Question for Hitech navigators.

graham

graham

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Any dazed kipper knows this is not the way to do it but On an average size tide on a yacht that can maintain 5 knots sail or motor if needed,How long would Bembridge to cherbourg take if you followed the Rhumb line compared to adding all the tides up and steering one heading most of the way over?

On a fast mobo it would be perfectly feasible and Im sure many moboers do exactly that.
 
I have a feeling it will be worse than that.On a spring tide you would be stopped dead stemming the tide at times if you caught the middle of the tide close to cborg
 
it would take carefull planning to arrive just at the worse time but if you did manage it the tidal stream atlas in reeds shows 28,56 3 hrs after HW Dover.

On an average tide you could make some progress all the time,painfull but possible. /forums/images/graemlins/grin.gif
 
Re: Question for any navigator with commonsense

Yes, you could end up dead into it. I've ended up 10 miles downtide off Cap de la Hague and motored against a Spring ebb at 5 knots for three hours - and not moved an inch until the tide slackened.

What, steer due East when you left home pointing Southwest?
Yep, got the T-shirt.

I'm now writing the book. This is it . . .
 
I make it: over 60NM, with 5 knots of boat speed and average of 2 knots of tide, it would take 13hrs and 6 mins if sailing the rhumb line, 12 hours if a steady compass course (assuming 6 hrs of tides each way)

If boat speed is 4 knots and average tide 2 knots then it would take 17hrs 19 mins along the rhumb line or 15 hrs sailing a compass course.

Any mathematicians correct me?
 
Depends when you start the Rhumb Line don't it? Get it right and you hit Cherbourg on the nose; get it wrong and you're downtide. Now if you are asking us to work out the worse time to start the Rhumb Line at least you might tell us why you want to know. Are you trying to work out whether to bother calculating the tides?
 
Nope... surely if you followed the Rhumb line the whole way you be stemming the tide a little the whole way....

Unless I misunderstood Graham was talking about following an exact course according to the GPS, and not ignoring the tide component at all... as opposed to what you are talking about which is choosing a CTS that is exactly the direct course to Cherbourg.....
 
Question was purely out of curiosity.And maybe to stimulate a bit of friendly debate.

i intended it to read steering a course directly down the rhumb line all the way adjusting the heading to stay on the line.Easy to do these days using XTE on the GPS even easier with a chart plotter.
 
Should say that that's from a vague recollection of O Level maths ie. Pythagoras triangle "A squared = B squared + C squared"

A being the long side of the triangle (speed and direction through the water)
B is the short side (tide vector averaged over six hours, then the same again in the opposite direction)
C being (course and speed made good to destination)

Any better suggestions?
 
Sadly that only works for a right angled triangle, and the tide isn't quite that helpfull!

You'd need to do it on perhap an hourly basis, with proper trignometric calculations to be more accurate.... but heck.... is it really worth the hassle? /forums/images/graemlins/grin.gif
 
But - fortunately the average tidal vector IS pretty much at right angles to a rhumb line from Bembridge to Cherbourg, at least close enough to give a good idea of the likely effect of adopting one approach or the other.
 
Re: Question for any navigator with commonsense

[ QUOTE ]
Lucky you didn't run into Alderney then!

[/ QUOTE ]

Didn't see it, poor vis. Besides, tongues were hanging out for Henri Ryst's duty-free booze!
 
And it's much more fun and huge sense of satisfaction calculating in advance where you're going to be, leaving at the most efficient time to catch the early tide and then... spending an extra half a dozen wet and weary hours clawing up tide because it turned out you had to tack all the way across against a gale which put all your calculations out... /forums/images/graemlins/frown.gif
 
There is an illustration of the differences in the back of my ancient Stanfords Tidal Atlas (English Channel West) based on Dover tide times. The illustration is of a trip from Chichester to Cherbourg and using the stay on track (ie continuously alter course to keep on a constant groundtrack) method the time taken at 6kts ordinary spring tides was 12hrs 20mins and doing it by preplanning a CTS was 11hrs 40mins so a nett difference of 40 minutes in 12hrs.

This is not the same discussion as on the other post which was suggesting ignoring the tides and steering the direct course most of the way and then sortiing out the error later.
 
Re: Question for any navigator with commonsense

[ QUOTE ]
I've ended up 10 miles downtide off Cap de la Hague and motored against a Spring ebb at 5 knots for three hours - and not moved an inch until the tide slackened.

Why not to Alderney then, a bear and tides turned and before you know you'r in Cherbourg?
 
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