Secondary port crocodiles - what's the point?

[BlueSkyNick]

To interpolate time and height adjustments for secondary ports, the 'RYA method' is to draw a horizontal line graduated with minutes or meters difference, and a second line touching the first at the left hand end and rising up at say 30degs to the right graduated with six hours. Then draw a line from the highest time on the sloping line down to the right most difference on the horizontal line. To find the difference at a particular time in between, draw another sloping line parallel to the first from required time down to the horizontal line and read off the difference. Easy, eh??

Much easier to draw one horizontal line, graduated with hours on the top and difference on the bottom and simply read off at the required time.

Or am I missing something obvious ???

 

[lw395]

The sloping line can be drawn on squared paper or to a suitable scale, you don't have to calculate the slope.
Whereas your line, you have to calculate how many units per inch or whatever to draw the two scales.
You can do the same thing algebraicly in your head. It's just a linear interpolation.
Or you say, 'sod it, within the error of these things, it's about half way between'. Or it's close to springs or neaps.

 

[onesea]

Ah but in the exam would you fail because your not following "the method"

 

[[70521]]

Ah but in the exam would you fail because your not following "the method"

Why? If it works and you can prove it works and the result is the same then I can't see that being a fail.

 

[lw395]

Why? If it works and you can prove it works and the result is the same then I can't see that being a fail.

I would hope the idea is to understand the method and why the answer is what it is.
I've known people with quite bizarre 'methods' that (mostly) work, but at the end of the day you have two data points to interpolate between, at work you'd use an excel spreadsheet, on a boat you have a blunt pencil, dividers, and a wet almanac.

 

[lw395]

And where do the crocodiles come into it?
How much rise of tide does a croc need?

 

[prv]

at the end of the day you have two data points to interpolate between, at work you'd use an excel spreadsheet, on a boat you have a blunt pencil, dividers, and a wet almanac.

Mostly on a boat I have an iPhone with the Imray tides app, to be honest

Pete

 

[fireball]

As with all things RYA - you have to demonstrate you know their method - even if you don't use it in practice yourself.

Had this problem doing the Dinghy level 5 course (just before doing instructors) - I didn't tack or gybe in the prescribed method because I was using racing methods which were far quicker to carry out - but I had to demonstrate that I could use the RYA method and tbh, it is a fallback that I can use when not racing or racing with a non-racing crew.

 

[DJE]

To interpolate time and height adjustments for secondary ports, the 'RYA method' is to draw a horizontal line graduated with minutes or meters difference, and a second line touching the first at the left hand end and rising up at say 30degs to the right graduated with six hours. Then draw a line from the highest time on the sloping line down to the right most difference on the horizontal line. To find the difference at a particular time in between, draw another sloping line parallel to the first from required time down to the horizontal line and read off the difference.

That's the method I was taught for dividing a line into equal parts when I did technical drawing in the 1970s!

 

[BlueSkyNick]

The sloping line can be drawn on squared paper or to a suitable scale, you don't have to calculate the slope.
Whereas your line, you have to calculate how many units per inch or whatever to draw the two scales.
You can do the same thing algebraicly in your head. It's just a linear interpolation.
Or you say, 'sod it, within the error of these things, it's about half way between'. Or it's close to springs or neaps.

Totally agree with your final point, it's perfectly fine. Unfortunately, I am preparing to have to prove that I can teach all this stuff to a YMI moderator in the not too distant future !!

On the lines, it doesn't matter if there are two at an angle, in parallel or one, they still have to be graduated and as Tim Bartlett writes in the RYA Navigation book they can be the width of a piece of chewing gum if necessary. I am finding a six inch ruler works pretty well!

 

[BlueSkyNick]

Mostly on a boat I have an iPhone with the Imray tides app, to be honest

Pete

Boatie is pretty useful, too!

 

[Ripster]

Mostly on a boat I have an iPhone with the Imray tides app, to be honest

Pete

Err yes, me too. Tides are on my garmin plotter, my ipad, iphone, laptop. I can unleash a battery of electronic devices for tides. If all these fail, then its tide tables and YM notes out!

 

[doug748]

In practice, I suggest we mostly just look at the figures, decide it does not matter and press on.

 

[lpdsn]

Ah but in the exam would you fail because your not following "the method"

I always do the calculations mathematically. I told the YM examiner exactly how I did it and demonstrated I could do it and he was perfectly happy with it. I made the comment that it was pretty easy for just about everywhere except Lymington and he replied "Yes, I know".

 

[LadyInBed]

That's the method I was taught for dividing a line into equal parts when I did technical drawing in the 1970s!

60's for me

 

[prv]

Err yes, me too. Tides are on my garmin plotter, my ipad, iphone, laptop. I can unleash a battery of electronic devices for tides. If all these fail, then its tide tables and YM notes out!

Although for some reason I do reach for the almanac when it comes to working out a cross-Channel course to steer. Just habit, I suppose.

Pete

 

[DJE]

Give up Nick. Do it the easy way!

 

[Hydrozoan]

Give up Nick. Do it the easy way!

Lost your copy of 'Four-Figure Tables' by Godfrey and Siddons?

 

[Mariner69]

Lost your copy of 'Four-Figure Tables' by Godfrey and Siddons?

I raise you Burtons 5 figure logs and a see you with a set of Haversines!

 

[VicS]

Give up Nick. Do it the easy way!

But thats only an image on a website.

Have you actually got one of those