Laying a mooring

[Scotty_Tradewind]

oops wrong posting....

 

[Allan]

Many thanks for that. The mooring in question is not drying, so it is not possible to dig it in at low water.
Allan

 

[lw395]

cos a 1 ton block is only "worth" 600-700kg underwater, our boat is 3 tons, and some others have moved in the bay on concrete blocks... tayvallich, sheltered for all except easterly gale, when waves do come in. but for the 30 years or so we been there, concrete would be ok for the summer.

as per above also, probably cheaper getting two anchors of your choice and laying them, and far easier to handle from your own boat.

we also have doubled the riser, so we can rotate it, cos almost all of the corrosion is in the top few feet, so easy to change riser. but not all anchorages will be like this.

When waves come in, the inertia of the block matters, as well as its buoyant weight. The reduced 600kgf of a 1 ton block is orders of magnitude more than the resistance claimed by Mr Rocna and friends. Either the friction is very low or something else is happening. Could be round 'blocks' rolling?
A 2T concrete block often takes 5T or more on Baker-Trayte's crane before it will move.
A pair of anchors on a chain can be moved relatively easily by a wind at right angles to the chain.
Few of us are able to use steel weights of the same mass as can easily/cheaply be done in concrete.
The strongest winds in memory in Portsmouth were in October (97). This may be considered to be part of the 'afloat' season. In such conditions I would have more faith in the mooring than the tender care of certain boatyards.
A lot of things to consider.
There is a lot to be said for following local practice, if it has evolved for the circumstances you are in. But beware of secondhand advice from people who have not actually got their hands dirty.
Whatever you have, it needs checking, a diver is often best.
At the end of the day, the professionals turned out to be good value in my experience.

 

[craigsmith]

What does the "600 kgf of a 1 ton block" have to do with its holding power? Nothing directly, you're only talking about weight, and besides, "the resistance claimed by Mr Rocna" reaches 600 kgf for a very small anchor indeed

If a 2 t concrete block takes 5 t to move via crane, then thats a weight-to-resistance ratio of 2.5 - hardly very efficient compared to any proper steel anchor. Three over-sized anchors rigged in a tri-linear pattern make an extremely effective mooring which is re-useable and considerably lighter and easier to both deploy and move at a later point than heavy alternatives.

 

[lw395]

What does the "600 kgf of a 1 ton block" have to do with its holding power? Nothing directly, you're only talking about weight, and besides, "the resistance claimed by Mr Rocna" reaches 600 kgf for a very small anchor indeed

If a 2 t concrete block takes 5 t to move via crane, then thats a weight-to-resistance ratio of 2.5 - hardly very efficient compared to any proper steel anchor. Three over-sized anchors rigged in a tri-linear pattern make an extremely effective mooring which is re-useable and considerably lighter and easier to both deploy and move at a later point than heavy alternatives.

Quite, Craig, that's why I would have a decent anchor in the locker of a yacht.

The 2.5x factor is for a vertical pull to lift a mooring block. It varies. Quite often the chain breaks before the block moves.
The lateral force to move it will depend on what cross-section area of seabed it is taking with it, it will have a large cross section compared to most anchors, provided it sinks into the bottom at least partly.
The full mass of the concrete also adds inertia, which is useful when you have a heavy boat jerking the chain in heavy weather.
Laying moorings using anchors is probably more 'efficient' than concrete blocks, but is not trivial if you have a small space that you are licensed to use and require strength in all directions. Moorings are usually on a much shorter scope than anchors need for their holding power to be good.
A concrete block is very quick to lay in the exact place you want it, minimising contractor costs. Personally I would generally be happy with a concrete block, provided I knew it was generous in size and had sunk into the seabed.
A system of anchors is going to take a lot more time to lay, which is OK if it's not time you are paying for. There is more to go wrong and I would generally only be happy if I (or a trusted diver) had seen it in situ on the bottom.
You have to be sure that all the anchors are properly located relative to each other, and properly set. Not trivial to see in a harbour with 1metre vis on a good day. The multiple anchors are also more at risk from interference as the cover a bigger area of seabed. The cost of inspection is also higher because of the greater amount of tackle involved. In general it must have more chain etc down there, so the annualised cost of chain corrosion will be more.
If I were starting again in Portsmouth harbour, personally I would want a square concrete block of about half the mass of the boat, then everything else designed to be serviced by diving. Less would be OK in a less exposed mooring.
I appreciate that in some harbours it is different because there are no contractors who can lay such clumps economically.
Another useful source of info is the gaelforce website iirc.
What you really want is a rock bottom and some rawlplugs

 

[cliffordpope]

How do you inspect a mooring that has sunk deep into the mud?

 

[lw395]

How do you inspect a mooring that has sunk deep into the mud?

If it's a concrete block mooring, you either:
a) get mooring barge to apply 3T of lift to it and declare it ok if it does not break
b) haul it out with a barge.
c) dive on it and dig down until you can see the top of the block, checking that the ring in the block is not corroded too much. Then check all of the chain, shackles etc. If the mooring has mostly had a boat on it, the chain will generally move around enough to keep the top of the block fairly accessible.
It may be easiest to change everything above the block, or just the riser, then inspect in detail ashore and re-use the good bits next time.

You adopt method a) at your risk! A bigger or smaller force may be right for you.

If it's a system of anchors, I think you need to lift the lot, maybe you can lift one leg and convince everybody that all the other bits will be in equally good nick. Maybe not!

What do others do?

 

[bikedaft]

dive and inspect (steel mushrooms/chain/shackles/riser)

or if too deep in mud to feel/find add another mooring down the (doubled) riser. until there are too many mushrooms off the riser to rotate it, then start again. sounds drastic but it ends up costing about £400 every 3 years for a new mooring, and changing the riser every 2nd year. and peace of mind is worth a lot

 

[jason -and the arguenauts]

best advice is not to ask on here but to ask round the clubs local to your area. for example, both portishead cruising club and newport uskmouth sailing club have lots of experience of what works in the bristol channel.

moorings are one of those things where what works in one area wont in another