rylevine Posted May 25, 2018 Share Posted May 25, 2018 A follow-on to the 4/14 (ebb) paddle occurred on 5/19 (flood). Both trip reports are attached, although there is some overlap. Previous to the paddles, during in-house sessions we reviewed vector constructions to predict drop down, ferry, and transit crossings in current. These are provided in the appendices. The 4/14 trip report is re-posted with some minor revisions and corrections. Thanks to all participants! Bob Trip_report_4_14_2018_Quincy_Houghs_Neck_Nav_Paddle_ebb_posted.pdf Trip_report_5_19_2018_Quincy_Houghs_Neck_Nav_Paddle_flood_posted.pdf Quote Link to comment Share on other sites More sharing options...
rfolster Posted May 25, 2018 Share Posted May 25, 2018 Wow, Bob! A lot to take in here. Haven't had a chance to read through it all, but I did notice one thing. In the 4/14 report, the vector calculation Figures #1 & #2 are both based on 1.8 knots ebb, but the blue lines representing the drift are not the same length. If they are both representing 1.8 knots, they should be the same length. Quote Link to comment Share on other sites More sharing options...
rylevine Posted May 25, 2018 Author Share Posted May 25, 2018 (edited) Rob, I think you mean Figures C1 and C2. The first case is a three knot paddler taking T=6 minutes to cross Hull Gap. The second is a two knot paddler taking T=8.23 minutes. The different lengths of the tidal drift vectors are due to the different transit times T. In calculating a ferry angle, we need to assume a paddle duration T, and then correct it afterwards by looking at the intersection of the paddler_water vector with the paddler ground track. The paddler ground track was assumed to be the same for both the three and two knot paddlers. The assumed T-value is somewhat arbitrary, and I just used the duration of the paddle speed on the crossing distance in each case (knowing that the ebb current was assisting in the crossing). The charts in Appendix C are not very well explained as they were just from notes presented at the in-house session. T-values can be insufficient, as seen in the 5_19 ferry angle calculation in Figure B2. Here the paddler is going against the flood current, and the initial assumed crossing duration is too short. Increasing the time of the crossing also increases the tidal drift (blue vector in Figure B2) but there is more time for the paddler velocity to overcome the effect. By the way, as I look at these charts again, I'm a little embarrassed by the number of digits written down. It is a personal OCD thing, but heck - it's a glorious feature, so might was well enjoy the power of a hand calculator! Bob Edited May 25, 2018 by rylevine Quote Link to comment Share on other sites More sharing options...
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