an improvement over michlet.

    I'm not sure why the overall response to that tool has been so muted. From what I understand it's the best tool for an amateur / novice like me. And it's free!
     
  4. Brian Alsum
    Joined: Nov 2018
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    Brian Alsum Junior Member

    I've played with prelimina my problem is i don't have a model, even a basic one that would match the boat I'm looking to get data on. if someone has a model they would share that would be awesome
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    au contraire

    For someone that wishes to learn, you're in one hell of a hurry to rush into a brick wall of your own making coupled to refusing to listen to the advice given.
     
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  6. Dolfiman
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    Dolfiman Senior Member

    I am developing a power catamaran VPP version, currently under test, a preliminary version is proposed here attached where I input data derived from the Leopard 48 brochure :
    ** Lw ~ Lhull = 14,75 m
    ** Lw/Bw assumed > 7 (necessary for the validity of the residuary drag)
    ** Displacement with half loading : 17,28 t >>> for each hull 8,4293 m3
    ** Wetted surface of each bare hull : here, without any linesplan and hydrostatics, I assumed Sw/D^(2/3) ~ 6,9 >>> Sw = 28,58 m2 for each bare hull
    ** No rudders neither keels taken into account for this computation.
    ** Space between hull axis ~ 5,2 m
    ** Saero : it is the estimation of the frontal area opposed to the apparent wind, estimated by B 7,64 m x H 3m ~ 23 m2
    ** Cx aero ~ 0,4
    ** Propulsion efficiency ~ 0,54 (here estimated by : propeller eff. 0,6 x mechanism eff. 0,9)
    >>>
    The file computes the drag components (friction, residuary, aero) in calm water, no wind conditions, the total drag and the power to install for each speed goal (from 1 to 18 Knots). Computation are detailed by columns , curves of the total drag and the "power to install" are shown.

    To note that the validity of the residuary drag computation, in addition to Lw/Bw > 7, is focused on Froude range Fn 0,4 to 0,9 : in the example attached , speeds 10 to 18 Knots are in this range.

    The file can be use with either Open Office or Libre Office, can be read only with Excel.

    By hoping this can be helpful ,
     

    Attached Files:

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  7. Mr Efficiency
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    Mr Efficiency Senior Member

    That's interesting Dolfiman, have you compared it to other existing similar tools ?
     
  8. Dolfiman
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    Dolfiman Senior Member

    Not yet, but I can tell you about my approach and my sources :

    For the residuary drag :
    I used the available output data of model tests as reported by :
    • P. van Oossanen in « Resistance prediction of small high-speed displacement vessels » where I picked up the ones for slender hulls (i.e. Lw/Bw > 7) in Series 64, SSPA series and NPL series.
    • Molland & al. in « Resistance experiments on a systematic series of high speed catamaran forms – Variation on length-displacement ratio and breadth-draught ratio », the Southampton series.
    The 2 papers are attached, I put all these data in same units and presented the results in the form of adimensional data and curves ready to use in « Data to estimate the residuary drag of a catamaran » also attached, from which I have developed the spreadsheet file.

    The key point for narrow monohulls of Lw/Bw > 7, highlighted by P. Oossanen in his paper and confirmed later on by Molland results was that, I quote P.Oossanen (page 219), «Up to Fn-value of about 0,9, the results for the 3 B/T values are almost identical, again leading to the observation that in the speed range of Fn 0,4 – 0,9 the length-displacement ratio is the only significant parameter », the LD ratio being Lw/Dc^1/3 (Lw waterline length, Dc displacement of the canoe body hull).

    The other crucial point is the amplification factor due to interaction between the 2 hulls, fully adressed by Southampton series / Molland experiences. In adimensional, Residuary drag : Dr/Mg (%) Cata = (1+K) * Dr/Mg (%) Monohull
    Here it is less simple to resume the respective role of Lw, Bw, Tc and Dc combined with S the space between hull axis. You can see the various cases in Fig. 3 to 12 of my paper, that we can average in Fig. 2 when keeping only S as main parameter. Currently for my spreadsheet, I use an enveloppe of all the "Molland" cases which actually look likes the Fig.2 with space S as unique parameter and is a priori a conservative approach, but I hope to do better for the final version.

    So in conclusion, one can estimate that way the residuary drag of a catamaran for Fn 0,4 – 0,9 (let'say 0,3 - 1,0 still acceptable) and under the condition that Lw/Bw > 7.

    Why no longer reliable for Fn > 0,9 -1,0 ? : because then the hulls are no longer primarily in displacement mode, dynamic lift forces occur, and then a lot of parameters like sections shape (circular or in U), Bw/Tc, trim, … can change the resulting drag.

    Why possibly inacurate for Fn < 0,3-0,4 ? : because the wave fields and their interaction are more complicated, but it should be noted that such speed is usually not a design objective except for slow by purpose catamaran.

    For the friction drag and the aero drag :
    State of the art formulations as for any boat project, for friction I use the ITTC57 formula for Cf with the Reynolds based on 0,7 Lw as recommended by Larsson & Eliasson in « Principles of Yacht design ». The key points being of course to have a right value of the wetted surface for the design displacement (for friction drag), of the frontal area and its global Cx (for the aero drag).
     
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  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Splendid post and attachments, enough material there to answer a lot of queries. I am curious about the threshold for L/B ratio (>7) and no scaling beyond that, is that an accurate method ?
     
  10. Dolfiman
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    Dolfiman Senior Member

    You mean what happens for L/B < 7 : I think it is for the same kind of reason as for Fn > 0,9 when L/B > 7, i.e. dynamic forces occur, we are then in a mix of displacement and dynamic lift forces which depends a lot of the hull sections shape. The fact to have narrow enough hull within the condition L/B > 7 (the usual catmaran case) could delay the appearance of the dynamic forces up to Fn 0,9.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    I was thinking we would get benefits by going more than 7:1, towards 12:1 or 13:1
     
  12. Dolfiman
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    Dolfiman Senior Member

    Ok, I understand your question : to increase the LB ratio at a given displacement of course increases the length-displacement one and so decreases the residuary drag. But at same time, the wetted surface usually becomes higher with high LB ratio. So only a VPP combined with a real early-stage project, inc. its design speed for the power to install, can identify an optimum. A design speed of Fn 0,6 or Fn 0,9 can lead to 2 different optimum. Yes, it is usual to consider that L/B 9:1 to 13:1 are good figures to start a project, but due to the relative importance of the residuary drag and of the friction drag in the total, only the approach with a VPP can answer precisely.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Good answer, and I look forward to more discussion along these lines, which is surely what the OP wanted, it did seem to me his 20:1 L/B ratio was heading off in the wrong direction with a lot of wetted surface area to contend with for the displacement involved. But I may be mistaken myself with that idea.
     
  14. Dolfiman
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    Dolfiman Senior Member


  15. Dragontri
    Joined: Jul 2021
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    Dragontri New Member

    Hi Brian,
    You've been getting some great info and the usual Curmudgeon's Observations. May I add another wrinkle: all the formulas address basics but I am missing out on considerations for a critical other aspect: useful info for adding wave and wind resistance, i.e. conditions when one is out there and really needs the power. I am in the same pickle: I am getting ready to put outboards on a 38 ft powertri and while I have the basic Power Displacement worksheet, I can't find anything easy (= plunk the relevant data in a spreadsheet) to sort this. Anybody?
     
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