Coastal Navigation Exercises; 5-2

You're mixing TWA (true wind angle) and TWD (true wind direction) :)

Sorry to bother. Should we estimate the leeway at 10 degrees since the wind is abeam at 15 kts and use that in the plot, or should we figure the angle of water track to TWD to get the TWA and then go to Leeway Table and use that number at the end?

The Leeway is always added at the end of the calculation. So you find your heading accounting for current then deal with the leeway.

My question was really about how to calculate the leeway, but I see that one of the leeway degrees used is 8 degrees, so that would have to come from the table and I would have had to calculate TWA from plotted water track and TWD. Thanks.

I'm also struggling a bit with the TWA and leeway calculation. I get that it is relative to your heading, and that leeway compensation is applied last. Looking at it from the perspective of actually transiting the passage, your final actual heading will compensate for leeway. So the TWA and leeway you will experience will be relative to your actual final heading net of the leeway compensation, not the water track heading. To me this suggests I should use a "guess and check" evaluation of the leeway table entries for fit. Try the closest TWA angles from the leeway table on for size and see which results in a final heading and TWA closest to the value in the leeway table. Taking the practice exercise problem 1-1 C46 to S22 the water track is 152 and TWD is 90 which would imply a TWA of 152 - 90 = 62, *if* one were to naively just take the water track heading and ignore leeway. 62 is (barely) closer to 90 than it is to 45 so I guess that is why the leeway table entry for TWA 90 was used in the answer key, I think. But you know you are *not* going to take that 152 water track heading. You are going to bear up some amount to compensate for leeway. So your heading with leeway compensation is going to either be 152 - 10 = 142 if you pick the entry for TWA 90, or it is going to be 152 - 12 = 140 if you pick the leeway table entry for TWA 45. 142 vs 140 is an immaterial difference but I'm trying to understand the correct approach and get the "right" answer, and with this method I keep wanting to pick the leeway value for TWA 45 which seems "wrong" vs. the answer key which says the leeway should be 10 (for TWA 90) rather than 12 (for TWA 45). My reasoning is that 142 - 90 gives a TWA of 52, which is significantly closer to 45 than it is to 90 and makes me think it is a mistake to use the leeway table entry for TWA 90. And it is even closer to TWA 45 if I switch gears to using the TWA 45 leeway entry to calculate and and take a heading of 140 for a TWA of 50, which is within 5 degrees of 45 and pretty far removed from TWA 90. Am I missing a fundamental concept here? Or is there an error in the material? Or is this just a case where the expected approach is to simply use the water track to calculate TWA and not be so fussy about checking it against the final heading? I agree with the other question here asking why the final heading for problem 1-2 is 106 rather than 118 - 10 = 108, which is the only reason I would be so bold as to suggest a possible error in the course material.

You are correct - the boat's true wind angle is closer to 45 degrees than 90 and so we adjusted the answer sheet to be 12 deg of leeway. This becomes especially more true because as you adjust the heading up to account for the leeway - the heading is even closer to 45 degs off the wind. Thanks for pointing this out.