SOG or STW for course to steer

[mtompson]

Hi
I'm a little confused which speed to use with course to steer calculations. SOG (speed over ground) or STW (log, or speed through the water)?
In various YouTube vids & websites, people mention both, sometimes saying never use the other!
Anyway, I'd appreciate anyone's theories or facts! on this one...
Many thanks...

[jeremiason]

I use speed over ground since it is your actual speed over the water. It is more accurate than Log Speed, since that is influenced by currents.

As an example, you could be sailing at 7 knots log speed with a 2 knot head current, meaning your speed over ground would be approximately 5 knots. Every hour your calculations would be as much as 2 nautical miles off in dead reckoning.

If you have the current books, you can calculate set and drift on paper and use log speed... But very few recreational boats do that.

Of course the easiest thing to do is turn on the GPS and relax :-D

[barnakiel]

I use SOG when I work on the COG vector. I use STW when I work on the CTW vector.

Since I get most of my data from GPS these days, I practically use only COG/SOG. (Much as I take mentally notice drift and set).

b.

[dugout]

Originally Posted by mtompson

Hi
I'm a little confused which speed to use with course to steer calculations. SOG (speed over ground) or STW (log, or speed through the water)?
In various YouTube vids & websites, people mention both, sometimes saying never use the other!
Anyway, I'd appreciate anyone's theories or facts! on this one...
Many thanks...

Navigating assumes reference to earth, (ground). Use SOG and COG.

STW and CTW, (Heading) are tactical, for lack of a better word, data points used for calculation or improvement of SOG and COG.
At one time all we had was STW and Magnetic Heading. COG and SOG where only available through good piloting skills, and carefully plotted as DR, or Dead Reckoning...

[mtompson]

Thanks for the replys so far, great stuff...

[muttnik]

Originally Posted by mtompson

I'm a little confused which speed to use with course to steer calculations. SOG (speed over ground) or STW (log, or speed through the water)?

Sorry to disagree with everyone but mtompso's UK location makes me guess that the question relates to course to steer calculations as someone studying for RYA Dayskipper might be attempting. Your COG/SOG is the sum of vectors representing tide/current speed and direction, boat speed and direction through the water, and leeway (speed and direction). You have a desired COG to end up with and the tweakable in your inputs is the course to steer (ie boat heading). Your SOG (in the combined output vector) is whatever it ends up as but the "boat speed" used in the "boat" input vector is "through the water"

[mtompson]

Thanks muttnik.
It does make sense to have no tidal vector before I add one anyway! If I understand correctly?
On paper (not on the water!) STW = SOG minus all currents & wind effects? (I understand that is simplistic, and that 'minus' is vector maths!)
Thanks for the input...

[boatman61]

Your spin log gives you speed through the water... have recorded 6kts while sitting looking at the Needles for 5hrs...

Your GPS gives you speed over ground... my log read 5.4kts but my GPS said I was traveling at 13kts... that was heading from Gurnsey up to Alderney....
Awesome tides...
Speed over ground is the one to use for the best results... along with tide/current data..

[mtompson]

thanks boatman61! yes Alderney is an extreme testing ground for this stuff!

[Pete7]

But get it right and the lift is huge. St Peter Port to Portsmouth saw us still carrying the tide well into the English Channel. However, for some reason it always means a 5am start to catch the right tide.



Pete

[muttnik]

Forgetting leeway and just assuming that COG+SOG being the sum of the effects of tidal flow (/current) speed and direction and boat speed through the water and heading I am guessing that the OP is following instructions which give the inputs to the CTS plot as:
- Desired COG
- tidal flow speed (from tidal atlas)
- tidal flow direction (ditto)
- Boat speed
...and the OP's question is whether this "boat speed" should be STW or SOG.

Knowing either one you can calculate the other and CTS, however I contend that the answer the OP is looking for in this instance is "STW" because:
1. You don't know SOG in advance. You *can* estimate STW given knowledge of the boat and the predicted conditions. Thus STW is the input which is useful in passage planning rather than dynamic course shaping
2. Tidal streams don't stay constant. You can stick the boat on a heading which gives you the desired COG (at which point you know your SOG) but if you maintain that heading in a tidal sea, your COG will change. Changing heading every hour to keep a constant COG crossing the English channel (for example) is a mug's game.
3. Elsewhere may be different, but if the OP's instruction source is a book based on the RYA syllabus it will be teaching the plotting of a course to steer based on STW.

Of course I am often wrong about many things and happy to be explained the error in my thinking :-) Perhaps mtompson could give a more detailed description of the question?

[jeremiason]

Originally Posted by muttnik

Forgetting leeway and just assuming that COG+SOG being the sum of the effects of tidal flow (/current) speed and direction and boat speed through the water and heading I am guessing that the OP is following instructions which give the inputs to the CTS plot as:
- Desired COG
- tidal flow speed (from tidal atlas)
- tidal flow direction (ditto)
- Boat speed
...and the OP's question is whether this "boat speed" should be STW or SOG.

Knowing either one you can calculate the other and CTS, however I contend that the answer the OP is looking for in this instance is "STW" because:
1. You don't know SOG in advance. You *can* estimate STW given knowledge of the boat and the predicted conditions. Thus STW is the input which is useful in passage planning rather than dynamic course shaping
2. Tidal streams don't stay constant. You can stick the boat on a heading which gives you the desired COG (at which point you know your SOG) but if you maintain that heading in a tidal sea, your COG will change. Changing heading every hour to keep a constant COG crossing the English channel (for example) is a mug's game.
3. Elsewhere may be different, but if the OP's instruction source is a book based on the RYA syllabus it will be teaching the plotting of a course to steer based on STW.

Of course I am often wrong about many things and happy to be explained the error in my thinking :-) Perhaps mtompson could give a more detailed description of the question?

You right for planning... But the OP said "To Steer" not to plan a route

[muttnik]

Originally Posted by jeremiason

You right for planning... But the OP said "To Steer" not to plan a route

A "route" to me is a succession of legs between waypoints, and that wasn't what I meant. I was talking about calculating a course to steer (ie heading) to take you directly from A to B given what the tide will be doing during that time.

What methodology for calculating CTS are you proposing that uses SOG?

[boatman61]

Originally Posted by muttnik

Forgetting leeway and just assuming that COG+SOG being the sum of the effects of tidal flow (/current) speed and direction and boat speed through the water and heading I am guessing that the OP is following instructions which give the inputs to the CTS plot as:
- Desired COG
- tidal flow speed (from tidal atlas)
- tidal flow direction (ditto)
- Boat speed
...and the OP's question is whether this "boat speed" should be STW or SOG.

Knowing either one you can calculate the other and CTS, however I contend that the answer the OP is looking for in this instance is "STW" because:
1. You don't know SOG in advance. You *can* estimate STW given knowledge of the boat and the predicted conditions. Thus STW is the input which is useful in passage planning rather than dynamic course shaping
2. Tidal streams don't stay constant. You can stick the boat on a heading which gives you the desired COG (at which point you know your SOG) but if you maintain that heading in a tidal sea, your COG will change. Changing heading every hour to keep a constant COG crossing the English channel (for example) is a mug's game.
3. Elsewhere may be different, but if the OP's instruction source is a book based on the RYA syllabus it will be teaching the plotting of a course to steer based on STW.

Of course I am often wrong about many things and happy to be explained the error in my thinking :-) Perhaps mtompson could give a more detailed description of the question?

If you can estimate your STW with the info you have and knowledge of the boat... you can do the same with speed over ground...
Your assuming a F5 for the day your planning for... or maybe a F3 after checking passageweather for 2-3 days ahead... no one plans just before they set off..
And as for crossing the Channel.. eg: Poole-Cherborg I'd allow say 12hrs (80-ish miles) and sail 1hr before HW and once level with Durlston Head I'll set a fixed course that will have me approaching land slightly W of Cherbourg on the last hour of the flood... if becalmed... engine on to maintain an average speed... no engine all plans are outa the window and its play it by ear..
In planning both are estimates...

[muttnik]

Originally Posted by boatman61

And as for crossing the Channel.. eg: Poole-Cherborg I'd allow say 12hrs (80-ish miles) and sail 1hr before HW and once level with Durlston Head I'll set a fixed course that will have me approaching land slightly W of Cherbourg on the last hour of the flood... if becalmed... engine on to maintain an average speed... no engine all plans are outa the window and its play it by ear..

I'm not dismissing any other method of navigation here. The OP was asking a specific question: what "speed" to use in calculating CTS. The method taught by the RYA in the UK (where the OP is located) and which appears in British books on navigation means "log speed" when it talks about "boat speed" in this context. For a multi hour passage there is an initial estimate of passage duration which clearly implicitly involves an SOG estimate, but that itself will be based on the combination of STW and tide. Your particular example is a peculiar case in that as an initial estimate of a 12 hour passage it's reasonable to assume tides cancel out and SOG == STW. You are using (as I would) a single CTS. I suspected some posters might think keeping COG constant was a good idea, which would obviously result in a longer passage: Hence my using cross channel as an example.

Once you have your duration estimate you can plot where you think the tides will push you each hour over the course of the passage. Given that tides aren't equal in different parts of the channel, this is where you find that things don't completely cancel out (although actually they mostly do). You then take your dividers, set them to the distance through the water your boat would have travelled in that time using the same scale as you did for the tidal plot, place one end on the end of the tidal plot, the other on the line representing your desired course (over ground), and presto, that's your course to steer to achieve the desired COG over the period you've plotted tides for. Books will tend to say the distance to set your dividers is "duration x boat speed" where "boat speed" means STW.

This is the "boat speed" I believe the OP was asking about, though perhaps s/he could clarify. I wasn't claiming that this is the only method of determining a course to steer.