Course to steer for crossing the channel

[NealB]

It very much depends on circumstances, but I have been on ships where we kept to a predetermined track (with bubble times) to ensure that we arrived at port x or spot x at the exact time arranged. I have also been on ships where the OOD was required to 'stay inside this box and avoid using more than so many degrees of helm' (The latter to ensure that everything didn't go flying as we made a handbrake turn.)

Thanks John!

I was given the info. about the RN approach some years ago, by a RN Commander from your neck of the woods.

A lovely chap, and, as you'd expect from someone responsible for hydrographic surveys, a real authority on anything to do with navigation.

He was, however, a relentless joker: I was never quite sure when he was being serious about anything.

If I were to mention his name, I'd have to nip down west and shoot you (I believe).

 

[FairweatherDave]

Well thanks all. Really useful stuff. Obviously the transition from a theoretical exercise to doing it for real is ripe with issues, and as johnalison has correctly determined, it is the use of GPS, compass and plotters (tablets) that is behind my questions. I'm really happy (although rusty) working out tides and vectors etc in the comfort of my home on a rainy day, but don't enjoy chartwork once bobbin up an down. So I'm really happy most people have a relaxed attitude until they got close to Cherbourg.

 

[capnsensible]

Well thanks all. Really useful stuff. Obviously the transition from a theoretical exercise to doing it for real is ripe with issues, and as johnalison has correctly determined, it is the use of GPS, compass and plotters (tablets) that is behind my questions. I'm really happy (although rusty) working out tides and vectors etc in the comfort of my home on a rainy day, but don't enjoy chartwork once bobbin up an down. So I'm really happy most people have a relaxed attitude until they got close to Cherbourg.

You got it. Its quite straightforward. like all things in sailing, have a go, its never as hard as you imagine!

 

[Angele]

But as part of calculating the net tidal effect you presumably generate a series of offsets - pushed 3 miles west this hour, 2 miles east that hour, etc? If so, it's then hardly a chore to pick off these distances from the rhumb line and pencil a little cross at each.

Indeed I do, but the question I was responding to was whether I would draw my curve IN ADVANCE, which I take to mean before I set off from my home berth. I most definitely don't, because it takes me an hour or two to get to either Bembridge Ledge or the Needles and, even then, I can't be exactly certain in advance whether I will actually cast off at 04:15 (or whatever the plan said).

 

[Angele]

I'm really happy (although rusty) working out tides and vectors etc in the comfort of my home on a rainy day, but don't enjoy chartwork once bobbin up an down. So I'm really happy most people have a relaxed attitude until they got close to Cherbourg.

Like you, I don't much enjoy chartwork when the boat is bouncing around, but there are some shortcuts that I use that reduces the need for me to do things at the chart table. (I'll probably get roasted for some of these, since they aren't the "RYA way", but here goes).

I write down on a scrap of paper (either before casting off, or whilst in the shelter of the Solent) the expected tidal stream for each hour of the passage, without worrying whether the direction is 260 deg or 280 deg. I simply call it west. Same for after the turn of the tide. I then simply calculate the net effect over the anticipated duration of the trip as x.x nm E or W.

I then go to my chart plotter (at the helm) and use the ruler function on it to draw a line from whichever entrance to Cherbourg I am aiming for, either W or E (as required) by however many miles I have calculated for the expected net tidal flow. I move the cursor to the end of that line and read off from the display the bearing to that point from my present position and that gives me my CTS. So, I can do it without spending 10 mins at the chart table. Handy in rough weather.

 

[East Cardinal]

Interesting thread. Can’t say I am convinced by some of the methods used by some to navigate by but if it works.......
I navigated warships and we certainly always had a navigation track to follow. The “box” that was referred to earlier was used on occasion to keep safe overnight as an alternative to anchoring. The OOW was free to manoeuvre as he wanted to keep clear of other vessels.
Wednesdays toast “ Ourselves” to you all.

 

[NealB]

Interesting thread. Can’t say I am convinced by some of the methods used by some to navigate by but if it works.......

Wednesdays toast “ Ourselves” to you all.

There's some fairly experienced people on this thread.

You seem to be missing the point that it's more efficient (ie less distance, quicker, less fuel used) to plan to steer a constant course.

Maybe your RN teaching didn't bother with that (as your vessels are rather speedier than ours), but that doesn't make the principle any less "convincing".

Here's a little exercise you might find interesting on a cold, blustery, winter's evening .....

Grab a Channel chart. Then plan a passage from say the Nab, to Cherbourg's eastern entrance, assuming a speed through the water of 5 knots.

In fact, do it twice:

- firstly where you make, say, hourly adjustments for tide (so you stay pretty close to a straight-line over the ground)

- secondly, where you steer a constant compass course (straight line through the water, but an S - curve plotted on the chart).

The second option is significantly quicker.

Take a look in any yachtsmen's navigation manual (the Reeve Foukes tidal atlas has a very thorough treatise on this) or take an RYA course: it's the recommended approach, because it makes absolute, logical sense.

Cheers!

 

[East Cardinal]

5 knots..........
Might as well stop both engines and wait for the world to move round....hahaha!
I’ll have a look at the chart as you say. How much time would it save with say a 3kn tidal stream equally westwards and eastwards over half the time period?

 

[NealB]

How much time would it save

Haha ... good try ... but that's YOUR homework! ( I tried it decades ago).

But might I suggest that your tidal assumptions are too simplistic ..... I did it using the tidal diamonds, so as to get a more more realistic tidal cycle (remember, the French tides tend to be stronger than our wimpish English ones).

The results are illuminating!

 

[harry potter]

There's some fairly experienced people on this thread.

You seem to be missing the point that it's more efficient (ie less distance, quicker, less fuel used) to plan to steer a constant course.

Maybe your RN teaching didn't bother with that (as your vessels are rather speedier than ours), but that doesn't make the principle any less "convincing".

Here's a little exercise you might find interesting on a cold, blustery, winter's evening .....

Grab a Channel chart. Then plan a passage from say the Nab, to Cherbourg's eastern entrance, assuming a speed through the water of 5 knots.

In fact, do it twice:

- firstly where you make, say, hourly adjustments for tide (so you stay pretty close to a straight-line over the ground)

- secondly, where you steer a constant compass course (straight line through the water, but an S - curve plotted on the chart).

The second option is significantly quicker.

Take a look in any yachtsmen's navigation manual (the Reeve Foukes tidal atlas has a very thorough treatise on this) or take an RYA course: it's the recommended approach, because it makes absolute, logical sense.

Cheers!

Gospel Reeve Foukes Tidal Atlas. Tides stronger on French side of Channel, also IOW inhibits taking full advantage of ebb tide. Bias to the east, especially on springs. Entry into Cherbourg tends to be 'interesting'.

 

[NealB]

Gospel Reeve Foukes Tidal Atlas.'.

Agreed!

For me, it was a seminal read.

 

[MikeCC]

Surely enough people have done it different ways and recorded the track on a chartplotter?

 

[East Cardinal]

Agreed!

For me, it was a seminal read.

I’ll see your Tidal atlas........
And raise you “Secrets of the Conquerer” by Stuart Prebble

 

[NealB]

I’ll see your Tidal atlas........
And raise you “Secrets of the Conquerer” by Stuart Prebble

Thank you!

I'm currently laid up in dry dock (broke my right leg last week), so am catching up on my reading list, to which this is now added.

 

[East Cardinal]

Haha ... good try ... but that's YOUR homework! ( I tried it decades ago).

But might I suggest that your tidal assumptions are too simplistic ..... I did it using the tidal diamonds, so as to get a more more realistic tidal cycle (remember, the French tides tend to be stronger than our wimpish English ones).

The results are illuminating!

You are right. At 5kn SOA in a yacht the S curve constant heading plan saves just under 1,5 hours, over the 65nm track.
A difference between 14hours and 12 hours 40 mins.
I would take the RIB and cross in 2 hours ��

 

[john_morris_uk]

You are right. At 5kn SOA in a yacht the S curve constant heading plan saves under 1,5 hours, over the 65nm track.
A difference between 14hours and 12 hours 40 mins.
I would take the RIB and cross in 2 hours ��

If all I wanted to do was get there, I’d take the ferry.

 

[Popeye & Olive]

We left Iles Chausey headed for Rotheneuf, didn't allow enough westerly course against the easterly tide. Swept passed the entrance, ended up hours later in Granville.
Realy miss tides and currents here in the med but was surprised to find some of the islands do have anything up to 7 knots sweeping around them in the right conditions.

 

[Angele]

ended up hours later in Granville.

I can think of worse places to fetch up.

 

[NealB]

You are right. At 5kn SOA in a yacht the S curve constant heading plan saves just under 1,5 hours, over the 65nm track.
A difference between 14hours and 12 hours 40 mins.
I would take the RIB and cross in 2 hours ��

That sounds about right.... And respect to you for doing it.

 

[BelleSerene]

Yes, I was replying only to the "rubbish" comment: like you, I've never plotted the s-curve in advance.

I've used it once or twice - eg from St Malo up to St Peter Port, Guernsey. I wanted to know how far east into the Minquiers hazards the tide would take me.

I've also used it to figure whether my course across the Channel would venture into the exclusion zone around a central cardinal mark.

If I'm using nav software to remove the tedium from the calculations or just fine-tune for departure time, it shows me the S curve for the selected departure time.