Cruisers Forum
 


Reply
  This discussion is proudly sponsored by:
Please support our sponsors and let them know you heard about their products on Cruisers Forums. Advertise Here
 
Thread Tools Search this Thread Rate Thread Display Modes
Old 05-02-2019, 14:33   #31
Registered User

Join Date: Aug 2018
Location: Hudson Valley, NY
Posts: 365
Re: Brain hurts - Working out course to steer

Quote:
Originally Posted by Andy Todd View Post
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.
You take the average, ie: if sailing due north 2 hours in a 2kt west current & 3 hours in a 4kt east means if you sailed the bearing to destination you'd be 8nm east of you target. You figure your CTS on that 8nm eastern drift.

And yes, the boat will sail a series of wide reversing arcs, but it's still the best way to the destination, unless you like to monitor cross track error on your GPS and correct to that every10-20 min.

And yes, conditions change while underway, so breaking a long voyage into 6-8hr stints and re-calculating if things aren't going as planned is necessary. (This is where the electronic nav aids come in: they get you an accurate location to start over.)

An autopilot that could hold a bearing to the destination and apply drift correction to maintain that should be possible, but I don't know of any. (someone correct me if I'm wrong)
Pete17C is offline   Reply With Quote
Old 05-02-2019, 14:52   #32
Registered User

Join Date: Dec 2010
Location: W Carib
Boat: Wildcat 35, Hobie 33
Posts: 13,480
Re: Brain hurts - Working out course to steer

Quote:
Originally Posted by Andy Todd View Post
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.
Be aware there is "SailGrib" (a GRIB viewer) and "SailGRIB WR" (the weather routing version), both are quite good and available at no charge (I think...or VERY inexpensive if not) on Google Play.
belizesailor is offline   Reply With Quote
Old 05-02-2019, 15:10   #33
Registered User

Join Date: Aug 2018
Location: Hudson Valley, NY
Posts: 365
Re: Brain hurts - Working out course to steer

Quote:
Originally Posted by billdre View Post
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.
If I ever end up sailing with you on your boat, please never loose site of land.

/electronics fail
//dead reckoning and pilotage are basic navigation skills, reading a chart is as well
///but you do you
Pete17C is offline   Reply With Quote
Old 05-02-2019, 15:47   #34
Registered User

Join Date: Jan 2017
Location: Fort Pierce FL
Posts: 322
Re: Brain hurts - Working out course to steer

Take courses ASA103 and 104.
conchaway is offline   Reply With Quote
Old 05-02-2019, 15:58   #35
Registered User

Join Date: Aug 2018
Location: Hudson Valley, NY
Posts: 365
Re: Brain hurts - Working out course to steer

Quote:
Originally Posted by billdre View Post
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.
Actually, sailing a boat involves getting from A to B in a planned manner. Trying to anchor in a dark, crowded harbor because you failed to plan for a 4kt head current and lighter than forecast winds got you there 3 hrs late is not good sailing, and not enjoyed by anyone.

/some folks sail boats
//others drive them
///\_(ツ)_/¯
Pete17C is offline   Reply With Quote
Old 05-02-2019, 16:18   #36
Registered User
 
danielamartindm's Avatar

Join Date: Jul 2011
Location: St. Petersburg, Florida
Boat: Leopard 39
Posts: 860
Re: Brain hurts - Working out course to steer

Quote:
Originally Posted by Pete17C View Post
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.

When you see that the AP magnetic course steered is slowly and consistently changing in a given direction, the idea is to correct any cross-track error and then to tweak the AP course a few degrees manually until it "holds" that steered course over time. When that happens, you'll be on course and cross track error will remain minimal because you have compensated (by crabbing) for the forces that wanted to veer you off. Obviously, if you don't do this periodically, you'll sail a huge arc rather than a straight line. That would be a rather oblivious thing to do.
danielamartindm is offline   Reply With Quote
Old 05-02-2019, 16:20   #37
Registered User

Join Date: Dec 2010
Location: W Carib
Boat: Wildcat 35, Hobie 33
Posts: 13,480
Re: Brain hurts - Working out course to steer

Quote:
Originally Posted by conchaway View Post
Take courses ASA103 and 104.
Coastal Navigstion is 105 (neitner 103 or 104 cover calculating CTS).
belizesailor is offline   Reply With Quote
Old 05-02-2019, 16:25   #38
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Denmark (Winter), Helsinki (Summer); Cruising the Baltic Sea this year!
Boat: Cutter-Rigged Moody 54
Posts: 33,764
Re: Brain hurts - Working out course to steer

Very many folks posting in this thread don't understand the problem.


If you have a constant current, then yes -- you can just steer until COG equals BTW and you're set. No need for any math.



But the problem the OP is talking about is different -- the case where, in the course of the passage, the tide sweeps you first one way, then the other way. This is typical Channel or North Sea crossing. If you steer to keep the boat on the rhumb line, you will end up sailing a great deal longer distance than you planned, and in extreme cases you can't even arrive. This is so inefficient in changing currents because the boat doesn't sail over ground -- it sails in water. The shortest distance across a moving body of water is always the shortest distance through the water -- not the shortest distance over ground. That is the whole idea of CTS -- course to steer. It's based on heading, not COG -- COG is useless in this situation. You attempt to calculate and AVERAGE OUT the set and drift over the whole passage, so that you steer a constant heading which is that one straight line through the water, which gets you to the destination without changing course. Your chart plotter cannot solve this problem.


The old thread linked to above has a pretty good explanation of the issue.




To the OP:



If the tide runs approximately perpendicular to the rhumb line, like the classical 60 miles from the Needles to C-bourg, then you can use the traditional method as taught by the RYA, and a paper chart isn't really needed -- you can do it with a tidal atlas. The traditional method is pretty simple and not all that laborious. It does not account for the exact direction of the tide, but since you cannot predict your speed with great certainty anyway, for most purposes this does not matter.


Where this breaks down however is when you you are crossing not perpendicular to the tides. When you have to add the direction of the tide at each hour, then you start to need to be doing trig and it becomes quite laborious, especially considering the desirability of doing several runs of this calculation for different possible average speeds, so that you know the limits of possible different CTS's resulting from variance between predicted and actual speed.


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. So not only is the direction of the tide taken into account -- the calculation is done on 6 minute intervals rather than one hour intervals. When I am sailing across tidal waters I start off with three possible average speeds based on the conditions -- 7.5, 8, and 8.5 knots, for example, in good sailing conditions -- and do three model runs, and get a range of possible CTS depending on how the speed actually turns out. You choose one of these but you can change it as you see how your average speed is developing. Then re-run the model every two or three hours and generate corrected CTS values. Neptune Plus allows you to plug in leeway, which is important if you are sailing anywhere near upwind.




Note that the importance of this procedure is greater, the slower your boat is, or rather, the greater the speed of the tide relative to your boat speed. It is possible to imagine situations where attempting to stay on the rhumb line can result in not ever reaching the destination, whereas the destination can be reached easily by steering a proper CTS. Some of our friends here who have not sailed in the Channel, have difficulty imagining the problem, which is one of the more interesting navigational puzzles affecting sailors.
__________________
"You sea! I resign myself to you also . . . . I guess what you mean,
I behold from the beach your crooked inviting fingers,
I believe you refuse to go back without feeling of me;
We must have a turn together . . . . I undress . . . . hurry me out of sight of the land,
Cushion me soft . . . . rock me in billowy drowse,
Dash me with amorous wet . . . . I can repay you."
Walt Whitman
Dockhead is offline   Reply With Quote
Old 05-02-2019, 16:26   #39
Registered User

Join Date: Dec 2010
Location: W Carib
Boat: Wildcat 35, Hobie 33
Posts: 13,480
Re: Brain hurts - Working out course to steer

Quote:
Originally Posted by Pete17C View Post
...

An autopilot that could hold a bearing to the destination and apply drift correction to maintain that should be possible, but I don't know of any. (someone correct me if I'm wrong)
Raymarine APs have a "track" mode in which they will steer to a waypoint and apply corrections as necessary to keep XTE below a preset level...but what fun is that! [emoji6]
belizesailor is offline   Reply With Quote
Old 05-02-2019, 16:30   #40
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Denmark (Winter), Helsinki (Summer); Cruising the Baltic Sea this year!
Boat: Cutter-Rigged Moody 54
Posts: 33,764
Re: Brain hurts - Working out course to steer

Quote:
Originally Posted by Pete17C View Post
. . . .

An autopilot that could hold a bearing to the destination and apply drift correction to maintain that should be possible, but I don't know of any. (someone correct me if I'm wrong)

Well, most if not all modern autopilots can steer a rhumb line -- called Nav Mode or Track Mode. That will give you a constant COG towards the target and keep you running in a straight line over ground, automatically correcting set and drift first one way and then the other way as the tide changes.



But this is not the right way to do it if the tide changes over the course of the passage -- you will be fighting the tide and sailing a much greater distance through the water, and thus you will get there much later, than if you sail a constant HEADING based on an accurately calculated Course To Steer, which AVERAGES the set and drift over the whole passage to find the shortest path through the water.


Here is a good simple explanation of the traditional method:


http://www.skysailtraining.co.uk/tid...e_to_steer.pdf


which is what you need to know to pass a Yachtmaster exam. Note that when you do this correctly, your boat will follow an S curve across the Channel, which looks like a long way over ground, but it's the shortest and fastest way across because you are sailing a straight line THROUGH WATER.
__________________
"You sea! I resign myself to you also . . . . I guess what you mean,
I behold from the beach your crooked inviting fingers,
I believe you refuse to go back without feeling of me;
We must have a turn together . . . . I undress . . . . hurry me out of sight of the land,
Cushion me soft . . . . rock me in billowy drowse,
Dash me with amorous wet . . . . I can repay you."
Walt Whitman
Dockhead is offline   Reply With Quote
Old 05-02-2019, 17:25   #41
Registered User

Join Date: Aug 2018
Location: Hudson Valley, NY
Posts: 365
Re: Brain hurts - Working out course to steer

Quote:
Originally Posted by danielamartindm View Post
When you see that the AP magnetic course steered is slowly and consistently changing in a given direction, the idea is to correct any cross-track error and then to tweak the AP course a few degrees manually until it "holds" that steered course over time. When that happens, you'll be on course and cross track error will remain minimal because you have compensated (by crabbing) for the forces that wanted to veer you off. Obviously, if you don't do this periodically, you'll sail a huge arc rather than a straight line. That would be a rather oblivious thing to do.
Yes, that's adjusting your plan (based on forecast) to actual conditions. But we're in the planning stages here.

ie: with forecasted conditions can I sail a heading at an approximate speed, and be where I need to be at a somewhat approximate time. (sailors are usually pretty flexible)

(BTW, I've only run one boat w/AP, on a week long charter, hold heading is all I was taught to use, and all I really needed)

/forecasted conditions
//it's more than the weather
Pete17C is offline   Reply With Quote
Old 05-02-2019, 18:08   #42
Registered User

Join Date: Aug 2018
Location: Hudson Valley, NY
Posts: 365
Re: Brain hurts - Working out course to steer

Quote:
Originally Posted by Dockhead View Post
Well, most if not all modern autopilots can steer a rhumb line -- called Nav Mode or Track Mode. That will give you a constant COG towards the target and keep you running in a straight line over ground, automatically correcting set and drift first one way and then the other way as the tide changes.



But this is not the right way to do it if the tide changes over the course of the passage -- you will be fighting the tide and sailing a much greater distance through the water, and thus you will get there much later, than if you sail a constant HEADING based on an accurately calculated Course To Steer, which AVERAGES the set and drift over the whole passage to find the shortest path through the water.


Here is a good simple explanation of the traditional method:


http://www.skysailtraining.co.uk/tid...e_to_steer.pdf


which is what you need to know to pass a Yachtmaster exam. Note that when you do this correctly, your boat will follow an S curve across the Channel, which looks like a long way over ground, but it's the shortest and fastest way across because you are sailing a straight line THROUGH WATER.
If you're sailing, the quickest way is never the shortest way, wind and all.

The S curves over ground are a result of you wanting to hold a constant heading and sail trim (good for sailing in a constant wind, remember we're planning, not actually sailing) and end up where you hoped you would, while the water moves you around.
Pete17C is offline   Reply With Quote
Old 05-02-2019, 18:12   #43
Registered User
 
Stu Jackson's Avatar

Join Date: Jan 2011
Location: Cowichan Bay, BC (Maple Bay Marina)
Posts: 9,704
Re: Brain hurts - Working out course to steer

Dockhead,


Thanks for chiming in. Happy New Year.


Your explanations continue to be enhanced over time. It really is an interesting subject and your "imagining it" presentation is superb.



I'd bookmarked the link to your great material so when this topic arises I can help folks find your work.


Thanks again, I just love this stuff. 'Specially during winter.
__________________
Stu Jackson
Catalina 34 #224 (1986) C34IA Secretary
Cowichan Bay, BC, SR/FK, M25, Rocna 10 (22#) (NZ model)
Stu Jackson is offline   Reply With Quote
Old 05-02-2019, 18:22   #44
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Denmark (Winter), Helsinki (Summer); Cruising the Baltic Sea this year!
Boat: Cutter-Rigged Moody 54
Posts: 33,764
Re: Brain hurts - Working out course to steer

Quote:
Originally Posted by Stu Jackson View Post
Dockhead,


Thanks for chiming in. Happy New Year.


Your explanations continue to be enhanced over time. It really is an interesting subject and your "imagining it" presentation is superb.



I'd bookmarked the link to your great material so when this topic arises I can help folks find your work.


Thanks again, I just love this stuff. 'Specially during winter.

Thanks, and cheers


It's a fun problem, which has been great fun to think about, and not the least because it is just about the last mathematical problem which our plotters can't solve for us, so one of our last chances to be real navigators instead of mere button pushers
__________________
"You sea! I resign myself to you also . . . . I guess what you mean,
I behold from the beach your crooked inviting fingers,
I believe you refuse to go back without feeling of me;
We must have a turn together . . . . I undress . . . . hurry me out of sight of the land,
Cushion me soft . . . . rock me in billowy drowse,
Dash me with amorous wet . . . . I can repay you."
Walt Whitman
Dockhead is offline   Reply With Quote
Old 05-02-2019, 18:30   #45
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Denmark (Winter), Helsinki (Summer); Cruising the Baltic Sea this year!
Boat: Cutter-Rigged Moody 54
Posts: 33,764
Re: Brain hurts - Working out course to steer

Quote:
Originally Posted by Pete17C View Post
If you're sailing, the quickest way is never the shortest way, wind and all.

The S curves over ground are a result of you wanting to hold a constant heading and sail trim (good for sailing in a constant wind, remember we're planning, not actually sailing) and end up where you hoped you would, while the water moves you around.

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.
__________________
"You sea! I resign myself to you also . . . . I guess what you mean,
I behold from the beach your crooked inviting fingers,
I believe you refuse to go back without feeling of me;
We must have a turn together . . . . I undress . . . . hurry me out of sight of the land,
Cushion me soft . . . . rock me in billowy drowse,
Dash me with amorous wet . . . . I can repay you."
Walt Whitman
Dockhead is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Want To Buy: Navman / Northstar Master Control unit / Brain / course computer Dream Maker Classifieds Archive 2 09-02-2015 13:13
SOG or STW for course to steer mtompson Navigation 21 18-09-2013 15:07
Survey surprise hurts FL plans Tom1340 Monohull Sailboats 24 01-01-2012 10:00
right brain left brain tyrntlzrdking Flotsam & Sailing Miscellany 39 20-09-2008 18:40
Question as old as the sea! First boat-what to buy? My head hurts! Donny_D Meets & Greets 16 12-05-2008 22:57

Advertise Here


All times are GMT -7. The time now is 10:35.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.