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.

.