Predicted squat and drag of a 6m hull

Discussion in 'Boat Design' started by Leo Lazauskas, Jan 5, 2010.

  1. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Attached is a brief note describing predictions of the squat and resistance of Rick Willoughby's V14 6m hull which he used in an endurance race down Australia's River Murray.

    I have put this in a thread of its own because some of the results have features that I think are general enough to be of interest to others. The hull is shown in a separate thread (Pedal Powered Boats).

    Note, for example, how the displaced volume changes with increasing speed. After it reaches a maximum at about 3 m/sec, the displacement begins to decrease back towards its initial value as speed increases. Some might disagree, but there is no way that I would describe this as planing, or even semi-planing.

    Also attached are two comma-separated text files of results which can be read by Excel. You should be able to reproduce the graphs in the report using these files. I hope the headers of each column are self-explanatory.

    File "report1_stat.txt" contains results for the hull assuming it is in a static attitude at all speeds; "report1_dyn.txt" is for the case when the hull is free to sink and trim.

    Rick, please tell me if there are any barbarous typos or mistakes in the report and I'll fix them.

    Leo.
     

    Attached Files:

    Last edited: Jan 5, 2010
  2. baeckmo
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    baeckmo Hydrodynamics

    Interesting, but would be more interesting if speeds and vertical positions were nondimensionalized; comparisons made easier.
     
  3. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Yes, but it would be easier for some and worse for others. Some want speed in m/sec, others in knots; best of all IMO is Froude number, but that immediately loses more people. Same with drag: Newtons, a drag coefficient (which one!) or the specific resistance?

    In any case, the raw data is there and people can scale it themselves :)

    All the best,
    Leo.
     
  4. Guest625101138

    Guest625101138 Previous Member

    Leo
    This result is impressive. It agrees very closely with the feel I have with the boat.

    Anyone back-tacking through the story on the pedal boat thread would believe you have contrived the results to match my observations or I knew the results before I wrote my observations.

    I do not notice the squat on the main hull but do see the aft end of the stabilisers come closer to the surface as I build to cruising speed.

    You may not consider the drag increase very much but I actually work out whether I carry 3, 4 or 5 litres of water depending on the distance and temperature of each day. The dipping rudders I use save about 3W. An extra 7W (say 9W at cranks) from sinkage is significant.

    I will have to back-track on the results you did on the sinkage of the V11 hull last year. I do not recall it lifting as the V14 hull does.

    NOW the obvious question- How would Flotilla change the shape for the lowest drag hull designed for 3.3m/s with a length constraint of 6m and chine constraint? Can I do better than V14?

    I am very pleased with the V14 hull. If nothing else it shows that an easily built hard chine hull has some advantages over a hull with rounded sections as the speed goes up.

    Rick W
     
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I'm not surprised that the results are good because your hull is thin in the waterplane. I have some remarkably good results for real rowing shells which are even thinner.

    I'm sure it's important for some races, and a minor matter in others. Again, in elite rowing 1% is a good winning margin so accounting for squat is important. There the matter is complicated by the rower's movement in the shell: at some stages of the rowing stroke the rowers CoM counters the bow-up moment, at others it increases that moment. That's a really fun challenge!

    Flotilla has no optimisation capabilities. Squat takes about 1sec to 10sec to calculate so it is still too slow to include in my optimisation routines. I am looking at other ways using Godzilla, for example, by allowing constraints on GML and GMT to guarantee minimum initial stability to reduce roll and pitching. If I come up with something better than your V14 I will most definitely pass it on.

    All I can suggest at the moment is to try to understand the delicate interplay between viscous drag and wave resistance. To do that you will need to heed Baeckmo's advice and non-dimensionalise the speed and the drag.

    For a start, I would replot whatever experimental results you have collected and calculated as functions of Froude number. Try comparing the drag of your vessels using a drag coefficient. For example, divide the drag by the usual 0.5 * rho * U^2 * A, where A is some reference area, U is speed and rho is water density.

    For skin-friction, use the wetted area as the reference area and see how your candidates compare. To better appreciate the wave drag behaviour, use beam squared as the reference area.

    All the best,
    Leo.
     
    Last edited: Jan 6, 2010
  6. Guest625101138

    Guest625101138 Previous Member

    Leo
    It would be interesting to see if optimising on Flotilla would drive toward flatter sections. The trim change and associated sinkage or lift brings in a third dimension in the drag determination. It is certainly going to be significant at higher froude numbers on any hull.

    It seems with Flotilla you now have an analytical tool that challenges Savitsky for the planing regime.

    The sinkage data for the V14 hull shows the "hole" to be around the design speed. I will be thinking about how I can overcome this. Losing 10% of my body weight is an unlikely option.

    Rick
     
  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    1. Trim and sinkage certainly complicate matters. As I said before, in many cases it is not important, and there is no need to include them in calculations. For applications where 1% or 2% performance gains are important they should be accounted for.

    2. Flotilla cannot handle planing. Planing is NOT the same as deep-water squat.

    Furthermore, Flotilla is a thin-ship code. It would not be accurate for planing hulls which typically have small L/B. I was working on a planing theory for slender hulls (e.g. surfboards, skis etc) with E.O. Tuck a couple of years ago. Unfortunately he died last year and I am on the verge of unemployment again so that project is on hold.

    3. The "hole" at the design speed does not depend on your weight. It is mostly a function of the hull shape and the length-based Froude number. Even if you lose weight the hull will still experience maximum sinkage at a Froude number of about Fr=0.4 to Fr=0.5.

    Leo.
     
  8. Guest625101138

    Guest625101138 Previous Member

    The weight comment was made to point out the significance of the sinkage in terms of my performance. If I wanted to be at the design power including sinkage then I need to be 10% lighter.

    In operation I do not sense a hole although I do sense the lift at higher speed.

    I can observe the sinkage on the outriggers but I am not certain this translates to increased wetted surface on the main hull. The bow wave reduces the water level down the side of the hull for a fair distance. I have never looked closely at where the actual waterline is in operation but it is something I could check. Would have been easy in the murky water in the Murray but more difficult in the clear water of the local lake.

    Rick
     
  9. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Or you could design the hull for a slightly larger displacement at the design speed.

    This is what Don Macpherson and others sometime refer to as the "moving dish problem": not only does the hull sink downwards, but the water level close to the hull also drops. It is not a simple matter to determine how much the wetted surface increases or decreases for small thin hulls. For example, does one include the meniscus on the hull in the wetted area? It's only 1.5mm or so, but that can be significant for a hull with a draft of less than 0.1m.

    Leo.
     

  10. Guest625101138

    Guest625101138 Previous Member

    The bow on V14 has a greater included angle than V11. It has a noticeably larger bow wave. This gets back to the question of form drag I expect. Is there benefit in reducing the dynamic wetted surface using this approach?

    The V11 hull had no noticeable dynamic qualities. Throughout my speed of interest it behaved as a displacement hull. It felt like I was approaching a brick wall at my power limit but the speed was almost 5m/s.

    The V14 hull seems to have some dynamic merit around my speed of interest. As noted the bow wave is noticeable, the trim change at cruising speed is perceptible and the lift at speed is noticeable.

    Irrespective I have been pleasantly surprised with the performance of the boat. So much so that I was actually disappointed when I did my last engine calibration because I expected my speed on the water would have translated to higher power output. I was doing better speed than I expected for my puny 120 to 130W.

    I will do some actual waterline measurement on the current hull before I change the design displacement for the next boat.

    For now I have to do paid work.

    Rick
     
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