Tidal streams

[Ed Colonna]

Having only just completed my day skipper I thought I'd put into practice what I'd learnt by making a real passage plan. When plotting the tide from the tidal streams atlas I'm aware that I have to split it into what was only taught to me as crocodile teeth (If I'd like to work out the tidal stream between 1045 and 1145 and high water is at 0915, then I know to plot the tidal stream for 2 hours after HW based on the fact that I have split the timings by half an hour. However lets say I wanted to plot the tidal stream for a passage between 1015 and 1115, surely I'd have to plot half an hours worth of tide from 1 hour after HW and half an hours worth of tide from 2 hours after high water? Please correct me if I'm wrong, just stumbling over this part. Thanks!

 

[prv]

I don't know what the official RYA answer is, but I would observe that given the inherent imprecision of tidal stream atlasses, the interpolation between neaps and springs, and your own ability to steer a course and accurately predict sailing speed, these sort of fine details don't make much difference in practice.

Perhaps add a little ad-hoc interpolation to give a stream which is halfway between the two atlas charts?

Pete

 

[PaulMcC]

Having only just completed my day skipper I thought I'd put into practice what I'd learnt by making a real passage plan. When plotting the tide from the tidal streams atlas I'm aware that I have to split it into what was only taught to me as crocodile teeth (If I'd like to work out the tidal stream between 1045 and 1145 and high water is at 0915, then I know to plot the tidal stream for 2 hours after HW based on the fact that I have split the timings by half an hour. However lets say I wanted to plot the tidal stream for a passage between 1015 and 1115, surely I'd have to plot half an hours worth of tide from 1 hour after HW and half an hours worth of tide from 2 hours after high water? Please correct me if I'm wrong, just stumbling over this part. Thanks!

Ed, Yes exactly right. Plotting the stream for half an hour is the same as plotting the stream for an hour at half the speed (if that makes sense) so look at the stream for HW+1 and plot a vector on the chart half the length of the stream in knts, and then add directly on the the end of that tidal vector the stream for half an hour at HW+2. The combined vector is the total tidal effect over the 1015 - 1115 hour.

 

[Neil]

I did exactly the same thing once - made a real passage plan and calculated the tidal streams for a theoretical passage. Despite the fact that there is a substantial tidal current up and down the east Irish coast, I was disappointed at the relatively minor effect it had on the process, unless one was going directly against on a spring.......

If I remember correctly, we didn't do the crocodile teeth for DS, but rather for Yachtmaster theory.

 

[jerrytug]

You are not wrong, Ed, you have grasped it correctly, half an hour of each! Learning the theory 'by the book' is a very good idea, then afterwards you can decide which bits are vital, and which you can guess/fudge. Pete is not wrong, but I think you will enjoy your sailing more if you get completely on top of the theory, that's what worked for me anyway, cheers Jerry.

 

[prv]

Despite the fact that there is a substantial tidal current up and down the east Irish coast, I was disappointed at the relatively minor effect it had on the process

On the other hand, the weekend before last I went through the process for the trip back from Alderney, set the resulting course into the autopilot as soon as we'd hoisted sail outside the breakwater, and didn't alter it by a single degree until we were abeam the Needles lighthouse, just inside the Bridge buoy, and it was time to turn up the channel. This despite a departure from the rhumb line of ten miles or so before swinging back. Certainly a lot of luck involved there, but it's very satisfying when it all works out.

Pete

 

[Twister_Ken]

On the other hand, the weekend before last I went through the process for the trip back from Alderney, set the resulting course into the autopilot as soon as we'd hoisted sail outside the breakwater, and didn't alter it by a single degree until we were abeam the Needles lighthouse, just inside the Bridge buoy, and it was time to turn up the channel. This despite a departure from the rhumb line of ten miles or so before swinging back. Certainly a lot of luck involved there, but it's very satisfying when it all works out.

Pete

What was you leeway allowance?

 

[prv]

What was you leeway allowance?

Luck

Wind on the quarter though, so won't have been much.

Pete

 

[LadyInBed]

Ed, what you describe is correct, but only as a classroom paper exercise. In practice it is as Pete described.
The way that I look at the tide stream is in the following order
It is with you or against you.
It is pushing you to Port or Starboard.
It is a combination of the above.
Depending on location, Spring and Neap rates can be as little as 0.2 knots difference or as great as 6 knots difference, so where are we in the lunar cycle.
For me, the most important thing I note is the time the tide turns (slack water), because as you may have noted, slack water time doesn't necessarily coincidence with local high and low water.
So my bottom line is that I try to plan a passage where for the majority of the time the tide stream is assisting me and my departure or arrival time, whichever is more critical, is at the most advantageous state of the tide, which might be flood, slack or ebb.

 

[MoodySabre]

I don't know what the official RYA answer is, but I would observe that given the inherent imprecision of tidal stream atlasses, the interpolation between neaps and springs, and your own ability to steer a course and accurately predict sailing speed, these sort of fine details don't make much difference in practice.

Perhaps add a little ad-hoc interpolation to give a stream which is halfway between the two atlas charts?

Pete

And leeway too.

 

[Buck Turgidson]

I don't know what the official RYA answer is, but I would observe that given the inherent imprecision of tidal stream atlasses, the interpolation between neaps and springs, and your own ability to steer a course and accurately predict sailing speed, these sort of fine details don't make much difference in practice.

Perhaps add a little ad-hoc interpolation to give a stream which is halfway between the two atlas charts?

Pete

Careful! averaging vectors isn't just adding and dividing by n. So ad-hoc interpolation isn't so easy unless you have a logarithmic brain. Some nice vector tools here :http://hyperphysics.phy-astr.gsu.edu/hbase/vect.html#vec8

 

[alant]

Having only just completed my day skipper I thought I'd put into practice what I'd learnt by making a real passage plan. When plotting the tide from the tidal streams atlas I'm aware that I have to split it into what was only taught to me as crocodile teeth (If I'd like to work out the tidal stream between 1045 and 1145 and high water is at 0915, then I know to plot the tidal stream for 2 hours after HW based on the fact that I have split the timings by half an hour. However lets say I wanted to plot the tidal stream for a passage between 1015 and 1115, surely I'd have to plot half an hours worth of tide from 1 hour after HW and half an hours worth of tide from 2 hours after high water? Please correct me if I'm wrong, just stumbling over this part. Thanks!

Yes, you are correct.
Use 1/2 of the +1 & 1/2 of the +2 hrs.

 

[alant]

I did exactly the same thing once - made a real passage plan and calculated the tidal streams for a theoretical passage. Despite the fact that there is a substantial tidal current up and down the east Irish coast, I was disappointed at the relatively minor effect it had on the process, unless one was going directly against on a spring.......

If I remember correctly, we didn't do the crocodile teeth for DS, but rather for Yachtmaster theory.

Think he's got the wrong "crocodiles teeth". Those on YM, are used for working out secondary port stuff.
What the OP is talking about, is usually (in my classes anyway), called a 'Tidal hour Ladder'.

 

[prv]

Careful! averaging vectors isn't just adding and dividing by n.

I'm certainly not talking about doing anything so mathematical. I literally mean looking at one chart that says there's 2 knots of eastgoing tide, and the next where it's a little stronger and has a bit of a northerly direction to it, and mentally averaging to "just over two knots, and slightly north of east". Unless you have a Solent racing tide-book that covers every square inch, you have to do loads of that kind of interpolation anyway between the widely-spaced Admiralty arrows and numbers.

Pete

 

[LadyInBed]

Ed, looks like you are running under two ID's.

 

[dslittle]

For me, the most important thing I note is the time the tide turns (slack water), because as you may have noted, slack water time doesn't necessarily coincidence with local high and low water.
So my bottom line is that I try to plan a passage where for the majority of the time the tide stream is assisting me and my departure or arrival time, whichever is more critical, is at the most advantageous state of the tide, which might be flood, slack or ebb.

Absolutely. It is the same as ascertaining if you theoretically have 0.1m clearance - I am more interested to know if the tide is rising as I go over the obstruction.

 

[alant]

Absolutely. It is the same as ascertaining if you theoretically have 0.1m clearance - I am more interested to know if the tide is rising as I go over the obstruction.

If you have clearance, does it matter?

 

[Neil]

Well, I did a classic secondary port tidal gate calculation and determined that I had an hour's clearance on a falling tide. I'd just got past the safe water mark when I brushed the bottom (sand!). Hoping, it was just a temporary sand bank, I pressed on to find that I was again dragging the keel through sand. In a panic, I turned round and headed back out (on a different line), not as fast as I'd have liked, but probably faster than was wise! You can do all the calculations you like, but you never know if the charts reflect the current state of the seabed.

 

[dslittle]

Well, I did a classic secondary port tidal gate calculation and determined that I had an hour's clearance on a falling tide. I'd just got past the safe water mark when I brushed the bottom (sand!). Hoping, it was just a temporary sand bank, I pressed on to find that I was again dragging the keel through sand. In a panic, I turned round and headed back out (on a different line), not as fast as I'd have liked, but probably faster than was wise! You can do all the calculations you like, but you never know if the charts reflect the current state of the seabed.

My point exactly. THEORETICAL calculations are well and good. REALITY is different. 0.1m is a ridiculously small margin of error even if the THEORY is correct.