Brain hurts - Working out course to steer

[Dockhead]

Originally Posted by StuM

That's a thought that has been recurring every time I come back to read the latest posts in this thread.


All of the discussion has used the premise that the boat will maintain the same speed through the water regardless of heading and wind direction (and can sail in any direction).

You have to start with that premise in order to eliminate extraneous variables, so that you can concentrate on the moving water problem.



Then, for real passages, you have to overlay sailing tactics on top of the CTS problem. It can become hellishly complex (especially if there is tacking involved), but you are fundamentally dealing with both sets of problems so that's just what it is. If you're cruising, you can throw up your hands and wing it, but if you are sailing a cross-Channel race, victory or defeat will depend on getting both of these tasks exactly right.



One thing which comes out of the interaction between CTS navigation and sailing tactics is the "lee bow effect", something so complicated that hardly anyone really understands it -- it's kind of the Rule Against Perpetuities of sailing*.


We discussed it here: http://www.cruisersforum.com/forums/...ll-115053.html


As to passage speed -- this is the biggest unknown for a vessel under sail, when doing a tidal vector calculation for CTS nav. We talked about this, and I described the technique used to deal with this upthread. This is much easier than dealing with sailing tactics, which create really complicated problems if there is upwind or downwind sailing involved.









* A legal doctrine which is supposedly so complicated,there was a statute which exempted lawyers from malpractice claims in cases where they got it wrong . Seriously.

[Q Xopa]

Originally Posted by Dockhead

You have to start with that premise in order to eliminate extraneous variables, so that you can concentrate on the moving water problem.



Then, for real passages, you have to overlay sailing tactics on top of the CTS problem. It can become hellishly complex (especially if there is tacking involved), but you are fundamentally dealing with both sets of problems so that's just what it is. If you're cruising, you can throw up your hands and wing it, but if you are sailing a cross-Channel race, victory or defeat will depend on getting both of these tasks exactly right.



One thing which comes out of the interaction between CTS navigation and sailing tactics is the "lee bow effect", something so complicated that hardly anyone really understands it -- it's kind of the Rule Against Perpetuities of sailing*.


We discussed it here: http://www.cruisersforum.com/forums/...ll-115053.html


As to passage speed -- this is the biggest unknown for a vessel under sail, when doing a tidal vector calculation for CTS nav. We talked about this, and I described the technique used to deal with this upthread. This is much easier than dealing with sailing tactics, which create really complicated problems if there is upwind or downwind sailing involved.









* A legal doctrine which is supposedly so complicated,there was a statute which exempted lawyers from malpractice claims in cases where they got it wrong . Seriously.

Yes and to add to the complexity I would include-
Performace polars, and
Weather routing
to your list. Im sure there are others.

But as you say it all boils down to sailing between 2 points as efficiently as possible, using moving water,
our boats best sailing configurations, and
favourable winds,
to our advantage as much as we can.

[cyan]

Originally Posted by Dockhead

OK, let me take a shot at that.


So let's imagine a simplified Gulf Stream crossing.


60 miles on 090 by the rhumb line, sailing at 6 knots on a beam reach.


The first 30 miles there is zero current -- still water. Then you hit the Stream -- 6 knots of current running N. 20 miles of this, and then 10 miles of still water again up to the destination.


Can you navigate this passage?


You set off sailing fat dumb and happy on heading 090, COG 090, STW 6.0, SOG 6.0 -- all is great. It's even the fastest VMC.


So is that enough information? Are you sailing the right way?


Think about what will happen in 5 hours.


I think you will understand, if you think about this -- and I don't think you even need to plot it -- why it's not enough to navigate according to what your instruments can tell you right now, or indeed what they can tell you at any time during the first 5 hours of this passage, and why you need the chart showing the currents ahead, to plan the passage properly, anticipating and taking account of changes in current, in setting your course.

That's a great (and quite patient) way to pose the question. In my experience, there can be a lot of time to ponder: Am I sailing the right way?
If a current has you drifting off course, it feels quite natural point the bow up-stream a bit to compensate. No magic to that. My kids can figure that out. The clever "trick" to CTS is that the fastest way across such (known or estimated) currents is actually a certain constant heading that can "feel" quite odd, with the bow pointing somewhere strange at times. That is obvious to no one at first glance, as the GPS and (and my wife) will disagree, but the clock does not lie. ("but we're heading the wrong way and the water is calm here!")

The non-intuitive physics are my favorite. Take a funnel and a ping pong ball. Blow air through the inverted funnel and place the ping pong ball up in it. Let go of the ball. It will defy gravity and stay up in the funnel even though you are blowing air directly downward at it. It does not make sense, until you study the Bernoulli effect. After all that, my brain still says that I'm blowing DOWNWARD at the ball, helping gravity, yet the ball stays UP inside the inverted funnel. Non-intuitive. Magic.

The ping pong ball does not lie, just like the clock. (well, unless travelling close to the speed of light, then Einstein and Newton don't agree any more)

[requiem]

Originally Posted by Q Xopa

I agree the more information, especially good and relevent, we have the better.

However, I think what we have is enough. Its like saying we need to have a predicted planned speed we are going to be commuting in our car along the expressway today. Thats fine, but the best method is to just look at your speedo. It just counts how fast our wheels are spinning.

Water movement, like car or sailboat movement is not that complicated.

But Im open to being shown the error of my thinking if you can explain to me the logic of why you are saying "its not enough".

Ok, here's a diagram:


Figure 1 shows the speed of a tidal current over time. For simplicity, assume the set of the current is at right angles to the bearing of your destination. The area of the shaded portions from the start of your trip at time 0 to your "arrival" represents the distance the current will have pushed you off course. Obviously, if you complete your trip at times A or C you will be mostly offset to one direction, but completing at time B largely negates that.

If you only have real-time data from your boat's instruments, you will know the current vector at time B only when the time has reached B. If you are only at time A, your boat's instruments will show a different value, and the same for any other point in time. What is unknown is where you are in the cycle; if you can spend a day or so collecting data then you'd have created your own tidal atlas but my impression is that would count as cheating.

Now consider figure 2. Let's say the horizontal axis represents the bearing to your destination and that the set of the current is again at right angles, so aligned with the vertical axis. If 'c' represents your velocity, it can be treated as separate components: 'b' being the part in line with the current, and 'a' being the part that progresses you toward your destination. The purpose of the CTS calculation is to make 'b' cancel the effects of the current, so that only 'a' remains.

(Of course, the more you steer away, the shorter 'a' becomes and so your trip time increases, meaning your current correction is slightly off, but I think this is often negligible and easily worked around.)

[Q Xopa]

Originally Posted by requiem

Ok, here's a diagram:


Figure 1 shows the speed of a tidal current over time. For simplicity, assume the set of the current is at right angles to the bearing of your destination. The area of the shaded portions from the start of your trip at time 0 to your "arrival" represents the distance the current will have pushed you off course. Obviously, if you complete your trip at times A or C you will be mostly offset to one direction, but completing at time B largely negates that.

If you only have real-time data from your boat's instruments, you will know the current vector at time B only when the time has reached B. If you are only at time A, your boat's instruments will show a different value, and the same for any other point in time. What is unknown is where you are in the cycle; if you can spend a day or so collecting data then you'd have created your own tidal atlas but my impression is that would count as cheating.

Now consider figure 2. Let's say the horizontal axis represents the bearing to your destination and that the set of the current is again at right angles, so aligned with the vertical axis. If 'c' represents your velocity, it can be treated as separate components: 'b' being the part in line with the current, and 'a' being the part that progresses you toward your destination. The purpose of the CTS calculation is to make 'b' cancel the effects of the current, so that only 'a' remains.

(Of course, the more you steer away, the shorter 'a' becomes and so your trip time increases, meaning your current correction is slightly off, but I think this is often negligible and easily worked around.)

Good explanation and diagrams thanks.