designing a fast rowboat

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

  1. Leo Lazauskas
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    Leo Lazauskas Senior Member

    A most excellent policy!

    ProTip: Remember that Michlet adds a boundary layer displacement thickness to
    the hull offsets before calculating the wave resistance if the value of Ntheta is
    positive. You can turn off BL effects by using a negative value for Ntheta.
     
  2. Clinton B Chase
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    Clinton B Chase Senior Member

    I think this is significant.

    The two boats I have in mind have similar principal dim's but different hull shapes, smooth-skinned/round bilged vs. v-bottom/chine construction.
     
  3. NoEyeDeer
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    NoEyeDeer Senior Member

    Just tried something yesterday, out of curiosity. Did a rough-but-fairly-accurate mockup of a Savo 575 and ran it through Michlet. Interesting result.

    I knew the Savo had a very low prismatic for a (relatively) high speed boat, and obviously this would mean higher wave drag towards the upper end of the speed range. The low prismatic does reduce wetted surface in the ends though, whch is important given that the Finnish boats have a beam restriction on their midships section.

    There's a limit to how much you can reduce BWL under this class's rules, which means you want to think about other ways of cutting wetted surface too. Low prismatic is the obvious choice, and this accounts for the distinct hollow in the waterlines of the Savo. IOW, the waterline shape of the Savo appears to have nothing to do with reducing wave drag, and to be purely a result of trying to reduce wetted surface.

    Anyway, this means the trade-off for the Savo is to take a wave drag penalty in exchange for less wetted surface. If compared to a more "normal" hull with similar BWL, length and displacement but with a higher prismatic, less draft, and a bit more wetted surface, the Savo ends up with less total resistance at low to moderate speeds but loses the advantage at higher speeds, where the two hulls are effectively equal in resistance. However, the fuller ended hull still has more stability than the Savo.

    This is interesting to me because of the way it illustrates a problem with rules and their effects, that being that the effects are often not what the rule makers intended. It seems clear that the intent behind the beam restrictions was to promote more stable craft better suited for a range of uses, and to avoid less stable craft intended solely for racing. However, because of the way the builders have exploited the lack of other restrictions, the result has been a hull form that is less stable while not being any better for racing.

    Go figure. :)

    ETA: In rough terms, it appears that for the same resistance at race speeds and the same DWL, you can get a metacentre about 100mm or so higher (which is a big difference on this sort of boat) with the drawback being 10-15% more resistance at speeds of 4 knots or less.
     
  4. Clinton B Chase
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    Clinton B Chase Senior Member

    I just figured out the word play in your screen name...why it too so long, I have noeyedeer.

    ON second reading, I followed you much better. My thing is fixed seat rowing, and the Savo, while being a sliding seat boat, may actually be fantastic for fixed seat as well with its low WSA. (I don't know the history of the development of this class in regards to whether it ever was a fixed seat rowboat, but when I was visiting there I did see some that appeard to be fixed seat...I also saw photos of people who had special short that allowed them to slide on top of the seats back and forth without chafe).

    It will be interesting for me to play with the hull shape that is in the water and draw something very pleasing to my eye above the water. In rough water conditions, the above the DWL shape is still important, though, not completely and aesthetics thing...windage, shape as the boat lifts (or doesn't) to waves, etc. David is studying hull shapes using Michlet and I look forward to seeing the results.

    In rough water, to keep the speed/momentum up, it seems that the important thing is keeping down resistance and making sure the boat is interacting with the waves/chop appropriately.

    I row on the Saco River a lot lately, and am feeling the need for a boat that will also slip easily through the current there, which can get quite strong. I look fwd to rowing the St Lawrence R. Skiff I have on the Saco. I have rowed Drake and did pretty well in the currents, but so hard to say in absolute terms what works better on the River.

    [​IMG]
    SacoRiver by Clint Chase Boatbuilder, on Flickr
    Here is a photo of my new rowing grounds. So, I need two rowboats now for sure, one for the River and one for the Bay!
     
  5. NoEyeDeer
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    NoEyeDeer Senior Member

    Oh damn. You need two rowboats? Life totally sucks sometimes. :D

    Anyway I remember reading some site somewhere, which said the Finnish classes were restricted to fixed seat originally, and changed to allowing sliding seat sometime in the early 2000's (IIRC). The slippery pants were an intermediate stage.

    ETA: Found it. http://koti.kapsi.fi/hvartial/scull/scull.htm

     
  6. Clinton B Chase
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    Clinton B Chase Senior Member

    At least two: when my quiver is complete it'll be at least 3.
     
  7. rowerwet
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    rowerwet Junior Member

    I worked in Portland for years, many times as I crossed the river I thought" what a great place to row", hardly ever saw a boat on it though, and never a rowboat. I tried to find a launch spot near the highway, but never did.
     
  8. keith66
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    keith66 Senior Member

    Three? I currently have a 16ft hamble river skiff, 18ft Skiff, 27ft Gig & i have just started setting up a stretched Cosine wherry, the Edwin monk stubby doesnt count as shes a tender, Guess i got it bad!
     
  9. Clinton B Chase
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    Clinton B Chase Senior Member

    The Fore River? Only place to really launch is at East End and row all the way around through the busy harbor. It is a fun way to go.

    I am missing my Drake more than I thought I would. I need to get busy in the next boat.
     
  10. El_Guero

    El_Guero Previous Member

    Use air ....

    I always wonder why people do not use air when they want boats to go faster .... ?

    This is a serious, 'why?'
     
  11. NoEyeDeer
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    NoEyeDeer Senior Member

    Well I tried that, but it didn't work. The problem seems to be that boats made of air tend to leak rather badly. :)
     
  12. El_Guero

    El_Guero Previous Member

    Well, my inquiry was SERIOUS.

    Forcing air underneath your hull in places, greatly reduces the WSA .... and it speeds up the boat, this would work wonders, shouldn't it?, on smaller, and slower boats as well as larger and faster boats.
     
  13. Clinton B Chase
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    Clinton B Chase Senior Member

    This brings up the whole lapstrake vs smooth skinned idea....which is faster anyway?
     
  14. Syncrowave
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    Syncrowave Junior Member


  15. NoEyeDeer
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    NoEyeDeer Senior Member

    It only has a 14 foot waterline, so would not be particularly fast. Nice boat though.


    If smooth skin isn't fastest, why do all Olympic classes use smooth skin?
     
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