Speed over the ground

D

Dougie_the_Mate

Well-Known Member
Joined
28 Mar 2004
Messages
318
Location
Scotland
Visit site
I am doing some chartwork revision prior to going off to do my Day Skipper Practical next week and I realise (again) that the one thing I cannot get my head around is how to calculate SOG.

Is there a simple formula that can be applied to work this out?

Thanks in anticipation
 
right!

Speed through the water is simple.. its the speed of the boat through the water .. OK so far.
The water is sometimes moving as well so its movement is relative to the ground and its rate and direction is measured by the tidal flow. So if the boat were sitting still in the water it might still be moving relative to the ground .. OK?

Next take the two vectors of boat through the water + tidal effect and you've got movement relative to ground (this is what a GPS will give you). So the tidal strength can be calculated from SOG - Speed through water.
 
My wife is currently 'enjoying' her DS course.

They most certainly do require tidal calculations, as explained by Rusty and Jimi!

Distance from start to finish as SOG will ensure a failed question.
 
I am aware of the principle and have no problem with the chart work, but it seems to me that if the tide is pushing you sideways as well as forwards or back, then the simple answer of addition or subtraction to the boat speed doesn't come to the correct conclusion.

Still puzzled.
 
Simplest method as above, just read it off the GPS. BUT that's just a snapshot. If you're doing it on a chart there are two ways depending on wheather you're working out a course to steer or an estimated position.

Everything's easy if you use 1 hour times.

Estimated position goes like this.
1. You know where were are so plot a fix.
2. Using your course steered & speed from the log, plot a new position. This is the DR (Dead Reconed) position.
3. Apply the tide for the time you ran. This is your EP (Estimated position).
4. Measure the distance from the starting fix to the EP & use that to calculate the SOG. Similary the bearing from the starting fix to the EP is the COG.
SOG = distance x 60/time (in minutes)

i.e. distance = 3m, time = 35 mins -------- SOG = 3 x 60/35 = 5.1kts

Course to Steer goes like this.
1. You know where you are so plot a fix.
2. You know where you want to go so plot that.
3. Draw a line from the fix to the destination making sure it runs well past the destination if it's a short run.
4. From the first fix, plot a vector for an hour's tide.
5. Set your dividers to the boat speed & measure from the end of the tide to the line between the starting fix & the desination. It may cross the line well after the destination if it's a short run.
6. The measurement from the starting fix to this point is your SOG.

You can scale the triangles by up or down according to the run. I haven't included leeway since I don't think that's used at DS level.
 
\"Look at the GPS\"...

...is not exactly the answer expected by a teacher upon such question, is it? /forums/images/graemlins/grin.gif
 
[ QUOTE ]
OR Look at the GPS.

[/ QUOTE ]

Shame on you Julie. And you a destuctor an' all. /forums/images/graemlins/shocked.gif /forums/images/graemlins/wink.gif /forums/images/graemlins/grin.gif
 
It's a vector, so no easy calculation you can write down here, as it depends on conditions you are out in. You need tide and wind, then plot, on a chart.
 
Powerskipper\'s dead right

I'm baffled by this question. In what part of 'sailing in waters familiar to the candidate by daylight' would you need to calculate SOG for chartwork? Suggest you keep your chartwork down to lists of bouys (for pilotage) the tidal heights, speeds and directions of flow you expect to meet in local waters and keeping a plot of sorts on the chart (in the cockpit if you don't feel confident about the chap/ess at the wheel). You won't need SOG except in the crudest way (how fast am I sailing and how much is the tide hindering or helping), plus there is a disadvantage... if you spend time below tiddling about calculating your SOG your instructor/examiner will wonder if you are fit to sail the boat. Now the classroom course, that's different. Hope you enjoy it. Not many people fail, so relax!
 
The easiest way to visualise it is to use graph paper. Select a scale and plot your course. Then plot the influence of tidal or wind elements. Eg you may be heading N at 6 knots but you know that you are going to make leeway at say 5°. So offset the course by that amount. However say you have a a westerly current of 3 knots then, starting from where you plotted the leeway position move this three knots to the left on the scale you chose - after one hour. You now have plotted your start and finish points and it is easy to determine your effective course and speed over the ground by simple measurement.

Or use GPS.

Or maths.

John
 
Re: Powerskipper\'s dead right

Thats my type of sailing the KISS method. I look at the log and if it says 5knts and I've got 10nm between the marks I reckon if hit the mark (as in bounce off it)in 90 mins my SOG was more than 5knts, if however after 150 mins I still can't see the mark through the bins, the SOG is less than 5?

BTW, Being the purist type of sailor that I'm known for, bretton cap, defaced ensign, brass dividers, oil lamps and pump up cooking stove if I get below 5 knts I always turn the engine on to improve things like SOG /forums/images/graemlins/smile.gif
 
It simple:

Draw a line (or series of lines) on the chart from your start point which represents you course and speed through the water.

At the end of this add a "tail" of tidal vectors which represent the tidal streams encountered during your passage.

Finally the line between the start point of the passage, and the end point of the tidal streams represents your cumlative course over ground and speed over ground.

Simply divide the length of the line by passage time to get your SOG. Easy.

Worth remembering that this is not your actual course over the gound, to plot this you would need to perform this process over smaller and smaller sections (though typically hourly sections are enough) so that the changing effects of the tide are brought out individually, rather than compounded together.

Regards, Jeff.
 
May be my english wasn't clear.... I thought it was...
"I think of it as a factor of where you start to where you actually finish, after allowing for tide.

DS questions won't need any complicated maths as they are usually 1 hour plots.

So distance from start to actual finish will most often be SOG."

So from where you start to where you finish...AFTER ALLOWING FOR TIDE.

Jim
 
Agreed, the only way of getting a running SOG, of any accuracy, is a GPS set. Otherwise it's the charted distance between two known points in NM / time in Hrs. Strictly speaking this will give you your speed made good, as there may have been some interruptions to the heading, ie. deviation to avoid traffic.
 
You must log in or register to reply here.

Share this page

Top