YM Exam - Best way to do Sec Port Calc

[jakey0]

Hi All,
I am getting ready for a coastal skipper theory test in a few weeks (any tips greatly appreciated). Almost all of the content has been understandable so far however interpolation, using the "RYA Way" crocodile graphs just isn't clicking.

The main issue Is the lack of precision I seem to be able to get from it. I understand how to draw it, lay it out and use it however my answer will be far enough off to be out of tolerance, but close enough that the method is still right. )

One issue I have identified I have is when the question uses a port with differences that aren't easy to divide up along the intervals (i.e the range is -0017 and -0043) I can't work out what to make the intervals and more importantly have a really hard time matching the two axis intervals so that each axis has the same number of intervals, but covers the entire range it needs to, if that makes sense. I imagine this is basic maths, but I clearly missed out on it.

I had great success with the android app for tidal calculations however Bluestacks has stopped working and more importantly, I'd rather be confident in myself knowing how to do it properly. On a side note, I am doing the exam online, however, will the practical exam require proof of competency in doing these as well?

If anyone has alternative ways of doing the calculations (analog or not) or has suggestions for what I am doing wrong, please share.

Many thanks

 

[WindyWindyWindy]

You don't need the same number of intervals.

Draw the two lines with whatever is convenient in each and use a parallel rule from the two greatest extents. Halve the ranges until you have something close. So 17-43 becomes 17-30-43 becomes 17-23-30-36-43 etc... If it's then between 17 and 23 for instance then it's 20.

Then split it down the other axis as necessary.

It's an estimate, the marking will involve a range. Although I would expect round numbers.

I was always disappointed in the lack of mathematical precision, until I used them in the real world and found the real world to be lacking precision.

There's a similar graph where you draw an X, but I think that's basically equivalent.

 

[LittleSister]

I was always disappointed in the lack of mathematical precision, until I used them in the real world and found the real world to be lacking precision.

 

[Skylark]

The so-called crocodile method is a convenient and visual way to interpolate (and extrapolate in the case of big tides) tidal heights and times. It assumes a linear relationship between the two axes.

Each axis can be any convenient length. 0000 to 0600 easily divides into 6 intervals. For -0017 to -0047 I would use either 2 minutes, 5 minutes or 10 minute intervals. Scribe a line between the 0600 and -0047 points and the relationship between the two axes will be lines parallel to the one drawn.

Another way is tabular. Care is needed with sign convention and it’s easier to make a mistake. The crocodile method has the benefit of being visual so easier to spot errors.

The difference between 0000 and 0600 is 6 hours (could even be 360 minutes). The difference between -0017 and -0047 is 30 minutes.

Question. What’s the time at secondary port when standard port is 0437?

0437 is 277 minutes. 277/360 x 30 = 23 minutes.

Add 23 to the -0017 gives a secondary port difference of -0040 (care needed with signs).

 

[laika]

Is this "crocodile graph" malarky a new thing? I don't recall it from my YM theory (albeit 17 years ago) and had to google it. It does seem a bit of an unnecessary palaver for simple linear interpolation.

In an exam I'd go with skylark's method (ie totally simple, straightforward and linear interpolation)

In reality, sitting at the chart table before pootling off somewhere it'd be:

0437 is slightly more than 3/4 of 6 hours
3/4 of the 30 minute difference in offset from -0017 to -0047 is 22.5 which rounded up (see "slightly more" above) is 23. So same result as skylark.

If we use the OP's 0043 rather than skylark's 0047, the difference between the 2 offsets is 25. slightly more than 3/4 of that is 19, so in the OP's example with skylark's 0437 the offset is -0036 (so 0401, aka "about 4am")

Using fuzzy maths sounds a bit shonky but you'll rarely be more than a few minutes out and:

• Linear interpolation is only an estimate
• Tide tables are themselves only an estimate

If anyone's navigation plans rely on them knowing the time of high water to the minute, or the height of tide to within 5cm, they're probably doing it wrong

 

[jakey0]

Each axis can be any convenient length. 0000 to 0600 easily divides into 6 intervals. For -0017 to -0047 I would use either 2 minutes, 5 minutes or 10 minute intervals.

This is fundamentally what I am struggling with. I can work out, through trial and error, what intervals to use but that basically means me filling in the graph a few times only to find that 5 min intervals mean I cover the range over only half of the axis (like pictured below) or I go massively over with intervals that are too small. Is there any mathematical way to work out the most logical intervals to use?

Its such a trivial thing but all the videos and instructions totally gloss over it and don't give any guidance as to how best to choose an interval range.

 

[mattonthesea]

Is this "crocodile graph" malarky a new thing? I don't recall it from my YM theory (albeit 17 years ago) and had to google it. It does seem a bit of an unnecessary palaver for simple linear interpolation.

In an exam I'd go with skylark's method (ie totally simple, straightforward and linear interpolation)

In reality, sitting at the chart table before pootling off somewhere it'd be:

0437 is slightly more than 3/4 of 6 hours
3/4 of the 30 minute difference in offset from -0017 to -0047 is 22.5 which rounded up (see "slightly more" above) is 23. So same result as skylark.

If we use the OP's 0043 rather than skylark's 0047, the difference between the 2 offsets is 25. slightly more than 3/4 of that is 19, so in the OP's example with skylark's 0437 the offset is -0036 (so 0401, aka "about 4am")

Using fuzzy maths sounds a bit shonky but you'll rarely be more than a few minutes out and:

• Linear interpolation is only an estimate
• Tide tables are themselves only an estimate

If anyone's navigation plans rely on them knowing the time of high water to the minute, or the height of tide to within 5cm, they're probably doing it wrong

Just a note to say that I passed with this approach.
And, given that I allow 50cm contingency for bars and anchoring, this has worked in practice for 15 years ?

 

[jlavery]

The crocodile graph is relatively new. I got my YM in 1991. First heard of this when studying for my Cruiser Instructor a couple of years ago.

For me, the mental linear interpolation works. But that's because I find maths easy. I can understand how some struggle with that. So the crocodile graph can be very useful for those who find the maths of interpolation/extrapolation hard.

@jakey0 - this is the sort of thing which some worked examples and practice can help with. PM me, and if you want we can go through some examples.

 

[glynd]

The crocodile approach seems best for 'visual' people - but in a cramped space the interpretation method works best - and is less fiddly.
I came across it as it was mentioned by The Cunliffe in 'The complete Yachtmaster' - and then threw away crocodiles for good.
Got me through my Ym also

 

[Praxinoscope]

I’m another who had to look up the ‘crocodile graph’, certainly hadn’t heard of it when I did my YM back in 1983.

 

[Sandy]

Hi All,
I am getting ready for a coastal skipper theory test in a few weeks (any tips greatly appreciated). Almost all of the content has been understandable so far however interpolation, using the "RYA Way" crocodile graphs just isn't clicking.

The main issue Is the lack of precision I seem to be able to get from it. I understand how to draw it, lay it out and use it however my answer will be far enough off to be out of tolerance, but close enough that the method is still right. )

One issue I have identified I have is when the question uses a port with differences that aren't easy to divide up along the intervals (i.e the range is -0017 and -0043) I can't work out what to make the intervals and more importantly have a really hard time matching the two axis intervals so that each axis has the same number of intervals, but covers the entire range it needs to, if that makes sense. I imagine this is basic maths, but I clearly missed out on it.

I had great success with the android app for tidal calculations however Bluestacks has stopped working and more importantly, I'd rather be confident in myself knowing how to do it properly. On a side note, I am doing the exam online, however, will the practical exam require proof of competency in doing these as well?

If anyone has alternative ways of doing the calculations (analog or not) or has suggestions for what I am doing wrong, please share.

Many thanks

Welcome to the forum and the fuzzy world of navigation.

Sea Chest in Plymouth sell a laminate with a secondary port triangle on it and instructions on how to use it. They also sell lots of laminated goodies.

Don't get too hung up on second and mm precision, the height of tide is a mathematical forecast.

Good luck with the exam.

 

[Chiara’s slave]

Welcome to the forum and the fuzzy world of navigation.

Sea Chest in Plymouth sell a laminate with a secondary port triangle on it and instructions on how to use it. They also sell lots of laminated goodies.

Don't get too hung up on second and mm precision, the height of tide is a mathematical forecast.

Good luck with the exam.

There are several places you can compare forecast with actual tidal rise. As you say, seconds an mm are completely irrelevant.

 

[jakey0]

Welcome to the forum and the fuzzy world of navigation.

Sea Chest in Plymouth sell a laminate with a secondary port triangle on it and instructions on how to use it. They also sell lots of laminated goodies.

Don't get too hung up on second and mm precision, the height of tide is a mathematical forecast.

Good luck with the exam.

Thank you for the suggestion, have ordered one along with a mini col regs book. Great selection of stuff they have.

 

[LadyInBed]

Is this of any help:
Tide Time at Secondary Ports Calculation : Monty Mariner

 

[capnsensible]

Is this of any help:
Tide Time at Secondary Ports Calculation : Monty Mariner

Excellent!

 

[john_morris_uk]

If you’re interested in one particular examiners approach, this is what I do. First of all, I might only start asking for secondary Port calculations if someone is looking a bit dodgy on their navigation and tides. Secondly, if you can interpolate in your head and show me you understand what you’re doing then I’ll be happy and move on. In reality I don’t really care what method you use so long as you understand it and it works.
However I’m exasperated by the number of candidates who will take 40 minutes to get an answer that’s sort of acceptable but then have no idea of how to apply it.
Eg. ‘You’re anchored in a secondary port area at xxx time and the echo sounder reads 5.4 metres. You draw 2 metres. How much clearance under your keel will you have at next LW? How much chain will you put out to ensure you’ve sufficient scope at next HW? ‘
This is EXACTLY the sort of everyday question yachtsmen face when sailing in tidal waters so it’s not exactly unfair to ask it… yet some YM candidates really struggle.

 

[jakey0]

If you’re interested in one particular examiners approach, this is what I do. First of all, I might only start asking for secondary Port calculations if someone is looking a bit dodgy on their navigation and tides. Secondly, if you can interpolate in your head and show me you understand what you’re doing then I’ll be happy and move on. In reality I don’t really care what method you use so long as you understand it and it works.
However I’m exasperated by the number of candidates who will take 40 minutes to get an answer that’s sort of acceptable but then have no idea of how to apply it.
Eg. ‘You’re anchored in a secondary port area at xxx time and the echo sounder reads 5.4 metres. You draw 2 metres. How much clearance under your keel will you have at next LW? How much chain will you put out to ensure you’ve sufficient scope at next HW? ‘
This is EXACTLY the sort of everyday question yachtsmen face when sailing in tidal waters so it’s not exactly unfair to ask it… yet some YM candidates really struggle.

Interesting to hear. I understand the application and the actual 'happenings' behind nav and tide, just struggle with doing the calculations quickly. Can't quite imagine myself interpolating in my head though.

In passage planning, it takes me ~5 mins to get a full day of tide converted to a secondary port, but that really seems like the sort of thing you would do properly in the planning phase so as not to have to do it in a hurry later on and potentially muck up.

 

[john_morris_uk]

Five minutes sounds very good and very fast! Interpolate in whatever way suits you. Just be ready to explain your answers…!

 

[Spirit (of Glenans)]

Is this "crocodile graph" malarky a new thing? I don't recall it from my YM theory (albeit 17 years ago) and had to google it. It does seem a bit of an unnecessary palaver for simple linear interpolation.

In an exam I'd go with skylark's method (ie totally simple, straightforward and linear interpolation)

In reality, sitting at the chart table before pootling off somewhere it'd be:

0437 is slightly more than 3/4 of 6 hours
3/4 of the 30 minute difference in offset from -0017 to -0047 is 22.5 which rounded up (see "slightly more" above) is 23. So same result as skylark.

If we use the OP's 0043 rather than skylark's 0047, the difference between the 2 offsets is 25. slightly more than 3/4 of that is 19, so in the OP's example with skylark's 0437 the offset is -0036 (so 0401, aka "about 4am")

Using fuzzy maths sounds a bit shonky but you'll rarely be more than a few minutes out and:

• Linear interpolation is only an estimate
• Tide tables are themselves only an estimate

If anyone's navigation plans rely on them knowing the time of high water to the minute, or the height of tide to within 5cm, they're probably doing it wrong

I had to google it, and discovered that this was the method I learned in (circa) 2003. I didn't know it was called a crocodile graph, though!
I keep a couple of photocopied sheets containing a series of those intersecting lines with my nav stuff. When I need a calculation I can fill in the relevant divisions on the lines.
To answer the OP, having decided what intervals to use, I would take my Portland Plotter or my ruler, and lay it on the axis and the chose units to suit the number of intervals that will fit on the axis as drawn, i.e. I might use centimetres, half centimetres, inches or half or quarter inches

Welcome to the forum and the fuzzy world of navigation.

Sea Chest in Plymouth sell a laminate with a secondary port triangle on it and instructions on how to use it. They also sell lots of laminated goodies.

Don't get too hung up on second and mm precision, the height of tide is a mathematical forecast.

Good luck with the exam.

Your advice to not "get hung up on second and mm precision" is not good, however, because the theory exam requires accuracy to two decimal places.

 

[john_morris_uk]

I had to google it, and discovered that this was the method I learned in (circa) 2003. I didn't know it was called a crocodile graph, though!
I keep a couple of photocopied sheets containing a series of those intersecting lines with my nav stuff. When I need a calculation I can fill in the relevant divisions on the lines.
To answer the OP, having decided what intervals to use, I would take my Portland Plotter or my ruler, and lay it on the axis and the chose units to suit the number of intervals that will fit on the axis as drawn, i.e. I might use centimetres, half centimetres, inches or half or quarter inches

Your advice to not "get hung up on second and mm precision" is not good, however, because the theory exam requires accuracy to two decimal places.

Not as far as I’m concerned. (And I’m authorised to teach theory and conduct and mark the theory exams). I’ve heard horror stories of some night school instructors in the past but I thought most of them had been weeded out by now

or perhaps there was a touch of hyperbole in your post?