mooring design

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tico

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Got the opportunity now for the spot to lay a mooring. Any suggestions as to design, lengths/weights of ground chains, anchors or sinker - how big etc etc
as a hint, max depth 10m tidal range 7m, boat 30' mobo weight 5 tonnes approx.
Help or design references appreciated.
Thx
 
Quote

'How the hell do you lift and move this kind of thing?'
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local moorings man has a small trawler type vessel, he just cranes one anchor in, motors a bit then cranes in the rest of the kit.

took it home in a 1ton army trailer on my landy

You need such kit in Holyhead, that why I'm in portugal /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif
 
Next time round I will use two oversize Danforth and 30mm chain with 12mm chain riser of course to buoy that has a thick strop through centre and eye each end with swivel
 
Building a mooring is a matter of what you can get cheaply and what you can manage to0 lift or manouvre yourself or with help.
The design starts with chain. As heavy as possible to give long life.
About 20 metres minumum of 1 inch or so chain is laid out with a shackle to the middle going up to the boat in chain perhaps lighter but long enough to reach the boat or buoy at highest tides plus waves and then a bit.
On the ends of the laid out chain you attach whatever weight you can get. Concrete is cheap but heavy to get into place. Anchors are best but expensive unless you make your own one fluke mooring anchors. If you have light weights or small anchors then you attach more horizontal chain in different directions with more weights. More weights in different directions will more precisely locate the mooring in various wind and tide pulls and make the whole mooring more reliable. Railway wheels or big gear wheels are often used.
I understand it is common in UK to lift moorings for maintenance where here it is more common to do maintenance in water replacing worn chain and shackles. The way you do maintenace might also dictate how you lay the mooring ie with a barge and crane or divers or both. I prefer diving and adding bits as necessary.
Don't forget a swivel shackle in the riser chain and a buoy able to support the weight of the riser chain. I prefer several ropes from the chain to the boat. good lcuk olewill
 
The ground tackle used for a mooring depends on the local conditions.
The first thing you must do id find what works in your mooring area.

My local conditions are, Depth max about 8 metres, Tidal range 6 Metres max, exposed conditions, muddy bottom, 33ft loa, weighing about 5 tons.

I do not clain to be an expert but this is what I have.

Ground tackle.

Two concrete filled lorry tyres with concave bases. 40mm rebar crossbraces inside each tyre.

Ground chain 44mm, 7metres long, weighing about 300Kg.
The end link of the chain is passed through the rebar in one tyre and welded to it.
A meter from the other end of the chain is joined to the other tyre in a similar manner.

Through time the two tyres and all the ground chaon bury themselves well under the muddy sea bed.

The riser

This is made in two parts. The first part is two metres of 25mm chain which is secured to the tail on the tyres using a very heavy marine shackle with the pin welded. This is really an extension of the ground chain but it allows access to to the shackle between it and the main riser at reasonably low tides.

The riser itself is 16mm chain and is 20 metres long.

This riser is inspected anually and at least every other year the top 5-8 metres needs replacing.

The top if the riser is connected to the mooring buoy with a rope grommet. The rope grommet has a galvanised sweged loop as it's core.

Pickup

A smaller chain is shackled near the top of the riser, this chain has a plastic sheath and a rope line to the pickup buoy.

My ground tackle chain is a bit of an overkill but it is just what was available fron the local scrapyard at the time.

As I said at the begining you must find out what works in your particular location before deciding what to do.

Cheers

Iain
 
I'd go for 1 1/2 inch ground chain if you can get it. Much heavier and will last longer especially as you will no doubt have lots of tide with a 7m tidal range. You will be surprised at how much wear is put on the tackle, at the very least beef up the ground chain either side of the riser where most wear will occur.
 
I regularly dive to check moorings in ramsey Sound, West Wales, with a similar tidal range. A couple of things I have noticed is the rate of corrosion. Even stainless seizing wire lasts no more than a couple of seasons. The odd thing I saw this week was some 5/8 riser chain, which was like new near the surface, but almost rusted through near the shackle to the ground chain. Seemed odd.

What we do with ours, is to remove the mooring buoy and drop the riser to the sea bed, with a rope to a small surface buoy over the winter, when the chain is likely to get the most wear. Of course, it's no problem for us to dive for it if it comes off. We've done this for the last three years and it has saved a hell of a lot of wear on the riser. Not an option if your boat is moored all year, of course.
 
Most people will use whatever chain they can get for the least cost. Certainly no advantage using galvanised for ground chain and not even sure they make galvanised greater than 1/2". On my mooring it's all coloured chain, riser is 3/4" which shold last much longer than the previous 1/2" galvanised did.

With regard to the last post about wear at the bottom of the riser, I expect that's again down to the grinding affect of the seabed when that part of the riser is lying on the bottom.
 
Certainly is. However this is the first year in the last 10 I've got the professionals in, not up to handling 50 odd foot of 1 1/2 chain by myself!!
 
Hi Tico,
I have in the past trawled the internet and locally for correct information as to the design of a swinging mooring,I have asked at Plymouth university, Plymouth harbour masters, local mooring companies(who guess it!) using a single block of granite, to no avail nobody can come up with the basic formula.... anyway to cut a very long story short and i have driven everyone down the pub mad with "mooring blocks" I can offer you a solution to your problem if you visit the following link you should find a local council who obviously take boating very seriously and are prepared to lay down standards and I wish there were more areas in the UK that would do the same..

Regards Millbrook

www.ecan.govt.nz enter search "swing moorings" 16 page document acrobat file
 
Again, thanks to all, interesting site from NZ

Am leaning towards 3 equally spaced legs of around 5metres of heavy chain, with a single fluke anchor of around 20kg dug in at the end of each leg. Riser 15-20 metres of 12mm chain.

That way I can dive and lay myself.

bearing in mind that its mainly for lunch stops or overnight in calm conditions. No intention of leaving unattended for long periods.

Any comments?
 
The lifeboat mooring at St Davids has a similar design, but larger, obviously. I think the key is getting good tension on the three lengths, and where they join in the middle.

As a diver too, all you need to do is drop in and check after a few days, then a few weeks and after a blow. Seems fine to me.
 
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