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Re: Course to steer at a given speed.
From: Andrew Seligman
Date: 2012 Nov 29, 12:42 -0500

***This just occurred to me while typing: Could you use the result from a calculation of "course to steer and speed to use" to correct a "course to steer at a given speed"? As in my previous example: With no drift, I could make it to port in one hour by steering a course of 270° at a speed of 10 kts. But with a set of 360° and a drift of 3 kts., I would need to steer 253° at a speed of 10 and a half kts. So does that mean that if I were in a vessel going 10 kts. (and without changing speed intentionally), when I turn to 253° my speed will be *reduced* by half a knot? (reflected in the need to increase speed by half a knot in the first problem.) Preliminary calculations seem to support this as I just drew it out and steering a course of 253° at 10 kts. in a 360° set/3 kt. drift results in a speed of 9 and a half kts. But that could be a fluke. I'll have to experiment more. [Incidentally, in order to make good a course of 270° at 9 and a half kts., one would need to steer to about 251°. Two degrees off from the initial calculation.]

You need to clearly define "speed to use" and "given speed"

The proper terms are:

Speed Made Good (SMG) defined as actual speed of the ground as you move through the water;
Speed Of Advance (SOA) defined as the speed a vessel would travel through the water excluding external forces such as Set, Drift, and Leeway;
Velocity Made Good (VMG) defined as the speed towards a vessels next waypoint regardless of course changes.

If you want a really good book/procedure for this calculation, I recommend the book Coastal Navigation by Mike Pyzel.

Captain Andrew F. Seligman
Certified ASA Sailing Instructor
215-514-6724

On Nov 29, 2012, at 10:11 AM, Sean C wrote:

"Ok, 1) Draw a 1 hour DR Plot;..." (etc.) -Andrew Seligman

I think this and other posts might answer my question. I'll have to work it through myself to grasp it fully.

Looking back, I think I could have asked the question more clearly. What I meant to say was: It is my understanding that when you measure the distance from your A.P. to your fix, you are measuring the sum of all errors, including leeway. Please correct me if I am wrong. So, IF you take the term "set and drift" to encompass all factors affecting course error, then, when figuring the course to steer at a *given speed*, would turning the sailboat into the drift reduce the given speed used to make the calculation, thereby changing the required course to steer?

I suppose one could make the first course correction, then check the speed on the new course and re-iterate the problem using the new given speed. I ask because in my example, the speed to use was 10 and a half knots [which didn't display correctly in my post]. Obviously if I were in a small sailboat I couldn't make that speed. I'd have to make due with what I could trim the sails to achieve, given the conditions. So I use the method in Bowditch (and described here) of swinging an arc to find the course to steer to make good a desired course. But in doing so, I must use the speed of my vessel now, not the speed it will be making when I turn into the drift. (Which I don't know yet.)

I'm not thinking in terms of meeting a schedule, although that was the problem that got me thinking about this. I'm only thinking about maintaining a desired course. I'm assuming that any change in course steered will affect speed and any change in speed will affect the course needed.

***This just occurred to me while typing: Could you use the result from a calculation of "course to steer and speed to use" to correct a "course to steer at a given speed"? As in my previous example: With no drift, I could make it to port in one hour by steering a course of 270° at a speed of 10 kts. But with a set of 360° and a drift of 3 kts., I would need to steer 253° at a speed of 10 and a half kts. So does that mean that if I were in a vessel going 10 kts. (and without changing speed intentionally), when I turn to 253° my speed will be *reduced* by half a knot? (reflected in the need to increase speed by half a knot in the first problem.) Preliminary calculations seem to support this as I just drew it out and steering a course of 253° at 10 kts. in a 360° set/3 kt. drift results in a speed of 9 and a half kts. But that could be a fluke. I'll have to experiment more. [Incidentally, in order to make good a course of 270° at 9 and a half kts., one would need to steer to about 251°. Two degrees off from the initial calculation.]

Thanks for all the replies!!

-Sean
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