Stern Extension to Help Rowing

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

  1. upchurchmr
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    upchurchmr Senior Member

    How about another picture of the stern with you in place, whenever you can.
    I wouldn't want to tell you "I told you so" about the skeg - Not Much anyway.:p;)

    The only real way to do an unbiased check is to tow the boat with and without the fairing, with a scale in line with the tow rope (you in the boat of course). Might be difficult to get the exact same speed, but with your system it might not be.
    You also have to have your row boat out of the direct line of the prop wake.

    Looks like an unqualified success. We can always "discuss" whether it could be better, of course, but I assume you are happy.
     
  2. Russ Kaiser
    Joined: Jul 2009
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    Location: Winston-Salem, NC

    Russ Kaiser Exuberant Amateur

    Real Test Data

    OK, so this morning I cut off the skeg and sprayed some spray paint on the wound. Then my son and I went out to the lake to try to do some scientific testing.

    First, while I was fresh, we went out without the spoiler on and figured out a couple of cadences that I was comfortable rowing using a metronome app on my phone. I chose 83 and 93 beats per minute with a 4 count and an accented beat. Rowing to those speeds, one stroke per accent, worked out to 20.75 and 23.25 strokes per minute.

    Next we rowed about .7 km at each of those strokes without the foil. Then we went back to shore, installed the foil and went back out and did the same two cadences with it in place.

    When I got back home I cropped the STRAVA (GPS) data segments so that we only used the middle of each run. Unfortunately, the boat does lose a good bit of directional stability with the foil on. It took some getting used to. Still I think the segments were pretty steady which you can see from how close the average and the top speed are for each run.

    During these tests my son stayed on board sitting on the back seat facing aft. He took video of the water coming off the back of the boat. I will attach links to these videos.

    [​IMG]No Spoiler 20.75 SPM by Russ Kaiser, on Flickr

    Video of wake
     
    Last edited: Aug 17, 2014
  3. Russ Kaiser
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    Location: Winston-Salem, NC

    Russ Kaiser Exuberant Amateur

    Last edited: Aug 17, 2014
  4. Russ Kaiser
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    Location: Winston-Salem, NC

    Russ Kaiser Exuberant Amateur

    Last edited: Aug 17, 2014
  5. Russ Kaiser
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    Location: Winston-Salem, NC

    Russ Kaiser Exuberant Amateur

    Last edited: Aug 17, 2014
  6. Russ Kaiser
    Joined: Jul 2009
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    Location: Winston-Salem, NC

    Russ Kaiser Exuberant Amateur

    Summary of today's testing.

    OK, here is how the data shakes out from today's testing.

    At 20.75 Strokes per minute.

    Without foil, Average Speed = 4.0 km per hour.
    With foil, Average Speed = 4.8 km per hour.
    Speed Increase - 20%

    At 23.25 Strokes per minute

    Without foil, Average Speed = 4.6 km per hour.
    With foil, Average Speed = 5.7 km per hour.
    Speed Increase - 23.9%

    The boat pulled in a completely different way with the spoiler in place, like I stated earlier it had less directional stability but it coasted much better and felt lighter. My son weighs about 130 lbs. and the without the spoiler I could really tell he was sitting in the back of the boat. You really need to watch the video links of the wakes to see what I'm talking about.

    I hope to go out and do this again solo. It was a beautiful Saturday here and I wanted to spend time with Stephen and he enjoyed helping take the videos and photos. My older son is doing his Order of the Arrow ordeal this weekend so Stephen was just lying about with no one to torment.

    At the end of our test, Stephen got out at a pier and shot a photo of me rowing with the spoiler in place to share with UpChurchMr.

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

    I guess the whole point of this frivolity is that if you have a sharp sterned boat that you occasionally row you can make something like this to make it behave better under oars.

    It also proves that it's extremely important to clean up the wake of a small boat that is going to move at or below displacement speed, whether it be powered by wind, oar, or a trolling motor; you don't want to be dragging a sharp ledge through the water.
     
  7. Manfred.pech
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    Manfred.pech Senior Member

    Thank you Russ for sharing your great research work with these astonishing results. When you look at the whole waterline (photo above) between bow and stern - there is no sign of a valley between them which means that there is no suction from the bottom (Bernoulli) - another benefit of your boat. Manfred
     
  8. NoEyeDeer
    Joined: Jun 2010
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    NoEyeDeer Senior Member

    I'm not sure what you mean here. If you mean the hull isn't generating a wave trough amidships, that would probably be because it is moving at a very low speed/length ratio. His average speed of around 3 knots (5.7 km/h) gives a ratio of less than 0.9, so wavelength will only be 40% or less of waterline length.

    Anyway, it's an interesting result. I wouldn't have expected that great a percentage increase in speed. For what it is, it's quite good. It may even do a bit better if the aft end of the extension was given some deadrise.

    I can say though that hulls like that, with such a blunt bow, do not like being rowed into any chop. Perhaps the next trick could be a pointy bow extension. :D
     

  9. Manfred.pech
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    Manfred.pech Senior Member

    Yes, you are right. I didn`t mean the hullspeed trough, but I didn`t say that clearly enough. The only indication could be found in the name Bernoulli.

    The hull of Russ seems to me a simple planing form with straight lines, no rocker and built for an outboard motor. These hulls do not generate suction as hulls with convex underwater shape do. This can be seen here.

    [​IMG]


    Between bow and stern of the CFD print there is no lift (Kein Auftrieb) and the blue and green colours indicate suction up to -0.942192. These areas will show a valley in the water between bow and stern at froude 0.316.

    [​IMG]

    There are designers who are used to work with this suction as for instance Ian Farrier who designed his trimarans with a remarkable rocker in the after part of the main hulls to develop suction and to prevent the bows from nosediving. "At high speed, the trimaran main hull planes early due to the low rocker, while the swept up aft section generates negative lift", which can actually lift the bows well out of the water. As a result, Farrier multihulls are very resistant to bow burying." http://www.f-boat.com/pages/introduction/features.html. Scroll down please to "Hull Lines" under the sketch.
     
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