floating a mooring

PabloPicasso

PabloPicasso

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I have this idea that i can cast a mooring sinker in cement with a hole in the middle through which i shall loop and shackle the riser chain. then i will use a 1ton builders bag (filled with air bags) to float the sinker off the slip as the tide rises.

Would bin liners do as airbags?
How big a cement sinker would an air filled 1ton builders bag lift?
Would the builders bag straps be strong enough?
 
Bin liners are not tough enough. The hydrostatic pressure, and roughness of the fabric weave will cause one to burst, then sequentially trigger the others. You need a tougher bladder and one which will enable you to gradually reduce lift under fine control.

One builder's bag is quite a bit less than a metre cube, and if full of air would lift - very roughly - 900 kg.

Concrete has an approx density of 2400 kg per metre cube, but underwater this is reduced by the amount it displaces e.g. 1000kg. So - again - roughly, a metre cube of concrete weighs 1400 kg under water.


Have a word with your local BSAC. They are likely to have buoyancy bags and expertise in lifting weights under water.

Bag straps will be strong enough to lift 1000kg, with a bit of a reserve, but any bag which has been in the sun for a while will be quite a bit weaker due to UV degradation.
 
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so

So a truck tyre filled with concrete can be floated off the slip with one, or perhaps two, air filled builders bags. I could make two sinkers and float them over one ata time i Guess?

What is a local BSAC?
 
BSAC British Sub Aqua Club.

A 30 inch dia truck tyre filled with concrete is going to be about 0.2 of a mtre cube, with an underwater mass of about 280kg, and a weight on land of about 500kg. Unless that is well buried, I'd have reservations about that holding your boat in fresh conditions.

Two tyre sinkers in a tandem configuration would be better. But what do other people already have in that area ?

You do need to give thought to how you are going to entrain air in a builder's bag, and also how you release the air to sink the tyre under control.
 
PabloPicasso said:
I have this idea that i can cast a mooring sinker in cement with a hole in the middle through which i shall loop and shackle the riser chain. then i will use a 1ton builders bag (filled with air bags) to float the sinker off the slip as the tide rises.

Would bin liners do as airbags?
How big a cement sinker would an air filled 1ton builders bag lift?
Would the builders bag straps be strong enough?
Click to expand...

We used to use two oil drums tied together with a length of 4x4 on top.
The sinker was hung below the drums using a strop over the 4x4.
The sinker was carried down the beach at low water using our tractor
And the drum floater attached, as the tide came in the sinker lifted and could be towed out to position and finely positioned in place using warps to nearby moorings. When all was ready using a bread knife on the end of a long boat hook the strop was cut and the sinker dropped into position.

Another permutation of this with bilge keelers was to hang the sinker over the bow of the boat at low water and let the rising tide do the work in a similar fashion. Sometimes the boat was so bows down the prop came out of the water so a second boat as a tug was used.

Finally I have also seen it done with two inflatable dinghies side by side with a beam between them.

We laid many moorings in these ways ... All before H&S of course.
 
Two ideas both of which I have used.
First you create a tank out of concrete-so beginning with a box made of chipboard you cast in a concrete base-you then creat an inner chipboard wall to create the tank-mine had walls aboult 2.5 iches thick.The result is a concrete block which has the buoyancy to float.When you get it to site just start filling it with bags of sand until it sinks.If the mooring is quite shallow you can end by dropping more bags of sand ito the tank.The sand bags add even more weight to the tank.
The other method is to caste concrete in a high walled chipboard box which again will float.
In both cases I built the box/tank in situ on my trailer so that you can float it off on the tide.
Chipboard because its cheapest sheet material with some structural integrity and once soaked for a while disintegrates !
Maths are quite simple as already outlined but in imperial are as follows.
A cubic foot of water weighs about 65 lbs.A cubic foot of air displaces a cubic foot of water and is therefore capable of neutralising its weight therefore a cubic foot of air will easily lift 63 lbs.
Concrete weighs in air about 150 lbs so aprox 2.5 cubic foot of air is needed to lift a cubic foot of concrete-so you need an airspace of 2.5 cubic feet for every cubic foot of concrete you want to lift.
An air weight one ton block sunk in soft mud will hold a small yacht.
Check out link to Canterbury Council site in NZ where moorings are strictly licenced by local authorities.
http://ecan.govt.nz/publications/General/MooringBookletfhmx.pdf
One other idea is to take some 45 gallon plastic drums-I can get used for about £20 each-one is capable of carrying about three cwt of concrete caste into it so six drums girdled together will give you a total weight of 18 cwt
 
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Go with Bluechips suggestion..............I do it virtually the same way except I have a purpose-made metal frame which takes the 2 oil drums.
Watch out when you cut the line holding the weight- the frame and drums launch themselves into the air with some enthusiasm !!

Jaba
 
Jabamusic said:
Go with Bluechips suggestion..............I do it virtually the same way except I have a purpose-made metal frame which takes the 2 oil drums.
Watch out when you cut the line holding the weight- the frame and drums launch themselves into the air with some enthusiasm !!

Jaba
Click to expand...

Me too -
and you can retrieve the weight with your float, though you might need 2 or 3 tides to get it ashore.
Can I add that those air weights assume that you are using just concrete (Obviously a ton of balsa wood in a bag would not make a very good clump) it's the weight in water that matters so the relative density is the main thing. Put as much scrap metal as you can muster in your truck tyre first and then fill up with concrete. I found enough between the tide lines for mine, including an absolutely huge swivel which I left sticking out of the centre of my weight..
A final observation. As Jaba says the reaction of the float when you release the weight can be a bit alarming. But if you take about 4 turns of rope (10mm polyrop. will do) through the eye and round the centre beam of your float and just tie the ends together, when you cut through just one thickness with your knife taped to a pole - the "surging" of the line will put the brakes on a bit.
 
I have Used builders " Dumpy" bags for floating mooring weights . just fill the bag with fenders/mooring buoys.
 
Concrete in Tyres are ok, but better if you can put a few coils of fairly heavy chain in the concrete and make an X across the tyre. The chain will also gives you an anchor point for the riser.
Don't use a shackle for the riser chain, get an eye welded into the end, then you can feed the riser chain round the X in the chain across the tyre and back through the riser eye.
 
LadyInBed said:
Concrete in Tyres are ok, but better if you can put a few coils of fairly heavy chain in the concrete and make an X across the tyre. The chain will also gives you an anchor point for the riser.
Don't use a shackle for the riser chain, get an eye welded into the end, then you can feed the riser chain round the X in the chain across the tyre and back through the riser eye.
Click to expand...
Concrete is not actually that heavy underwater, though it feels heavy in air. The more metal you can incorporate into the concrete the better to add real weight underwater. If it sinks into soft mud you can get away with a lesser weight underwater, but on a hard bottom you need a very big concrete block indeed to be any use as a mooring.

Usually whatever others use in the area is a good guide....
 
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