Fortress / Danforth mooring

I'm looking to make a long term, yet removable mooring out of 3 fluke anchors for dense mud bottom. My 44ft Marine Trader is supposedly in the 27500 lb range with high windage. I want something that I can trust for everything but tropical storms/hurricanes in a location with 2 miles of fetch.

When it comes to anchors, I am a go oversized kind of guy. I'm here to get a reasonable range so that I do not go OVERLY oversized (haha). What size do you recommend?

I should note I already have a Danforth 35 High Tensile (rated 3800lb holding power) as a backup anchor. If starting from scratch, my impulse is certainly to go bigger, but if you think that is actually adequate for one leg, let me know.

Additionally, I would never typically spend the big money for a Fortress, however I spy a gigantic one selling on the used market selling for as cheap or cheaper than a comparable danforth. My only concern is would an aluminum anchor be more prone to corrosion in this situation where it will be semi-permanently sunk in conjunction with a galvanized chain?

My only concerns with Fortress/Danforth type anchors is resetting. I much prefer the Rocna and Mantus anchors.

I am going to assume the mooring is the traditional 120 degree spread of fluke anchors.

For a permanent mooring, setting or resetting isn't the issue in this style mooring, corrosion and wear usually are. I too would be concerned over longevity of aluminum components, but there are very resistant alloys. A call to fortress asking if their anchors are tested for long term submersion and with what components to prevent mixed metal corrosion might be in order.

Bottom line is the initial setup should be optically checked by diver or camera and at least annually. The problem with small component moorings I would guess.... is too much is buried and now no longer easily visually checked.

Thinking you might be able to retrieve this kind of mooring is worthy of a lot of thought. Duration of deployment and forces expected during that period could make retrieval of one or all anchors a salvage operation over just going out and hauling them. Then....when retrieved, are the suitable for further service?

As usual with most long term anchoring/mooring situations, location often trumps gear.... the best setup often fails because of factors it just doesn't work well with.

psneeld said:

I am going to assume the mooring is the traditional 120 degree spread of fluke anchors.

For a permanent mooring, setting or resetting isn't the issue in this style mooring, corrosion and wear usually are. I too would be concerned over longevity of aluminum components, but there are very resistant alloys. A call to fortress asking if their anchors are tested for long term submersion and with what components to prevent mixed metal corrosion might be in order.

Bottom line is the initial setup should be optically checked by diver or camera and at least annually. The problem with small component moorings I would guess.... is too much is buried and now no longer easily visually checked.

Thinking you might be able to retrieve this kind of mooring is worthy of a lot of thought. Duration of deployment and forces expected during that period could make retrieval of one or all anchors a salvage operation over just going out and hauling them. Then....when retrieved, are the suitable for further service?

As usual with most long term anchoring/mooring situations, location often trumps gear.... the best setup often fails because of factors it just doesn't work well with.

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Yeah i did wonder if a fluke anchor would bend if i had to power unset it backwards. Then again is that anchor style really prone to a deep diving bury in dense mud -- even my mantus still has the shank exposed after 9 months. Seems like a fluke would stay close to the surface. Regardless, i expect i could just go down there and dig it out by hand if necessary. Also could attach marker bouys on them and trip them that way, to avoid bending.

Buoys are a nuisance and depending on the bottom as I posted before can cause all sorts of headaches from install to use to retrieval.

The best stuck retrieval method I have experienced is to attach drum(s) and let the tide and wave action work the anchor out slowly.

ERTF said:

Yeah i did wonder if a fluke anchor would bend if i had to power unset it backwards. Then again is that anchor style really prone to a deep diving bury in dense mud -- even my mantus still has the shank exposed after 9 months. Seems like a fluke would stay close to the surface. Regardless, i expect i could just go down there and dig it out by hand if necessary. Also could attach marker bouys on them and trip them that way, to avoid bending.

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In a triple setup where there's no direction change I'd expect deeper burying, as they'll only be sitting or being pulled on to bury deeper, nothing to cause them to un bury.

It depends on the bottom too. Judging by mud caked on, in soft mud I've managed to bury my 73 lb Vulcan all the way up the shank and a couple feet up the chain. Retrieval confirmed this, as I still had a few extra feet of chain below the water (compared to the depth) when it went vertical and tight on retrieval and I was able to take in several feet as it worked its way out before the anchor actually broke free of the bottom.

ERTF said:

I'm looking to make a long term, yet removable mooring out of 3 fluke anchors for dense mud bottom. My 44ft Marine Trader is supposedly in the 27500 lb range with high windage. I want something that I can trust for everything but tropical storms/hurricanes in a location with 2 miles of fetch.
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This article was published on Morgan's Cloud recently about hurricane moorings. While the article is about more permanent moorings and for hurricanes, it would be well worth the read. There are links to other discussions that might be useful too.

The links is behind a paywall but well worth the money.

I used a high tensile Danforth 35 as my main with all chain on my 40 Albin for 10 plus years. It held fine in all kinds of situations but nothing over about 60 mph. The Albin was supposedly 26 k lbs empty, so close to your Trader. Full flying bridge enclosure as well.

I have explained the mathematical calculations once before, you can find it in this topic.

What it comes down to is calculating your wind surface, which is usually the frontal part of the boat, from sea level to the highest point. That will give you a certain number of square meters (or square feet if you want to calculated like that.
Then you calculate the wind force at which the boat should still be safe. If you never encounter winds above 6 Bft on anchor there is no reason to calculate for 12 Bft. If you do encounter stronger winds or wind gusts obviously you will need to calculate with those windspeeds.
Usually the winds are given at 10 mtrs high, so winds at sea level are around 70 % of that on the 10 mtr level. However, if you find yourself in open sea, with not a lot of protection, then calculating the full wind speed is the best option.
4 Bft = 21.5 N/m2
5 Bft = 40 N/m2
6 Bft = 70 N/m2
7 Bft = 110 N/m2
8 Bft = 151 N/m2
9 Bft = 224 N/m2
10 Bft = 290 N/m2

Again, if you feel comfortable calculating with 70 % of that then feel free to do so.

The forces you read above now need to be multiplied by the total square meters of the frontal surface of the boat.
So, let's say frontal surface is 15 m2 then at 8 Bft you would encounter a force of 2250 N on the boat.
In order to avoid dragging you would need to counter that force with a minimum of 2250 N via holding power of the anchor plus chain.
The holding power for each type of sea bed should be provided by the anchor manufacturer, so that one is a given.
As for the anchor chain, you can find the weight per meter for every diameter of anchor chain real easy, but otherwise the supplier of the chain should be able to provide that.
The weight of the chain in kilogram is multiplied by 10 to get to Newton and to that you add the weight of the anchor, also in kilograms, multiplied by 10.
E.g. my chain is 4 kg per meter, my anchor is 50 kg. So at 50 meters I have 250 kg in total in the water, for a total of 2500 N and that is without the holding power of the anchor.
In theory this should be enough to not start dragging at 8 Bft however we have to take the depth of the water into consideration and the result that has on the angle of the chain (and anchor shank) to the sea bed. Because, as soon as the angle of the shank reaches 25 degrees off the sea bed, the anchor has zero holding power, other than the weight of the total anchor plus chain.

Large vessels don't rely on the holding power of the anchor, they basically anchor based on the weight of the chain and anchor. Smaller vessels need the holding power of the anchor to ensure safe anchoring, since many boats don't have heavy chains. In fact, many boats have undersized chains and anchors, making calculating the required holding power very important.

Based on the above you can decide at which wind speed you still want to feel safe and comfortable. I know that nobody, in his right mind, is going to anchor in 10 Bft, but a heavy thunderstorm or squal line can bring winds of 60 kts. Even when that lasts for only 15 min I think that all of us will want to be safe.
So, instead of guessing how heavy the anchor or how thick the chain should be you can actually calculate what you need in which situation.
Of course there are other considerations, such as current, wave height, tides etc, but also those are mathematical calculations which will lead to a solution.

Hope it helps you.

Danforth High Tinsel accept no substitutes IMHO

Go to the auto scrap yard and buy three burned out V-8 engine blocks. Since this is not a hurricane/storm buoy, chaining these beasts to your central point will yield a pretty good soft mud system. If winds and seas happened to get out of hand and pulled hard enough to drag one leg, I'll bet you next month's paycheck it would not drag all three. Renew the chain as it corrodes beyond use, but the blocks will be there long after the need for a mooring is gone.

Brooksie said:

Danforth High Tinsel accept no substitutes IMHO

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I'm curious about the ratings on the Standard vs HT. For example, a 35HT has smaller flukes than a 70S. Yet the 35HT quotes a higher holding power. The only way that makes sense to me is if the Standard is expected to bend at 3000lbs as opposed to pull out the bottom.

And, I'm assuming these numbers are for sand. So I'm wondering if the 35HT would still have higher holding power in mud, or if the larger fluke area on the 70S would be superior in that application.

The standards are all mild steel thus flukes are thicker & not sharpened so don't reset as good as HiTinsel. Resetting in a wind/tide change is key to good times ashore and good sleep. Being mild steel standards will bend their stock and flukes. Hi Tinsel will not.