Mooring block weight suggestion.

[greeny]

I'm considering putting a mooring block down for my small day sail boat. 21 ft, 1.4 tonne. lift keel. In the river 1 to 2 knots tide flow. 20 to 30 knts wind max. water depth 8 to 15 ft. Only used in the summer. Not used in winter. I've got a lot of construction work going on so considering casting a concrete weight block for the mooring. Anyone got any idea what weight would be the minimum for the block.

 

[Neeves]

I was under the impression rail wagon wheels were the de rigour 'block' in the UK.

Jonathan

 

[earlybird]

My experience of self-laid moorings in a tidal estuary suggests that concrete isn't the best material unless buried in a fairly deep excavation. It's not that dense, so "loses" weight in water and currents tend to scour it out. Cast-iron, perhaps in the form of a fairly large engine block, is better. As for weight, I'd suggest around 100Kg minimum and heavy chain, esp if the current reverses with the tide. Where I was, chain was eroded very quickly by the sandy water.
If the mooring dries, there's a risk of the boat settling on an exposed weight.
Edit Neeves suggestion is good if you can get them.

 

[Graham376]

Presumably only intended for use by you but what about uninvited visitors with far larger boats using and dragging it? Our concrete block is 2.5 tons dry, just over half that weight wet.

 

[Wansworth]

As mentioned concrete almost floats in sea water,made a concrete keel for a boat and it floated a good two inched higher

 

[rogerthebodger]

If you use steel punching in place of the stone this makes a much more dense concrete.

The balast in my steel boat is steel punching set in tar in mt hollow keel

 

[Baltika_no_9]

I was under the impression rail wagon wheels were the de rigour 'block' in the UK.

Jonathan

May not be de rigueur but would certainly provide the rigour required.

 

[srm]

Add scrap iron/steel to the casting to improve density. Make the block streamlined with much greater diameter than height and a domed top. A concave base will improve suction in the sediment. There is a risk with simple block moorings that the boat can drag it in a series of short jerks.

I had a larger boat and used fabricated anchors intended for salmon farm moorings so can not advise on weight of a concrete block.

 

[chris-s]

Our previous boat, a 23ft Pegasus 700 on a swing mooring in an exposed location in the river Fal had a 3/4 ton granite block, held her fine even in a few rough southerly blow.

Our new boat, same sort of location 32ft 4 ton Beneteau has 1 & 3/4 tonne split between two granite blocks.

As mentioned above, concrete isn’t great as it is not as dense as you might think.


Hth

Chris

 

[Poignard]

Austin 7 cylinder block?

 

[Tranona]

Stuart Turner better as it is "marine".

 

[neil_s]

For a 28 foot boat in a similar situation, I have a mooring clump cast in concrete that weighs around 400 Kg. It has steel reinforcement and added iron ballast in the form of scrap chain. Make it a low truncated pyramid shape for best holding and minimum damage if your boat sits on it. Allow scope of 3:1. Most of my clump has disappeared into the mud, only the flat top showing. I still have and use the mooring - chain has been replaced several times.

 

[andsarkit]

I once made a mooring from two 5m lengths of I beam welded into a cross. Probable weight was about 200kg.
It held a 35ft boat in quite windy conditions but was not used for long enough for a real test.

 

[greeny]

Austin 7 cylinder block?

Can't find one in Portugal .

 

[greeny]

I was under the impression rail wagon wheels were the de rigour 'block' in the UK.

Jonathan

Fortunately not in UK. lol

 

[Poignard]

Can't find one in Portugal .

 

[greeny]

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.

 

[greeny]

Woops, are we going into another anchor thread. OMG. Nooooooo!

 

[langstonelayabout]

Our sailing club uses cast reinforced concrete weights for moorings. Each weight is cast ashore and weighs approximately 3 tonnes. The weight is installed so that the top surface is about 1metre below the surface of the mud bottom. They usually work well but have sometimes surfaced after a number of years.

 

[andsarkit]

When I had a mooring on the Exe I used two anchors joined by a heavy ground chain with a riser chain at the centre. The anchors were welded up locally with no fluke projecting upwards to damage the boat at low tide. If laid in line with the tide the anchors take the strain in turn with no need to reset and the ground chain lifts off the seabed in rough weather to reduce shock loadings.
The ground chain was about 25m of 3/4" chain which could be carried in the dinghy. I laid it all at low spring tide walking alongside the dinghy in about 4ft of water paying out the chain as I went. Quite easy to do as you only have to handle about 10kg of chain at a time. You could do it all from the dinghy if the water is deeper but you need to check that the anchors are properly set.
This setup lasted for 10 years for a 8m 2.6 ton boat in a quite exposed location with negligible wear on the ground chain once it sank into the mud. The riser was replaced a couple of times.
With a smaller boat you could probably use a couple of Danforth anchors and 10mm ground chain for fairly moderate cost.
The only problem now is that insurers may require a 'qualified' and insured person to lay and check the mooring.