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SeaTalks about Electronic Navigation and Communications

Forum - Electronic Navigation and Communications

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Student
Robert Neyman
GAINESVILLE, GA, USA
Crew Level 0
Status: Sailor
Qualified days: 0
True wind deviation in 2nd drawing
Posted 2021, Mar 08 21:17
In the diagram accounting for leeway why has the true wind direction changed from north? Does the direction of the boat due to leeway change the true wind direction?
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Student
Onur Aydemir
Dubai, Dubai, United Arab Emirates
Crew Level 0
Status: Sailor
Qualified days: 0
HDG vs COG
Posted 2023, Jun 11 17:02
No doubt that these are great exercises but I suppose HDG and COG values on the display need to be exchanged or alternatively the title and color of lines.
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Student
mark newell
Seattle, WA, USA
Bareboat Charter Master Level IV
Status: Big Time Sailor
Qualified days: 169
Calculating TTW with velocity or boat speed?
Posted 2024, May 02 03:55
Appreciating the difference between boat speed and velocity, I'm wondering is 'velocity' is the correct word in the following paragraph about TTW. Since DTW includes an implied direction, "straight line", would "boat speed" be the correct input for determining TTW? "In this case, your sailing distance will be much longer than your DTW. Thus you can not always calculate the time to your destination by simply dividing your DTW by your velocity. Some units...." Feedback very appreciated.
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Student
Dorian Swerdlow
White Plains, NY, USA
Captain Level IV
Status: Really Big Time Sailor
Qualified days: 169
An observation about optimum downwind VMG
Posted 2025, Jan 05 23:08
In the example where the optimum downwind VMG is 135 deg TWA, the VMG decreases only slightly for greater TWAs but it decreases rapidly for smaller TWAs. Thus it may be prudent to slightly increase the TWA. You won't lose much VMG, but you won't risk a dramatic drop in VMG if you fail to keep the target TWA locked in. Plus, you'll stay closer to your desired downwind waypoint in case the wind shifts. I'm not an experienced racer, but the above strategy seems apparent from the polar diagram.
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Dorian Swerdlow
White Plains, NY, USA
Captain Level IV
Status: Really Big Time Sailor
Qualified days: 169
The graphic looks a bit off
Posted 2025, Jan 06 16:18
Assuming the red line is the intended course, the boat looks like it's about 0.16 nm off course. That's judging by the placement of the 0.10 nm limit line. The display, however, indicates 0.33 nm. Am I not understanding how the diagram works?
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Dorian Swerdlow
White Plains, NY, USA
Captain Level IV
Status: Really Big Time Sailor
Qualified days: 169
Bending of VHF radio waves
Posted 2025, Mar 09 18:23
If we really want to get technical, the VHF waves do not bend around the curvature of the earth due to gravity. The slight observed bend is due to "tropospheric ducting" which is refraction due to temperature gradients in the atmosphere. See https://en.wikipedia.org/wiki/Very_high_frequency Also, the text mentions signals going around land to some extent by "refracting off land masses". It would be more accurate to say "diffracting around land masses".
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