Help ! Calculation of Rates ?

[Boo2]

Hi,

Can someone please give me a step-by-step guide to calculation of rates using the blank graph in the almanac ?

I am confused as to what refers to the local rate (eg from the tide atlas or chart diamond) and what refers to the reference port so if you could be so kind as to put rate_d for diamond, rate_r for reference port etc in your reply then I would be very grateful.

(It's for a test so I do need to actually do it and can't just guess them.)

Many thanks,

Boo2

 

[VicS]

This help at all ?

 

[Skylark]

Hi,

Can someone please give me a step-by-step guide to calculation of rates using the blank graph in the almanac ?

I am confused as to what refers to the local rate (eg from the tide atlas or chart diamond) and what refers to the reference port so if you could be so kind as to put rate_d for diamond, rate_r for reference port etc in your reply then I would be very grateful.

(It's for a test so I do need to actually do it and can't just guess them.)

Many thanks,

Boo2

Which Almanac? Page 62 of Reeds 2013?

That Computation of Rate, same as those on Admiralty Tidal Stream Atlas, generally refer to Dover as the Standard Port. Each of the Tidal Streams shown within also refers to Dover. See also page 262 to get MHWS and MHWN.

If you are using diamonds on a chart, look to see the chart reference port and use MHWS and MHWN for that standard port.

Does that help?

 

[superheat6k]

This help at all ?

Oh yes I can see why the OP sees it is about as clear as mud !

Alternatively estimate the stream speed and direction past a pot marker or similar !

I appreciate this does not help him with his theory course, but not sure this awful table does either. Great in a classroom, but not on a rolling chart table in a force 5-6, with less than 30 seconds below deck before throwing up or needing to re-check the REAL situation around you.

 

[lpdsn]

I find it easier to work them out mathematically. Here's an example, please ignore if you really need to use the computational chart.

Say the tidal diamond says 1.4 knots for neaps and 3.0 knots for springs. It'll refer to a particular port, so what you will need for that port is the mean Neap and spring ranges. Neap range = MHWN - MLWN and Spring range = MHWS - MLWS. Let's say you get answers of 2.8m & 4.4m.

You then find out your range for this period. If the primary port is local, you can actually use the range for the current flood or ebb, if it is Dover and you're nowhere near Dover, use the average for the day and accept there may be some inaccuracy (not that there isn't inaccuracy anyway). Let's say you find the range to be 4.8m, a bigger than average spring.

If you consider the difference between 4.4m and 2.8m (1.6m) to be 100%, then you can consider 'today' to be 125% (4.8m-2.8m = 2.0m, which is 125% of 1.6)

Next step is that you add 125 % of the spring rate - neap rate to the neap rate to get the rate for the day in question. So 1.6 Knots X 125% = 2.0 knts. Add 1.4 knots and the calculated rate is 3.4knots.

Once you get used to it, it's easy to work it out in your head. It's even easier to type the tidal diamond into a spreadsheet and work out the rate for each hour.

Hope this helps.

 

[jac]

Also do it in my head but tend to work in 1/4s. So 1/4 more than neaps or whatever. Of course if maths makes 3rds easier I may do that!

Key point though is that these are estimates as is all planning. You may plan on 5 knots and make 5.3 or 4.6. The helm may get distracted and be a few degrees off. You may delay reefing and make 10degrees leeway not the 5 you expected.

So great to know how to do it but in real life the difference between a quick and dirty and accurate to 2 decimal places will be obscured by all the other planning assumptions getting blown out of the water because you left 30 mins late.

 

[Boo2]

This help at all ?

Yes, thanks Vic, that does help.

So I just take the hour's tide rates from the atlas or diamond and plot the line on neaps and springs then use the range at the reference port to select the actual tide rate for the hour ? I think that's clear enough. Can you tell me where that text is from ?

Thanks again

Boo2

 

[davidej]

I don't know why the RHA still make us do this rubbish.

Tides are so variable based on weather winds etc, that a rule or thumb (eg rule of tenths) plus an intelligent fudge factor for the weather is far more likely to be right than an interpolation to two decimal places

 

[Spirit of Glenans]

The OP mentions that this is required for a test, therefore is required to be accurate to, I think, one decimal place, so mental arithmetic is out. The explanation in Post#2 explains it in very simple terms.

 

[VicS]

Yes, thanks Vic, that does help.

So I just take the hour's tide rates from the atlas or diamond and plot the line on neaps and springs then use the range at the reference port to select the actual tide rate for the hour ? I think that's clear enough. Can you tell me where that text is from ?

Thanks again

Boo2

Yes you seem to have got it.

Note the reference port for the range is Dover, not the standard port from which local tide times and heights are calculated.

I dont think I have ever used the method in practice.

Copied from an old edition of the Macmillan & Silk Cut Almanac. The first one I think, before they split it into two books anyway, which I keep on the bookshelf as a reference book ( for answering questions on forums )

 

[lpdsn]

The OP mentions that this is required for a test, therefore is required to be accurate to, I think, one decimal place, so mental arithmetic is out.

What makes you think mental arithematic can't be done to one decimal place.

I admit though, that not everyone is comfortable with it.