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Old 04-02-2019, 17:44   #1
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Brain hurts - Working out course to steer

If I work out CTS on a paper chart over multiple tides and hours.... its an easy vector diagram exercise. But say I do all my nav on a tab, and I can't draw a vector diagram, how do I work out average drift over the whole passage. Does anyone have a spread sheet to do it or is there a 'trick'?


It's been a long time since O level math for me!!
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Old 04-02-2019, 17:52   #2
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Re: Brain hurts - Working out course to steer

Im not very clear on your objective, what is the problem you are trying to solve/avoid? Whats a tab? Tablet? If so, then nav software can give you CTS.

If you want to not do it graphically for some reason then it can be all done with trig functions too.

Pragmaticaly over a long passage you dont work it out for the whole passage because current/leeway wont be consistent across the entire route. You work out a new CTS each time you update your position (fix). So, its a lot of little triangles, or lots of little trig problems, or lots of little SWAGs.

There is also Weather Routing software which will work it out for you over longer runs with varying current. See SailGrib WR and the plug in for OpenCPN.
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Old 04-02-2019, 17:55   #3
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Re: Brain hurts - Working out course to steer

You should be able to find info on your question here, and in the links within the topic:


CTS vs Following Rhumb Line - DILUTED THREAD
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Old 04-02-2019, 18:42   #4
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Re: Brain hurts - Working out course to steer

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.
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Old 04-02-2019, 18:48   #5
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Re: Brain hurts - Working out course to steer

... BTW the CTS vs Following Rhumb Line - DILUTED THREAD greatly simplifies the problem by assuming due east and due west. It gets more complicated when you're not crossing at right angles to the tide, and the direction of the drift changes as you cross, and your SOG changes as a result of the drift.
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Old 04-02-2019, 19:38   #6
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Re: Brain hurts - Working out course to steer

I thought this was easy. ...just found this on Penzance Sailing Club site. ...attached ... it's not.
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Old 04-02-2019, 20:20   #7
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Re: Brain hurts - Working out course to steer

You'll be dealing with leeway as well as current, and both will likely change many times during the voyage. If you're offshore, you'll probably be fixing your position with GPS and/or celestial. Setting a series of waypoints along your rhumb line(s) divides the trip into smaller segments more attuned to local conditions, and the course that you have to steer to stay on the rhumb line provides valuable information about current and leeway as your voyage progresses. A good autopilot will do this for you and provide cross track error, but the average course steered between waypoints should be noted on your chart as a deduced reckoning log in the event that one is needed. You can draw triangles and do trig, but what you're really needing to know at any given time is what magnetic course to steer, with all present factors considered, to keep you on the rhumb line, right? A chart plotter or GPS readily provides that information, and updates it constantly.
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Old 04-02-2019, 23:01   #8
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Re: Brain hurts - Working out course to steer

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belizesailor: yes 'tab' = tablet. Spot on... it's the trig function I need. Example... How do I cross the English Channel.
For a slow displacement boat...if you are figuring a 6hr run, start 3hr before slack and point her in the right direction and keep a steady heading. Odds are pretty good that the currents will switch part way thru and line you back up with your destination.

Use the free charting software on your tablet to check periodically you aren't too far off.

Problem with coastal situations is things change too much and to unpredictably...If you run the math assuming a 6kt cruising speed but the wind only gives you 4kt...all your math is going to be wrong because you won't be in the same current. Also in most areas, you won't know the exact current each combination of time and location anyway. A little closer to a shoal and the current could be faster or slower and may even change direction.

In the end, you make a rough guesstimate and adjust based on what happened.
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Old 05-02-2019, 02:06   #9
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Re: Brain hurts - Working out course to steer

danielamartindm: That's the wrong way to do it. The rhumb line across multiple tides is significantly longer (distance and time) than CTS. See Stu's suggested thread... CTS vs Following Rhumb Line - DILUTED THREAD


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.


Drawing a vector diagram on a paper chart makes it easy, but how do I do it in software? Predictwind Pro is the only thing I've found that does it, but that's $500 a year subscription.
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Old 05-02-2019, 02:21   #10
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Re: Brain hurts - Working out course to steer

These 3 should help:

Trig shortcuts for quick nav solutions - Ocean Navigator - January/February 2003

https://www.rya.org.uk/knowledge-adv...av-skills.aspx

Coastal Navigation Mathematics | Sailing Blog by NauticEd
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Old 05-02-2019, 02:51   #11
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Re: Brain hurts - Working out course to steer

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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.
You mis-read. You pick a heading and stay on that heading (as in what direction is the compass heading matching where the bow pointing, not the GPS calculated heading). The chart plotter just gives you an idea if it's working out reasonably well.

Crossing the Channel this works because it's a fairly straight forward problem. 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.

Though actually it's pretty easy to do CTS with a chart plotter. Compare the GPS heading to your compass heading...I usually keep track of it as a rough assessment mentally.
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Old 05-02-2019, 04:33   #12
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Re: Brain hurts - Working out course to steer

I'm not sure about average drift over the whole passage, but if you know your current set and drift, you could drop a way point in your nav program in that direction and distance from your current position. Drop a second way point along the course from your current position a number of nautical miles away which matches your speed made good. The course from the first way point to the second is the course to steer. The length of that course is your speed through the current.


IOW, draw the vector diagram in your nav program. That's the best I can come up with.
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Old 05-02-2019, 06:06   #13
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Re: Brain hurts - Working out course to steer

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There must be a lot of people sailing far further than they need to.
But that's a good thing!

If you are trying to get from A to B, almost any different mode of transport will do so more efficiently.

Yes, (half) joking
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Old 05-02-2019, 09:39   #14
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Re: Brain hurts - Working out course to steer

Electronics will help monitor this by giving instantaneous COG but this problem is so much simpler on paper that is the way I would continue to do it. Not aware of any nave program that can do it.
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Old 05-02-2019, 09:50   #15
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Re: Brain hurts - Working out course to steer

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.
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