Calculating tidal differences for secondary ports

[Bottle_of_Rum]

Ym theory.

Does anyone have any clever ways of calculating tidal differences at secondary ports. i.e,.. differences when the time is between the figures in the tables.

Graphs or a ready made calculation.

I realise that in practice a rough guess will often be ok. But I'm trying to get my head around it for the YM shore based.

 

[DJE]

I'm in the middle of this myself at the moment. There is a graphical method in the RYA Yachmaster book on pages 16 to 18. It is simple straight line interpolation and easy enough to do at sea with a parallel rule or Breton plotter. (If you haven't got the book PM me and I will send you an example.)
In practice the differences for most of the places I have been are so small that I can do it in my head.

 

[iangrant]

For the theory there is a small and simple interpolation table method in the RYA book for finite calculations in the exams.

In real life however, too many other things play a part in tidal height predictions so it becomes a bit academic in truth. Good to know how to do it though....!!

Ian

 

[Superstrath]

Interpolation, while relatively easy, can be a pain. I thought it might be challenging to write a spreadsheet that could do it for secondary ports. If anyones's interested in the results, I'll dig it out and send it to you.
In the beginning it got things back to front occasionally, and although I've tested it a fair bit, it might do it again, so it would be helpful to have a few testers to iron out the bugs.
It would come with a disclaimer as long as your arm.

Alistair

 

[wooslehunter]

It's just linear interpolation and can be done purely mathematically. But this does meen that unless any other navigators on the boat are familiar with the method, there's no easy way to hand over and also it's not as easy to spot an error.

I prefer a table method since it doesn't involve any drawing. It does need a few simple sums though.
Here's an example:

High Water
0300 0900
1500 2100

-0005 +0011

Say we want to calculate the time of HW when HW at the stadard port is 04:15GMT.

First calculate the total difference time = 16mins
Now split this up into 6 hour intervals = 16/6 mins per hour = 2.67
(or for how ever many hours there are on the table of differences)

Now we can construct the table as below:

0300 0400 0500 0600 0700 0800 0900
-5 -2.33 +0.34 +3.01 +5.68 +8.35 +11

Numbers are rounded up or down to the nearest minute to give:

0300 0400 0500 0600 0700 0800 0900
-5 -2 0 +3 +6 +8 +11

For 04:15 GMT we can use the the difference for 0400 so we get -2mins.

This method is not as accurate as the graphical method but the error is only going to be 1min. You could always not round & use the first table plus 1/4 of the hourly for the 15mins. In this case it would give -1.66mins so we still use -2 mins anyway. This is going too far though. The graphical method relies on the ability to plot accurately and you end up with around the same errors anyway.

One other nice advantage of this method is that once you have the table, you can use it for any time 0300 - 0900 and 1500 - 2100. In this case it can also be used around the clock. Often though the differences are not plotted for 6 hours. In this case you just have to make two tables and you have the differences for all times.

Take your pick no-one's going to complain about a 1min error especially if they can see where it came from.

 

[Birdseye]

[ QUOTE ]
Ym theory.

Does anyone have any clever ways of calculating tidal differences at secondary ports. i.e,.. differences when the time is between the figures in the tables.

Graphs or a ready made calculation.

I realise that in practice a rough guess will often be ok. But I'm trying to get my head around it for the YM shore based.

[/ QUOTE ]

Just stick to simple mental arithmetic. If the gap you are trying to interpolate is 6 hours then each hour is one sixth of the change. Or if you dont want to divide by 6, multipl by .16. (If the gap is 7 hours divide by 7 or multiply by 0.14)

So if the difference goes from -10 mins to +20 mins, then the total change is 30 mins and one sixth is 5 mins. If the time you are interested in is 3 hours away from the -10 mins time, then the difference is -10 plus 3 x 5 = +5

Messing around with plotters and graphs down below in a moving boat is asking for nausea to my mind. On the other hand, the level of mental arithmetic required is probably what you did in junior school.

 

[iangrant]

Agree totally with the keep it simple for the practical, and it's worth taking a look at the barometer and the wind direction!

For the theory you need to show your workings on paper, I'm pretty sure you need to show the RYA graph for the exam too.

Ian

 

[jimi]

Agree .. however if you can do it in your head its no great shakes to write it down. However if you are looking for a mid tidal situation eg earliest entry time when there's a bar or sill etc then I've always applied that to the relevant tidal curve of the primary port which is rarely linear.

 

[Goodge]

When I did YM prep the skipper was a fairly recent Fast track type chappy.

On the first morning before we even left Hamble I had to do secondary port calculation. Did by simple maths and gave him the answer.

He looked perplxed and asked where was my graph ?

He had to do a graph himself to check it and then he realised I was right.

It seems the easier option to me.

 

[KevB]

buy one of these Tidal Wheel

 

[iangrant]

Oh, and if you are going into Bembridge in a fin keel boat that draws 2.0 Metres do it on a rising tide one hour before HW.

That rule seems to work in lots of places!
Ian

 

[jimi]

yeah .. but you & I both know how to do it properly if need be. We all use these rules of thumb in familiar places .. but entering an unfamiliar place I (and I'm sure you) certainly want to know a bit more accurately the margin of error .. or if we can sneak in and make the pub.

 

[Anonymous]

I draw 2.0m and it didn't look to me as though Bembridge was possible - is Bembridge Marina suitable for a 42' 2m?

 

[iangrant]

Never yet not been able to get into Bembridge. Springs or neaps. About 2.5 hours before on a big spring but you do bottom out on the north end of the pontoon at LW for an hour.

Look at the tide gauge (pesimistic) call up the HM and he will tell you how much water inside. DON'T cut the corners. The least water is by the beach at the entrance on the port side.

Tie up and note the time, look at the tide guage on the pontoon so that you can confidently leave at the right time.

Ian

 

[jimi]

I draw 1.7m and have gone down to 0.1 under the keel when entering earlyish(2.5 hours before), I'd say you'd have no problem 1 hr before HW but check the tide gauge at the entrance and you'll almost certainly sit on the mud at LW in the Marina (but its OK as its just soft squishy mud). Probably be OK on the visitors pontoons just inside.

 

[iangrant]

You're right Jimi, now and again a dust off of the old brain cells and it is still there with a bit of a struggle.

Now then, collision regs ... Hmmm - last vessel in a tow, you are runing into danger. Hmmm., maybe time to get the old book out 'cause i've forgotten some!

Ian

 

[tcm]

Secondary port info - WARNING

ah, yes indeedy, you need an almanac. Now, looking inside the almanac, there's various numbers which the tidal heights differences and time differtences can be calculated from the standard port, itself having a separate list somewhere else of the high tides allowing you to use a tidal curve to eventually plot the tidal heights at all the times of the particular day in question.

BUT in the same almanac, there might be another number in the almanac. It's a telephone number. You MUST NOT EVER ring this telephone number with a vhf radio or especially not with a newfangled mobile phone and say "hi there Mr HM when it will be okay to come in the the harbour oh and by the way can we have a berth please?" - cos that's flippin CHEATING, ok?

 

[jimi]

Re: Secondary port info - WARNING

and what 'appens if you get some daft frog on the ither end whae disnae underon a wird o' guid engleesh?

 

[jimbouy]

Re: Secondary port info - WARNING

Is this a recognised RYA method? /forums/images/graemlins/confused.gif

I think not!!! /forums/images/graemlins/mad.gif

Certainly my suggestion, in the weather paper for YM theory, to ring one of the weather services was not received very well. /forums/images/graemlins/grin.gif


But it seems that you learn RYA methods for the exams then go off and use real world methods for sailing.

Jimbouy

 

[tcm]

Re: Secondary port info - WARNING

much easier to learn french than do the secondry port calcs! Key thing is to listen out for them asking for "tirant d'eau". But they normally speak english at Bembridge...