gps speed question

[RupertW]

Does anyone actually draw tidal vector diagrams, outside of the classroom?

Obviously in this case the SOG is the hypoteneuse but I always used to draw the vectors for each hour when crossing the channel, particularly at the western end as there was no other way I had for knowing where I was. Then got GPS in 93 so gratefully stopped

I still do it approximately when planning though, as otherwise I won't know what compasss course to start out with to produce that lovely S-shaped course over the ground that saves an hour or three in passage time.

 

[onesea]

Does anyone actually draw tidal vector diagrams, outside of the classroom?

Some times yes free hand helps if you are getting yourself confused, or showing some one who is...

 

[john_morris_uk]

In reality a best guess is usually good enough and that is what I suspect most of us do. It might be OK for aviators as well, but I don't suppose it is what the MCA or Yachtmaster examiners want to hear.

Does anyone actually draw tidal vector diagrams, outside of the classroom?

Well as you probably know, I am a YM examiner and I quite often just guess. I hope the guess is an educated one though....
I don't even mind if candidates guess things, although they ought to know how to calculate things properly as well. It just a matter of knowing when a guess is good enough!

And yes, I am very happy to admit I still draw tidal vectors. Why wouldn't you? If you are crossing the channel or similar it's the best way of determining your course to steer across several hours of changing tide.

 

[JayBee]

I am certainly not knocking the practice of drawing tidal vectors. I asked the question in a genuine spirit of enquiry.

Crossing the channel from, say, the Chenal du Four to the Lizard, I find it perfectly satisfactory to sum the hours of eastgoing and westgoing tide and allow for the excess of one over the other. A simple calculation involving the rate of the tide as a fraction of boat speed and 6 degree increments of offset does the rest.

In any case, the tidal stream atlases are only approximations to the actuality on any particular day and set of weather conditions. Classroom type tidal vectors may be better than nothing, but are of doubtful precision in the real world. All IMHO, of course.

 

[sailorman]

If, according to the GPS I am making 6 knots SOG but have a 3 knot cross tide on my beam, how much of the 6 knots is forward speed as against sideways slip due to the tide?

do a tidal vector & find out

 

[JayBee]

do a tidal vector & find out

.

You really should read the whole thread from the beginning, or is this the killer answer?

 

[sailorman]

.

You really should read the whole thread from the beginning, or is this the killer answer?

i clicked on the first i did not know of the subsequent 25 posts. i did get the answer in one though

 

[john_morris_uk]

I am certainly not knocking the practice of drawing tidal vectors. I asked the question in a genuine spirit of enquiry.

Crossing the channel from, say, the Chenal du Four to the Lizard, I find it perfectly satisfactory to sum the hours of eastgoing and westgoing tide and allow for the excess of one over the other. A simple calculation involving the rate of the tide as a fraction of boat speed and 6 degree increments of offset does the rest.

In any case, the tidal stream atlases are only approximations to the actuality on any particular day and set of weather conditions. Classroom type tidal vectors may be better than nothing, but are of doubtful precision in the real world. All IMHO, of course.

But that's almost exactly what I do - its still a tidal vector though. I thought you were implying that everyone just steers what the bearing says to steer on the GPS nowadays... Some do, but it's a slow way to sail. And my apologies as this is drifting the thread.

 

[JayBee]

But that's almost exactly what I do - its still a tidal vector though. I thought you were implying that everyone just steers what the bearing says to steer on the GPS nowadays... Some do, but it's a slow way to sail. And my apologies as this is drifting the thread.

Yes, drifting the thread towards comprehension and the inadequacies of the English language, perhaps. We are probably on the same page, even if drawing and roughly calculating mean slightly different things to me.

Cheers

JB

 

[Little Five]

.

You really should read the whole thread from the beginning, or is this the killer answer?

Good comment JB.

 

[sailor211]

I am happy doing right angle triangles om my head and using a educated guess to asjust for the degrees off a RAT and usually am good enough.

I once sailled with a Maths expert and was explaining where the 3kt tide was taking us and our 5 kt speed together 1/cos tide angle (or something similar) 37 degrees up tide and it worked. He became very useful at these pilotage calcs in his head . Mostly sopt on and no disasters.

 

[nigelmercier]

clock code for me...

Just looked clock code up. I didn't realise it worked up to 60°, cool!

On the other hand, it's probably easiest to do a diagram on a scrap of paper.