Brain hurts - Working out course to steer

[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







But if you are going thru the channels in Georgia, it's a near impossible task as the current can be coming from any direction over the course of a day and it will vary wildly in terms of strength both as it relates to the state of the tide and where you are in the channels.

[grantmc]

Originally Posted by Andy Todd

Stu: interesting thread. ... blows me away that sailors with any level of experience don't get the concept of sailing through a moving body of water. There must be a lot of people sailing far further than they need to.

Quite agree. I crewed with a supposedly professional US skipper (Fiji to NZ) recently who didn't know the there was a difference between magnetic and true. He'd never heard of either variation or deviation.

[stormalong]

You are all over thinking this. Set a waypoint on your GPS. Initially steer the bearing to waypoint and then adjust for cross track error.

Set and drift calculations are for navigation class. In the real world use the best tools you have. The best tool for this is a simple GPS. Carry a few - they are so cheap today it is best not to be caught without one.

[belizesailor]

Originally Posted by Andy Todd

belizesailor: yes 'tab' = tablet. Spot on... it's the trig function I need. Example... How do I cross the English Channel (like this

at 8:20), but using trig rather than vector diagrams? I.e. what's my initial heading.



Stu: interesting thread. ... blows me away that sailors with any level of experience don't get the concept of sailing through a moving body of water. There must be a lot of people sailing far further than they need to.

Nautical Ed has a section specifically on the trig behind navigation vector diagrams:

http://www.nauticed.org/sailing-blog...ath-behind-it/

And there are a few navigation calculators that will do it for you:

https://www.google.com/url?sa=t&sour...awUJBlVIgZAaKX

[boatman61]

I used to do the Poole to Cherbourg and back regularly in the past.. set my course from Old Harry Rocks to the W breakwater entrance and head out from Poole 1 hr before HW.. twelve hours later would see me approaching the entrance.. the tide would sweep me W for 6hrs then back E for the next 6hrs.. secret to accuracy was maintaining the speed needed to do the 60nm in twelve hours.. 5kts gives one a nice lazy ) .
If not then you find yourself fighting against a strong tide pushing you E or W.. depending on the state of tide at departure
If your say crossing Dover to Calais then you only have one direction of tide to worry about so you take the average set for the time estimated to cross then steer the course - + degrees to compensate for the drift.. fine tune as you close if you have not managed a constant speed.
Used to stress it with hourly fixes but soon dropped that habit..

[Tricolor]

Yep, if you do this from IJmuiden to Lowestof (270º) and plot every hour on the paper chart, you end up with a nice sinus from the tide/current.

[Pete17C]

Originally Posted by Dooglas

As danielamartindm observed, an autopilot solves this problem in the best possible way by taking into account the actual effects of wind, current and other factors in real time. Trying to estimates these influences from forecasts, tables, measurements, etc will always only be a general approximation. The autopilot is actually making corrections based on how these factors are affecting your boat right now. If you wish to do paper or tablet calculations as a double check, that will be interesting. It will also show you the difference between such estimates and real time information on boat performance.

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.

[goeasy123]

Dooglas: I don't thing you understand my question. roland stockham: I think you do.



It's not a question of what you do when you're on passage. That's easy. It's when your sitting at your kitchen table, on land and your trying to work out your initial heading from your departure point. Roland, the very expensive pro version of Predictwind does it. It could be a relatively simple spreadsheet app for someone who knows a bit about cos, sin, radian and tan.



An extreme example might be a trip from Madeira to the Isle of Man, a straight line through a whole bunch of tides on all sorts of points. What heading do I start with on any given departure time/date. Yes, I will have to replot as as I go along and things change.

[Pete17C]

Originally Posted by Andy Todd

danielamartindm:

valhalla360: Charting software doesn't give you a CTS which is a big series of curves across the current flow change. Sure, following the straight line on a plotter with gets you to where you want to be, but it's the long way to go.
.

The CTS is not a big series of curves, it is a correction for crossdrift. The boat follows a straight line, just that the compass heading is not the same as the bearing to the WP.

If you follow a line on your plotter from one WP to another, and stay on the line, you are sailing a straight line and the shortest distance to the WP, but the bearing to the WP most likely (unless there is no current) will not be the same as course steered.

Now if you just take a bearing to the WP and steer that, and keep correcting for drift along the way, that's the long way. This is what an autopilot will do if you set it to nav to a WP instead of entering a CTS.

[boatman61]

Originally Posted by Pete17C

The CTS is not a big series of curves, it is a correction for crossdrift. The boat follows a straight line, just that the compass heading is not the same as the bearing to the WP.

If you follow a line on your plotter from one WP to another, and stay on the line, you are sailing a straight line and the shortest distance to the WP, but the bearing to the WP most likely (unless there is no current) will not be the same as course steered.

Now if you just take a bearing to the WP and steer that, and keep correcting for drift along the way, that's the long way.

[goeasy123]

Originally Posted by Pete17C

The CTS is not a big series of curves, it is a correction for crossdrift. The boat follows a straight line, just that the compass heading is not the same as the bearing to the WP.

If you follow a line on your plotter from one WP to another, and stay on the line, you are sailing a straight line and the shortest distance to the WP, but the bearing to the WP most likely (unless there is no current) will not be the same as course steered.

Now if you just take a bearing to the WP and steer that, and keep correcting for drift along the way, that's the long way. This is what an autopilot will do if you set it to nav to a WP instead of entering a CTS.

So are you saying that if I set a single CTS across, say three 180 degree current changes I'll follow a straight line? To stay straight I'd correct for the subsequent tides and change my bearing for each tide. And as you say in your last para this is the wrong thing to do. You might be right if it's a single current. How do I work out the CTS for the whole, multi-tide passage?

Here's a reference to the 'curve theory' https://www.pbo.co.uk/seamanship/nav...straight-32181 As you see it's easy to work out on a paper chart, but if you're doing it all digitally you need, either expensive nav software or a spreadsheet that resolves polar coordinates.

[belizesailor]

Originally Posted by Andy Todd

Dooglas: I don't thing you understand my question. roland stockham: I think you do.



It's not a question of what you do when you're on passage. That's easy. It's when your sitting at your kitchen table, on land and your trying to work out your initial heading from your departure point. Roland, the very expensive pro version of Predictwind does it. It could be a relatively simple spreadsheet app for someone who knows a bit about cos, sin, radian and tan.



An extreme example might be a trip from Madeira to the Isle of Man, a straight line through a whole bunch of tides on all sorts of points. What heading do I start with on any given departure time/date. Yes, I will have to replot as as I go along and things change.

Ah, ok.

If doing this with vector or trig then you either have to make a simplifying assumption like: current will average 2 knots over the route, and then correct as you get actual fixes and can compute actual set & drift for that time period, or work a LOT of problems in sequence in advance. The first approach I think is traditionally the most common: calculate an initial approximate CTS and then correct versus actuals as you move along. Thats how I do it.

If you want to crank it all out ahead of time why build a spreadsheet, use a weather routing app and let it do all the crunching...most are WAY slicker than a spreadsheet will ever be anyway. I mostly use SailGrib WR, it calculates using GRIBs for current and wind, you can load polars, play back routing on screen, generate way points of calculated route...tons of features. How is a spreadsheet better than that for planning?

Screen shot attached.

Note that even if you use WR software, or whatever, up front then you are still going to in effect revert to the first approach above in reality because actuals will almost certainly vary to some degree.

[goeasy123]

Thanks belizesailor. And thanks for the 'steer' to SailGRIB. I bought the wrong s/w. didn't do that and the only others I could find were very pricing.... Or OpenCPN, which I haven't got my head round yet.


Mind you I still think a lil ol' spreadsheet would be handy.

[boatman61]

Originally Posted by Andy Todd

So are you saying that if I set a single CTS across, say three 180 degree current changes I'll follow a straight line? To stay straight I'd correct for the subsequent tides and change my bearing for each tide. And as you say in your last para this is the wrong thing to do. You might be right if it's a single current. How do I work out the CTS for the whole, multi-tide passage?

Here's a reference to the 'curve theory' https://www.pbo.co.uk/seamanship/nav...straight-32181 As you see it's easy to work out on a paper chart, but if you're doing it all digitally you need, either expensive nav software or a spreadsheet that resolves polar coordinates.

No.. you do not change your bearing/CTS at the turn of each tide.. if you do you've wasted time working out the course to steer.. the tide will push you one way then the other.. you just have to calculate optimum departure time/speed and CTS.
(«
>)
(«
Your COG will look something like this.. sorry, the above is the closest I could get to the effect of the tides on your direct CTS over two ebbs and one flood, or vice versa.

[billdre]

I find this very entertaining. I can appreciate the theory discussion but is anyone interested in actually sailing from A to B. Then a good chartplotter and a set of waypoints will get you there. Also actually sailing to boat means adjusting to conditions as they develop, i.e. current, wind, etc. Sailing is about enjoying the journey not plotting the definitive route. Enjoy the math if you want I'll take being on the boat and going there.