VPP with measured Drag

Discussion in 'Hydrodynamics and Aerodynamics' started by bitfield, Jul 10, 2022.

  1. bitfield
    Joined: Jul 2022
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    bitfield Junior Member

    Hello,

    I am designing a VPP model (using ORC equ) that will help me build and optimized sails for my boat (Outremer 51, Catamaran). However, instead of modeling the boat, I would like to use empirical measurements to come up with the hydrodynamic forces.

    I was thinking of using a simple drag equation like F_drag = C*V^2 and finding out C by measuring the time the boat decelerates in the water. Knowing the mass of the boat, I can then calculate C using F_drag = m*a . I can do these experiments on the boat at a time with no wind, no current and no sails. I can probably also incorporate the wind and direction at a latter time to come up with a model that is wind related.

    Is there a better way of doing this. I tried to search a bit, but could not find much on people using measurements to build the hydrodynamic forces on a boat for VPP.
     
  2. Remmlinger
    Joined: Jan 2011
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    Remmlinger engineer

    Your coefficient "C" is a function of the Froude-number. You have to do steady state towing experiments at different Froude-numbers to determine the drag over the entire speed range. The correct drag coefficient is: C = F-drag / (0.5*rho*V^2*reference-area).
     
  3. bitfield
    Joined: Jul 2022
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    bitfield Junior Member

    Towing the boat at various velocities would be a bit difficult (its a 51 foot catamaran). I was hoping to achieve the same thing by getting the boat up to about 7kts of speed, kill the engines and let it stop all while logging boat speed over time (repeating the tests several times).

    Given the mass of the boat, I should be able to solve for C. Am I missing something?
     
  4. jehardiman
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    jehardiman Senior Member

    Yes, you are missing something. A vessels drag is comprised of two parts, the wetted surface friction (Cf), and the form drag (Cr); i.e. total drag Ct = Cf+Cr. Furthermore, the form drag is proportional to two factors, the body shape and the "wave" resistance.
    Realistically, the "wave" resistance is the interaction between the shape of the forebody and the shape of the afterbody (google Wigley hull); and for a cat the interaction between the two hull wakes. This leads "humps" of higher resistance and "hollows" of lower resistance as speed and length interact.
    So there is no one constant value of "C", it changes with speed.

    Edit: Read this paper.
    https://www.boatdesign.net/attachments/residuary-drag-of-sailing-catamaran-pdf.149315/
     
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  5. bitfield
    Joined: Jul 2022
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    bitfield Junior Member

    Thank you so much for the paper. It explains a lot. So the equation is really non linear with C being actually C_v where a different constant value is used for various boat speeds.

    Could I assume that C_v is linear within a given range (like 1kt) and just measure the time it takes to drop 1kts? Then calculate a different C for other ranges. So 7 to 6, 6 to 5, etc.
    Any other suggestions on how I can measure that without towing?
     
  6. Skyak
    Joined: Jul 2012
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    Skyak Senior Member


    Isn't it a M&M design? Couldn't you just ask for the VPP that they filed away when they validated the design? Then you could just swap in your sail terms.
     
  7. DCockey
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    DCockey Senior Member

    The force of the water on a boat accelerating or decelerating is different than the force on a boat at steady speed due to what is called "added mass". The added mass effect is proportional to the mass of the displaced water and the acceleration rate. Added mass - Wikipedia https://en.wikipedia.org/wiki/Added_mass
     
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  8. DCockey
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    DCockey Senior Member

    My guess is the accuracy of drag derived from an open water deceleration test would be comparable to the accuracy of a drag estimate using an empirical regression based formula.
     
  9. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    I am the author of this paper, and here attached is the last version V3 of it.

    From this investigation based on all model tests results of slender monohulls and catamarans I could found, I derived a practical short version of data and approach to use for a direct estimation of the residuary drag of a catamaran, also attached.

    And finally, I integrated all that in a specific VPP for a sailing catamaran, so taken into account the other components of the drag (friction, aero, ..) and the specificity of a sailing catamaran meaning two hulls (windward and leeward) at different immersion in the water. For the input data for this VPP, you first need a modelisation of the hulls and of the sailplan with an ad hoc software. I can propose also the speadsheet application Gene-Hull Catamaran for that, although reverse engineering is not the goal of the application, but for a VPP objective, a good approximation of the hulls bodies can be sufficient. Applications, details and guide to use them are in :
    Gene-Hull Catamaran 3.0 and SA-VPP catamaran 1.0 | Boat Design Net

    I have not up to now the opportunity to compare the VPP results with scale one data, so if you continue in this investigation, I am of course interested by your results.
     

    Attached Files:

  10. bitfield
    Joined: Jul 2022
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    bitfield Junior Member

    Thank you so much for the updated papers and the context.

    For the VPP software, I have forked off Python-VPP and have been adding the catamaran eq.
    Python-VPP/LICENSE at master ยท marinlauber/Python-VPP https://github.com/marinlauber/Python-VPP/blob/master/LICENSE

    I would love to share my results with you and perform any updates that you see fit.

    One note, the boat is fitted with load sensors of the shrouds and sheets and I am able to capture some of the forces for the hydrodynamic part.

    I am going to run a few tests this weekend. I can share the data with you if you want. Let me know if there are any specific tests you think I should run.
     
  11. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    You've got the boat, tow it and plot it's drag curve.
    This is a no brainer, no?
     
  12. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    What can be done at first should be the most simple to interpret, i.e. :
    - some motoring runs (with no sails up), on flat water as much as possible, with wind from beam (to minimize wind interference), where you can record the boat speed and the motors rpm, providing you have the rpm/power curves for the motors + info on propeller efficiency if possible >>> one can then have an order of magnitude of the total drag and compare with the one from the VPP (I did also a motoring version of the VPP Catamaran).
    - beam reaching runs with mainsail and genoa, and to record the wind speed, the boat speed, the boat heel >>> comparaison with the values given by the VPP.
    + ... your shroud and sheet tensions but with all the details (geometry, angles, ...) >>> to be able to relate them to sails thrust and side forces, but indeed not easy to do that accurately, I think.

    On my side, I can try at first to build a numerical catamaran with Gene-Hull as close as possible to the Outremer 51, and then to prepare the input data for the VPP.

    I suggest you to continue on the details for all that by emails, I will send you a PM.
     
  13. Dolfiman
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    Dolfiman Senior Member

  14. bitfield
    Joined: Jul 2022
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    Location: Oxnard

    bitfield Junior Member

    Unfortunately its not that easy. Towing a 51 foot boat at these speeds is impracticable and a bit dangerous. I need to find another way to measure the data.
     

  15. bitfield
    Joined: Jul 2022
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    Location: Oxnard

    bitfield Junior Member


    Thank you so much for the response. I will run some experiments this weekend.
    I also just emailed you on a separate note.
     
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