designing a fast rowboat

Discussion in 'Boat Design' started by nordvindcrew, Oct 13, 2006.

  1. Clinton B Chase
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    Clinton B Chase Senior Member

    I'm skeptical that the chines are an issue. The Finnish boats are fast (I've rowed them and in Finland) and they are most definitely chined (glued lap, 4-strake).

    For the Finnish boat, its all about absolutely minimum wetted surf and a displacement that is closely matched to the rower. The Cp is not crazy high on the boats either, I think its 0.52-0.53. They are essentially double ended on the waterline. If you watch the videos of two strong tandem rowers, you can see the boat getting overpowered, so they are not all out speedboats, but rather designed for more marathon type races on the lakes there.
     
  2. NoEyeDeer
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    NoEyeDeer Senior Member

    On the Savo, the laps are arranged such that they will cause little or no turbulence. This is not so much the case with the "traditional" single chine hull that has a more or less shallow deadrise amidships. These tend to have quite sharp kinks in their fore and aft lines (waterlines, diagonals, etc) which may be an indication of higher drag.

    Incidentally, I wonder about the assumptions behind the Savo's. What I mean here is that Michlet allows for the use of form factors, and IIRC the Savo hull was designed with assistance from Michlet. The form factor equation commonly used for viscous resistance (skin friction) is highly sensitive to the entry angle. This probably doesn't matter so much for hulls that share the same basic fore and aft shape, like rowing eights. These don't have hollow waterlines, so the entry angle will be more or less directly related to the length/beam ratio. I know Leo has used such a form factor to get Michlet results for eights that very closely follow the actual results for full-sized boats.

    However, if one were designing a hull that was significantly wider, and if one started employing hollow waterlines and calculating the form factor on that basis, then the result would be a significantly lower calculated viscous resistance. However, the boat still has to get much beamier anyway, so the angles some way back from the bow will be greater to compensate for the finer entrance. This would probably mean that the calculated form factor, based on the hollow entry, was artificially low.

    Short version: if using a viscous resistance form factor and hollow waterlines, Michlet may lead to a case of GIGO.

    I don't know if the designer of the Savo was using form factors, or how they were worked out if they were used. I am curious though.
     
  3. flo-mo
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    flo-mo Junior Member

    Here is a little comparison of Firefly and Savo 650 Double.

    It is problematic though because I only have the lines of the Firefly 18, which is the single. So I stretched length to 650 cm to equal Savo lengthwise.
    Anyway I think the lines are close enough to point out the differences of the two designs.


    Savo 650 Double:
    Displacement: 225 kg
    LWL: 640 cm
    BWL: 77 cm
    Wetted Surface: 3,54 m2
    Hull Surface: 8,66 m2
    cp: 0,57
    [​IMG]


    Stretched Firefly 18:
    Displacement: 225 kg
    LWL: 584 cm (566 cm without skeg)
    BWL: 76 cm
    Wetted Surface: 3,45 m2 (3,27 m2 without skeg)
    Hull Surface: 6,1 m2
    cp: 0,52
    [​IMG]
     
    Last edited: Aug 7, 2014
  4. Tallman
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    Tallman Junior Member

    Wow, great drawing Flo-Mo!
    What is the effect of a higher or lower CP?
    The Firefly Double I have is stretched, in my case, to 23 feet 4" overall, with a corresponding longer waterline. The waterline beam is the same as the Firefly 18. The top strake is 1" wider. I went a bit more vertical with the transom so the angle is less dory-like but I don't suspect that matters.

    Does the added length change the comparison much?

    The Savo lines look smoother overall, and no chine kink.

    I need to try more outings with a gps speed gauge -- I'm wondering if the one reading my friend took was accurate.
     
  5. flo-mo
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    flo-mo Junior Member

  6. Leo Lazauskas
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    Leo Lazauskas Senior Member

    The form factor equations devised by Nelson and Scragg have been used by
    some people for other types of boats, but I wouldn't recommend it.

    I also agree that hollow lines (and transom sterns!) could give misleading
    predictions.

    OTOH, as AdHoc has advised repeatedly, in design we should be looking at
    trends, not absolutes, so a small, rationally-based form factor is probably Ok.

    Form factors are a blunt object and I dislike using them. Flotilla 6.2 has a
    simple one devised by Grigson, but I have dumped it in version 7.x. in favour
    of the viscous pressure resistance. I'll release that version in the next few
    months.
     
  7. NoEyeDeer
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    NoEyeDeer Senior Member

    Just had a brainwave and compared the lines of the Savo 650 single and double. The single has the exaggerated hollow in the waterlines and the lower prismatic, while the double has a higher prismatic and waterlines with only a very slight hollow.

    This makes it pretty clear that the single's shape has nothing to do with calculated form factors, and is purely a result of trying to reduce wetted surface and displacement within the class minimum beam restrictions (they are measured a certain distance up from the keel). This makes sense, since wetted surface is going to be the vast majority to the total resistance, so reducing that rather than the wave drag (via higher prismatic) would be the better option.

    The chart of optimum prismatic isn't relevant when you start accounting for other factors like this.
     
  8. NoEyeDeer
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    NoEyeDeer Senior Member

    Just for fun, here's where I'm up to with my long-delayed beastie.

    It's going to be fixed seat, and the leading dimensions are much the same as a Savo 575. I'd expect speeds to be similar (assuming both were rowed fixed seat, of course). Mine has a smidgeon more wetted surface, but lower predicted wave drag at race speeds, as well as being a bit lighter. Should also have slightly better initial stability.

    LOA - 5790 mm (19' 0")

    DWL - 5693 mm (18' 8") at 0.112 m3 immersed volume

    Bmax - 1173mm (3' 10") to outside of planking

    BWL - 680 mm (2' 2.75")

    Draught - 67mm (2.6")

    Wetted surface - 2.544 m2 (27.4 sq. ft)

    Prismatic - 0.583

    The 0.112 m3 volume is probably a slight overestimate, or at least it will be once I get back into shape. :D
     

    Attached Files:

  9. GER407
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    GER407 Junior Member

    Do you have more pictures?
     
  10. NoEyeDeer
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    NoEyeDeer Senior Member

    Yes. Loads of them, but I probably wont bother posting them here. The boat is almost ready to flip and I'm sick of looking at it upside down. I'll post a few shots once it's right way up. :)
     
  11. NoEyeDeer
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    NoEyeDeer Senior Member

    Ok, so I feel like cheating a bit, just to show off. :p

    Instead of turning the boat right side up, I turned the world upside down. This was easier. ;)

    I had to do a clean up anyway, and was feeling so chuffed that I thought a few pix might be in order. Apart from anything else, they're good motivation for me.
     

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  12. NoEyeDeer
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    NoEyeDeer Senior Member

    I was idly thinking about this immersed transom idea, as used on Ben Booth's boat. Olympic sculls are exactly the sort of boat that could use such a stern. They are always rowed flat out, and at speeds higher than any fixed seat boat will reach. They also presumably want to minimise pitching, since the sliding seats make them pitch a lot.

    Given all that, I'd expect sculls would use a transom if there was any benefit in it. I'd also expect that research has been done on this as it's too obvious to overlook, and those guys are always trying to think of ways to go faster. Does anyone have any stuff on what effect transoms have on sculls, and what the trade-offs are?
     
  13. DCockey
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    DCockey Senior Member

    I vaguely recall an online conversation with Leo on this topic. My recollection is they have been tried but were not an improvement.
     
  14. Clinton B Chase
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    Clinton B Chase Senior Member

    She is looking great! Nice work. Look forward to hearing sea trial reports and comparing actual numbers to preliminary numbers!
     

  15. NoEyeDeer
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    NoEyeDeer Senior Member

    Welllllllllll, TBH if you know where to look (and I do) the work isn't always as nice as it should be. :D

    It'll pass though. Shouldn't fall apart, which is the main thing, and will mostly give a good impression.
     
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