Mooring block weight suggestion.

[wallacebob]

The crucial factor is the sea or river bed. If mud, then the suction of the mooring is improved; we use lorry tyres/wheels filled with stone/concrete, dug well into mud. They don’t shift . A simple block or anchor will drag if the weight is insufficient. Tidal range is other factor; mooring can be lifted by boat movement at high tides.
Bradney guide to mooring and anchoring is useful

 

[Neeves]

Using an anchor, or 3 (set opposing each other) seems an easy option - but as they settle into the seabed they become sufficiently buried as impossible to retrieve using amateur kit. Danforth would be the anchors of choice as they lie flat when set.

Mind you lifting a half ton block (or engine casting) well settled into the mud would hardly be a bundle of laughs to retrieve either.

You will need to access the chain and any swivel as they will abrade.

It might need a bit more thought to design so as to be user friendly but a series of strings of redundant CQRs, of which there must be many, set in 3 opposing strings might offer both the hold and be easier to retrieve for servicing.

It is unlikely anything you design would be accepted by an insurance company. and as servicing will be necessary and something of a nightmare - paying a professional looks a very sensible option.

Jonathan

 

[William_H]

To answer the initial question my little boat in almost identical conditions has a 250kg railway wheel as mass. Mostly buried. Installed by professionals. (mandatory here including 2 yearly inspection) Another professional at one stage reckoned it was marginal but to me no signs of movement.
However I would be keen on DIY if it was permitted. It all depends on you capabilities to handle the mass. Both to the waters edge and to location. I would find a single mass heavy enough just too hard to manage. Far better for DIY is the heavy chain with riser from the centre and anchor at each end. Anchor can be fabricated fisherman style with one fluke or anything else you can find. Far better again is another chain laid at right angles to the first with another 2 anchors. This means that no matter what direction of pull anchors do not turn or move. Now all this can be laid from above but far better if you can get in the water (dive) to get anchors set and correctly located. To set from above you need a longer anchor to anchor chain and accept that you may not get it taught. So centre point may move a little.
As always heaviest chain is best and inspection is vital at least every 2 years preferably each year. ol'will

 

[Rappey]

A pyramid block is preferred as a rectangular or square block runs the risk of the chain wrapping around the block.
Anchors run the risk of a fishing vessel towing gear and snagging the anchor, pulling it out.
I think its a 43% weight loss for concrete in sea water.
Steel loses 13%.
A tidal mooring is easier to deal with as it can be drug in at low water.

 

[geem]

A 1m3 block of concrete weighs 2.5t approximately. In water it weighs only 1.5t as it displaces 1m3 of water so this comes off the weight of the concrete

 

[srm]

So back to plan B. Back to oversize anchor and trust it resets as tide turns.
Everything suggested is pretty much outside my capability to lift, transport and set.

Two anchors on a heavy chain span aligned with tide flow is an established mooring technique with a riser from the middle of the span. Avoids the resetting problem. As suggested danforth or other anchors without sticking up bits would be preferable. For a boat of your size its probably possible to lay the mooring from a dinghy if you have enough power to keep the ground span straight with a slip rope on the crown of one of the anchors.

 

[Daydream believer]

Engineers work on 143 lbs/ft3 for reinforced concrete.( Or they did when I was a student doing engineering) 1 ft3 water weighs 62.41 lbs. Therefore, 1 ft3 concrete in water weighs 143-62.41= (say) 82 lbs/ft3 * 35.31 (ft3/M3) = 2895 lbs = 1.313 Tonnes

We place sinkers up to 2.5 tonnes but find them totally unnecessary for most of our boats. here is a picture of some sinkers on the beach waiting for the tackle to be refurbished. They are for boats 24-28ft on shingle/mud bottom in an exposed area with tides up to 2kts. We have a 40 ft boat which has a sinker 1.6 metres diam * 460mm deep & has not dragged over 5-6 years. It is on the most exposed site. We have a lot of sinkers identical to the nearest on in the picture & this is used for boats up to 30 ft.They are placed & left for 14 days before a boat is placed on them so they settle a bit first.

 

[fisherman]

Big engine flywheel from a scrapyard for me. We had it lifted, in the van, rolled out low water, lifted it under the boat. The big flywheel on a Ford Dover weighs 600lb, it was an optional extra on a Sabre.
Lot round here use a block of granite with a large bar through a hole.

 

[Stemar]

Anything that doesn't lie flat is OK if they're buried in the mud, but I wouldn't want it on a firm bottom. It would take a rare combination of wind and tide plus a good helping of bad luck, but you don't want your boat to land on something like this

when the tide goes out.

My mooring's on gloopy mud, and is a railway wheel sunk about 4' deep. As long as the chain's in good nick, it ain't going anywhere.

 

[Daydream believer]

My mooring's on gloopy mud, and is a railway wheel sunk about 4' deep. As long as the chain's in good nick, it ain't going anywhere.

How did you get it 4 ft in the mud & how do you service the tackle.

 

[Farmer Piles]

We are blessed with granite quarries near us in Falmouth which does make the job easy. I have an inner mooring which is in very similar conditions to those you describe A 400kg granite block with 2m of 25mm chain and a 12mm chain riser. Our harbourmaster rates that for up to a 22ft boat.., It is sat in shallow mud.
Are there no quarries inland from you that you could get a lorry and hyab to deliver it to the waterfront?

 

[Stemar]

How did you get it 4 ft in the mud & how do you service the tackle.

I didn't. The previous owner of the mooring carried it out under his boat, waited for low tide, then got VERY muddy

As for servicing, only the top of the riser wears, the rest is buried in anaerobic mud, so has a long life - best part of 20 years so far. They guy who services my mooring (the club Commodore!) digs down to a shackle in the mud when the top bit of riser needs changing.

 

[Quiddle]

Two anchors on a heavy chain span aligned with tide flow is an established mooring technique with a riser from the middle of the span. Avoids the resetting problem. As suggested danforth or other anchors without sticking up bits would be preferable. For a boat of your size its probably possible to lay the mooring from a dinghy if you have enough power to keep the ground span straight with a slip rope on the crown of one of the anchors.

That's what I used for a 2.2t, 27ft boat in an exposed tidal estuary. 25kg Fortress both ends of 30m of 16mm long link ground chain aligned with tide and weather. 10m 12mm shortlink riser in in the middle. As recommended by Gaelforce marine and it lasted 5 seasons until I sold the boat. The anchors do sink though my neighbour recovered his by lashing a 10l drum to the groundchain, which was submerged on every tide, gradually easing the anchor out.

 

[William_H]

I didn't. The previous owner of the mooring carried it out under his boat, waited for low tide, then got VERY muddy

As for servicing, only the top of the riser wears, the rest is buried in anaerobic mud, so has a long life - best part of 20 years so far. They guy who services my mooring (the club Commodore!) digs down to a shackle in the mud when the top bit of riser needs changing.

Be very careful. IMHO it is the movement of the chain that causes wear not because it is anaerobic. (might be wrong) Being burried in mud may mean not much movement of course. So heavy chain that does not move much will last a long time but riser chain constantly being tugged on wears rapidly. ol'will

 

[Daydream believer]

In our area we would not allow mooring holders to use 12mm chain as risers. It would not last more than 18 months & the boats would be on a necklace. The sinkers are lifted every 3 years & normally the 19mm riser chain needs replacing as it would not last another 3 years.
It usually gets donated to the club for the race marks. These are lifted annually & we can see from the wear that it would not be advisable to use as a yacht mooring for another term. We use 16mm on the top 1.5 metres & this definately only lasts 3 years at best.

 

[Stemar]

Be very careful. IMHO it is the movement of the chain that causes wear not because it is anaerobic. (might be wrong) Being burried in mud may mean not much movement of course. So heavy chain that does not move much will last a long time but riser chain constantly being tugged on wears rapidly. ol'will

I am careful. I get the mooring checked professionally every year or two. It used to be the chain strop that wore quickest, but I've switched to twin polysteel strops, which seem to last better, but I keep a good eye on them and the top of the riser, which is the bit that wears quickest now.

 

[andsarkit]

The riser definitely rusts more at the top where there is more oxygen in the water. I had some success using a zinc anode attached near the top of the riser. MG Duff used to sell a chunk of zinc moulded onto a short length of chain for this purpose. I didn't want to break the riser and insert the anode so I just shackled it to the outside of the riser. This seemed to work well and definitely extended the life of the riser.
I suppose you could also use a collection of used hull anodes that you can usually find discarded in any boatyard.

 

[fisherman]

Be very careful. IMHO it is the movement of the chain that causes wear not because it is anaerobic. (might be wrong) Being burried in mud may mean not much movement of course. So heavy chain that does not move much will last a long time but riser chain constantly being tugged on wears rapidly. ol'will

My mooring here was fore and aft. Outer harbour at Porthleven. 1m to 6m. Either end would hold the boat, and no swivels to fail. Also the the whole thing stayed on the bottom, the risers were 25mm and 20mm dia Nylon, pickup a small ball on 10mm leaded. It lasted years, where others had a light chain riser, and would wear out in one summer season as they were up and down twice daily and jangled in the exposed location. But they did what was traditional. I used to buy scrap risers from Falmouth Harbour, always with a middle section worn out, ends good, (just bundled up for anchor weights on gear). So if possible I would leave any mooring on the bottom and have a light rope pickup. Not always possible, obs with bigger kit.