A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Patrick Goold
Date: 2012 Nov 28, 15:26 -0500
To pick up on Lu’s point:
Recently I sailed from Portsmouth to Cape Charles Virginia. I an hour after high tide so that the ebb would carry me up the Elizabeth and out into the Bay. The course from Old Point Comfort to Cape Charles is roughly 45 degrees true. The wind forecast I used to plan the trip did not bear out, however, and instead of NW I was seeing almost NNE. Close-hauled on a port tack I was almost able to make my course at first but then as I got out further into the bay, the outgoing current really took hold. CMG was about 65 degrees. The question then was not what course to steer (that being as close to the wind as possible) but when to tack. Just to test it, I tacked just east of the Baltimore channel after crossing somewhere between buoys 16 and 18. That put the current on the nose. Boat speed fell and with it my ability to point. Instead of something decent like 320 true, the new course was more like 300 and making barely three knots. (Payback for the sea-keeping qualities of that traditional full keel) I resumed a port tack, expecting the tide to turn soon. I hoped then to tack and ride the incoming tide north. Events reminded me that the SE corner of the bay, I guess because of the shape of the bottom and the shore, sees a variety of late flows and strange counter-currents. Eventually the current did become favorable but by that time the wind had also picked up. 25mph winds blowing counter to the current across a considerable fetch raised a steep swell. It was difficult to make any headway at all. I ended up motoring into Cape Charles.
There are several websites that give very detailed information about currents in the Chesapeake Bay. It is interesting to see how variable and how local they can be. Even an ocean current like the Gulf Stream can have massive eddies and bulges that mock one’s calculations.
Ah, but how does one "calculate leeway" to adjust course? Every single text I've seen, from Bowditch to something like Sweet's "Weekend Navigator" all assume you know that leeway will be and then simply tell you to subtract or add it to course steered to get CMG (or vice versa). Leeway depends on one's point of sail -- maximum when close-hauled, non-existent when going downwind. More important, it also depends on the boat's underwater configuration. Sailboats are supposed to go straight ahead and not sideways, so I suspect this has been an issue for naval architects ever since they started doing scientific design of boats.As a long-time navigation instructor, I've always found current calculations a bit like a solution searching for a problem. While potentially appropriate while well offshore, in coastal waters current will shift with the tide. So if I do the standard calculation for set and drift based on the difference between my DR and Fix, how is that relevant except as determining an average value over the exact period of time and tidal conditions for the calculation? If I'm trying to determine course-to-steer (Track) as in Sean's problem, how do I know what value to use for Drift (speed of current) when it's changing minute-by-minute over a current cycle?Let me make it clear that, as someone with a graduate degree in engineering, vector math (which current calculations are) do not intimidate me, they were part of my education from my freshman year onward. So my comments above are absolutely not because I can't do the problems, it's just that magic number called Drift that's the problem. And its cousin Leeway.Lu
From: Andrew Seligman <firstname.lastname@example.org>
To: "NavList@fer3.com" <NavList@fer3.com>
Cc: "NavList@fer3.com" <NavList@fer3.com>
Sent: Wednesday, November 28, 2012 8:18 AM
Subject: [NavList] Re: Course to steer at a given speed.
You need to determine Speed Through the Water to obtain a Course Made Good; You also have to calculate Leeway and compensate for it. If the sailboat is on a Starboard Tack, then you Add the Leeway Angle to the boats heading; Conversely, if the sailboat is on a Port Tack, you subtract the Leeway Angle from the boats heading.
Andrew F. SeligmanUSCG Licensed MasterASA Sailing Instructor
On Nov 28, 2012, at 11:01 AM, "Greg Rudzinski" <email@example.com> wrote:Sean,Sailboats do not have the luxury of scheduling :( If you have a sloop then you can point 30* off the apparent wind which complicates making a mark, waypoint, or destination. Then there are tidal currents which in some cases will be resulting in a negative headway. To avoid this a sailor needs to time arrival for slack water or have the tidal current directed toward the destination.Greg Rudzinski
[NavList] Course to steer at a given speed.
From: Sean C
Date: 28 Nov 2012 04:10
A while back, I was reading the chapter in Bowditch dealing with dead reckoning. I was especially interested in the section dealing with course and speed made good. Well, last night on my lunch break (I work nights), I decided to solve a quick problem I made up:"If I were on a ship which was 10nm due East of port, with a set of 360° and drift of 3 kts., and I needed to arrive in port in exactly one hour, what course and speed should I use?"I figured the answer to be 253° ---5 kts. But then I started to wonder: That's all well and good for a vessel under engine power, but what about a sailboat? I know the way to figure course to steer at a given speed, but wouldn't turning a sailboat into the current (as in my example) actually slow the speed through the water enough to change the necessary course to steer? Or is this error so small as to be negligible? Or is there some other way to factor that in?Regards,
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