Brain hurts - Working out course to steer

[boatman61]

Originally Posted by Dockhead

Sure. But also the result of wanting to get there in the most efficient manner (arriving before the pubs close in Cherbourg, for example )


Note that contrary to some things posted in this thread, the most efficient way is NOT indeed that way, which minimizes XTE. XTE will be all over the place if the tide is changing and you're sailing the correct constant heading.


As an aside -- this situation is also a good illustration of ground wind vs. true wind. With big changing tidal streams, a constant ground wind gives changing true wind -- both speed and direction. This is another challenge of English Channel crossing and a very complicated tactical problem for cross-Channel racing. If the wind speed is relatively small, the change in the True Wind, when the tide changes, can be dramatic.

This comes under the heading of Local Knowledge and Experience..

[Pete17C]

Originally Posted by Dockhead

Sure. But also the result of wanting to get there in the most efficient manner (arriving before the pubs close in Cherbourg, for example )


Note that contrary to some things posted in this thread, the most efficient way is NOT indeed that way, which minimizes XTE. XTE will be all over the place if the tide is changing and you're sailing the correct constant heading.


As an aside -- this situation is also a good illustration of ground wind vs. true wind. With big changing tidal streams, a constant ground wind gives changing true wind -- both speed and direction. This is another challenge of English Channel crossing and a very complicated tactical problem for cross-Channel racing. If the wind speed is relatively small, the change in the True Wind, when the tide changes, can be dramatic.

Ok, you lost me. Does ground wind =apparent wind? And I guess with big temperature swings the water temp might have an effect on the wind, but in any channel the wind usually funnels between the land masses, following the coast lines.

[NewMoon]

. Both

[Stu Jackson]

Originally Posted by Pete17C

Ok, you lost me. Does ground wind =apparent wind? And I guess with big temperature swings the water temp might have an effect on the wind, but in any channel the wind usually funnels between the land masses, following the coast lines.

Both do happen, but I believe the point Dockhead was making is apparent to what one would experience with strong current sailing, where the boat is moved by the current and "makes" its own wind. True wind, ground wind created by the current moving the boat, and then apparent wind is what your masthead instrument is showing and your sails are seeing.

I did a lot of current sailing on SF Bay in winters. Light wind, current moves boat, creates enough wind to sail. Really fun to learn how to do.
But that was a single current carrying me in the direction I wanted to go, not changing current directions (and strengths, of course) at varying wind angles to your heading.
As you might have seen from the earlier link discussion, it's also important to define heading, bearing, and the other basics. Some folks interchange GPS heading with compass headings, for example.

[valhalla360]

Originally Posted by anacapaisland42

This is a very important point.......if you are in a narrow channel, sailing from buoy to buoy and you somehow get out of channel, it is NOT safe to assume that if you head for the middle of the next set of buoys that you are OK. You may well run aground, generally you should head back to the center of the channel for your current position.......
I didn't explain that very well :-(
Bill

This is where using GPS COG makes sense.

But you can tell the impact of current by comparing GPS COG to what direction your bow is pointing. Trying to plot it out ahead of time...too many variables.

[valhalla360]

Originally Posted by Pete17C

But you don't follow a straight line when done this way... it's like using a NDB when flying, if you don't correct for crosswind (current) you fly (sail) on an elliptical arc to the destination, increasing the distance traveled.

This assumes a single constant cross current. In that case, you turn slightly into the current until your COG is directly to the destination and that's your fastest route and shortest distance.

If the current reverses, you are best off keeping a steady compass heading as boatman explained. Yes, you cover more ground but it's actually faster. If you constantly try to quarter into the current to stay on the shortest distance, you waste a lot of distance thru the water, fighting current.

[Dockhead]

Originally Posted by Pete17C

Ok, you lost me. Does ground wind =apparent wind? And I guess with big temperature swings the water temp might have an effect on the wind, but in any channel the wind usually funnels between the land masses, following the coast lines.

OK, sorry for the thread drift! But here's the answer --


Ground wind is wind referenced to the ground. What the weather forecast tells you; what is observed from land or from a boat which is not moving with any current.



True wind is wind referenced to the water. What your instruments tell you is true wind.



Apparent wind is wind referenced to your boat, so adding boat speed and heading into the equation.




So what I was talking about here, is the wind shift you would feel when the tide changes, nothing to do with temperature or coast line.


Let's say you're crossing the Channel from the Needles, with CTS of 180, so you are maintaining a constant heading of 180. You head out into the Channel on the flood and at springs, with the tide running roughly East at 4 knots or so at peak flow. Your instruments tell you you have 20 knots of true wind -- F5.



When the tide changes and goes West, the true wind will 8 KNOTS STRONGER, 28 knots, at peak flow, or already a weak F7. A big difference requiring reefing or a sail change. The direction will change too if the wind direction is not exactly parallel to the current.



All that is assuming the ground wind remains constant at 24 knots. You have to consider this in your sailing tactics, and it is one element of the misunderstood but real "lee bow effect".





I just mentioned this because you commented that sailing a constant heading could be good if you want to sail pleasantly on the same point of sail etc. In fact changing tides create their own wind shifts, wind shifts which are entirely predictable.

[valhalla360]

Originally Posted by Dockhead

OK, sorry for the thread drift! But here's the answer --


Ground wind is.....

Interesting. Learned something new (and yes, I googled it to confirm you weren't making things up).

What's odd (until you look at the historical context) is "true" would normally imply the least changeable, which would be wind relative to ground.

But historically there was no good way to measure wind relative to ground on a boat so "true" was assigned to relative to the water which could be estimated by taking apparent wind and speed thru the water (if you measure on land, there is no current and thus "true" and "ground" will be identical, so meteorologists usually don't care about the terminology). Since the term "true" was already taken, they must have settled on "ground" as an alternative.

[Dockhead]

Originally Posted by valhalla360

Interesting. Learned something new (and yes, I googled it to confirm you weren't making things up).

What's odd (until you look at the historical context) is "true" would normally imply the least changeable, which would be wind relative to ground.

But historically there was no good way to measure wind relative to ground on a boat so "true" was assigned to relative to the water which could be estimated by taking apparent wind and speed thru the water (if you measure on land, there is no current and thus "true" and "ground" will be identical, so meteorologists usually don't care about the terminology). Since the term "true" was already taken, they must have settled on "ground" as an alternative.

Yes, that's right exactly.


Note that there are different concepts of "true" wind. Non-sailors will call wind in relation to the ground "true wind", for the reason you stated.



"Ground wind" is a fairly recently invented concept, which exists from about the time that we have reasonably good instruments.


We sail in the interface, or the sheer, if you like, between air and water. The relative motion between wind and water -- and not between wind and the earth -- is what drives our boats.



I just looked this up and was amazed to find this explanation in the Raymarine Knowledge Base:



http://www.raymarine.co.uk/knowledge....cfm?view=7595


"Before GPS, it was impossible to calculate Ground Wind. That is because the only speed data we had was STW. Possibly the idea of True Wind versus Ground Wind arose because instrument makers who suddenly had access to SOG data on the network needed some way to differentiate true wind calculated that way versus True Wind calculated using STW.

"Don't be confused by the fact that meteorologists use True Wind to describe the difference between wind and land. Of course they do -- they are sitting on land, not on water. That in no way contradicts our usage of True Wind as being the difference between wind and water."




Those are my words, taken from a 2011 thread from CF! http://www.cruisersforum.com/forums/...ion-73563.html


Ha! That's pleasant.

[Pete7]

Originally Posted by Dockhead

If you are sailing in UK waters, there's a real simple solution to this problem -- the program Neptune Plus, which is what I use. This produces a vastly more accurate calculation based on the (I believe) six-minutely Admiralty tidal model, using computer power to do what would be unrealistically laborious by hand.

Indeed, there was an even cheaper program that only worked for the South Coast of England, but it seems to have disappeared sadly. The big advantage I think is the ability to change to start time and speed to see if departing earlier or later has an effect and it does. From memory offering 30 - 60 minutes savings for a 10 - 11 hour passage.

For the OP who mentioned multiple tidal changes, this would suggest several days. In the UK (and perhaps the PNW) this gets super complicated because the weather is certainly going to change during that time. Also any forecast is just that, a forecast averaging over quite a big area. Add some headlands with tidal gates plus wind shifts and you almost need a full time nav handcuffed to the chart table. Certainly make a plan and run through a series of options for times and speed etc, then stay flexible. A tweak of the course as you approach your destination so you don't end up down tide during a big Spring would be prudent.

[Pete17C]

Originally Posted by valhalla360

This is where using GPS COG makes sense.

But you can tell the impact of current by comparing GPS COG to what direction your bow is pointing. Trying to plot it out ahead of time...too many variables.

This only gets you a correction for CTS. If you have a current running directly with or against you this doesn't work. You won't know if it's slowing you down (or speeding you up).

How can you plan to be somewhere before nightfall if you don't know if you'll be fighting a head current?

It's an estimate with many variables, but you have to have a plan. Then be ready to adjust as needed.

[goeasy123]

To Dockhead from the OP, Thanks for clarifying the problem domain for everyone. And, more specifically for me I'm looking for a simple digital solution.



BTW I'm a pilot (air type) where we 'fly the air mass', so I fully understand the problem.


It amazes me the number of sailors I've spoken to that don't get CTS. The advent of the 'tear along the dotted line' plotters has a lot to answer for.



I note that SailGrib Free (and in common with many other products) doesn't have tidal atlases for European coasts which makes its weather routing pointless and maybe dangerous for the ab initio navigator. It works for US waters 'cause the authorities make them free issue.

[Tricolor]

Originally Posted by Andy Todd

............BTW I'm a pilot (air type) where we 'fly the air mass', so I fully understand the problem.


It amazes me the number of sailors I've spoken to that don't get CTS. The advent of the 'tear along the dotted line' plotters has a lot to answer for...............

People don't learn anymore to navigate, its now more like driving a car with a route planner.

People my age learned to sail with the use of tidal atlas and paper charts to predict their position as accurate as possible. So tidal information and current was important. Now with the use of GPS, Plotters with moving charts etc. most sailors just input a waypoint and have no knowledge how to interpretate the results.

Thats why i still use paper and plot every hour my position, doing this keeps me allert, and gives me something to do.... and yes i still shoot a sun at 12 o'clock...

PS. I fly a cessna 152

[Dockhead]

Originally Posted by Tricolor

People don't learn anymore to navigate, its now more like driving a car with a route planner.

People my age learned to sail with the use of tidal atlas and paper charts to predict their position as accurate as possible. So tidal information and current was important. Now with the use of GPS, Plotters with moving charts etc. most sailors just input a waypoint and have no knowledge how to interpretate the results.

Thats why i still use paper and plot every hour my position, doing this keeps me allert, and gives me something to do.... and yes i still shoot a sun at 12 o'clock...

PS. I fly a cessna 152

Traditional navigation skills are extremely useful, even if you don't do hourly plots and do most of your work on a plotter. At least to understand what the computer is doing for you.


But tidal vector calculations is something which no plotter and no general navigation program I have ever seen will do for you. And you are basically screwed trying to cross a rapidly moving body of water like the English Channel, or the North Sea, or the Gulf Stream, without understand how to do this. I think it's an essential skill. I don't think you can pass any of the RYA Yachtmaster exams without it.

[valhalla360]

Originally Posted by Pete17C

This only gets you a correction for CTS. If you have a current running directly with or against you this doesn't work. You won't know if it's slowing you down (or speeding you up).

How can you plan to be somewhere before nightfall if you don't know if you'll be fighting a head current?

It's an estimate with many variables, but you have to have a plan. Then be ready to adjust as needed.

If you current is running for or against you, SOG is what will tell you when you will arrive...plus I have a pretty good feel for STW for normal cruising RPM.

It gets you the correct CTS in real time.