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

[boatone]

Re: I think not

Does it really matter? The principle is right and we were supposed to be encouraging a channel virgin here, not tying the poor [no thanks] up in dodgy sheetbends /forums/images/graemlins/grin.gif

 

[Deleted User YDKXO]

Re: I think not

Powerskipper, thanks. Thats what I've been trying to say. Only, if you calculate a single heading for the crossing using an avarage of the tidal vectors, I still dont think the boat will track in a smooth curve, more likely a wobbly track depending on the tidal rate at each point but you'll still end up roughly where you want to
You could demonstrate this just by using the Track facility on the plotter

 

[tcm]

Powerskipper isn\'t agreeing with you Deleted User...!

um, she's actually agreeing with me, not you! The fastest tracvk is a curve over the ground. Yep, the curve isn't mathematically smooth but it's very very smooth if seen from the air where you could see the whole 80nm.

Your xte track-following ignores the tide, so descrivbes thousand of little a triangles, always fighting the actual tide. You travel further.

Can someone tellthe raggies to stop tittering....

 

[tcm]

er, yes it does matter

In the grand sceme of the universe, not much.

But if he's going to cross the channel this way, he oughta do it correctly, and this applies to the nav as well as to lots of other things too.

The most efficient way is to take the shortest amount of time give the capability of the boat. We've already established that he's nervous so his crew will be apprehensive. So yep, taking praps 20-30 mins less than the auotpilot tracking endpoint with tight xte option advocated elsewhere is worthwhile.

More importantly - sposed summink happened? Lose an engine or whatever. With the abiklity to roughly work the tides, he would know how to recaluclate and still be able to get into port, rather risk being downtide with no buttons working and not appreciate that heading directly won't gettim there. Spose both engine go - then what - and so on.

It's easy to drive a car or a boat and lots of other stuff - the skill bit is when things don't go according to inital plan. So, yes, this is important.

 

[boatone]

Re: er, yes it does matter

No No No NO nO........course the principle matters !!
I meant does it matter whether its a perfect curve or a bit dodgy one which is what Deleted User is intent on pursuing /forums/images/graemlins/grin.gif

In fact, I'd always prefer to plan to be a bit uptide if poss.......

 

[Deleted User YDKXO]

Re: Thankyou! sorry Deleted User and stop the screaming

tcm, I thought we agreed to disagree on this. I'm quite happy to carry this on if you want but all it amounts to is I'm saying I'm right and you're saying you're right and since we're both pig headed, we're not going to get anywhere. Can I suggest that we both take a time out and sit down with a piece of paper to work out a) the heading that Sami should take and b) his actual position at hourly intervals. Lets make it as easy as possible and assume an avarage east going tidal rate of 3 knots for the whole trip (although I'm quite happy to make it more complicated if you want) @ 90deg to Sami's course. Constant boat speed of 18knots. Assume total distance 80miles and course 150deg although neither is critical
Then PM me and we'll discuss what comes out

 

[tcm]

jeez

yep the puter will sort it out.

But the tidal effect at springs is high - overall, if he didn't turn engine on for four hours, he'd end up about 16nm from where he started. Draw a triangle wiv the short side 16 and the long sides 80 and work out the angle - that gives the overall correction. Yep occassionaly bit faster, bit slower but over the whole time that's the effect. The (not very) complicated bit is working out that angle. 6.3ish radians make 360 degrees, so 1.05 rads make 60 degrees so 1/5 radian as in this triangle makes 12ish degrees. round to nearest 5 degrees - hence 15 degrees.

 

[Deleted User YDKXO]

Re: er, yes it does matter

On this point I will agree with you tcm. If his electronics go blank then he should know about calculating tidal vectors but since this is going to be his one and only channel crossing and assuming that he has a back up hand held GPS on board, on balance the risk is v small

 

[tcm]

ah, agreed

agreed yes - the curvature of the curve is not constant. Agreed - it hardly matters. Agreed the principle is important. Agreed ending up uptide

 

[duncan]

Re: er, yes it does matter

as usual a somewhat bizarre thread..............
I agree with both Deleted User and tcm pretty much all the way through, and I don't think our raggie friend ( /forums/images/graemlins/smile.gif) has said anything wrong - so why aren't you all agreeing with each other?

If you set off on a single compass bearing, calculated for the average tidal vector, but the tides aren't constant to that average........
1. you will cover the least distance through the water, use the least fuel and arrive in the least time.
2. the path you travel over the ground will not be a straight line

If you calculate seperate tidal vectors for periods of constant tidal flow (although in practice this would have to be very small time periods!) and steer a constant bearing for each you will travel in a single straight line over the ground. Effectively you are saying where do I have to point htis thing to travel in that dierection for ths tidal flow. YOu will travel further through the water and will get there later having used more fuel - following the track on your plotter will do exactly this for you anyway.

The only argument seems to be around what constitutes a smooth curve...............

finally, I am concerned that tcm started this with as much of a steer ( /forums/images/graemlins/crazy.gif) to (a) travel at a time that has wind and tide in the same direction and (b) keep upwind and uptide while conditions allow because if they detiriorate this will give you a smoother run in the worsening conditions - and this excellent advice is getting smothered.

 

[tcm]

so....stage 1

um, at least we can agree that if the tide was exactly against him running at 4 knots for the four hours - he's actually take five hours, yes? It wd've been like going 82miles PLUS 16 miles. Can we agree that?

Can we also agree that if the tide is entirely behind him- he'd be at sea for a lot less time?

(clever clever types: yes, ok we oughta allow for more tidal effect in example one if against the tide and less effect if with the tide cos in one example he'll see (say)five hours of tide, in tother he'll see (say) three hours of tide)

 

[tcm]

afaik - Deleted User want to run along a straight ground track using xte

...was what i though was the main issue of the argu- erm i mean discussion. I dion't agre with Deleted User at all on this. Deleted User doesn't agree with me. Or at least, he doesn't agree yet. Hence disagreement.

 

[ShipsWoofy]

Re: afaik - Deleted User want to run along a straight ground track using xte

Where is Tomes funky cross channel plot from last years Cherbourg trip, this made an easy to understand graphic which should explain all your (everybody's) arguments quite well.

Does anyone know Tomes web address, I bet it is still there!

 

[tcm]

Re: so....stage 2

now, this bit is about a very small trip. In fact the trip is only 20 metres! Our cross-track error is usually a lot more than this so it won't kick in or do anything. The trip is right on top of a tidal diamond at exctly the time for which we have a tidal stream value given. Now, we want to end up dead on a positon 20m due south. The tide is running west to east.

If we drive due south, we'll actually end up not quite inthe right place, agreed?

We'll end up a little bit to the east of it, yes? Hey don't worry about this just yet - its tiny effect - if where doing 20 knots and the tide is doing 4 knots the once we're level with where we need to be , we're only 4 metres off. So we won't worry about it yet, or perhaps even notice.

 

[duncan]

Re: example

you mean this one?

channel track

 

[MedDreamer]

Re: I\'ve only done the theory on this but.....

Shouldn't he also adjust his track to cross the shipping lanes in the right manner (at a right angle if I remember correctly) or is this something else that MOBO's can ignore?

Interesting thread for us novices this helping us`undrerstand how the colassroom theory is practically applied.

 

[Deleted User YDKXO]

Re: jeez

Umm, are we talking 4 knots or 3 knots tide now? Anyway, what the hell. The method you have used whilst being basically correct is not the accepted method for calculating a heading
The correct way is this. Draw a vector at the start point to represent the speed and direction of the tide and from the end of the vector draw another line to intersect the base line to represent the speed of the boat. To make this easy I said the tide was at 90deg to the track. So what you get is a right angled triangle with the short side 3 or 4knots and the hypoteneuse 18knots. The actual heading of track is given by the sine of 3/18 or 4/18 so for 3knots tide, the heading off is 9degrees and 4knots tide its 13degrees
Now to calculate the actual track the boat will follow you need to calculate an estimated position (EP) based on the heading you've just calculated. This is the opposite of what you've just done. Lets do it for 1 hour although it doesnt matter over what intervals you do it. Draw a line at 9 or 13 degrees off the track 18nm long (to represent the distance and direction the boat has travelled in 1 hour). From the end of that line draw a vector of 3nm or 4nm at 90degs to the track (NOT the heading) to represent the tidal flow in 1 hour. What you end up with is an EP exactly on the original track proving that in this case the boat will follow the straight track but crabbing at 9 or 13degs off, but not a curve
Now obviously if the tide is not exactly 3 or 4 knots at every point, the actual position of the boat will not coincide exactly with the EP. As I've already said, what you will actually get in this case is an irregular track but definitely not some kind of curve
What happens with a plotter linked to a pilot is roughly this. Lets assume the max XTE is set to 0.5nm. The boat will veer off to the east until the XTE is 0.5nm whereupon the pilot will calculate a revised heading to the next waypoint (ie 80miles away). This will result in a small correction and these corrections will get larger and larger the closer you get to the waypoint. Some pilots are more sophisticated than this and will over correct but the principle is the same. The boat will follow the track probably a bit east of it and making increasingly larger corrections as the next waypoint is approached. Now I dont think that the time taken for this kind of plotter guided navigation will be much more if any more than a heading as calculated above using a tidal vector because your tidal vector calculations are never correct because nobody can precisely predict the tide at every point on the track

 

[Deleted User YDKXO]

Re: example

This is not the same situation. It seems to show a change of tide in mid channel and takes account of leeway (wind) as well. What I think it shows is the best way to take advantage of a change in tide by not steering into the tide

 

[Nick_H]

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

[ QUOTE ]
I could do as mike f suggests and just go over on the chart plotter gps and auto pilot but as you say I will be pushed up channel and the auto pilot will constantly be correcting the course

[/ QUOTE ]

Isn't that what auto pilots are supposed to do?

 

[Nick_H]

Handy trick for collision avoidance

The most useful tip I was given for dealing with big ships going up and down channel:

When you first see it sit upright and draw an imaginary line vertically downwards from the ship to a point on your boat. Do the same a few times at sensible intervals. If the point on your boat that the imaginary line hits is moving toward the bow the ship will pass in front of you. If the point is moving towards the stern the ship will pass behind you. If the point is staying same you are on a collision course and best to assume that it is you that needs to change course regardless of any rules of the road. In reality of course in a fast motorboat you're not going to collide unless your downstairs making the tea, but its good seamanship to alter course early so the ship knows what your doing.

All above assumes that both vessels hold a steady course and speed, but ships do tend to.