Looking for help determine speed and power requirements of potential power catamaran

Discussion in 'Hydrodynamics and Aerodynamics' started by massandspace, Nov 8, 2017.

  1. sigurd
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    sigurd Pompuous Pangolin

    I'll compare your numbers to what I got then. Meanwhile, would it be interesting for you to run this smoother hull of the same general shape? Edit: Sorry, I couldn't close it to become solid to measure volume, not sure what I did wrong. Let me know if I should fix it. Edit: Your measurements match mine but I didn't do anything -hydro with it.
     

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  2. TANSL
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    TANSL Senior Member

    I will try to make some numbers with your hulls and I will show them to you. I have to tell you that I do not calculate with Rhino.
     
  3. TANSL
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    TANSL Senior Member

  4. Ad Hoc
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    Ad Hoc Naval Architect

    See below, in kW:

    Power Cat DB Forum.jpg
     
    Last edited: Nov 12, 2017
  5. sigurd
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    sigurd Pompuous Pangolin

    Thanks TANSL, I assumed you used Maxsurf?
    Anyone know if there is a way to get .mlt from Rhino? It's been a while since I used Michlet but it would be cool to compare since you and Ad Hoc already have presented several solutions. I only have a script for exporting 2d foils to text though. Also, if both of you had the resistance and or power numbers very conveniently accessible I could put them in the same OOO graph.
     
    Last edited: Nov 12, 2017
  6. sigurd
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    sigurd Pompuous Pangolin

    Here is what Hullcalc spat out.
    That's brake and effective power, and friction and total drag.
    Edit: I fudged the prop efficiency and correlation factor so that it correlated with my old cat, a 6m 350kg going 10-11 kt with 6hp. This also includes 5m^2 frontal area in the air with Cd 0.4 and some drag from the engine struts. The prop eff used is 0.6.
     

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    Last edited: Nov 12, 2017
  7. TANSL
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    TANSL Senior Member

    Yes, indeed, I have used for this boat Hullspeed (Maxsurf).
    These are the results that I get, with an efficiency of 60% (not exposed area).
    Regarding obtaining .mlt, I have no idea on what data are required or how to obtain them, I'm sorry.
     

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  8. sigurd
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    sigurd Pompuous Pangolin

    Yikes. Just to be sure, that was my hull right?

    I had to replace all the commas with dots for OO Calc, so I didn't bother with all the data points. I used Enguage digitizer for AdHoc's graph. My data is from Mal Smith's Hullcalc.xls.
    For the OP: My data is very sketchy with a big correlation factor since without it Hullcalc underpredicted the power of my boat by a lot. I believe AdHoc designs boats for a living, so I would guess his method is well tried. Anyway, it was interesting to see the spread.

    Incidently I am not sure high thrust outboards come in all sizes, but I do think they come in the 9.9hp variety, which used to be a stifled 15hp so it was easy to remove the clog and get 15hp. MaxSurf, AdHoc and Hullcalc 7p3m cat graph.png
     
  9. TANSL
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    TANSL Senior Member

    Yes, it is.
     
  10. sigurd
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    sigurd Pompuous Pangolin

    Interestingly you predict it to have a higher drag than the first one. For this graph I digitized both yours. So it is a comparison of the original, and then my hull version. The axis are HP and knots in this one.
    Original hull vs sigurds Tspline hull.jpg
     
  11. TANSL
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    TANSL Senior Member

    In the original I used an efficiency of 65%.
    On the other hand, the wet surface of your hulls is slightly higher than those of the original hulls and the coefficients of the hull (Cp, Cb, ...) in yours are greater than in the original (compare the hydrostatic values of each one). The resistance to the advance, therefore, should be greater in yours.
     
  12. sigurd
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    sigurd Pompuous Pangolin

    Makes sense. Thanks.
     
  13. massandspace
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    massandspace Junior Member

    Hello.

    I am the person who originally posted the question....I really appreciate the help but am a bit confused on the 4 lines on the chart....what the heck is a "FUNG"?

    I drew the hulls as best as I could by adding a pieces of tape every 2 feet that went from the waterline down to the keel....used a level taped to a piece of wood and measured in to the hull about every 2 inches down...drew points at those points in Rhino...connected the points to make curves, lofted all those curves into a surface, mirrored the surface, then copied the entire hull and spaced it apart from the first hull. Sorry, I am not an expert on Rhino and the best I could do! I tried to "smooth" the surface after drawing it but don't really know how to perfect that Rhino command.

    It is my understanding that the hull spacing is important in multihulls as the bow wave will impact on the other hull....I assume the calculations were done using a single hull...and not sure if they will work as asuch?

    Can someone explain the different lines....what is the highest FUNG line....is that where I am at? Or the lower ones?

    And, does the graph take into account the rocker in the rear of the hull? I understand a flatter section back there would be better, yes, but the hulls are what they are and just trying to get a rough idea of power requirements. I have thought about building small foils ack there.....if I did that the bows would not rise so much as they ar doing now....and I assume would greatly flatten the go curve, no? If I drew the foils and added them to the surface would the software understand that and spit out new curves?

    And....the bisggest engines I can put on are two 20 hps....so 40 ho total. That is because they only make "high thrust" props for motors up to 20hp....and I need those "5 pitch" props to not waste energy due to the prop not biting in.

    Thanks again for you're help.....David
     
    Last edited: Nov 14, 2017
  14. TANSL
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    TANSL Senior Member

    Hi David,
    "FUNG" is the name of one of the methods used to calculate the power. Another method, perhaps more suitable for your boat, is the so-called "Compton".
    The hulls of your boat are very far apart and the maximum speed that you will reach will not be higher than 10 knots so the interaction between the hulls could be neglected.
    You have done a good job in obtaining the shapes of the boat and so you have to recognize it but you need to soften the hull a bit more if you want to perform the calculations correctly.
    The software takes into account everything that is included in the 3D model.
     

  15. sigurd
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    sigurd Pompuous Pangolin

    Hi mass... I think all the power graphs (3 from TANSL, one from Ad Hoc and one from me) that models your hull is trying to do the same - predict the power/speed. I was worried that the bulges might add significant drag in his sim, that is why I drew a smoother, similar shape for TANSL to run. But I no longer think that is a significant issue? My smooth hull had more drag than yours in the end.

    So, which prediction should you believe? Like I said, my numbers are only correlated to one other boat, which was very different to yours. Also I have not studied the formulas in the spreadsheet I use. Hence, I would not have put much trust in my numbers, but it was interesting to see that they weren't that far from Ad Hoc's numbers in the relevant speed range. Yes, my numbers take into account the rocker but WL shape and other shape factors are only approximate.
    Like I alluded to, I would not be surprised if Ad Hoc has tested his method against many real boats - if it was me, I would go with his numbers.

    I don't believe the hull-hull interference is going to be a very big deal, as mentioned by TANSL too.

    I'm pretty sure a foil is a good idea, but I'm not able to model what effect it would have on speed.
    I had one 6hp to one side of the 6m cat, and it had a shelf just above the propeller where I mounted two windsurfer fins. It helped a tiny bit on speed, but it was actually faster to take them off and ballast the bows.
    These were the downfalls of my foil as I see it:
    Too short span
    Too shallow
    Big lift discontinuity in the center where the engine strut was
    Symmetric sections.
     
    Last edited: Nov 14, 2017
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