Anchor Dragging in Rivers

[peasea]

I often anchor overnight in West Country rivers, using a plough and all chain -- say 20 mtrs in 4 mtrs. depth depending on swinging room etc. -- but despite motoring the anchor well in going astern when laying the anchor, we have sometimes dragged later when the tide has turned.
Usually in the middle of the night when it is p****** down. Do other folks have this problem. ? Is it worth laying a kedge astern to stop the swing.?

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[qsiv]

It is a fact of life when anchoring. Personally, I dont mind getting up to babysit the turn of tide if required - but SWMBO doesnt agree.

I wouldnt consider laying out a stern anchor - it would need to be as big or bigger than your main - and youd have to be careful of the loads on the rudder and steering gear.

If you want to do it 'properly', then you could consider mooring (two anchors from the bow) - but again I wouldnt be that keen, as I dont carry a suitably rated swivel, and I have once, just once, seen the tangle that can ensue without a swivel. I worked at the local sailing school, and we spent a merry half hour towing this poor chap round in circles unravelling the twists he had generated in his cables. If it's just for one tide, then you might risk it.

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[LadyInBed]

River mud is very soft and fine. I reckon you will have to use heavier tackle and spend more energy digging it in.
If you are on a river and not in an estury, then putting a kedge out would be worth a try, but again it needs to be reasonably hefty.
The other alternative is to get further up the river out of the tidal effect/forums/images/icons/wink.gif.

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[Mirelle]

Tidal currents in shoal water

The force exerted on the hull by the tide in shoal water is much greater than in deeper water. This is particularly true for shoal draft boats, but all boats are affected. I have figures for ships where the increase in loading is some three fold. This might offer some sort of explanation.

A further thought - I know this sounds silly, but are you sure that you are actually dragging, rather than merely ranging the chain out over the bottom in the opposite direction - the noise can sound very similar!

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[ean_p]

Re: Tidal currents in shoal water

Whats the cause of this effect Mirelle ?....intuativly (?!!) you'd expect it to be the same .....all things except depth been equal, and is the effect graduated inversly to depth or is there no direct realtionship ?........

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[Jacket]

Re: Tidal currents in shoal water

At a guess, for a boat that's relatively wide compared to its depth, the effect if due to the hull forcing surface water underneath it. If you're anchored in 30 feet of water, the fact that your hull (lets say it has a depth of 2 foot) is forcing the top 2 feet of water underneath it. So the 28 feet depth under your boat now has 30 foots worth of water travelling through it. No big deal. But say now that you're anchored in 6 feet of water. The hull is now forcing 6 foots worth of water through a gap 4 feet deep. As water is incompressible, it means that the water is forced to speed up by 50%. The increased force on the hull is a result of the hull forcing the water to increase velocity.

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[hylas]

This is a very well known phenomena..

Most anchors when pulled 180° from the initial setting.. break out and sometime never reset...

On the test done by the US magazine "Practical sailor" only two anchors re-align themselves without breaking free: the US model Super MAX and the SPADE..
(www.spade-ancor.com and www.Oceane-anchor.com)

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[AndrewB]

If you are anchoring in 4m charted with maybe 3m of tidal range and a typical strongish tide stream then 20m chain scope is a bit minimal, I'd normally put out nearer 30m.

Without engaging in the 'which anchor is best' debate, I've found plough types benefit from decent weight for an efficient re-set. Is your anchor big enough?

Fore and aft anchoring works badly in my experience. It leaves you vulnerable to a side wind. However, I've used a 'Bahamian' moor with success in rivers. This is two anchors both set from the bows, one upstream, the other downstream ready for the change of tide. It's good if the anchorage is tight and you haven't room to swing much. Don't use it among other yachts anchored on a single anchor though.

Mirelle is quite right, the characteristic rumbling as you go round sounds just like dragging. I normally expect to be wakened by this and do a brief anchor watch on the turn of tide.