How heavy a concrete block?

[mike_bryon]

Does anyone know the size of block we should use as a permanent mooring for our 60’ schooner. The displacement with all tanks full, stores and so on is stated in the yachts stability booklet as 26.569 tonnes

We have swung on two anchors for a couple of months now in Grand Bay, Canouan in the Grenadines and have obtained agreement to lay a mooring. The local construction company has agreed to build it and we are waiting to hear if the diver operator can move it into position and set it. We understand a cone shape is the best ideally with a concave bottom.

if you would like an indication of where it will be then visit www.canouan.com. We are anchored in sand right in front of the peer of the Tamarind Beach Hotel. On the home page of the hotel’s website there is a picture looking down their peer out to sea we plan to set the mooring right there!

All advice welcome.

 

[Ludd]

Don't know how easy to get there, but I'd consider using a "cloverleeaf" of large truck brakedrums held together by rods.Being on sand,they will sink in due to the vents in the drums. Three would give you about 60kilos.Use more if necessary.

 

[Bajansailor]

Forget concrete blocks - they weigh very little underwater compared to above because of their density.
Steel engine parts as Ludd suggest are better - or better still in my opinion would be to use 2 anchors again, perhaps one size larger than the two anchors you are currently using.
OK, it will cost more to buy the anchors, but it will be infinitely easier and safer to lay (compared to chucking out a tonne or two of engine bits).
And you could perhaps take the anchors with you when you move on, and keep them as hurricane anchors?

The mooring for our 35' boat here just has 2 x 40 lb anchors on it, and this has held her quite happily for the last 12 years.

 

[daveyw]

Here they reckon on a ton for a 25' boat http://www.inamarmarine.com/pdf/Moorings.pdf

 

[Bajansailor]

You could also consider a helix type mooring anchor like these - http://www.helixmooring.com/index.php/boatmoorings
I think that some of them can be installed manually by divers, perhaps using a large bar for leverage to screw them into the bottom (?).
One advantage of these helix moorings is that you can get away with much less scope than with a traditional anchor or deadweight.
And they are much more environmentally friendly to coral as a result - 5:1 scope of chain ranging across the bottom will wipe out any little coral reefs in the vicinity very effectively!

Changing tack, Mike, I am sure that most folk on here would love to read an account of your transatlantic passage (and see some nice photos), and your experiences of living in the Grenadines. I certainly would!

 

[sarabande]

Forget concrete blocks - they weigh very little underwater compared to above because of their density.

Errr, ummm...

Concrete has a density of about 2400 kg/m³. That is 2.4 tonnes. OK so it displaces 1 tonne of sea water, which means the block is still very heavy, but its mass remains at 2.4 tonnes. The skill in using concrete as a sinker is in the design and that will depend on the seabed , currents, etc.

The USCG, who no longer use concrete as their main mooring method, have a spec sheet which might be of use
http://www.uscg.mil/hq/cg4/cg432/docs/specifications/407-A.pdf

 

[rivonia]

Does anyone know the size of block we should use as a permanent mooring for our 60’ schooner. The displacement with all tanks full, stores and so on is stated in the yachts stability booklet as 26.569 tonnes

We have swung on two anchors for a couple of months now in Grand Bay, Canouan in the Grenadines and have obtained agreement to lay a mooring. The local construction company has agreed to build it and we are waiting to hear if the diver operator can move it into position and set it. We understand a cone shape is the best ideally with a concave bottom.

if you would like an indication of where it will be then visit www.canouan.com. We are anchored in sand right in front of the peer of the Tamarind Beach Hotel. On the home page of the hotel’s website there is a picture looking down their peer out to sea we plan to set the mooring right there!

All advice welcome.

Hi we were in St.Maarten for a couple of years and put down a mooring. It was two 45 gall oil drums filled with bits of scrap metal and concrete. We put a scaffold pole through each one first. These were to secure them by thick chain linked to a huge shackle onto ships rope to a mooring bouy.

Good luck

Peter

 

[mike_bryon]

Thanks for some very useful links and ideas

Here scrap steel is hard to come by so I think we are stuck with a concrete block. We might be able to use both a block and anchors (we brought some big ones especially for the job). I guess the weight of the block is somewhat limited by what the diver can handle so as Peter suggests maybe two or three blocks linked by chain once positioned will be what we end up with. I will find out about the density of the aggregate that the builder plans to use.

Regarding the change of tack. We left Venice in July and spent the summer in Greece then onto Sardinia for two months. We set off the end of October endured strong head winds in the med some gale force but the Atlantic was kind perhaps too kind as we did not find the trades until the Cape Verde. We arrived on the 15th December as agreed with my wife’s new employer and started work and school the next day.

We are really enjoying our new life in the New World. The children are settled in at school. Our son has joined the football club and one daughter has joined the steel band. Live is simple; a police station, post office, a few small shops and that about it apart from the resort, casino, 5 star golf course and airport for the private jets! My wife is happy at work and I am writing away somewhere close to paradise. We have made a couple of trips to Tobago Cays and visited St Vincent, otherwise we spend our Sundays at the beach watching the pelicans dive for fish.

Our preparations for Dark Horse were about right. Except that our 4 x 70 watt panels aren’t enough so we have ordered more that will double the watts. Hopefully then we can run the fridge!

Peter can you estimate the weight of your drums when filled and give me an idea of the boat size displacement that you attached. Thanks

 

[rivonia]

Here scrap steel is hard to come by so I think we are stuck with a concrete block. We might be able to use both a block and anchors (we brought some big ones especially for the job). I guess the weight of the block is somewhat limited by what the diver can handle so as Peter suggests maybe two or three blocks linked by chain once positioned will be what we end up with. I will find out about the density of the aggregate that the builder plans to use.

Regarding the change of tack. We left Venice in July and spent the summer in Greece then onto Sardinia for two months. We set off the end of October endured strong head winds in the med some gale force but the Atlantic was kind perhaps too kind as we did not find the trades until the Cape Verde. We arrived on the 15th December as agreed with my wife’s new employer and started work and school the next day.

We are really enjoying our new life in the New World. The children are settled in at school. Our son has joined the football club and one daughter has joined the steel band. Live is simple; a police station, post office, a few small shops and that about it apart from the resort, casino, 5 star golf course and airport for the private jets! My wife is happy at work and I am writing away somewhere close to paradise. We have made a couple of trips to Tobago Cays and visited St Vincent, otherwise we spend our Sundays at the beach watching the pelicans dive for fish.

Our preparations for Dark Horse were about right. Except that our 4 x 70 watt panels aren’t enough so we have ordered more that will double the watts. Hopefully then we can run the fridge!

Peter can you estimate the weight of your drums when filled and give me an idea of the boat size displacement that you attached. Thanks

Hi, we loved it when we were in Granada as well. We shall return hopefully one day. Sorry but unable to help with regards to the weight. Our yacht only weighed in at 15 tons fully laden, however we sold the mooring to an Oyster which was HUGE compared to us. No idea of his weight, but he was happy with it for the hurricane season. Hope this helps. By the way visit all or any mechanics places and you will find bits of metal scrap as we did.

Good luck

Peter

 

[sarabande]

a 45 gal / 220 litre oil drum filled with concrete will weigh approx 525 kg.

 

[Bajansailor]

Mike, it sounds like you do now have quite an idyllic life in Canouan! Albeit a fairly hectic crossing, arriving just in time to start work.

Re concrete blocks, Rivonia's mooring in St Maarten was probably in mud in the lagoon (?) where a block would sink in fairly easily - Dark Horse is anchored in (probably hard) sand, hence any concrete block would take a long time to sink in.
And as Sarabande points out, a 2.4 tonne cube of concrete only effectively weighs 1.4 tonnes in the water - you could probably drag this cube along the bottom with your dinghy and outboard motor.

I reckon that by the time you have arranged for the local construction company to cast a suitably sized block, and for the divers to take it out and drop it, the cost will be such that buying a couple of big anchors could be cheaper.
Mike, you said you brought some big anchors specially for the job - how large is your standard bower anchor for Dark Horse, and how big are these extra anchors?

If you do need to get some larger anchors, it might be worth while sailing down to Budget Marine in Grenada - they do 100 lb Danforth anchors for about US$ 700 each.
http://www.budgetmarine.com/catalog...+Screws/Anchors/Danforth+Anchors/product.aspx

There is also an Island Water World chandlery in Grenada - they have 88 lb Delta anchors for about US$ 650 each.
http://www.islandwaterworld.com/browse.cfm/anchoring-docking/anchors/delta-fast-set/2,1077.html

 

[mike_bryon]

I really appreciate all the assistance, thanks to everyone.

My wife works for the company who will build the block or blocks and they have offered to provide them and deliver them to the town jetty at no cost to us. We just need to let them know what we want and pay the diver. Our bow anchor is a 75lb CRQ, I brought with us a 120lb Danforth and an approx 70lb CQR copy to use as semi-permanent moorings. We are swinging to the CQR and CQR copy at the moment.

We really enjoyed the crossing and chose to spend extra days in Barbados before crossing to the Grenadines so it was not as hectic as it might seem. We loved Barbados and are still drinking the water we bought there.

 

[sarabande]

sorry BajanS

"you could probably drag this cube along the bottom with your dinghy and outboard motor"

Not in this world with its current rules of physics. There's such an animal as friction.

 

[AndersG]

Some ideas about moorings in http://www.harbormasters.org/rockport/mooring.shtml
deadweight mooring for a 45 - 55 boat: 5000lbs weight in water.

 

[tri39]

When I was crewing on a reef boat out of Port Douglas, we made a mooring with an oil drum full of concrete, it also had steel bars sticking out at an angle. This easily held a high passenger ferry in the trade winds out at Low Island.
I suppose if you want to play safe you could fill two oil barrels, then chain them together b4 attaching your riser. Barrels can be rolled and are readily available.

 

[mandlmaunder]

Hi Mike
Hope you continue to enjoy canouan.
When we first looked into mooring our 18,000lb cat we investigated blocks and Helix sand screws and chose the sand screws for environmental as well as ease of installation.
Our sand screws are the short 6ft with a 10inch blade diameter and can be installed by 1 person and a long bar. Set in a 3 or 4 point star.
Cost approx $120 at Budget Marine, longer and larger are available

The concrete block design we found to be the best alround was about 7ft sq 12inches at the edges and 24inches at the center with a 6 inch dish on the under side. The dish is to form a vacume when any upward movement is applied, and will work the block into the sea bed.
Fit the largest eye bolt you can find for the concrete option .A shackle for either mooring system and rope not chain as this will help protect the area around your mooring and last longer than chain.
All mine own opinion and can be ignored

 

[mike_bryon]

Thanks for the great advice everyone

I can’t tell you how much we are enjoying living at anchor through a Caribbean winter compared with a European marina berth. For the last six winters we hunkered down, covered the boat, erected the Kabola drip feed heater chimney and stayed put until school finished. Here, every other weekend we have gone for a day sail. Sunday we had a great sail for the 6 miles or so to another island. We took a group of our new friends one of whom was a local fisherman. He and our son went fishing and returned with a box full of lobster for lunch.

I have proposed to the guy who will build our mooring block that we meet over a beer with the diver to discuss what he can safely manage. From the advice given here I have offered the following pointers The concrete block design that seems to work best is about 7ft sq, 12inches at the edges and 24inches at the centre with a 6 inch dish on the under side. The dish is to form a vacuum when any upward movement is applied, and will work the block into the sea bed. We should fit the largest eye bolt we can find.

A 60’ boat such as Dark Horse needs 6,000lb submerged dead weight which if using concrete = required dry weight of 10,908lbs (calculation for concrete 1000 lbs wet weight ./.55 = 1818 lbs dry weight. Granite = required weight /.64, steel better still)

The following is stated by the US Coast guard in the construction of their concrete sinkers.
3.2.1 Cement. Cement used in the concrete mixture shall meet the requirements of ASTM C150, Type 1.
3.2.2 Aggregate.
3.2.2.1 Coarse Aggregate. Coarse aggregate shall be uniformly graded from 1½ inch diameter to No. 4 sieve and shall meet the requirements of ASTM C33
3.2.2.2 Fine Aggregate. Fine aggregate shall be uniformly graded from No. 4 to No. 100 sieve and shall meet the requirements of ASTM C33.
3.3.3 Concrete Mix. The concrete mix shall be proportioned by volume as follows: 1 part cement, 2 parts fine aggregate, 4 parts coarse aggregate. Water shall be 6 gallons per 1 cubic foot (94 lb. sack) of cement.

I will keep you posted on how we get on.

Thanks again

 

[impact30]

cast ground chain into mooring block

Hi Mike
Most of what you have summarised is good information, however I would cast 'ground chain' into the block with a good stout reinforcing bar through the last link to anchor it in there. I would use a length of ground chain long enough to reach the surface for servicing swivels, shackles, etc. Diver can also do this.
Ground chain suitable for this displacement boat would be so heavy that it could only be lifted 3 links at a time!!! Old ships anchor chain is the usual.
The concrete block in my opinion needs to be at least 5 tonnes if no metal is to be cast into it. I used over a tonne of metal in mine, total weight 4 tonnes for 50 ft ketch, 24 tonnes displacement.
A diver is not going to handle this! You need a substantial workboat or tug to simply drop it into position. It will be possible to drag this if the chain is got near vertical (a feat in itself) but you will need a big strong boat that can take 2 tonnes hanging off the bow and 100 hp + engine. Be aware of the catastrophic effects of galvanic action on the chains. Ground chain should be good for at least 10 years, riser chain maybe 2, and all other fittings 1 year.
When casting simply mound some sand at bottom to create the concave effect. Plastic sheet can be used as liner. 2m x 2m x .6m average height is approximately what will achieve this. Remember though steel loses only 1/8 of its weight in water whereas concrete loses almost 1/2 so addition of scrap reduces concrete requirement drastically.
Enjoy your stay
Padraic

 

[Bajansailor]

I cannot understand this obsession to use a concrete block(s) for the mooring, especially as it will be located on relatively hard sand.
Soft mud ok, then it will sink in nicely, and have reasonable holding from the suction effect.
But hard sand - it won't take too much effort to drag it along the surface (yes Sara, I am familiar with coefficients of friction! )

It will be interesting to see what the final cost of this mooring will be, including the cost of manufacturing the block (which will be borne by Mrs Bryon's employers) and the cost of a suitable workboat / barge with HIAB (or similar) crane with at least 5 tonne lifting capacity along with crew and diver.

Mr Maunder has mentioned sand screws above - they would definitely be worth considering as an alternative. Especially as they can be used with much less scope than conventional anchors would require. Port St Charles marina here have a couple of moorings for super yachts, and they are just screws in the sea bed, on about 1.5 : 1 scope. The largest yacht I have seen on these moorings was over 200'.

 

[mike_bryon]

I’m obsessed with a concrete block why couldn’t it be a Swan 65!

The idea of a great big lump (or series of lumps) on the sea bed appeals because it seems foolproof. I have not discounted the idea of the sea screws and I may go for them if the builder and diver can’t agree. I’ve also noted the names of a couple of ships calling at the town jetty with cranes. The master of one of them may be willing to undertake a small diversion for a reasonable sum of EC dollars.