MOORING LUMP

[JIM_TEAL]

I am casting a new concrete lump for my swinging mooring, using an old Truck tyre as a mould. It will hold approximately 0.3 cubic metre of concrete. According to my research that should give a weight of about 800 Kg. My mooring contractor says he dislikes concrete as it is "lighter in water" than steel/iron. Of course it is lighter. Surely the key element is the weight of the lump (concave base assumed)? It is for a 23ft Ketch with a displacement of 2400 LBS.
Is my estimate of weight about right? Will that be heavy enough using 3metre of 1inch ground chain in 5 metres max depth? Comments greatly appreciated.

 

[EdWingfield]

Looking at the scrap stockpile in Tyne Dock I was drooling at the sight of railway wheels - surely the ultimate in mooring lumps? Sadly, much to heavy to manhandle. I settled for 2 truck brake drums, dug them in to the mud and connected them with heavy chain. Luckily, I'm not scrutinised by a moorings Authority.

 

[Ecosse120]

I put mine in last week and think that digging them into the mud is as is important as overall weight.

 

[Viking]

Try it. you could always add another one by passing the mooring chain through the centre of the tyre/conrete. so it would drop on the exisitng one?

 

[dur]

I did one a few years back - used hardboard to make a circular mould. I have a feeling that it was about 75 cm diameter x 75 high which would make it about 1/3 of a cubic metre. It was v heavy but I think 400-500 kg not 800. The important bit is that the top surface was about 3' below the surface on the ground when planted. I have a pic somewhere which might give a better clue to the size. Caught a lift with a fishing boat to plant it. Nice big drum winch on the side

 

[Topcat47]

This is a simple question to ask but can be extremely complicated to answer fully.

The fundametals are straightforward. The net weight of your "anchor" is the gross weight, less the buoyancy provided by water. Thus as 800kgs of steel displace less water (much less) than 800kgs of concrete, the effective mass will be less. If your concrete weight has a 0.3 cubic meter volume and an 800kg mass, in water it's net weight will only be 500kg. But you actually bury your anchors in the mud. (I hope) so the effective weight will be even less. Of couse the mud is more viscous than water and will exert more drag.

The pull on the anchor is not verticle, and the greater bulk of the concrete anchor will give a little more lateral resistance on the seabed.

I've not laid a swinging mooring for many years, but the one I did lay had three concrete "anchors" connected by ground chains to a riser. We lashed the anchors underneath a tender and floated them aout one at a time, then dug them in at low tide (it wasn't a full tide mooring).

Hope this helps.

 

[Niander]

how about 4 car tyres [in a cross layout]buried in mud @ [low tide]
all connected by chain to a central riser...dya think that would be ok ?
thinking about it for my solution

 

[Bajansailor]

Rather than going to all the hassle of casting big lumps of concrete, and then worrying about how to get it out to the designated mooring location, and dropping it successfully - IMHO the simplest (and safest when it comes to laying it) solution is to instead use conventional anchor(s) a couple of sizes larger than what your bower anchor is.

 

[gardenshed]

Get the old counterweight from a fork lift truck. Smallest is typically 1.0t and the effect of it being immersed is far less than that on concrete.
I got two for €100 each and then chained them together with a riser chain from the middle. Given the irregular shape of these things, there is far less chance that they will pull out from the seabed as railway wheels can get pulled up to the surface and dragged along if the chain is attached from the centre and then around the outer rim.
HEre is a weight just about to be dropped into the sea. The chain is through the middle of it so no welds to be broken and a "hammerlock" used to join the chain (as used by trawlers to tow their nets).

 

[geoffatstanpit]

The mooring for my 1 tonne Parker 21 weighs less than 100 Kg. But it is 2 x 20Kg Danforth anchors 10m apart and dug in a metre or so down into the mud, linked by 15mm chain to a swivel and 15mm riser. What it lacks in weight I feel it gains in science. It hasn't moved when sundry "lumps" have dragged across the creek into the wall.

 

[hylass]

[ QUOTE ]
<span style="color:blue"> 23ft Ketch with a displacement of 2400 LBS. </span>


[/ QUOTE ]

For a 23 ft boat I will suggest a bloc of concrete of 810 kg or (0,324 cubic meter..)

Safe mooring,

Alain

 

[dur]

It depends also on where it is and what you are going to hang on it. This was mine. In the second (rubbish) pic you can see that the top link of chain is just at "ground" level. The ground is 6" of mud then a layer of chalk with clay underneath.

 

[GMac]

Steel weighs 87% of it's in the air weight when under water.

Concrete less then 1/2 it's in air weight. This can vary depending on how much spudding it got when poured. A 'spud' in this case is like the girlfriends vibrator, it buzzes away and draws all the air trapped in the concrete to to top hence the underwater weight will increase. Kicking or whacking with a hammer or similar when pouring the concrete sort of does the same thing, do it lots though.

Disk shaped with a concaved bottom will hold better then a lump. On bottoms like the photos above the vacuum will be large, this is a good thing. Also the lower profile will help at low tide, less hight for keel to bump if it's that close.

Whatever weight you make put reinforcing steel in it and leave a nice yet strong hole for the chain to past through. If you cast the chain into it when the chain runs out all you have is a lump of concrete and have to start all over again.

Train wheels rock big time. The average smaller spoked ones weigh around 220kg and are close to the best weight you can get. They have good places for the chain to go around and will last ages. A well sunk train wheel will hold similar to a well shaped 1000kg concrete block sometimes a lot more.

A few comments from someone who until very recently looked after 6000 odd moorings.

 

[smeaks]

great thread..... how do you sink the block in deeper water? where access for digging it in is not possible?

 

[dur]

My concrete lump seems to be fine for the location (up a creek some miles from the sea) and ground conditions etc. etc. I just copied what the local yard was doing but went a bit bigger but don't pretend it is the best solution. If there had been any old trains lying around I definitely would have had the wheels off them. The heavy chain shows very very little rot because it is always below the mud level and there's no air. The 3/4" riser chain has virtually no wear at the connection point for the same reason and because in all except v. high tides or windy conditions, doesn't take any strain.
But these lumps can be pulled out if the swivel on the mooring seizes. Then over time the chain is wound tighter and tighter until it is tight with every tide. Then some bad weather comes and the vertical jerking starts to work the lump upwards (assuming something on the boat doesn't break).

The mooring I replaced with the lump was three van wheels, filled with concrete and chained together. They were only just buried and came out by making fast on a short scope and then allowing the tide to do the work. This was over the bow of a 19' open boat.

I think the message is to make sure that the sinker is appropraite for your boat and the particular local conditions.

 

[Bajansailor]

I notice that everyone has studiously ignored the comments above (eg Geoff has 2 x 20 kg Danforths for his boat) re using anchors rather than 'lumps' for moorings.......

I would go for science, ie anchors, any time rather than lumps, especially if I was doing the job myself.

 

[Jonny_H]

But obviously ... you would use a Rocna wouldn't you!! /forums/images/graemlins/laugh.gif

Which anchor would be best for a permanent mooring?! /forums/images/graemlins/tongue.gif

[ducks and runs for cover]

 

[Bajansailor]

Can't get Rocknas here mate....... /forums/images/graemlins/laugh.gif

Unless Rocky Craig would like to appoint us as a dealer for Barbados, and send us a few demos..... /forums/images/graemlins/smile.gif

Sorry, couldnt resist...... Rocknas do seem to be very nice anchors, for sure.

IMHO I reckon ANY anchor (of an appropriate size) would be better for a permanent mooring, rather than using a Lump.

After all, a Lump is basically the same as Hylas' favourite pre-historic Killick anchor, but without the appendages sticking out.

 

[tyce]

[ QUOTE ]
I notice that everyone has studiously ignored the comments above (eg Geoff has 2 x 20 kg Danforths for his boat) re using anchors rather than 'lumps' for moorings.......

I would go for science, ie anchors, any time rather than lumps, especially if I was doing the job myself.

[/ QUOTE ]

Beleive me, if i used the anchor arrangement he uses where my boat is moored it would not be there for very long

 

[GMac]

[ QUOTE ]
I reckon ANY anchor (of an appropriate size) would be better for a permanent mooring, rather than using a Lump.

[/ QUOTE ]

A couple of BIG anchors plus the extra bits to tie them together plus mucking around setting them properly and so on. Total time and money $$BIG

One bloody big lump of something just dropped over the side. A lot less $$ and will probably work better and longer.

The reason I say Big anchors is small one will rot away relativity quickly and you have to start again. On a mooring the bigger it is the better it holds and longer it lasts.

We've done a few moorings with actual anchors on the end. All of the anchors have been bloody huge and purchased real cheap buy the owners. Generally speaking using real anchors on moorings is un-economic and gives a poorer end result.