Crossing the channel 1st time wed 25th may bit nervous

[Guest]

Re: example

Aaaarrrgh! /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif

Leeway is a complete red herring, it has nothing to do with it. There is no indication in this track whether there was any wind blowing and if so from what direction (except if it was a sailing boat it wasn't from the south, 'cos it wasn't tacking).

As for the change of tide direction, it makes no difference, the same principle applies provided the tidal stream is varying in speed - and tidal stream always DOES vary in speed.

 

[jfm]

Re: I think not

Powerskipper, the whole point is you are not trying to get a straight ground track. If you did, your journey would be longer thru the water than if you had a curved groundtrack

 

[Deleted User YDKXO]

Re: example

Rubbish. The screen clearly shows wind arrows (the purple ones) so I assume the program also takes account of leeway otherwise whats the point in showing them? But as you say, its not relevant to this thread but certainly is relevant in practise.The tide is shown by the black arrows. None of us know what parameters were used to calculate this track but I believe it shows the most efficient track over a change of tide which as I keep saying is not relevant to this thread either

 

[Bergman]

Yeah

Don't bug ger about with all these angles and diamonds and things.

Set a waypoint for somewhere in spitting distance of Le Harvre or where ever, put the autopilot on and open the taps - sort things out when you get to other end.

You're doing 10 to 20 x tide speed so won't make much difference either way.

Lifes too short

(Sits back tittering quietly)

 

[Guest]

Re: example

Oh yes, I see the diagram shows there was a wind blowing. But I'm glad we can agree that leeway is a red herring for our discussion.

 

[mjf]

s shape

what i have always done on ocean passages, and in offshore sailing races, is to look at the projected tides/winds for the entire passage. If needed apply a little course correction to neutralize any inbalance of set. The object is to start on the Rhumb line and end on it too.

Thus you will kind of track an S shape over the course line. ie don't fight the tide/wind use it to advantage.

With modern GPS/Plotters (if the rules allow their use) you can monitor your XTE and if it looks like you initial calcs are crap you adjust a liitle

 

[powerskipper]

Re: I think not

I know, /forums/images/graemlins/grin.gif
I just explained how you could do either.

 

[GrahamR]

Re: yes re east upchannel : no, re autopilot

Well if he wasn't nervous about going across he will now, best read on here in ages!!!

 

[jfm]

Re: I think not

Oops ok, sorry, just re-read your post. I wasn't paying attention properly /forums/images/graemlins/grin.gif

 

[tcm]

nice, but wrong

well a moment ago you had problem with tidal vectors, yet you calcs now use them! at least we're getting somewhere. You've not ever sailed a boat x-ch have you? Nor indeed, have ever had a boat thatgoes undeer 10 knots it wd seem.

Provided that you calcs transferred the total tidal vectors to the start, then by setting a pair of compasses to represent the time taken to travel in the four hours, and the angle of the line from the end of the tidal vectors to tyhe point of intersection being the heading - we agree.

at no point did i caluclate an ep - and all you did was calculate an ep being the... destination in one hour. This is too simplistic. Your idealised trip had steady 4knots, tide.

But this actual boat sets of at the botom of the tide and experiences tides up to 5 knots plus. In an extreme example, it might have no tide at all for the first 3 hours, then 1 hour of 16 knots. Yes, lets call it 4 knots on average.

The curviness of the ground track curve is cos the heading (held for entire trip) smooths all these effects - 3 hours in, frexmaple, in the example of the tide being a 1-hour shot of 16knots sideways it is 12 miles off course even no tide.

You chplotter bit is not right. Is there any point at which you tel a chplotter the tides? Nope, never. The apilot rumbles along, each time correcting - with the assumption that for the REST OF THE TRIP there will be no tide. Opps but there is!... so it chases back to the track, every so often and then phew! settles back on the dead heading and...oops more tide..and so on.

 

[Deleted User YDKXO]

Re: nice, but wrong

Seems there has been a misuderstanding. Just to be clear, I'm not saying that navigating by plotter guide autpilot takes account of tides other than correcting for them

 

[duncan]

Re: example

mike - I was simply responding to the request by shipswoofy that's all, and the track shown was the actual course over the ground of Amaya whilst she was basically kept on the same heading for the entire trip.