Passage plan Plymouth- Lezardrieux

[Akestor]

Hi!
For educational purposes, I created a passage plan for the leg. After calculating the tidal streams for each tidal hour at the hypothetical position of the boat at that time of the trip, I came up with a single vector of all streams and got a 2.6kn stream towards 285 degrees.
The ground track is 151 true, and calculating the CTS with 5.5 kn speed got 130 degrees.
The problem is that with the stream, the average SOG decreases to 3.2kn, which kills the whole plan since the boat will not be at the hourly positions assumed with the 5.5kn speed.
Extending the Ground track to 5.5 miles ( to the point the boat is expected to be in 1 hour) and connecting the edge of the tidal vector with the new end at 5.5 miles, found that the average speed through the water should be 7.6 kn to be at the area of expected streams, and the new CTS around 135'

Is this process acceptable? assuming no use of GPS.

 

[Sandy]

I am not sure with what you mean by acceptable.

The SOG in any Channel crossing I have done does include 3.5 knots. I always look at an average SOG when doing a passage plan. Like most yachts I use 5 knots. If you look at your track on the plotter/chart it will be S shaped.

On a passage plan of this length I ignore hourly changes in SOG. Once clear any coastal hazard, assuming you are leaving Plymouth via the Western Entrance you should miss the Gt Mewestone. Just a matter of popping out from behind the breakwater, setting your course and trimming your sails.

 

[doug748]

On a long cross tide trip like this it is better to work out the net tide offset for the whole projected sail and apply that at the start. As Sandy has said, you will not sail a straight course but it works and you will sail less distance through the water. Short term offsets can be ok on a short, one tide, passage but over a longer distance you end up fighting the tide and slowing progress. Tom Cunliffe covers this in the introduction to the Shell Channel Pilot ( or at least he does in my edition ).
A lot of coarse navigators more or less point towards where they would like to be and see what happens which you can get away with modern nav aids.

Of more importance on this notional trip, is arriving up tide of the destination which has some of the fastest cross tides in Brittany (no mean accolade). You would need to make sure you are entering the last 10/15 miles with the tide turning in your favour or you could be out there all day, this is where the planning comes in, some of which will happen as you check your actual progress on passage.

.

 

[Sandy]

Another thing that comes to mind, and this is purely a personal thing, is shipping lanes.

On my Western Channel chart I have lines drawn from the TSS at Casquets to the one at Brest as the ships steam in straight lines between them. I treat the area between the lines as being TSSs.

 

[Akestor]

Have you been in a position where the plan didn't go well because of wind direction? For example, waited for a beam reach but it turned out to be closed hauled in some parts of the leg, causing leeway, reducing speed, and not making it on time in a favorable tide stream near the destination?
What do you do on this occasion? I assume just spending the day outside until the stream turns again?

 

[Daydream believer]

What do you do on this occasion? I assume just spending the day outside until the stream turns again?

Start the engine and motor sail. Get a move on.
Your proposed destination is a rocky coastline & not good for a night or foggy arrival. I have done Leziardrieux in both & it can be difficult first time with strong cross tides.
Passage plan should have get out plan half way. ie Guernsey- for a rest etc
Being a SH sailor I would probably have made Guernsey a stop off point anyway.
110 Miles is a nice 20 hour trip without over doing it & St PP is a good 24 hour port of entry from the N or S via the Little Russel Channel
Then head south passing the Roche Douvres to the west

 

[doug748]

Have you been in a position where the plan didn't go well because of wind direction? For example, waited for a beam reach but it turned out to be closed hauled in some parts of the leg, causing leeway, reducing speed, and not making it on time in a favorable tide stream near the destination?
What do you do on this occasion? I assume just spending the day outside until the stream turns again?

It's a good question, you have to have a plan B. On that trip I think you would do well to keep somewhat to the West of the target unless you are very sure of your timing.

The main problem will usually be on the ebb, ie when the tide is running West, it will also mostly be running out of the river Trieux as well, meaning a long 1kt slog with the depths falling over all the shortcuts. Being to the West, If you do find tide against, there are options open to you and you can divert to Port Blanc, or anchor/moor around Perros Guirec etc ).
On the other hand, if you end up to the west on the flood tide, no bother, with the tide in your favour, you will fly in.

Finding yourself well to East and facing the flood tide your choices are less and, even if you had a fair ebb tide under you in open water, it will likely be ebbing out of the river as well.

Anything is doable but, in a nutshell, hit the coast somewhere to the West and try to engineer a flooding tide to waft you right up to Lezardrieux. If not, find somewhere for a little rest and food.

.

 

[Nimrod18]

“After calculating the tidal streams for each tidal hour at the hypothetical position of the boat at that time of the trip, I came up with a single vector of all streams and got a 2.6kn stream towards 285 degrees… The ground track is 151 true, and calculating the CTS with 5.5 kn speed got 130 degrees.”

Akestor, I sense that your calculations are a bit off. If I’m wrong, mi dispiachi.

My interpretation of your words is that you’ve added (say) 18 or so hourly tidal vectors, the resulting sum of which gives 2.6/285. That is the resulting tidal offset after (say) 18 hours, and If that is true then you need to apportion that vector pro-rata to each hour of sailing by dividing 2.6 by 18 (or whatever your assumed crossing time was). In turn that will reduce the track / heading difference, which your calculation has as 21 degrees, which is fairly extreme (and is also the source of your extreme ground speed worries). I suspect that the true cross track error will be nearer 2 degrees and the boat speed / COG difference will be very small. I’d get my Portland plotter out, but I’m currently pinned on the sofa under the dog.

 

[MontyMariner]

OK it's an exercise and the next destination isn't stated, but practically it's not a destination I would head for from Plymouth, difficult access and strong tides at destination.
Pre Brexit, my preferred destination in that area was Anse du Diben. From Plymouth about 96m at about 170M. Easy 24hr access, no strong tidal issues and visitors buoys are free (maybe not free, but I've never been charged). These days, it's only a stones throw from Roscoff so that would be my French entry point.

 

[Akestor]

“After calculating the tidal streams for each tidal hour at the hypothetical position of the boat at that time of the trip, I came up with a single vector of all streams and got a 2.6kn stream towards 285 degrees… The ground track is 151 true, and calculating the CTS with 5.5 kn speed got 130 degrees.”

Akestor, I sense that your calculations are a bit off. If I’m wrong, mi dispiachi.

My interpretation of your words is that you’ve added (say) 18 or so hourly tidal vectors, the resulting sum of which gives 2.6/285. That is the resulting tidal offset after (say) 18 hours, and If that is true then you need to apportion that vector pro-rata to each hour of sailing by dividing 2.6 by 18 (or whatever your assumed crossing time was). In turn that will reduce the track / heading difference, which your calculation has as 21 degrees, which is fairly extreme (and is also the source of your extreme ground speed worries). I suspect that the true cross track error will be nearer 2 degrees and the boat speed / COG difference will be very small. I’d get my Portland plotter out, but I’m currently pinned on the sofa under the dog.

You are correct! The net tidal vector is for 18 hours, but I applied it as an hourly vector, which is completely wrong. So the average hourly vector would be 0.15kn/h at 285.
New calculations are CTS=150 ( just 1 degree from 151T)
And the average speed will decrease only to 5.4kn from 5.5kn

Great input from everyone many thanks

 

[Nimrod18]

You are correct! The net tidal vector is for 18 hours, but I applied it as an hourly vector, which is completely wrong. So the average hourly vector would be 0.15kn/h at 285.
New calculations are CTS=150 ( just 1 degree from 151T)
And the average speed will decrease only to 5.4kn from 5.5kn

Great input from everyone many thanks

I can neither confirm nor deny that I did exactly the same on my practice-practice plan for my YM. Plymouth to St Peter Port.

A technique shared with me during my similarly academic exercise was to have a decision point (decision ‘line’ more correctly) approximately 6 hours prior to arrival (interpret the corresponding distance as you see fit). Don’t make any error corrections prior to that decision point / line but, on arrival at whatever time / cross track error you have achieved in practice, work out what the last 6 hours of tide will do to you and where you need to steer as a result. Might not work for all situations, but saves skipper / crew chasing the needles during the core of the crossing.

Oh, and bias the last 6 hours to arrive on the up-tide side of the destination; that was another technique shared with me.

 

[B27]

....

A technique shared with me during my similarly academic exercise was to have a decision point (decision ‘line’ more correctly) approximately 6 hours prior to arrival (interpret the corresponding distance as you see fit). Don’t make any error corrections prior to that decision point / line but, on arrival at whatever time / cross track error you have achieved in practice, work out what the last 6 hours of tide will do to you and where you need to steer as a result. Might not work for all situations, but saves skipper / crew chasing the needles during the core of the crossing.

Oh, and bias the last 6 hours to arrive on the up-tide side of the destination; that was another technique shared with me.

Our technique tended to be similar, we would tweak the plan a couple of times in a long crossing.

Planning to arrive up-tide is attractive, but also it's good not to be dead up or down wind of your target.

So, shaping your course somewhat to allow for predicted wind shifts is good.
That could be sailing high on a reach, with the aim to get the kite up later.

For racing there is obviously kudos in choosing the fastest passage plan, but some people like to take a gamble on doing really well, while others want to be sure of a reliable 'good' place.
For cruising, we tend to aim for somewhere, perhaps over-optimistic, but with it in mind that if the wind is turning less favourable, we'll happily adopt 'Plan B'.
If you are not racing, nobody actually cares if it takes you an hour or two longer than optimum, sometimes the quickest passage is not the least stressful.

I've done a few passages this year with light winds in the morning. Deciding when to set off can be complex, how early should I set off under motor, expecting fair breeze later?

The other thing is, if it's wavy or or upwind then I don't always worry too much about the compass or CTS, just keep the boat moving as well as possible and keep an eye on the XTE. If you know what the XTE should roughly be hour by hour because of f cross-tide, then you can keep your track about right or know you're on your chosen side of it. Or get an indication that the kite is not taking you where you hoped to go....

The simple 'textbook' approach is not often much good on a sailing boat IMHO, but it's a start of understanding the question.

 

[Chiara’s slave]

Our technique tended to be similar, we would tweak the plan a couple of times in a long crossing.

Planning to arrive up-tide is attractive, but also it's good not to be dead up or down wind of your target.

So, shaping your course somewhat to allow for predicted wind shifts is good.
That could be sailing high on a reach, with the aim to get the kite up later.

For racing there is obviously kudos in choosing the fastest passage plan, but some people like to take a gamble on doing really well, while others want to be sure of a reliable 'good' place.
For cruising, we tend to aim for somewhere, perhaps over-optimistic, but with it in mind that if the wind is turning less favourable, we'll happily adopt 'Plan B'.
If you are not racing, nobody actually cares if it takes you an hour or two longer than optimum, sometimes the quickest passage is not the least stressful.

I've done a few passages this year with light winds in the morning. Deciding when to set off can be complex, how early should I set off under motor, expecting fair breeze later?

The other thing is, if it's wavy or or upwind then I don't always worry too much about the compass or CTS, just keep the boat moving as well as possible and keep an eye on the XTE. If you know what the XTE should roughly be hour by hour because of f cross-tide, then you can keep your track about right or know you're on your chosen side of it. Or get an indication that the kite is not taking you where you hoped to go....

The simple 'textbook' approach is not often much good on a sailing boat IMHO, but it's a start of understanding the question.

That’s what we do, an hour by hour XTE. Then if you’re looking quicker or slower, you can adjust your heading accordingly. Helpful for us as our speed varies enormously depending on wind speed and direction. We can passage plan all we like, but if you’ve got 6kn of wind right on the nose…..

 

[Daydream believer]

That’s what we do, an hour by hour XTE. Then if you’re looking quicker or slower, you can adjust your heading accordingly. Helpful for us as our speed varies enormously depending on wind speed and direction. We can passage plan all we like, but if you’ve got 6kn of wind right on the nose…..

Sensible. I prepare tidal sheet showing tidal directions at varying points throughout the day.That means that I can make mental adjustments on the fly when under way.
But one comment I would make, is that on that crossing I would like to keep a bit east so I can use land as a bolt hole in emergency. That can be anything- In my case extreme sea sickness- but weed on prop etc or rudder can be an issue which I have encountered when on passage from Roscoff to St Peter Port. The weed can be very long & strong roots & difficult to get off the keel

 

[Chiara’s slave]

Sensible. I prepare tidal sheet showing tidal directions at varying points throughout the day.That means that I can make mental adjustments on the fly when under way.
But one comment I would make, is that on that crossing I would like to keep a bit east so I can use land as a bolt hole in emergency. That can be anything- In my case extreme sea sickness- but weed on prop etc or rudder can be an issue which I have encountered when on passage from Roscoff to St Peter Port. The weed can be very long & strong roots & difficult to get off the keel

Weed is not a problem when your prop, keel and rudder all lift out when required, even under way.