Anchorage Tidal calcs

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Alifonz

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I've recently passed the YM / CS theory and I have a nagging question that even Tom Cunliffe doesn't answer. When you are calculating appropriate tidal hieghts for an anchorage that is not obviously related to a particular std or secondary port, do you just use the tidal data for the port geographically closest to the given anchorage.......may seem obvious but this is never covered in books or exams and I want to know for sure!
 
yes and no..........

base it on the nearest but take into account if this is an increasing or decreasing direction down the coast ie you may need to adjust up a touch or be comfortable based on that one range wise (I wouldn't adjust down!)
 
What you do is calculate what you think it should be using the nearest port, then go there, having worked out what the water depth ought to be in a particular spot, and measure it with your depth sounder. Unless said anchorage is likely to have significant time difference, e.g. is a long way up a narrow creek, then the height difference can be applied as an addition. Repeat for several states of tide and at springs and neaps. Then go into the business of writing your own pilot guide ..... in practice I suspect that the number you first came up with will be so close as to be indistinguishable from the final depth, unless you're trying to scrape over the last few centimetres of the bar.

Oh, and don't forget to factor in atmospheric pressure if that's what you're doing. I believe 1cm per millibar away from 1000mb is a good rule of thumb, though I can't lay my hands on the paper that told me that at the moment so will stand corrected if wrong.
 
Tidal heights are not a precise science but an approx one. Actual height varies with wind direction and pressure.

Anchoring is an even less precise science so I would work out worst case of nearest port, decide how long I wanted to remain at anchor compared to risk of swinging and hitting another boat (outside Yarmouth Iof W you can witness the way various boats answer to wind while others predominately answer to tide) and chuck out the maximum I consider safe.

I have once had to keep anchor watch all night in Spain in a F8 on a lee shore and while when we anchored there were 5 boats around us in the morning there were only 2! noticably the 2 british flagged boats!

I hope you are not solely concerned with maximum tidal height and a multiple of chain/rope thereof as I would consider the holding ability of the bottom and the risk of chafe perhaps more important. In any overnight anchor I always take the anchor tension with a rolling hitch on the anchor chain/rope and protect the rope with a polythene tube where it goes through the fairlead.

I think we can often worry about being precise when seamanship factors are far more important.
 
How much water is there under the keel now (from sounder) + or - tide left to rise or fall, from nearest std port. If on a falling tide is there enough water at LW?
 
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I have once had to keep anchor watch all night in Spain in a F8 on a lee shore and while when we anchored there were 5 boats around us in the morning there were only 2! noticably the 2 british flagged boats!
I think we can often worry about being precise when seamanship factors are far more important.

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I suppose silly Johnny foreigner bu88ered of to somewhere stupid .. like a safer anchorage leaving you and your stiff upper lip Brit mates keeping anchor watch in a F8 on a lee shore .. how unsporting of the chaps .. no moral fibre .. what ho!
 
Agreed - although I'm interested in discussion of how you ascertain a safe depth to anchor in (I know the theory when you have the tidal data btw!) when there isn't an obvious port to get the tidal information from - ie if your chose anchorage is a bay on the coast between a couple of ports that have tidal data but none is published for the bay / anchorage istelf......
 
Essentially you can't (possible local factors)! You just need to make a rough calculation based on the rule of twelths based on the estimated time of HW and work from that, needless to say a generous margin of safety should be apllied
 
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Agreed - although I'm interested in discussion of how you ascertain a safe depth to anchor in (I know the theory when you have the tidal data btw!) when there isn't an obvious port to get the tidal information from - ie if your chose anchorage is a bay on the coast between a couple of ports that have tidal data but none is published for the bay / anchorage istelf......

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Welcome to the forums BTW!

Simple answer is use the absolute worst case of the nearest data sources and allow an extra 3ft drop just in case. In the unlikely event that you are not awake all night worrying about it you will otherwise have set the alarm to get you up and check. That way also a bit of swell rolling in will not get your worry wort cells going either.
 
No the local marina refused to let anyone in but towed in all their own boats off their moorings parking them in solid so that no bertholder could get in or out. There was simply no room.

One boat started dragging its anchor at 20.00 and went up river. Another 2 dragged their anchors in the night. One was kindly squeezed into the marina and the other just barged in. The marina clearly stated that they NEVER take visitors. With language difficulties and shorthanded in a strange port in darkness I decided that while not ideal we were safest staying put. The next morning once we confirmed the winds would remain for a few days we went to a marina up river. I should add that the strong overnight winds were not forecast.

Our problem was that when they first occurred we did not know how long these unexpected winds would last. No other boat elected to move until they were forced to. With hindsight if I expected these winds I would go up river even if a strange port but it was nice to know that anchoring as recommended in the RYA booklets worked for us.

Looking back our previous UK sailing experience was just a prep course for the many experiences we has sailing across the Bay of Biscay to Spain. Any trip can soon become a new learning experience.

Its also nice to know you monitor my posts to keep me on my toes.
 
Yeah I sit here all day waiting for you to liven up my dull day by posting .. in fact I use the "ignore everyone but this user button" to ensure I don't miss any of your pearls of wisdom
 
Thanks - what a great source of knowledge- I'm sure there will be more scenarios that I'll think of to quizz you all with!! BTW it was interesting reading the threads on YM exams etc. As a student of all things RYA I totally accept while doing the courses / exams gives you a great foundation and fills you with excitement for adventurous sailing you do feel like you really still have a hell of allot to learn and although you know the theory it's all in the confidence of the execution in real situations.....
 
If you really want to be precise work out the two ports either side and interpolate between them based on how far you are from each one...thats the theory however I doubt I or anyone else would bother with that and would go with the failsafe options listed above ie go for the one that gives you least water then you don't get a nasty surprise...Iain
 
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