Anchoring/tides

lustyd said:
Yes that would work with one huge caveat. You said your boat reads below the 1.8m keel, so when you read 5m it's actually 6.8m so you need 8m of chain because the bow roller is 1m above the surface. Make this simpler for yourself and just add 3 to the depth reading :D
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Oops! forgot about that bit.

Regarding the setting of the echo sounder. We have ours offset to read from the bottom of the keel. the rational here was simply that the keel is the bit that will hit the bottom first (normal circumstances prevailing).
 
Don't know what you do about passage planning, but it is really helpful to collect relevant data before you go. I always print out the Easytide data for the relevant area and times. This is the Hydrographic Office data and you can access it from the BBC weather site when you are getting the forecast. You can print out anything up to 7 days and it gives the times of HW and LW and heights, but also a graphical representation of the curves for each cycle plotted against time. This gives you a quick way of identifying the state of the tide at the time when you want to be at a particular place.

As you have already discovered there are many, many ways of using the data, which in turn is available from many different sources. You just have to find one that suits you, test it out and when you are happy it gives you consistent good results stick with it.
 
Tranona said:
Don't know what you do about passage planning, but it is really helpful to collect relevant data before you go. I always print out the Easytide data for the relevant area and times. This is the Hydrographic Office data and you can access it from the BBC weather site when you are getting the forecast. You can print out anything up to 7 days and it gives the times of HW and LW and heights, but also a graphical representation of the curves for each cycle plotted against time. This gives you a quick way of identifying the state of the tide at the time when you want to be at a particular place.

As you have already discovered there are many, many ways of using the data, which in turn is available from many different sources. You just have to find one that suits you, test it out and when you are happy it gives you consistent good results stick with it.
Click to expand...

Wow. Sounds like a lot of work when you could jsut carry all the data with you on a phone.:D
 
I know I am going on a bit - 'woman' I hear you say! But I want to get a simple method clear i my mind for anchoring in recognised anchorages (for a start).

(n.b Echo sounder reads from bottom of keel)

Echo sounder shows 5m on arrival
Tide charts show range of 2m
Maximum echo sounder depth possible below keel is therefore 7m
So, 7m x 4:1 ratio = 28m +1.8m offset + 1m to bow roller = say 31m chain


Am I right?
 
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Almost there! Actually you need to add the offset and 1m to roller before multiplying by three:
(7 + 1.8 = 1) x 3 = 29.4

But having allowed for the range in the wrong direction, you will have a much wider swinging circle at low tide which could be a problem in a crowded anchorage or if it allows you to swing into overly shallow water. Also, if you did it the other way round (arriving at low water and adding the offset and height of bow roller) you would compromise the scope available to create a safe catenary in the rode at high water which might cause you to drag. Best to know, at least roughly, where you are in the tidal cycle - you'll already know this as it applies to navigating the tidal streams.

Of course, if you're likely to swing into shallow water, then chances are the wind is onshore which increases the risks anyway and you'd probably choose a different anchorage with shelter from the wind.

Rob.

P.S. Everyone learns and memorises things in different ways, so keep looking for a system that suits you! I have trouble teaching people to dance - some have pictorial memories, others have to count the steps...GOK what the others are doing!
 
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You've got the principle but I'd echo the advice of several of the posters on here. Get yourselves on a DS theory course, doesn't have to involve a classroom, I did my Coastal Skipper theory one-to-one with an instructor I found through the RYA. I paid slightly more for my course but it was well worth it.
Things like this will be shown to you in depth for one or two sessions, probably less time than you've spent on these posts! :D

I should add, it's good fun too.
 
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lustyd said:
Yes that would work with one huge caveat. You said your boat reads below the 1.8m keel, so when you read 5m it's actually 6.8m so you need 8m of chain because the bow roller is 1m above the surface. Make this simpler for yourself and just add 3 to the depth reading :)
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Or recalibrate the sounder to actual depth! See what I mean, it is way too confusing any other way.
 
Ex-SolentBoy said:
Wow. Sounds like a lot of work when you could jsut carry all the data with you on a phone.:D
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Not everybody has the need for or likes a phone. It is not a lot of work and you have a written record for your log. Sometimes, some people prefer simplicity and making appropriate use of technology for their needs.

As I said most people find a method that suits them, makes sure it is reliable and sticks with it. No need to change just because there are other methods available.
 
rob2 said:
Almost there! Actually you need to add the offset and 1m to roller before multiplying by three:
(7 + 1.8 = 1) x 3 = 29.4

But having allowed for the range in the wrong direction, you will have a much wider swinging circle at low tide which could be a problem in a crowded anchorage or if it allows you to swing into overly shallow water. Also, if you did it the other way round (arriving at low water and adding the offset and height of bow roller) you would compromise the scope available to create a safe catenary in the rode at high water which might cause you to drag. Best to know, at least roughly, where you are in the tidal cycle - you'll already know this as it applies to navigating the tidal streams.

Of course, if you're likely to swing into shallow water, then chances are the wind is onshore which increases the risks anyway and you'd probably choose a different anchorage with shelter from the wind.
Click to expand...

Having now got the correct sequence of sums - (your first point)
and surprisingly I did spot the crazy result it would bring about at low tide - (your second point).

I will now factor in the actual state of the tide on arrival and throughout our stay. As you say, we have to be aware of it anyway in the Dorus Mor (my spelling) and other tidal gates.

Thanks Rob and everyone else for all your patience and good advice. Now I wonder if I could trade in the boat for a caravan!:D
 
castoffandgone said:
Oops! forgot about that bit.

Regarding the setting of the echo sounder. We have ours offset to read from the bottom of the keel. the rational here was simply that the keel is the bit that will hit the bottom first (normal circumstances prevailing).
Click to expand...

And you're quite right that it makes it easier as a collision avoidance device. Unfortunately as you've seen for planning purposes it's simpler to have it read the actual depth of water (and you'll find most boats do this).

With it set to the depth of water, and chain marked every 3-4m you can just count out the same number of marks as your depth guage reads "and one for luck" and avoid pretty much all the maths :)
 
>Scope on your boat will be charted depth plus high water height plus 1m (for the bit between water and bow roller) times 3 for chain

Using chain our aim was to get the right weight of catenary to not pull on the anchor in the current wind conditions. Then the anchor will hold if the wind picks up.

In five metres we used 10 X depth, 15 metres 7 x, 30 metres 3 x. There is no fixed multiple.
 
lustyd said:
Unfortunately as you've seen for planning purposes it's simpler to have it read the actual depth of water (and you'll find most boats do this).
Click to expand...

I think we need a poll on this. The only boat I have ever been on with the sounder set that way was a sailing school boat.

Every single boat I have been on since then has set the sounder to read below the keel.

I have to disagree with this "it's easier for planning". I know my boat draws 2m. I just plan to sail where there is more than 2m of water. Once I am going all that matters is what is actually below the keel, not what I think should be below the keel.
 
Tranona said:
Not everybody has the need for or likes a phone. It is not a lot of work and you have a written record for your log. Sometimes, some people prefer simplicity and making appropriate use of technology for their needs.

As I said most people find a method that suits them, makes sure it is reliable and sticks with it. No need to change just because there are other methods available.
Click to expand...

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