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Old 06-07-2020, 23:47   #1
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How best to calculate weight needed for a new mooring?

Anika J is a 48 foot wooden boat with a beam of 14'6", and you can see a small image of her design in my avatar. This design has significant windage if beam on to the prevailing wind. She weighs about 35 tons.

Seabiscuit, currently moored in the Crookhaven river, weighs 12 tons, and is 40' with the same beam. Two 450kg weights (concrete, disk-shaped with sloping sides, narrowing upwards, each with a centre steel loop for fixing to; both are chained together with a centre attachment point for the mooring line), has proven secure in winds of 60Kn, on a number of occasions over the last year.

So, to my question: what are people's suggestions for weight needed to secure Anika J in the same conditions? My feeling is that Anika J and Seabiscuit present very similar profiles to the wind (the bow and cabin area, as a wind would feel it, are almost identical, although Anika J's cabin is a foot or 18" wider than Seabiscuit's).

I do not know of any formulae (or even guiding principles) in this regard, and welcome input.
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Old 07-07-2020, 00:11   #2
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Re: How best to calculate weight needed for a new mooring?

Quote:
Originally Posted by Kit_L View Post
Anika J is a 48 foot wooden boat with a beam of 14'6", and you can see a small image of her design in my avatar. This design has significant windage if beam on to the prevailing wind. She weighs about 35 tons.

Seabiscuit, currently moored in the Crookhaven river, weighs 12 tons, and is 40' with the same beam. Two 450kg weights (concrete, disk-shaped with sloping sides, narrowing upwards, each with a centre steel loop for fixing to; both are chained together with a centre attachment point for the mooring line), has proven secure in winds of 60Kn, on a number of occasions over the last year.

So, to my question: what are people's suggestions for weight needed to secure Anika J in the same conditions? My feeling is that Anika J and Seabiscuit present very similar profiles to the wind (the bow and cabin area, as a wind would feel it, are almost identical, although Anika J's cabin is a foot or 18" wider than Seabiscuit's).

I do not know of any formulae (or even guiding principles) in this regard, and welcome input.

Your mooring is way to small

Concrete looses half of its weight in seawater

For a definitive answer contact your insurance company

https://www.gowrie.com/pdfs/INAMAR%2...20Moorings.pdf

The insurance company and the port authority are the final authority concerning mooring specifications

They will have a mooring design table ....holding power needed ....material choice , iron , concrete , sea screw.. as well as chain diameter, pennant, shackle .. strength and length
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Old 07-07-2020, 00:29   #3
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Re: How best to calculate weight needed for a new mooring?

Slug has it exactly.

I’d add putting in a sea screw is the way to go. Don’t risk it dragging in any conditions at all.
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Old 07-07-2020, 01:01   #4
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Re: How best to calculate weight needed for a new mooring?

A lot will depend on the bottom and the length of the riser. I broke my "insurance" specified mooring and after paying for repairs they went and specified the exact same thing. I quizzed the local fishermen doubled the length of the riser and the mooring was bullet proof thereafter.


The bottom determines the type of "anchor" and the riser provides the shock absorber.
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Old 07-07-2020, 12:21   #5
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Re: How best to calculate weight needed for a new mooring?

Freshwater weighs appx. 62 lbs. per cubic foot. Concrete weighs appx. 150 lbs. per cubic foot depending on the mix (crushed rock, sand, cement, etc.) and rebar within. Another factor is how much does your concrete mooring actually displace? One cubic foot is one cubic foot, water or concrete, but if your concrete is not rectangular the water displacement calculation may be off. Also a concrete mooring sunk in mud has better holding power than one sitting on a hard bottom but that is difficult or impossible to calculate.
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Old 07-07-2020, 16:31   #6
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Re: How best to calculate weight needed for a new mooring?

Thanks to everyone; but as far as I know, sea screws are virtually unknown here. And how are these put in?
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Old 07-07-2020, 16:39   #7
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Re: How best to calculate weight needed for a new mooring?

Hi Kit_L,
Looks like your mooring is not adequate.

https://api.ecan.govt.nz/TrimPublicA...wnload/3203763

Across the ditch in NZ they do have some ideas. Maybe you can get an idea from their site as I did. So it comes down to Weight, ground chain, chain and mooring line or chain and scope. Also what waves you are likely to get. I don't think you get much in the way of waves where you are. But maybe flooding, trees coming down river etc.
I like to think the cheapest insurance you have is the mooring so make it a real good one. Plus annual lifting and inspection is essential I think and possibly a requirement of your insurer.
RMS Maritime will tell you to rely on your mooring contractor. As “Maritime” don’t have a clew and only collect your money. They don’t even have a harbour master to point you in the right direction.

I have a 41 ft yacht with 14ft beam and mast and rigging Im using two one ton blocks, heavy ground chain, riser chain and mooring line with a good scope. In a recent storm it held while a Cat next to me dragged onto my mooring and became entangled, my mooring held both boats in up to 60kt winds for about 8 hrs. With waves Up to two metres. There was extensive damage, but neither boat ended up on nearby rocks.
Happy moorings
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Old 07-07-2020, 17:02   #8
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Re: How best to calculate weight needed for a new mooring?

First, I’ve got no answer but a reference that doesn’t extend to your boats size.
Marblehead is a jam packed harbor with lousy protection to the northeast which is the prevailing storm direction. Mooring sizes required by the harbormaster depend on location and protection from the seas. At the mouth in a bad nor’easter you can see 6’ easily. Mid harbor in a tropical storm?

Not pretty.
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https://www.marblehead.org/sites/g/f...ead_waters.pdf
Moorings here are concrete. And they are much heavier than your neighbor’s.
Chain size is beefy too.
My 18 ton boat has about 8000 pounds (if I recall correctly) of concrete and very heavy bottom chain and heavy top chain.
The mooring sizes here used to be too small. Since the regulations ( a collective effort by the town and the mooring companies) the big enemy has become chafe from poor leads, and chafe from anchors on pulpits that’s are not clear of the pendants.
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Old 07-07-2020, 17:18   #9
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Re: How best to calculate weight needed for a new mooring?

Here in the NE US, dormor anchors are considered the best. Cast iron pyramid with thick, short shank. No wraps....

I have a 2000lb on my 20 ton boat wh8ch is admittedly overkill, but will hold in a hurricane.
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Old 07-07-2020, 17:22   #10
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Re: How best to calculate weight needed for a new mooring?

Formula for calculating mooring weights?
As said above, often local authorities have minimum requirements, but often these requirement are not calculated, just fixed numbers, thought of by someone in an office, and because of that, another person doubles those figures to make sure....... my cynical view on those requirements, but.... one has to abide by that.

Like with anchoring, there are 2 types of loads on the mooring: static (from wind) and dynamic (boat is moving/veering or because of wind and waves). The static load can be calculated, as the pressure of wind and area presented to the wind.
Here is an internet calculator:
https://www.engineeringtoolbox.com/w...ad-d_1775.html

In the OP case, if both boats have a similar profile presenting to the wind, the mooring weight is for both both the same if only static loads are taken into account.
However when dynamic loads come into play, stopping a 35 ton boat, will put lots more force on the mooring, therefore the mooring needs to be much larger. I don't know a formula for that.

Re weights, hmmm, concrete in general is a bad idea for a mooring, yes it is dirt cheap and can be made at home, but with a specific gravity of around 2.3 to 2.5 submersed in water there is only 1.3 to 1.4 (tons per M3) left to hold the boat, nearly half of its weight is 'lost'. Specific gravity of steel is ~7.8, and submersed still 6.8 (tons per M3) will hold the boat. So the physical size of a steel mooring can be a factor of 5 or 6 smaller than a concrete one.

There is another issue with concrete blocks for moorings: because they need to be of enormous size they often stick out above the seabed, at least a foot......providing a solid mass for a deep keeled boat to hit at low tide, or to wrap the chain or other lines around it.

Regarding the sizes you mentioned (900 kg of concrete for a 12 ton boat). Empirically I would at least double that if using concrete. My previous 11 ton 34 ft boat had a concrete mooring block of 2.6 tons (weight on land). Furthermore, does the area get a lot of storms, and what is the max wind expected, is the area protected from waves and soforth, If some of these questions are affirmative, than even 1800 kg of concrete block will not be enough.

Lastly is the length of the chain; locally here it is accepted that 3 times the depth of water (at high tide) is OK. Chain consists of 3 parts: very heavy for bottom 1/3, medium another 1/3, and top chain is similar to, or one size up from your anchor chain size.

Yes, if the chain is made longer, then the mooring will hold better, but in a tight mooring area there lots of boats moored; all boats need the same amount of chain, or they will swing in a different way and hit each other.

Lastly it is good practice to have 3 or 4 anchors out from the main block, that will stop the block from dragging.

Very last point: screw anchor/moorings can be used if the bottom is not too rocky. It is used locally in sensitive (coral) areas, with a shortish chain and or floating line, that does not drag a circle around the block, on the sea bottom like a normal chain does.

Did I mention swivels?
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Old 07-07-2020, 17:34   #11
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Re: How best to calculate weight needed for a new mooring?

Try this company. They are in Northern NSW but advertise and exhibit on the Gold Coast so must work up your way. Their website is not so clear but they make screw pile moorings.

Segrass Moorings - Home
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Old 07-07-2020, 21:40   #12
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Re: How best to calculate weight needed for a new mooring?

Following. I need to build a mooring for my Taswell 43 (approx 32,000lbs in cruising condition).
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Old 07-07-2020, 22:54   #13
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Re: How best to calculate weight needed for a new mooring?

@RexSnr: I tried to call Seagrass Moorings, but the number on their site is disconnected. The demo. video (where a decent sized tug tries to pull different types of moorings out), is very interesting though. I left an inquiry on their site, anyway.

Thanks to everyone who has replied. I should have clarified in my first post that I was intending to put an extra two tons of weight down for Anika J, in addition to the original 900Kg, for the reasons @HankOnthewater mentioned above (dynamic loading).

Although the mooring area next to Shaw's Creek (adjoins the much larger tidal Shoalhaven River) is exposed to the prevailing NE winds, it's the massive Westerlies (60+Kn) that blow the hardest in this area. Seabiscuit, the 12 ton wooden boat, a smaller sister in design to the much heavier Anika J, held without moving an inch on that small weight, but having read all the references folk have posted here, I think 1) I have been lucky, and 2) I am inclining away from concrete weights, for the many good reasons given.

Today, I have secured a berth at the fishermen's wharf here (just waiting for the paperwork to be sorted), so will dock there while I investigate whether anyone local can put one of those sea screws in—the tug demonstration I mentioned showed over 2 tons pull was not enough to dislodge one of these, both in mud and sand bottoms, but the concrete weights in the test were able to be moved by a force of just over one ton (measured via strain gauge in the tow line). That's hard evidence, for me.

@goodoldwoody: yes, no massive waves on the river (but the fetch is considerable, so we do get serious chop) and we do get floods, regularly. So debris, including big trees, is not uncommon.

@slug: unfortunately, I have found the insurance companies here are the last place to get reliable information. And we have no port authority nor harbourmaster here, either.

Sincere thanks to everyone who has replied: very much appreciated.
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Old 07-07-2020, 23:14   #14
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Re: How best to calculate weight needed for a new mooring?

Here in Tassie old railroad wheels are commonly used as mooring anchors. They come in various sizes and weights and often several are chained together. They settle somewhat into the mud and "suction" helps beyond gravity alone to aid their holding power. The usual combinations of chain are usually coupled to a ~1 inch diameter polypropylene riser (something I'm not so fond of) that seems to last a couple of years.

Dunno if the Shoalhaven guys do this...

Jim
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Old 08-07-2020, 02:25   #15
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Re: How best to calculate weight needed for a new mooring?

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Dunno if the Shoalhaven guys do this...
We do, we do—but if anyone can point me to where I can buy such wheels, I will be very grateful. Thanks Jim.
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