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

[boatone]

Re: er is this a windup?

[ QUOTE ]
Tide hardly ever reaches one knot mid channel.

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Bloody hell Haydn.....which channel you in??????

 

[Deleted User YDKXO]

No, its not

I know perfectly well how to calculate tidal vectors but I'm struggling to understand why you should want to use this method of navigation when you've got a GPS and autopilot. IMHO you would only calculate a course using tidal vectors if you were using traditional navigation methods ie a compass. Obviously you could'nt set off in a straight line between the Nab and Le Havre because, as you say, you'll end some 15 miles or so east of Le Havre assuming a 3knot constant east going tide. But I cant see how this is quicker than just following the straight line track using a GPS

 

[tcm]

Re: damn! he has used my trick of getting others to do the planning

much nicer offshore - less to hit!

But there are ships. Best to imagine that these sort of have right of way - hence not a fat lot of point in going as far as tidal diamonds cos that's for raggies sitting on same course fer ages - you have to divert at shippy lanes and carefully carefuly course calc go to rats- as you crossem due south. The rule is that if they line up and hit your stanchions, there is definitely a risk of collision er, or summink. Oops no, it's if they hold the same relative bearing, so turn off and aim to their stern, or the stern of the next one.

But anyway, wotever bearing you make it and at constant maintainable speed, take a note of that bearing on the compass in case the lectroins goes bang - cos although Deleted User's method show you are (were) somewhere along that line, using isn't quickest and the lack of lectronix doesnae get you to le havre - the lectronix-only method doesn't show how to get to le havre without er another gps. Go to standby across shippy lanes and turn back on the bearing.

If you have radar and are familair with it (and you should have) then the manual method needs no gps - cos you will see the coastal outline from afar.

If that goes bang as well, use the depth to decide exactly how shallow you can get to be within range to see some lights (like 20m contour or whatever, allow extra for tide a bit) and then head south or north along than contour to find some things that you can see off the chart.

Before that, as you close the fr coast frexample, a true bearing with handbearing compass of a headland plus depth will show fairly reasonably where you are - try it!

Tralah! x-channel is not too hard. and great fun.

 

[Guest]

Re: No, its not

[ QUOTE ]
Hurry up someone, the raggies will start sniggering ....

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We're watching! /forums/images/graemlins/wink.gif

 

[tcm]

another customer for powerskipper or suncoast

um, well.

B1 gives an extreme example. But let's simplify by going from um London to Manchester - at a time in the future when the whole lot is under water. Let's also assume that there is a er 10knot tide ripping across the submerged country from east to west. Rightho, agreed, plug in Manchester to the gps and off we go. That works no question. We blam along at 50 knots (cheap diesel , no trucks see) and arrive there in exactly four hours.

But all the whiole we (or ok the apilot) has been crrecting for the tide, when it could have been using it.

Imagine that the course to manchester is the hypotenuse of a rightangleish triangle. Your gps course runs up that hypotenuse.

Instead of going that way, we could have gone due north (and not quite as far through the water) and let the tide pull us sideways in the meantime. You can imagine this as whamming up to er about wakefield (which is nearer than manchester) and then pressing an instant button called "tide" which kerzunk moves you instantly whatever the tide wd've moved the boat - the effect is the same. The tide obv actually works over the whole time so it's a curve over the ground but less THROUGH THE WATER - and your route with gpes down a track corrects a zillion little triaangles all the time going up the hypotenuse and then oops across toward the NE.

needs some thinking about i admit. Praps instructors are better at this...

 

[Guest]

Re: another customer for powerskipper or suncoast

To put it another way, instead of spending your speed fighting the tide, let it carry you back and forth, and only correct for the minimum you have to at the end.

It's best understood by reduction ad absurdum: if your boat does 5 knots across to France and the tide is 4 knots first East then West after the tide turns, then, if you try to keep on a direct line between leaving point and destination, you will be aiming almost due West and then almost due Eest after the tide turns. Progress South crabbing sideways will be less than a knot. If however you ignore the tide and let it carry you sideways first one way then the other, then you will make 5 knots progress South.

 

[Deleted User YDKXO]

Re: another customer for powerskipper or suncoast

We're not talking about a change of tide in which case I agree with you as I've already said in a previous post but the situation we're talking about is a crossing over a single tide in the same direction

 

[Guest]

Re: another customer for powerskipper or suncoast

In that is the case a rough course can be calculated on the basis of the average tidal rate in that direction. Even over 3-4 hours the tidal rate is likely to vary considerably.

 

[Deleted User YDKXO]

I think not

tcm, rest assured I understand what you're on about. Firstly, if you do calculate a course to steer using a tidal vector and assuming a constant tide rate and direction, the boat will not describe a curved track between Nab and Le Havre or London and Manchester or wherever. It will follow the straight line track between the 2 points but the heading of the boat will be off the track by the angle you have calculated from your tidal vector calculation. Basically the boat will crab along the straight line; it can never be a curve because we have assumed the tide rate and direction is constant. If you dont believe me, look at the RYA YM course notes that you presumably got from Suncoast. Pages 8 and 24 in my copy but mine may be older
If the tidal rate and direction is not constant then you will have to apply a number of tidal vectors to the calculation which may result in the boat describing a non linear track but it certainly wont be a smooth curve
As you have pointed out, with a GPS linked autopilot the pilot will be applying regular corrections but if you set the cross track error (XTE) to be small, the boat will follow an almost straight track. In fact it will crab in pretty much the same way as had you set a course using the tidal vector calculation

 

[tcm]

suncoast info

we'll have to agree to disagree!

For fastest trip, the heading will be constant and the track *over ground* is curved, and a fairly smooth curve - although this aspect a bit irrelavasnt and best not thouggt about too hard. I didn't get suncoast's info, i spect he will comment...

 

[Deleted User YDKXO]

Re: suncoast info

OK, will agree to disagree but no way will the ground track be curved. The boat is crabbing along a straight line. All assuming constant tidal rate and direction

 

[Guest]

Re: I think not

[ QUOTE ]
it can never be a curve because we have assumed the tide rate and direction is constant.

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That's just what you DON'T assume. If it were constant then there would be no difference in the two approaches. But it ISN'T constant.

[ QUOTE ]
If the tidal rate and direction is not constant then you will have to apply a number of tidal vectors to the calculation which may result in the boat describing a non linear track but it certainly wont be a smooth curve

[/ QUOTE ]

It WILL be a smooth curve over the ground with TCM's approach, because you are not correcting each hour. Basically you are calculating the net distance that the tide will set you off over the entire course. You then work out a single compass bearing that will carry you to your destination. You stick to the compass bearing over the entire journey, only applying a small correction at the end to adjust for any errors that have crept in.

The point of doing this is that you arrive at you destination faster, having travelled less distance through the water. On a sailing boat it makes hours difference to the duration of a cross channel, on a fast motor boat less, but I suppose still significant.

 

[Deleted User YDKXO]

Re: I think not

Aaargh, I think I'm going to scream. tcm's second post referred to a tidal rate of 3 nm per hour (as he put it ie knots) ie a constant rate and thats what we've been discussing. We all know the bloody tide isnt constant and in practice you would have to plot a series of tidal vectors
And, no, even with a constant tidal rate, the boat would not describe a curve and, no, you will not arrive any faster. Think about it, the boat speed is constant, the tide is constant, you cant go in anything other than a straight line.In a previous post you described a change of tide or several changes of tide over a trip and, yes, under these circumstances it is quicker to go with the tide rather than fight it as the next tide will bring you back on course

 

[rich]

Re: I think not

Has Anyone here Actuly Done A nav coause. or do we leave it to the rag n stick brigade /forums/images/graemlins/laugh.gif

and forget about the spelin!

 

[Guest]

Re: I think not

[ QUOTE ]
Think about it, the boat speed is constant, the tide is constant, you cant go in anything other than a straight line.

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No, I read TCM's post meaning that the tide will get up to 3 knots. I don't think he mentioned anything about it being constant. It isn't constant, it is changing all the time, so why would anyone do their navigation on the basis that it were constant? It would make no sense.

Even if it's not changing direction (because you manage to get there in one tide) it is changing its rate all the time, varying between, say, 0 knots and 3 knots.

 

[Oldhand]

Re: er is this a windup?

<Tide hardly ever reaches one knot mid channel.>

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Wrong, tide can easily reach 3 knots in the middle of the English Channel, I guess you go too fast to notice or haven't interpreted your tidal atlas very well. /forums/images/graemlins/confused.gif

 

[BrendanS]

Re: I think not

Yep, and even have an A grade 'o' level in air navigation, taken whilst a teenager - the courses I've taken since are a doddle by comparison. Yachtmaster theory amongst them. Try navigating across Dartmoor in fog with a hand held compass before GPS, and hitting target Tor - which I had to do as teenager navigator in Ten Tors and trips beforehand. You can see 20 feet most - send someone ahead on compass bearing, and move them left or right, then keep that up over 10 miles - if you missed Tor by 100 feet you were lost in featureless landscape. Gives you lots of confidence navigating later in life

In Iditarod, don't take tides into account (cross channel - but they feature heavily in coastal trips round headlands cos of wind against tide) as the hull is rarely in the water. gulp!

Though in saily boats it makes a great difference - having said that, first time I helmed a saily boat cross channel back from Cherbourg, skipper gave me a heading then him and brother went down below to sleep (Hi guys) and left me to own devices - I tried various combinations, and found one heading gave best speed and most comfortable ride - some hours later they came back up, looked at plotter, and said, keep that heading, it looks great. Sometimes gut instinct plays a part, but thats more noticeable in sail than motor

 

[powerskipper]

Re: I think not

the only way you will get a strait line groud track on a long passage over the channel is by calculating a separate tidal vector for each hour, and if the info is correct, [remember its called am tidal prediction] you will get a strait ground track.

Much more relaxing to plot bearing for whole crossing, ie putting all the tide on at the start and calculating an average bearing, this does mean that at some hours the bearing will not be accurate enough to keep you on your original ground track, hence the curve TCM mentioned correctly but in the end it will get you where you want to go.
Try it, work out your course and then use GPS and the XTE to track where you go.

 

[Deleted User YDKXO]

Re: I think not

Well thats not how his subsequent London to Manchester scenario reads. In any case, even if the tidal rate varies, you would be very unlikely to track a smooth curve, more like a wobbly line depending on the tidal rate at each point

 

[tcm]

Thankyou! sorry Deleted User and stop the screaming

Yep, it's a curve over the ground, and can't be anything else: if you set off into the tide then for the first bit (over ground) you head slightly south (say) of the target, and as the tide works its effect it bring you to that target. Yeah, the tide works bit by bit gradually like - hence a curve over the ground. It's a constant heading, but not a striaght line over the ground.

Tidewise, you add the tidal vectors and these are indeed averaged in direction and magnitude for each hour, for the number of hours that you plan to be at sea. The more hours (or the more detailed the info you can find eg for each 30 mins but unlikely) the more vectors. They might or might not cancel each other out. They can be assumed constant (averaged) over an hour, no problem, and indeed can be averaged/smoothed over the whole trip - that's what the vector addition does.