mooring weight

PabloPicasso

PabloPicasso

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How heavy should a swinging mooring sinker be? Is there an accepted ratio of sinker weight to boat weight?

Boat is 3.5 to 4 tons, 27ft sailing boat iin a well proteceted tidal river/harbour.
 
it's not just the mass of the sinker; some shapes (e.g. old railway wheels) seem to dig into the gorund better than e.g. chunks of concrete.

Local knowledge is your first port of call. As SM suggests.
 
Local knowledge is needed. There are too many variables for any general rule of thumb. What works safely in soft mud may not hold at all in shingle, or sand, or even in clay. Is the bottom subject to scouring? The river may be 'sheltered' but is there a wind direction which can cause problems? Many even quite sheltered moorings have 'dangerous' quarter, where a particular slant of wind causes problems.

I used to moor in an estuary on the West coast which gave near perfect shelter - except in strong Easterly winds which 'funneled' down the vally in huge gusts which would rapidly find out any mooring tackle under par - usually with the loss of the attached boat.

Find out what others use.
 
We tried using railway wheels with a 4' stock but settled for purpose cast mushroom anchors, the main reason for using purpose cast mushroom anchors was that they dug in and laid down on the seabed whereas the railway wheel could stand up and in one instance in shallow water, punched a hole in the bottom of the boat, We found that just looping a chain through the centre of the wheel could slide along the bottom and the stud link chain we used had a very short life. We had reasonable success with concrete sinkers for smaller craft, these were pyramid shaped with a domed bottom (an old dustbin lid makes a good former) and an eye formed in 3/4" bar with the tails turned out and buried in the base of the concrete.
This was our findings on a sandy /gravel type bottom, where you are mooring could be totally different, local knowledge is worth its weight in gold!!
 
For 28' I have 1.5 ton granite block with 2 lengths heavy ground chain shackled side by side. 5/8 riser chain. All that weight give a good spring. In a gale it is less likely to snatch directly onto block. Estuary mooring open to the east about 1 mile fetch.
 
My 34ft boat is moored on the upper Tamar on a swinging mooring I built. The block was cast using a large tractor tyre and about 1100kg of aggragate. I made sure that the base was cast slightly concave. The chain to the low water level (2 meters) is ships cable and therefore quite weighty. It is cast into the sinker with about 6 lengths of 10mm reinforcing bar criss crossed through from one side of the tyre to the other. I guess the total mass is about 2 tonnes for a 7 ton boat.

yoda
 
Mine is about 400 Kg cast into a truncated pyramid with a flat bottom, 42 inches square. It rests on clay/shingle with a covering of about 6 inches of mud. It has held a 28 foot boat every summer since 2006 in a sheltered creek. When I had to move it a couple of years ago, it offered good resistance to the 50 foot fishing boat which did the job! The mooring design was recommended to me by the local boatyard that has it's own moorings elsewhere in the creek.

Neil
 
I would advocate that you manufacture anchors out of iron. These can be a lot easier to position and can be designed to dig in to the bottom so give a lot more holding for the weight. You do need some heavy chain to join the anchors (preferably 4) or even a small weight at the bottom of the riser to act like an anchor angel.
For design of anchors all you need do is copy the fishermans anchor style. You only need one fluke and a cross member to keep it facing down. A piece of girder or railway line with a fluke welded on would be ideal. The idea being that if it drags it pulls itself further under the sand. It does help to use divers or low tide to place anchors correctly.
In my case I surveyed the area under water and found some suitable large weights. These were connected together by chain to give me a mooring in the correct place. You might be lucky.
In the end you need a mooring that leaves you feeling confident in a storm. good luck olewill
 
Some approximate sums

It's possibly worth doing a 'back of fag packet' type calculation as a starting point before seeking local knowledge. Start with the worst case of a single sinker, reliant solely on weight, ie just a block not an anchor, on hard sand or rock, ie not dug in at all.

A riser chain of 1.4 to 1.5 x depth at HWS is a normal rule of thumb, so when fully extended in a storm it's at around 45°.

Then the horizontal force the block can take is its weight in water, M, less the upwards pull, Fy, times the coefficient of friction, which we can assume is around 1 for concrete on rock. This says that the maximum horizontal force Fx < M - Fy. Due to the riser being at 45°, Fx = Fy, and so we can rearrange this to give:

M > 2 Fx

Ie the sinker must weigh twice the maximum horizontal pull. A 27' boat will have a horizontal pull around 425kg in 50kts of wind (data on this is scarce: the ABYC guidelines say 750kg but measured data I have, and also that on Alain Fraysse's web site, both suggest that's pessimistic and 425kg is about right for a 27' monohull in 50kts of wind).

So you need around 850kg of weight. It has to be that in the water 'tho, so in air needs to be more: about 70% more if pure concrete and about 14% more if pure iron.

Putting it all together says that ~1 tonne of iron loaded concrete will be around what you need.

Two sinkers joined by a chain would mean the total weight could be almost halved.

Using anchors like William_H suggests mean that only 2 x 35kg anchors would do (see Bradney Chain's web site for instance).

However I suspect that what really saves most situations is that the sinkers don't just sink through water but into the mud as well, achieving a much bigger effective coefficient of friction, maybe 3 or so, which now gives M > 4/3 Fx. It's this factor which is so dependent on local conditions and thus local knowledge.
 
2 train wheels with a big chunky ground chain will be pleanty especially if it is soft ground i am using one wheel for my 1800kg motor boat and have tried to move/test it with a fair bit of power but it wont budge.
 
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