Stern Extension to Help Rowing

Discussion in 'Boat Design' started by Russ Kaiser, Jul 15, 2014.

  1. Skyak
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    Skyak Senior Member

    Manfred,
    I see the defining characteristic of the design you are showing is the "S" shape of a longitudinal section through the keel. The exit angle causes lift that increases with speed -essentially trying to act like trim tabs on planing hulls.
    -what is the exit angle at the transom?
    -do you have drag/speed curves to go with these designs?
    -this is a way to lift the transom, but I think we are skeptical that this is a better way than trim by shifting weight or buoyancy with flat or positive rocker at the low power of this case.
     
  2. Manfred.pech
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    Manfred.pech Senior Member

    Skyak, this is the last boat of a shipwright and scientist for himself and his son before he retired this month. There was no interest in drag/speed curves. In my opinion the exit angle is zero. The shape is far more complicated than an "S" and it needs a computer and a special software to create the lines for the design. It reminds me a bit to the underside of the wing of a modern aeroplane, but ... that is by far not all. The shape is created to get a well balanced proportion of lift and suction at the underbody of a boat/ship. It took years and a lot of tank tests and CFD to get the results.

    [​IMG]

    I think you will develop a good solution for Russ with weight shifting and/or added buoyancy. Kind regards, Manfred
     
  3. Russ Kaiser
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    Russ Kaiser Exuberant Amateur

    Manfred, do you have a better picture of that sailboat hull - it's very hard to tell what it looks like from the side. We are looking at it "bottom-up" correct?
     
  4. Manfred.pech
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    Manfred.pech Senior Member

    Yes, "bottom-up". It is impossible to copy it. But you can try a slight hollow section and test it.

    [​IMG]
     
  5. Russ Kaiser
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    Russ Kaiser Exuberant Amateur

    Manfred,

    I was not interested in copying it, just interested in general.

    I'm confused. It looks like you've shown pictures of two different hulls. The first hull that is white looks unfinished.
     
  6. Manfred.pech
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    Manfred.pech Senior Member

    The first hull is the latest development. The yellow one is more than 20 years old. Another foto can help to estimate the hollow part of the hull.
    [​IMG]

    Here you can find another design with hollow lines: http://www.atkinboatplans.com/Utilities/MorningStar.html
     
  7. Russ Kaiser
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    Russ Kaiser Exuberant Amateur

    Work Progresses - New Photos

    OK,

    So I finally got to work with the foam and learned some valuable lessons.

    TiteBond III is not good for laminating sheet foam if you are going to have to trim it significantly and here is why:

    Titebond when used with wood relies on the low moisture content of the joined pieces to dewater the glue joint and aid in drying. In a closed-cell foam lamination when the pieces are fairly broad (in this case about six inches across) it would probably take days for the glue to completely dry since air can't possibly get to the joint.

    In my case the block appeared dry when I unclamped it after two days. When I started rough cutting the foam, however, the interior of the glue joint was still wet. It hadn't dried at all. After it was trimmed I let the new edges dry some more before I proceeded.

    If you were gluing foam and the lamination was very close to its final dimensions you would never notice this because the first inch or so of the joint is dry. I image all wood glues would perform similarly.

    At any rate, here are some new photos. First the blocks setting in the spoiler after the two main edges were cut with the band saw.

    [​IMG]FoamBlockRough by Russ Kaiser, on Flickr

    Next is a photo after the block was shaped and after an initial coat of epoxy prior to filling and covering with glass.

    [​IMG]FoamBlockShaped by Russ Kaiser, on Flickr

    I have started filling the gaps with epoxy fairing material and I will post a photo when I get the glass on the bottom. I am trying to decide if I will glass any other portion of this prior to testing. It would be a lot of wasted work if it does not pan out. If it does work well, I will put a layer of glass on the part that touches the boat. I will probably just finish everything else with a couple coats of polyurethane and then paint the whole thing to protect it from the sun.
     
  8. Skyak
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    Skyak Senior Member

    The picture of pressures on a wing don't really relate to what is going on on this hull because the hull is operating on a water surface not a continuous fluid. If these hollowed hulls achieve better efficiency it is by modifying the wave generation so you would have to look at the displacement curve against the wave it creates. Done right it might achieve reduced thrust at some speed below wavelength. This is not very useful because everyone is satisfied with the thrust it takes at that speed but they are not willing to go that slow or buy a boat long enough to be satisfied with that speed. I suspect there are penalties at speeds differing from the optimum.

    I think we are still interested to see speed/thrust curves but I don't think anyone is willing to do the work for the gain it provides. The reason Russ is going so slow seems to be emotional attachment to his hull;)

    BTW, the tunnel in the river boat hull is for prop clearance/shallow draft.
     
  9. Manfred.pech
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    Manfred.pech Senior Member

    Hi Skyak, first of all, please where is the "tunnel in the river boat hull" http://www.atkinboatplans.com/Utilities/MorningStar.html Or do you mean the tunnel hulls from the Atkins as for instance the design SANDPIPER http://www.atkinboatplans.com/Cruisers/SandPiper.html? or this one http://www.atkinboatplans.com/Cruisers/RiverBelle.html

    Second - I do not want to open an old can of worms. The discussion about boats with concave sections has been in Europa in the eighties. The best arguements were from Prof. Dr. Antony Ceslaw Marchaj, Prof, Dr. I.J. Gerritsma (Delft, NL), Mr. Strepp from german YACHT, Dr. Burghard Müller-Graf, Berlin (Tank Test Center) Prof. Dr. Peter van Oossanen (NL) and of course from the developer of the hulls shown above.
    Mr. van Oossanen himself developed an eighteenfooter with a hollow for the two times worldmaster Robert Brown who has won with it 1984 and 1985 in Australia. [​IMG]

    Third - Manfred is interested in the hydrodynamics of boats and their history - I am not interested to promote the developments of one special designer.

    Fourth - For me Russ Kaiser is one of the guys who is young and courageous enough to be interested in ideas aside the mainstream and is willing to invest time and money to try these "new" ideas - which in my experience is very seldom.

    Fifth - If Russ wants to build a new boat, the best one in my opinion is the catamaran designed, built and well tested from Upchurchmr - not a monohull with a hollow.

    Skyak I´m not interested to fight with old arguements for an idea which is well accepted and for instance applied on latest and fastest Trimarans with a slight hollow in the end of their floats. Regards, Manfred
     
  10. Russ Kaiser
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    Russ Kaiser Exuberant Amateur

    Hot Wire Cutting Foam with an epoxy joint.

    When I was shaping the blue foam for the spoiler I did most of the cutting with my band saw. After that I used my hot wire rig to cut off some of the larger remaining chunks before I started sanding.

    My hot wire jig is made with some nichrome wire, a power supply, and a voltage regulator. The beauty of making your own tool is you can make it any depth and size you need as long as you have enough current to get it up to temperature. I use a 15 amp 24 volt supply and a circuit board mounted voltage regulator with a digital voltage readout that I bought for 5 dollars on eBay.

    As I was cutting I remembered an earlier conversation about cutting through an epoxy joint with a hot wire. It just so happened that I had a scrap of laminated foam from a previous project that had an epoxy joint so I thought I would do some testing.

    Here is what I learned: Like most of you speculated the epoxy glue joint does not cut well with a hot wire.

    If you are cutting perpendicular to the join you can force the wire through it. If you try to go through the glue joint at any kind of angle, the wire will want to deflect along the glue joint instead of cutting through the epoxy in the intended direction. This might not be an issue with a table mounted wire, but with a free hand jig it makes getting accurate cuts nearly impossible.

    Here is my test piece. It looks like it worked better than it did in reality.

    [​IMG]HotWire by Russ Kaiser, on Flickr
     
  11. Skyak
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    Skyak Senior Member

    Thanks for the info Manfred. My interest is just in quantifying what these features can do -not a fight. Sorry if it sounded otherwise.
     
  12. Manfred.pech
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    Manfred.pech Senior Member

    Thank you, Skyak.
     
  13. Russ Kaiser
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    Russ Kaiser Exuberant Amateur

  14. Russ Kaiser
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    Russ Kaiser Exuberant Amateur

    The proof is in the pudding

    I went for a 10 km row, pretty much the longest I have done since I started capturing data on Strava (great program BWT).

    The boat immediately felt different and sounded different going through the water. The skeg made it almost impossible to turn so I think I am going to significantly trim it or whack it off altogether.

    Here is the overview data from tonight's row:

    [​IMG]First test with foil by Russ Kaiser, on Flickr
     

  15. Russ Kaiser
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    Russ Kaiser Exuberant Amateur

    Data without the spoiler

    And here is, believe me, a really great session from a while back without the spoiler.

    [​IMG]Very good row without foil by Russ Kaiser, on Flickr

    I can honestly say I wasn't pulling that hard on tonight's test row and even stopped to take a couple of photos. Strava should factor those out since it quickly determines if you're stationary.

    What it won't factor out were the two or three times I really got out of position because I couldn't turn properly and ended up in the weeds. As it is, I picked up about 4 tenths of a km in my average speed.

    I would like to do some more scientific testing so I need a methodology. Perhaps duplicating segments rowing at a particular cadence would work to compare the performance with and without the spoiler. I am open to suggestions.

    BTW, this is one of the photos I took.

    [​IMG]2014-08-15_071558_006 by Russ Kaiser, on Flickr
     
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