Speed over the ground

[Cotillion]

Re: Powerskipper\'s dead right

"Not many people fail, so relax!"

When I did my DS their was a candidate (not me I hasten to add) who ran us aground on our night sail. Spent almost an hour going around the Nass beacon looking for the channel for Tollesbury (the instructor had to take over else we wouldn't have made it over the sill). Had the rest of us in fits of laughter with his comments during the engine discussion. I wouldn't go out with him in a row boat on Wanstead flats duck pond. But he got the certificate.

Kim

 

[Dougie_the_Mate]

Re: Powerskipper\'s dead right

It was not my intention to spend lots of time below working this out. I have already done both day skipper and yachtmaster (coastal) theory and was simply doing some revision of the RYA examples prior to next week.

My query was based on the fact that all the other bits and bobs of the calculations are correct, but my answer to SOG is never exactly right - close - but rarely a cigar. I simply wondered if I was missing something obvious which might help me get the right answer.

/forums/images/graemlins/confused.gif

 

[StugeronSteve]

Re: Yes indeed, but.....

.....one never knows the true tidal stream, therefore, one can only truly determine SOG/SMG by using employing a means of measurement ie, GPS, Radar etc, or by timing the passage over a measured distance. An eta can be determined by trigonometry but not a true SOG.

 

[Danny_Labrador]

Looking at whats been given above as helpful ......

Poor bugger…. He’s trying to pass an exam! not explore the minutiae of advance nautical trigonometry.

Dougie the Mate, my advice is get a past test paper book and work through it – they generally give worked examples and it will present the method that will get you a pass, method is as important for the exam as the result, (you could afford to be a little out).

 

[ashanta]

The formula you seek is :

S = D
--
T

Speed equals distance over time. This has been explained in many ways here but this is the formula. It takes in all factors such tide etc. If you set of from point A and get to point B measure the distance and how long it took and you have your SOG.

Regards.

Peter.

 

[ashanta]

Re: Powerskipper\'s dead right

Dougie,

For an exam it's theoretical so you have to accept that there are tidal and leeway influences just as you would when plotting a course and plotting vectors and calculating a ETA. If you are able to do this then the formula of speed = distance over time gives you your SOG for that passage.
As opposed to calculating SOG in retrospect having completed the passage. Please PM if you need any further help.

regards.

Peter

 

[alant]

Re: Speed over the ground - will this simplify?

Stop thinking its complicated & relax.

When you draw your speed vector triangle (either for CTS or EP), using any, but always same scale units, just make sure you mark each with the correct arrows.
Tide=3 arrows
Water speed (also water track)=1 arrow
Ground speed (also ground track)=2 arrows (easy to remember, cos you put 2 feet on the ground-a DS student taught me this).

Measure this side of the triangle remembering its the same speed units as used for tide & boat speed thru the water.
For CST, the SOG vector will be measured along the vector drawn from start thru the destination. >>
For EP, the SOG vector will be measured along the vector drawn from start to end of tide vector. Still >>.

Distance, only to be measured from the vertical edge of the chart, does not enter the argument, unless working out an ETA.

Usual beginners error in these, is to draw a CTS vector from the end of the tide >>> direct to destination.

 

[Dougie_the_Mate]

Re: Speed over the ground - will this simplify?

Fellow sailors

I realise that coming from Caledonia our pre-disposition to using accurate English can often be confusing to our Southern neighbours, however to clarify:

a) I am not trying to pass an exam.
b) I am simply doing these exercises out of interest and to revise the proper chartwork procedures
c) I already have more than enough examples from the theory courses, the exam papers and the RYA book of Navigation exercises.
d) All of these things suggested I can do.

The problem was that when I checked the solutions the chartwork would be almost always bang on - allowing that is for the over 50's shake - which can also affect my putting - the course to steer whether true magnetic or compass would be right, but the answer sought for SOG was always a bit out in terms of the answer sheet.

I will peruse these excellent pieces of advice and apply the words of wisdom offered in real time and if I arrive at Whitehaven an hour early because my ETA is wrong having miscalculated the likely SOG I'll let you know.

/forums/images/graemlins/smile.gif /forums/images/graemlins/smile.gif

 

[Gunfleet]

Re: Powerskipper\'s dead right

<<breton cap, defaced ensign, brass dividers, oil lamps and pump up cooking stove>> If you leave out the ensign business, exactly. I think we were at school together sir.


(Unfortunately abt 50 years ago!)

 

[Gunfleet]

Re: Powerskipper\'s dead right

It just so happens you're referring to a stretch of water I know very well. I have run aground in Tollesbury Fleet plenty of times, and so has everyone else I know who uses it. Most embarrassingly I had a friend who was born there and sailed there all his life (well over 60 years) on the rudder when we grounded just after the bifurcation from Mersea. Going into Tollesbury at night with no moon is for locals, experts and the brave only!

 

[TheBoatman]

Wonderful!!!!

It's nice to know that there are still some people out there that appreciate the finer points of cruiser sailing!
SOG is something that should never be calculated by a cruising yottie. It should always be left in reserve as a suprise for discussion when tied up, to slow and you can talk about what you should have done to set the boat up better, how the "maps" you were using were "off" on tidal predictions etc. To fast and you can congratulate yourself on how well you set the sails, on how good a sailor you are and that your hours of pouring over almanacs and "maps" has paid off. All this with a beer in hand!

God don't you just luv our sport?

Peter. /forums/images/graemlins/smile.gif