Horray anchor question :-) Tidal water. Falling a-dry.

[Franziska]

Hi,

I know everyone has been looking for the next anchor thread ;-)

Got a tidal related question.

I will be very likely cruising in Brittany soon. Some areas there have up to 13m (=39ft) tidal range.
Bottom is very often rock strewn with sand patches.


Here's the scenario:

You arrive with say 7m under the keel and cannot clearly see the bottom.
The spot you anchor will fall a-dry in the tidal cycle.
Not areal worry as your multihull (or lifting keel mono) can settle relaxed on the bottom.
You throw the anchor and pay out about 4-5 times the max waterdepth for the location as rode.
Let's assume 10m max leading to 50m rode.

The anchor settles nicely, because it hit a sandy patch by luck.

Your a single-handed exhausted and happy to sleep a deep sleep.

Few hours later the water falls and with falling water your boat swings around and moves aft (= result of depth reduction with constant rode length).

How can you make sure you do not fall a-dry on one of the few rocks strewn onto the sand patch by the almighty?
Just relying on the charts bottom indication seems not too clever.

Let's assume there are other boats at anchor too.
Do they all use huge swinging mooring circles?

How would you as a very experienced tidal area captain handle this?

One approach would be two anchors I guess.
Only anchoring in deep water is a thought but not the solution I am after.

Other thoughts?

Looking forward to them.

Many thanks & Merci!

Franziska

[cruisersfarm]

You can't be sure where your boat will settle and should find an alternative.

This is a very, very risky situation for your boat.

Find a better anchorage, dock/quay or any other alternative.

[Franziska]

I agree it's risky and going some better place is surely advisable.
Still I see many French boats doing it and wonder how they cut the risk down.

[Panope]

Arrive at low tide. Anchor temporarily and get to shore. Walk the area marking all hazards on handheld gps. Also mark nice places to set the anchor(s).

Steve

[skipmac]

I like Steve's suggestion but then what would you do if you arrive very late in the day and it isn't low tide?

If forced to anchor in the situation you describe and no other options I would set one or more alarms. A depth alarm to wake me while there is still sufficient water to maneuver and move if necessary. An alarm clock to wake me to shorten scope as the tide drops. An anchor alarm to wake me if the boat swings too far.

[cruisersfarm]

Originally Posted by Franziska

I agree it's risky and going some better place is surely advisable.
Still I see many French boats doing it and wonder how they cut the risk down.

Are they very stout steel boats?

Maybe a LOT of local knowledge?

To me, this is an unacceptable risk for damaging the boat.

If it's the kind of bottom I'm picturing from the northern usa in the same type of latitude, there are huge, sharp rocks strewn randomly across the whole anchorage, a boat length apart at maximum.

If this is the case, the people are nuts, ha ha ha

If there are clusters of good/bad areas, I'd say local knowledge and observation at low tide before you venture in.

[Cheechako]

I used to belong to a boat club for a popular 21 footer. One member took the boat to Alaska. The tide changes are huge there too. He had a picture of his boat high and dry balanced on top of a big boulder maybe 6 ft off the bottom surrounding it.. That's where it settled at low tide!

[double u]

I'd NEVER EVER dry out voluntarily in a spot I haven't inspected thoroughly before! to me it's playing russian roulette with the boat!

[skipmac]

Originally Posted by double u

I'd NEVER EVER dry out voluntarily in a spot I haven't inspected thoroughly before! to me it's playing russian roulette with the boat!

That's my feeling on the subject as well.

[Franziska]

Well in the sands of the Dutch and German Waddenzee that's normally no problem.
Still I wonder what the folks in Brittany and Normandy do.
Anyone around?

www.ladyrover.jimdo.com

[skipmac]

Originally Posted by Franziska

Well in the sands of the Dutch and German Waddenzee that's normally no problem.
Still I wonder what the folks in Brittany and Normandy do.
Anyone around?

Welcome to the Woods Mira 35 - Catamaran Lady Rovers Website!

Even in an area that is know to have sand or other soft bottom isn't there still a chance of debris on the bottom or even the occasional rock? Even if there is nothing hard on the bottom a dip, hole or uneven spot could put a strain on a cat if not level and the weight all sitting on one hull or one area causing a twist to the structure.

I would be very reluctant to let a boat sit on any bottom that I could not see or inspect in some way first.

[Franziska]

Fair enough, still having been local in the Waddenzee for the last three years I never saw someone with issues arising.

I fully agree though beeing cautious is good and Brittany is not the Waddenzee.

www.ladyrover.jimdo.com

[skipmac]

Originally Posted by Franziska

Fair enough, still having been local in the Waddenzee for the last three years I never saw someone with issues arising.

I fully agree though beeing cautious is good and Brittany is not the Waddenzee.

Welcome to the Woods Mira 35 - Catamaran Lady Rovers Website!

And I've never been on the Waddenzee so I confess to giving advice on something I know nothing at all about (but hey, it is the internet so that's allowed, right?).

However, even places I know well and know to have nice, flat sandy bottoms I would be cautious about what might be underneath.

[Pete7]

I think you are over analysing this. Sure there are big tides which are great for cruising if you plan to use them. 12 knots through the swinge in 10 knots of wind really knocks the miles off a journey.

As to anchoring whilst there are areas that dry out, equally due to the high tidal range the French have developed harbours were either it doesn't matter because they are deep water like Cherborg or then have taken it into account and built a cill. St Malo and Piriac Sur Mer are two that spring to mind. Elsewhere the rivers Rance and La Villaine have been dammed with locks.

Places like Sauzon on Belle Elle dries but it is large flat pebbles on the bottom were all the local boats dry each tide. Isle De Chausey is probably the most challenging yet the channel is marked and the water so clear you can see the rocks against the sand.

Alderney is interesting, you are not allowed to dry out on the beach in Braye Harbour. Instead we anchor just off the beach in 7m of water in a patch of sand 50m across. Big boats with deep draft keeps can't get in close but we can. Longis Bay on the French side of Alderney is a recognised anchorage. Folk tend to anchor just off the beach but out of the tide through the race and dinghy ashore.

Shell beach on Herm is one you must dry out on, its 5miles of sand so flat you could play boules on it.

Also unlike the South Pacific, charts and GPS align remarkably well and are accurate. These coasts are a well trodden path so just follow what the locals do and enjoy a superb cruising area.

This is Sauzon and Chausey.

[Franziska]

Hey Pete,

awesome just the sort of info I am after.

Thanks.

www.ladyrover.jimdo.com