scope in tidal waters

[Charlie]

Hi All:

I am going to be sailing in Ireland and the UK this summer. There are locations where the tides are going to have a range of 10 meters. If you are anchored in a tight anchorage how do you decide how much scope to release? Do you make adjustments during the tide changes to maintain the proper scope for the depth of the water?

[SailingHarmonie]

Most people set scope for the highest expected tide, and do not try to adjust it twice a day. There is no holding penalty for having extra chain on the bottom. If there is not room to swing at low tide, another anchorage would be my plan.

Do note that less scope is needed in deeper water to generate the same holding, so it might not be as challenging as you think.

[Muaddib1116]

That is a monstrous tide. I've been cruising in Maine where the tides are around 10ft and that has it's own level of challenge, but it's not a huge deal. I set the scope for high tide, and if I don't feel like the anchorage is big enough for that much chain at low tide, I wouldn't use it. It would certainly be a lot more challenging in a 10m tide. I think Sailing Harmonie's point is good about deep water scope, but my primary advice would be to use roomier anchorages and go with enough scope for high tide.

[Ziggy]

Originally Posted by Charlie

Hi All:

I am going to be sailing in Ireland and the UK this summer. There are locations where the tides are going to have a range of 10 meters. If you are anchored in a tight anchorage how do you decide how much scope to release? Do you make adjustments during the tide changes to maintain the proper scope for the depth of the water?

It really depends on how much swinging room you have. If plenty, then, just put out as much scope as you need for high tide. Otherwise, you probably should adjust.



I've never sailed in the British Isles, but based on my experience in the Bay of Fundy, in tight harbors there's usually no room to anchor. Most people tie up to a wharf, or to another boat tied to a wharf. In the few real wilderness areas I visited, there was plenty of swinging room to let out all the scope I needed.

[Kelkara]

In Alaska I did once duck into a very small anchorage to escape some weather that was worse than I was expecting ... with a tide of 7m and a shore tie being impractical, I got up every 3hr through the night to check and adjust my swing radius as needed.

[PippaB]

I probably prefer more scope than many here, but rather than be anti-social and put out 6:1 in an anchorage where everyone else is swinging to 3:1 I’ll put out two anchors in a Bahamian moor. It’s a PITA and definitely more work, but in a restricted space you can use it to set your swinging circle to pretty much any size you want.

This allows me to put out the scope I prefer, and by managing the slack in each rode I can manage the swing circle to match my neighbors. Also allows anchoring in some very tight locations that might otherwise be too small.

[JPA Cate]

Charlie,

First off, SUPER that you're going to sail in Ireland. Good on ya, mate.

I suggest asking someone local what they do in the different places you'll be going. The feedback you get may vary with the bottoms and the tidal streams. Local knowledge rules!

If for some reason, local knowledge is unavailable, then, yes, ev. 3 hrs. depending on the situation. It is only a small interruption to sleep, and helps to improve situational awareness.

Have a wonderful time!

Ann

[wrwakefield]

The 7+ meter tidal variations in our present cruising grounds truly have negligible effect on the diameter of the circle of swing at anchor in deeper anchorages.

Use Pythagoras' Theorem to demonstrate this to yourself...

What do I mean by 'deeper' waters? In areas of more extreme tidal variations (10m in your example; 7m in my present waters) I loosely define 'deeper' as at least 1.5 times the max tidal variation at zero tide- as my own rule-of-thumb. i.e., 10m x 1.5 at zero tide = 15 meters (at zero tide...) in your case.

Since less scope is required in 'deeper' waters, we typically deploy 2.5-3:1 scope (calculated at highest state of tide during our planned stay) using a proper size 'modern' anchor, and sleep well. (Here is our ground tackle inventory for reference.)

However, in shallower anchorages [rare for us...] the scope ratio is affected more when tidal fluctuation is a significant percentage of the overall depth. (i.e., a higher scope ratio is needed in shallower water- leading to a larger swing circle percentage-wise compared to deep water anchoring)

You can perform all the 'what-if' analysis you like (including circle of swing at different depths) using this excellent Anchor Chain Calculator developed by a CF member... (He also made smart device apps...)

Have fun in Ireland!

Sláinte, Bill

[Ziggy]

Originally Posted by wrwakefield

The 7+ meter tidal variations in our present cruising grounds truly have negligible effect on the diameter of the circle of swing at anchor in deeper anchorages.

Agreed

Originally Posted by wrwakefield

However, in shallower anchorages [rare for us...] the scope ratio is affected more when tidal fluctuation is a significant percentage of the overall depth. (i.e., a higher scope ratio is needed in shallower water- leading to a larger swing circle percentage-wise compared to deep water anchoring)

Also agreed. For example, last summer I anchored in Cows Yard in Maine, where the tidal range is about 15 feet (~5 meters). This is not especially great, but the low water depth in the anchorage is about 6 feet. If I just let out 120' of rode (5:1 scope at high tide), I would have 12:1 scope at low. No big deal, as I was the only boat there, but you get the idea.

[derfy]

I recently created this chart (attached - FOR MY BOAT - NOT FOR GENERAL USE) for an upcoming trip into an area of 10 ft tides, narrow coves, and rocky steep-to bottoms, and I just repainted my anchor chain marks at 50, 100, 150, and 200 ft. Wish I had another 50 or 100 ft.

I like to read depth in ft below the keel, so I have a 6 ft Depth Gauge Offset on my gauge (my draft is 5'8" - so I have a 4" margin when I read zero feet under keel - saved me more than once).

My "perch" - the height of the anchor roller, is 5 ft.

So, when I anchor, I just read my gauge, and read the chain length I need off the chart for the target scope. Add any additional tidal flood to the depth gauge reading.

Near high tide in deeper water, I go with less scope, maybe 3 or 4:1. More or much more if there is not much tide swing, or a lot of wind.

This is not as sophisticated as the cool calculator (which is very cool), but it's quick - I don't have a lot of time to make calculations while piloting in tight anchorages with changing depth, wind, and current. It gets me into the ball park, and requires no batteries, data entry, or Internet connection. A simple sailor, I.

The worksheet to make your own chart is also attached.

The scope you choose in your decision and yours alone.

Fair Winds in Ireland!