designing a fast rowboat

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

  1. nordvindcrew
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    nordvindcrew Senior Member

    wintech boat

    please follow up on that Wintech boat. if you find a site, post it for us. We made the changes to our 8'-9" oars: moved the buttons out-board 2". The set up was awful. We moved our stretchers back 3-1/2" and everything fell into place. The boat rowed well and we both felt very comfortable and were able to get out of the boat with no crippled legs. No GPS aboard so I don't know how fast we were going but it felt good and maybe approaching race speed. Winning time now is to be able to do 9 minute miles. Tough for us old guys. The boat is an ever evolving entity. Right now we think that because of the very deep fore-foot and stern post the boat is self-steering when it heels over. The easy fix will be to put a lot of rocker into the stern and bow so that they are only in the water by 1-2" instead of the 6" they are currently. This may negate the need for the rudder, and will certainly help the boat to turn better. An easy fix, maybe one day and all done. I hope we can do this in my brothers garage this winter to be ready for spring.
     
  2. rowerwet
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    rowerwet Junior Member

    on the catamaran design ideas, a few years ago there were some pictures in MAIB (messing about in boats) of a rowing cat' that used PVC pipes as the hulls, it was 18' or 20' long with straps around the pipes to bind the rowing seat to the hulls, according to those who used it it was super fast, rather wet (there was no shield under the rower) and turned like a battle ship, as the only rocker was whatever the unsupported ends of the pipes curved up to. the pipe ends were sealed with inflated beach balls
     
  3. HJS
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    HJS Member

    A fast rowboat

    This study presents the results of the design of a new bottom for rowboats. The usual opinion is that the traditional hull shape with fine ends would be the optimal for rowboats for exercise and sport. This study shows that it is possible to create much more easily driven and seaworthy hull than the common.

    In comparative tests in flat water, it appears that the new hull shape is about 20% faster than the traditional hull. In rough seas, the difference may be even greater. This means that the new boat can go nearly one knot faster with the same energy input.

    js
     

    Attached Files:

  4. DCockey
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    DCockey Senior Member

    What are the lengths of the boats? Wetted area?
     
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Wining margins in the Olympics and World Championships are very small, of the order of a few metres.
    If this boat is 20% faster than the current rowing shells, it would beat them by about 400 metres over a 2000 metre course.
    Does that sound realistic to you?
     
  6. DCockey
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    DCockey Senior Member

    Could be interesting to see how the new design does in competition versus some of the faster open water sliding seat boats.
     
  7. HJS
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    HJS Member

    Leo:
    no, this is not realistic
    please look at the illustrations
    these boats are no olympic rowing shells

    DCockey:
    length in water is 5,3-5,4 m
    displacement 0,14 m^3
    wetted surface 2,6-2,7 m^2

    js
     
  8. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I looked before I posted :)
    I still have the same doubts: in a race over a long distance
    the new design would win by hundreds of metres. I find that
    extremely unlikely for a fairly slender hull.
     
  9. HJS
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    HJS Member

    the traditional hull was not that slender as I thought
    the prismatic coefficient is low as 0,49
    the new design has a prismatic coefficient as 0,58
    and it behaves completely different in the water
    the bow and rear is not moving up and down as in a traditional boat with very slender bow and aft

    js
     
  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I'd want to see results of independent tests before I would accept
    a 20% speed improvement. Sorry, but it smells like I'm being
    offered snake-oil at the moment.
     
  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    I too have my doubts. I'm not really a rowing man but here is my impression FWIIW. I suspect the speed difference is much less and mostly results from the greater volume aft. The claimed speeds for either boat don’t seem especially fast, but we are not told what the boat length is.

    The traditional Whitehall style heart-shaped or wineglass transom has a somewhat canoe shape underwater with more capacity above the waterline and was used to improve capacity for a given length and also to maximise unloaded speed IMHO. The rational for that is, a boat plying for hire had to get to the customer faster than the compertition to get his business, after that if the stern is loaded down it didn't matter so much as the rower would have earned his supper. The newer transom design may be an improvement in a following sea, but might not fare as well in the original competitive commercial application.

    I don’t see the advantage in flat water of the schooner bow, but again it might help in open water. I wonder if a Baidarka stem would improve speed even more . . .

    The more rounded underwater shape at bow and stern would make sense to reduce wetted area but that seems to be a feature of the alternative “racing” design. Since the study states that wetted area is the same for both the new design and the traditional design I am a bit confused.

    The concept of the developed plywood design is interesting.
     
  12. river runner
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    river runner baker

    Lcb

    In a performance displacement hull, the LCB should be about 55% (+or-) aft of the cutwater. But the LCB isn't necessarily at the widest point on the wateline. Depth is as much a factor as width. Since the boat will track better and still turn if you give it some rocker in the bow, but little or no rocker in the stern, then the widest point can be moved forward. You would need to use a hydrostatics program to get these balanced. I would be inclined to put the widest point at 50% and play with the bow and stern rocker.
    Since the oarlocks need to be at least 4' apart and this is for potentially rough water, it would seem prudent to me to make the boat at least 4 feet wide rather than design a narrow boat and use outriggers. You can still make it narrow at the waterline.
    On a decked design, I would simply glue a wooden block under the side decking and drill holes for the oarlock sockets.
     
  13. river runner
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    river runner baker

    dories and oars

    A while back, Tim Tebo was voted the most over-rated player in the NFL. I don't agree with that, but I do think that the Banks Dory (and all it's offspring) is the most over-rated boat design in history. If you build some sort of Banks dory offspring, you will be shooting yourself in the foot.
    The more oar there is outside of the oarlocks, the higher the potential tip speed, but leverage is working against you. For those of thus that aren't Olympic rowers, a long oar with a small blade area will probably give you the fastest boat speeds for this purpose (ocean racer rowed by seniors). Oar length is also related to expected speed. With a rowing shell, with expected high speed, you'd use a longer oar than you would on a Jon boat. The faster and more efficient the hull, the longer the oar. The slower and less efficient the hull, the shorter the oar. So, you would want an oar longer than what you'd use on a fishing boat, but shorter than what the olympians use.
    But don't listen to me, I don't even belong on this forum.
     
  14. Leo Lazauskas
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    Leo Lazauskas Senior Member

    If the wetted areas are truly the same, then that means the new boat
    has an incredibly lower residuary resistance.
    I can't see that being achievable unless the original boat is not being
    fairly assessed, for example, by running with the transom running partly
    wet at the same time the new boat has a more dry transom. The original
    boat might perform better if the rower moves forward in order to
    keep more of the transom out of the water.

    I wish the advantage gained was true, but big claims (and 20% speed
    increase is a big claim) require big supporting evidence. The assessment
    I see does not have enough science behind it to convince me yet.

    Claiming that the same energy has been expended in rowing the two boats
    is also very subjective. I'd like to see some oarhandle force measurements,
    but that is probably beyond the resources of amateur designers/builders.
     

  15. HJS
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    HJS Member

    Nobody here seems to study how the boats move in a stroke. Most people just guess.

    Referring to racing boats with much greater length, lower weight, higher speeds and minimal seaworthiness I consider to be inappropriate.

    Nor can we refer traditional rowing boats fitted with roller seat to the usual boats characteristics and computational models. The boats are not optimized for roller seats and constant load.

    Rower moves back and forth and moves the system's center of gravity thus approximately + / - 0.25 meters during one stroke.

    For boats intended for sports and long distance probably the speed variation will be approximately + / - 1 knot during one stroke.

    The tested the boats moves also in very critical speeds.
    FnL 0,25 FnV 0,81 3,5 knots
    0,32 1,05 4,5
    0,39 1,28 5,5
    0,46 1,51 6,5
    The highest speeds above are those usually called semi-planing.

    All this obviously affects the boat's character significantly.
    Longitudinal movements can be very different depending on the design and weight distribution. These movements takes energy from the horizontal forward speed.

    Although if the difference between the tested boats would be somewhat less because some details, it is still a big difference.

    No modern design has a narrow stern today. There is a reason for this. Could it be that the boats are more easily driven?

    The difference between those who have doubts and me is that I know that the new boat is much more easily driven because of the completely different design. Take it or leave it.

    Although I doubted my calculations and assumptions before I got the confirmation. I myself am convinced that the plywood version with its longitudinal chine can be even better.

    Any doubters are welcome to try out themselves. Remember to bring your own instruments. Take the opportunity before the ice is coming. Winter will be here soon, even if global warming makes itself known also far up here in the north.


    Welcome aboard

    js
     
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