NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Set, drift, and leeway
From: Bill B
Date: 2006 Jan 28, 17:19 -0500
From: Bill B
Date: 2006 Jan 28, 17:19 -0500
This initially started out as a quest to see if it mattered which order leeway, and set and drift were applied when calculating the new course to steer to make good a rhumb line course to destination. So far, it does not seem to matter. The lovely part of the vector diagrams and equations in my texts is that they assume constant speed, or the ability to increase speed in some situations. It occurs to me that a sailboat is usually running at the best speed she can make, conditions permitting. A change of course, assuming wind direction remains constant, will alter speed unless she is able to run at hull speed on all points of sail. This is where red flags started popping up. Assume a wind from the north, 0d, 10 knots. Rhumb-line course to destination 90d. She is on a screaming beam reach at 7 knots. Leeway 6d. Set 180d, drift 3. To make good the intended track of 90d, she will have to come up approximately 29d, steering a course of approx. 61d. She would also have to increase speed through the water. But by coming up towards a close reach, she loses speed through the water. As she loses speed, the angle from the rhumb line to compensate for leeway, and set and drift, increases. As the angle increases she goes to yet a higher point of sail, so speed decreases again....and on and on. Which brings up the question, is there some slick way to solve this problem? Bill