The Offshore Voyaging Reference Site

Storm Mooring

JHH5-12306I have written in the past about our distrust of moorings and how we generally prefer to be on our own anchor when the winds blow hard. However, there is one exception to that rule: our own mooring at our Base Camp. We just had it checked and took some photographs of the process, which show how the mooring that Morgan’s Cloud has ridden out three hurricanes on is built.

Checking It Right

We believe that the only right way to check a mooring that is sunk in soft mud, like ours, is to lift the whole thing every five years.

Granite is Good

In our area, where the use of old metal parts like fork lift counter weights (our mooring in Bermuda) is banned, many people use concrete blocks, but we prefer granite because it loses less of its weight when submerged.


A hole was drilled right through each of the boulders to take a 1-1/2” (38 mm) diameter steel bar with an eye on one end that the ground chain is attached to with shackles that we had welded closed—no plastic wire-ties here.

Laid Carefully

We carefully laid the two boulders about 20-feet (6 meters) apart in-line with the one nautical mile longest fetch. The ground chain is attached to the up chain, then to the first boulder and from thence to the second boulder. We debated setting up the mooring with the up chain attached at the midpoint between the boulders but decided that, in our case where the fetch from all but one direction is a matter of yards, in-line arrangement would be better.

The mooring is in 20-feet of water at high water springs and we allowed for another 5-feet of storm surge.

All Chain is Not Equal

The ground chain is salvaged ship’s anchor chain that has lasted for five years with almost no deterioration.

When we initially laid the mooring we were only able to source, here in Nova Scotia, some no-name chain for the up chain. It was amazing how quickly it corroded; so the next time we drove to Maine, we picked up some American-made Acco chain, which, after three years, still looks brand new. All the shackles are American-made from Crosby and have held up well.

Annual Check

We check down to the joint between the ground and up chains every year by hauling it to the surface using our massive windlass. We attached the swivel at the top of the up chain, just under the buoy, where we can check it frequently, rather than at the joint between the ground and up chains, which is more common practice but harder to check. (I have seen two moorings fail due to the weld holding the nut on the swivel corroding through.)


We winterize by removing the mooring ball and attaching a small and robust fish-farm float—which stands up well to the up to three feet of ice that we can get in our inlet—on a length of line so that all the chain is lying in the mud, which seems to result in significantly less wear and corrosion on the up chain than leaving the mooring in its summer configuration, as many do.

Guard Against a Fumble

I have attached a deep water trawl float on 4-feet (1.25 meters) of line to the joint between the ground and up chains in case we ever lose the upper end while winterizing the mooring. An eventuality that, without this buoy that could be easily found by a diver, would certainly result in losing the whole works forever in the deep mud.

Now you know why we say that the Morgan’s Cloud motto is “if it’s worth doing, it’s worth doing to excess”.

How is your mooring constructed? Please leave a comment.

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Perfect no-nonsense mooring. Big deal, really!!

A suggestion. If using a swivel, how about above the buoy, dangling in the air between the water and Morgan Cloud’s stem?

I test my mooring 2x a year by going forward, then going in reverse at full power. At tether’s end, the bow snaps down quite a bit. At this point the force exerted by the 16 ton boat must be quite considerable.

Also I dive every other year, but cannot inspect or retrieve the 6 ton concrete block buried in mud. Ship’s chain that is visible tells me that ship’s chain in the mud is OK.

Matt Marsh

Re. surge test (backing down the boat at full throttle)

If the mooring fails this test, the answer is obvious.

If the mooring holds, though, that doesn’t necessarily mean it’s up to snuff.

Let’s say you put a 12 m (40′) sloop in full reverse, and her bow drops by two feet when the chain snaps tight. Typical moment to trim on a boat this size would be around 130 kg.m/cm, and the chain pulls on the bow, 6 m from the centre of flotation. So the moment is (130kg.m/cm * 60cm * 9.81m/s2) = 77 kN.m, and the bow-d0wn force on the anchor roller is (77kN.m / 6m) = 13 kN. We might have roughly 1.4 to 1 scope (~45 degrees) for a mooring, so that gives (13 / sin45) = 18 kN of tension on the chain….

….which will have no visible effect on anything heftier than 7mm (1/4″) G40 chain. Most of the boat’s kinetic energy was used up in lifting the chain off the seabed, and in forcing water out of the way to let the bow down.

Now, granted, the peak dynamic load will be a bit higher than in this static analysis, but the point is this: The boat under engine can create only a fraction of the loads that a 6-foot wave will create when it lifts her up and back, breaking over the bulwarks as the chain goes bar-tight. The only way I know of to simulate that load is to have a much larger boat pull on the chain so hard that the mooring block lifts off the bottom…. at which point you’re already hauling the thing up, so you may as well look at it.

Matt Marsh

PS – I wouldn’t be worried about shock loading the components by doing a surge test in full reverse. If your boat under engine can create loads- even shock loads- anywhere close to the yield point of the steel, the mooring will be so obviously undersized (or so badly corroded) that you’d be afraid to tie off to it in the first place.


Wow, thank you Matt & John. This is absolutely excellent stuff. Yes it is obvious that 6′ surging waves will far exceed the force exerted by going in full reverse.

In F11 at my mooring the waves don’t go over 2 or 3 feet, which still is a lot more forceful (wind plus waves) than going in full reverse.

Guess I need to send a diver down, to dig his way thru the mud down to the shackle connecting the cement block to the chain, for inspection. We don’t have the facilities for mooring haulout in this small town.

Geir Ove

Lower grade steel that is used, the less it will rust, High grade steel, rust a lot faster,
Just go up in dim, and down in grade, it will be OK, for a very long time.

Arne Mogstad

Hey, old post, but I’m trying anyway. I’m in the process of making a mooring myself for a 40 foot Ovni here in Northern Norway. My idea is to let the boat live on the mooring rather than in a marina, and to therefore size it for the job of our weather (which you know so well what it can be like). The place is rather sheltered for waves and swell, but not so much for wind. Do you have any tips on sizing, and your thoughts on leaving the boat “unattended” on a mooring like that? And just general thoughts on how to arrange the mooring ball etc? Also, I get a feeling that there is a trend towards rope instead of chain as a riser, and it kind of makes sense to me, as it could double as a snubber. Do you have any experience on it? And it’s easier (cheaper) to replace more frequently. I will be diving the mooring myself rather frequently.

Arne 🙂

Eric Klem

Hi Arne,

I continue to be amazed at how little sizing data and information there is out there for moorings, at least that I can find. The different harbors here in the US regulate what is required and there is huge variation in what is required and also some of the recommendations clearly show a lack of understanding of the physics going on. Here is the methodology that I used for sizing. It is far from perfect and I am certainly open to suggestions but hopefully it is helpful. Ideally, it would be an integrated dynamic model where you can play with trade-offs but that is much trickier and with the size of some of the other assumptions, it doesn’t make sense.

I started with getting a design load. This meant picking the conditions that I needed it to work in. The state where I live has a disaster preparedness plan that actually goes through all the recorded severe weather events for the last 150 years. Our mooring has had 2 direct category 1 hurricane hits (one in my lifetime where we had 3 boats on moorings which all survived) in this time and several tropical storm hits. From this, I developed a sustained windspeed target I was comfortable with (this is tricky as you are trying to figure out how far out on a tail of a distribution you feel comfortable with and increasing from say 60 to 70 knots is a huge difference) which I could then correlate to the ABYC load tables. These tables are known to give higher numbers than measured in the real world if you deal with dynamics so I took a look at the work by Drew Frye on the subject and adjusted the number. In our case, we can have some waves from 1 direction so I did not adjust down nearly as much as I might have in other situations.  

For the anchor, I applied a safety factor of 2. For our application when I was accounting for wave action, interestingly this put me basically back at the ABYC design load so I could have not done any of the knocking it down to real values and then adding a safety factor. I first looked at granite slabs and used the rough rule of thumb that the holding power is equal to the submerged weight. Unfortunately, the local mooring barge would have been unable to pick that up so I had to look at other options. I am not comfortable with the mushroom design in most bottoms so used a Dor-Mor pyramid. Unfortunately while the data for holding power is scarce on most anchors, it is a single data point on these so I only gave the holding power as half of the manufacturer’s claim. This may be very conservative and effectively give me a safety factor of 4 but it is what I needed to feel comfortable.

For chain, I sized the chain such that the catenary would store the energy equivalent of the boat moving at 2 knots by the time it hit my calculated working load. How I decided this would be the design point is convoluted and a little long for this but it works for me. If I were in a place that was wind only, I would be willing to lower it. Another method that I know of that some people like is to calculate the chain so that it is always pulling horizontally on the anchor at maximum expected load. Unlike anchoring gear, mooring gear really does benefit from catenary if you have enough length and use a big enough size.

For pendants, I used a safety factor of 10 to my calculated maximum load. Nylon does particularly poorly in fatigue applications and 10 gives you the ability to deal with the fatigue and have some safety factor. If there are prolonged storm force conditions, then I replace the working pendant. I discussed our pendant construction in another comment to one of John’s posts, pendants are a very important subject and one that it seems most people around here gloss over far too quickly.

Finally, I gut checked the whole thing against what I know are the local mooring standards and how often boats drag ashore. I did end up with sizing that is significantly more than most but it actually correlates pretty well to the number of boats we see dragging in gales and storms.

I realize that this involved several calculations not shown here and a somewhat scary amount of assumptions. Ideally, we all have some magic calculator that we plug into and get the sizing out but I am unaware of one. Around here, the towns are supposed to provide the info for you but as I mentioned earlier, they don’t seem to be working to the same assumptions. Also, the simplified way they do it won’t work as for example, the shorter your length of chain the heavier it needs to be but they just give a blanket chain size based on boat size and then base length on water depth. This may be no surprise but the 2 heaviest moorings where we are belong to us and our mooring guy, everyone else has much lighter gear. We both want our boats to survive another event like Hurricane Bob, everyone else seems to either feel that it is only a problem rarely or that moorings are just another expense to be minimized.


Arne Mogstad

Hey, so thanks a ton both John and Eric! I have fine read this article and what both of you have written now embarrassingly many times… but this is (as you guys are mentioning too), a subject that I feel is hard to get “healthy” information on. I just don’t trust most of what I read on forums (except maybe those that was too poorly designed), and the same with suppliers of gear etc, as none of them seem to take any responsibility of anything they “advice”.

Well, you mention winds of 60 knots…. that’s a bit of wind. Problem is, you can keep the number, but change the unit to m/s and we’re talking about right for somewhat of an average winter blow. People might start putting the brakes on the baby-stroller when the kid is sleeping alone outside…… I’m not sure we have very extreme weather here compared to other places, but I’m pretty sure that the weather we have on a normal basis, is quite hefty by most standards, and sustained winds of 30 to 40 m/s happens many times a year. My point is, I need something that I KNOW can withstand heavy weather on a regular basis, because I know it will come many times a year.

So John, I would very much like some specifics on pendants and arrangement (or where I can find it if you have written about it already? I have read through so much lately that I can’t remember where and if I’ve seen it.) I feel that if I, to a large extent, copy your setup, my boat with half the displacement will be quite safe, and I can maybe even downsize a little.

Eric, could you give any numbers for what you ended up with as your mooring? (And you boat size?)

And what are the specifics at the top of the riser regarding attachments of the pendant and so on?

I feel science and engineering will get me so far, but real life proof is at least as good to keep me asleep! 🙂

Just to give an idea of what my initial plans was/is:

I plan on putting the boat on a spot that is 10 meter deep, and there will be 100 meters free radius in the 2 “worst” directions.

Depending a bit on what I can have lifted, 2 or 3 blocks of concrete totaling a weight of 3-4 ton. Attached at a middle point (as opposed to what you did John) since I have two opposing directions of weather and fetch (think fjord).

20 meter of bottom chain 25 mm (about 1 inch). Connected to a riser that I have not decided on yet, but I’m thinking somewhere around 12 mm (1/2 inch), then a swivel, a buoy, pendant, and maybe non-stretch line on the boat to avoid chafe through the fairleads.

I realize the bottom chain should just be as substantial as possible. The riser can be whatever I feel safe with, but details regarding the rest I feel more iffy on…

Again thanks so much for great answers and also a truly great site! And congratulations and condolences on your boat John! 🙂

Eric Klem

Hi Arne,

I would be hesitant to use a combination of multiple mooring anchors that are intended to share the load because it is extremely difficult to actually get them to share the load reliably. Unless you are truly mooring on a smooth, hard, flat surface (not a good place for holding), then you will find that usually once a mooring block starts moving, its holding power is decreased. Imagine 2 blocks with a chain between them and your bottom chain coming off the middle of that chain. If the pull is in line with the blocks, the further away block will take all of the load until it has dragged to be even with the other block. At this point, it is no longer embedded so it will have significantly reduced holding power which is very dependent on the bottom but may be something like 50% of the original. So you end up with a holding power of only 75% of what you expected. The other issue is that as you veer, you will keep loading only one at a time so it is possible to now load the other one singly and get it moving and you could be down to 50% overall with the ability to slowly walk it further by uneven loading, not good. This is highly bottom and setup dependent but gives you an idea of the danger of failing to load equalize. The only time that I can understand multiple anchors is for a multi-point mooring because you are using several light anchors that are only reliable in a single direction of pull.

60 m/s is a whole lot of wind, I am not sure I could stomach keeping a boat on a mooring if that was expected on a normal basis. Do you mean this as peak wind instead of steady? Then the steady wind would likely be more in the range of 40-45 m/s (78-87 knots) which is a lot but something that should be survivable with an appropriate location and gear.

It sounds like you are worried about the ability to set so much weight (without running any numbers, the weight you have given looks light to me)? That can be really limiting and sometimes you need to hire equipment from further away to do it. Other times, you can still meet your requirements by changing your design. I am a big fan of properly sized granite blocks as they work in just about any substrate, the only difference being that in some they can actually sink in and generate more holding power (side note, Prock Marine in Maine actually digs a hole and drops the block in which I like too). Mushroom anchors can have very high holding and much lighter but it is a pretty rare seabed where they can actually sit vertical and fully buried, they usually just sit on their side with 1/3-1/2 of the bell buried generating fairly little holding power and likely oriented wrong for the load. For about 40 years, my parents had a daysailer on a mushroom that buried properly in the type of mud that was completely impossible to walk in but only 200′ away, there was enough sand in the mud that they won’t bury. For an application like this, I like the Dor-Mor pyramid but I don’t know if you can get something equivalent where you are. For both of those in the correct bottom, I am willing to go about 8X lighter than granite dry weight. The lightest of all is a helix that is screwed in but that takes special equipment which may or may not be available locally. The final thing that comes to mind is a multi-point mooring which can work in certain instances but take real knowledge and care to set up right. With any of these setups, I try to only lift it when it is not going to be tested by a storm soon, in our case that means lifting it out in the fall for inspection not the spring.

In our case, we are mooring a 36 foot average size and windage sloop that weighs in the 17-18,000 lb range dependent on tank levels. Our chosen design point was 60 knots sustained with some waves. Our setup is a 1000 lb Dor-mor, 30′ of 1-1/4″ mooring chain, 30′ of 5/8″ mooring chain, 12′ of 7/8″ Novabraid double braid nylon strop hitched to 6′ of 3/4″ Amsteel for a primary pendant and 22′ of 3/4″ Amsteel for a backup pendant. We have a buoy where the chain comes through it and there is a shackle on top. We run a large enough shackle that 2 thimbles fit side by side with the 2 pendants. The pendants are lashed together every ~2′ with a little slack built in on the backup pendant so that it never takes the load. There are several floats along the pendants which prevent them from wrapping under the mooring ball which will rapidly chafe them. There was some amount of rounding up here as the calculated sizes often fall between real sizes. I am sure that improvements are possible but I am comfortable with the sizing and while it may be a little on the conservative side, I don’t think it is unreasonably so.

By the way, I would definitely recommend following local regulations and also seeing what other people are doing and what weather it is capable of surviving. I would also recommend going and figuring out what the bottom is where you want to moor, if you have an anchor which tends to bring up a lot of bottom, just set it hard a dozen times and see what comes up on it. In my case, I always pay attention to the local papers which love to cover boats breaking free and I sometimes go take a peek as to why. I also created a spreadsheet of all the local town regulations and paid particular attention to places which have a lot of fetch like South Dartmouth, MA, Mattapoisett, MA, Rockland, ME and Belfast, ME.


Eric Klem

Here is a crummy picture of our pendant setup (while I share the same love of sailing as John, this should make it clear that my interest and skill level in photography is not there, this was taken with a cell phone several years ago that had a pretty bad camera). As you can see, this is not when the pendants were brand new but it gives you an idea of what they are. We replace the nylon sections after 5 years with 5 months of usage each year or after a big storm and the dyneema sections at 10.

I hope all of this is helpful. So far this has all worked for me but there are many pitfalls in setting a mooring up so you have to make sure that whatever you do makes you comfortable. The uncertainties are quite high so it is hard to say what will work with certainty. While I have given some thought to all of this, I would not consider myself an expert on it.


mooring pendants.jpg
Arne Mogstad

This is very brilliant and good info! Appreciate you guys taking the time to write all of it! 🙂

Granite have a density of around 2600 kg/m3 (the number vary a bit by the type of granite). Concrete can be constructed to have a more predictable density of 2400 kg/m3. I definitely see your point of using as high a density material as possible. I will however most likely be much more able to get a lot of concrete than a granite boulder that size, so I may have to stick to concrete. I may however be able to get a barge with a huge excavator on it and be able to put down a many ton block.

The bottom is sand, and the longest fetch realistically is about 7 km (if I let the waves make a turn, it will be around 40 km, but then they will have to make a slight turn and pass between a few narrow places with small islands sticking out of the water, so reducing their size).

I absolutely see your argument of the anchors not being equalized and then dragging in a leapfrogging way as the boat swingings from side to side. I did not think of that! But with a big lump of granite/concrete I feel it can still move a little with some leverage from the attachment being a bit to the side, or at least enough to dislodge it? With concrete it can be shaped fairly flat and therefore the attachment in the middle, making it hard to get any off-center drag compared with a slab that is basically round? However, ideally it should be sized large/heavy enough to not budge at all.

I have not been able to find any of those types of anchors you have in the USA. Only copies that are so small I would not really trust a dinghy to it. So industrial type drilled ship-mooring is the only option, but not a very realistic one due to mainly the price being about the same as a new boat.

Regarding wind strength I have the numbers from the local airport wind reporting, and I’m unsure about sustained or gusting. But 40-45 m/s sustained sounds correct. Luckily not on a weekly basis, but for sure several times a year.

And the pendant info is brilliant, and looks like a real nice setup! I have not been able to find mooring balls that let the chain pass through, but they have those with a 16 mm galvanized steel rod and a loop on each side. I may have to order something from abroad. Where do you put the swivel? I feel that your setup John with it directly under the ball is a good one both for inspection and function.

Arne Mogstad

That’s a great idea that I did not really think of! Pulling a 10 ton flat-bottomed boat out should not be so difficult to arrange. I think I will still opt for a pretty massive mooring, but make my own slip as you say, and use that for the longer “storage” periods and predicted bad weather. Anyway, thanks a lot for many great inputs! 🙂 Highly appreciated!

Eric Klem

Hi Arne,

Just reading this and before I got to John’s reply I had a similar thought to him which is that with a 45m/s wind, I would consider anything more than about 1km to be too much fetch and I would not consider a mooring viable with 7km fetch. The only other possibilities to me would be inside a true breakwater or out of the water completely. Building your own marine railway would be a significant project but would work, be aware that they usually take at least a few people to run and can only be used in fair weather. Alternatively if the surface is paved or otherwise hard enough, you could make a massive winched dolly avoiding the need to source tracks.

Out of curiosity, I plugged into one of the many online wave size calculators (I can’t vouch for it being right) 45m/s and 7km fetch and got 3.3m waves at 5s which is really brutal steep waves. Even if you limited to 1km, you could have 1.2m waves which will have the boat moving around a decent but probably manageable amount. I used to live and work on a harbor that sees 2m waves most years and occasionally 2.5m waves both at slightly longer period and it was quite brutal on the boats on moorings, a large percentage had major damage or broke free. Pendants in particular did not hold up well, they tended to fail at the fairlead but blocks dragged a bunch as well and decks were even torn apart by cleats occasionally. One boat I worked on was on a pier that faced into this and we really had to work to keep all of the lines in good shape during these events. If the boat had been shorter and therefore pitched more similar to the cruising boats in the harbor, it would have been very tricky to keep lines intact and would have been very dangerous too.


Eric Klem

Hi John,

Rockland, ME in this case. When you get a nor’easter, there is basically always some part of the harbor with an ~8NM long fetch right across to North Haven/ Vinalhaven. Where we were in the harbor, the exposure was during the earlier part of the storm when it was blowing SE to E and it would really get going and then at some point, you would notice it was rapidly dying down even though the wind seemed the same and that would be when it would swing a little more N and the breakwater would come into play and it would die right down in only maybe 20 minutes.

If you are talking Rockport, ME, I can’t imagine being there in a backhand storm where the breeze swings through south and those usually have the most wind too although they are rare. And if you mean Rockport, MA, when the storms get going here we often go out to Halibut point which is at the tip to watch the waves roll in, that place is hopeless in a storm unless you are in the inner harbor but there is still surge. I have always thought it was too bad that they never finished the outer breakwater, it is an unbelievable hazard to navigation and doesn’t knock the waves down much.


Eric Klem

Hi John,

We like Rockland a lot too and while it has gotten fancier over the years, it still has its industrial side (Camden by the sea, Rockland by the smell). I too have spent some weather there, probably at least 5 nor’easters and 3 tropical storms anchored out plus all the fall/winter nor’easters tending boats on the dock. For TS Irene, a friend had already taken their boat to our normal spot and called us and said that someone had told all the yachts from Portland to go there so my wife and I went to the south end (backhand storm) of Rockland with 2 other boats owned by friends and had a perfectly fine time. The radio was non-stop with people in trouble, many of whom were where we had planned to go including Mirabella V. As you say, Rockland has a place that works for every direction and the other advantage is that is isn’t crowded with other boats like Pulpit, Perry Creek, Seal Bay or some of the other well known places to hide.

Your aversion to crowds must be stronger than mine which is not weak, we just avoid south of the Cape from July 4 until a week before Labor Day. Actually, most years we do a quick 10 day cruise down there at the end of June and pretty much always have been able to go wherever we want. We also love sailing in Maine in October but by November, we stick to sea kayaks.


William Murdoch

If granite weighs 2.7 g/cm3 in air, it should weigh 1.7 g/cm3 underwater losing 37% of its weight underwater. If concrete weighs 2.4 g/cm3 in air, it should weigh 1.4 g/cm3 underwater losing 42% of its weight underwater.

Iron would easily beat both. At 7.9 g/cm3, it should weigh 6.9 g/cm3 underwater losing only 13% of its weight underwater.

Neil Clifford

Hello John and others, I’ve been reading your posting and the great input regarding different Mooring set ups, materials and specs on blocks, chains, shackles, swivels, mooring balls and pendants. All this in relation to local weather and mooring location.

I am a new owner of a Boreal 47, which I will say weighs 30,000 lbs fully loaded. I want to err on the heavy side, and have her safe in the possible heaviest conditions I could ever expect in Glen Margaret, where my property, dock and soon to be mooring lies 1km down at the bottom of the channel running northwest from St. Margaret’s Bay. This would be the only direction of real fetch but I am told that the hurricanes come from offshore and not down the channel.

I recently spoke with a concrete company outside of Chester, Nova Scotia. They can pour a 4500 pound marine concrete block with a seven-eighths inch steel loop in it. I believe it’s the heaviest they can pour. My understanding is that marine concrete has a different composition than regular concrete and holds up better in salt water.

I am hoping, (and with further research, like who and what will move, lift and drop such a heavy block?) that this one block will suffice to withstand any category hurricane that might come whipping into Glen Margaret.

I understand it is a sandy and muddy sea bed in about 25 feet of water. I have to have a certain concern for scope and swing because there is another Mooring not too far away and in a fully stretched out chain I have to be sure I’m not going to be whacking somebody else’s boat.

My question to those in the know, is will this block suffice on its own, or will I require a heavier setup? perhaps a combo of two 3500 lbs blocks. This way very little will move one block and to move both would require a particularly nasty act of god.

In this situation I hold true to John’s comment that “if it’s worth doing, it’s worth doing to excess.”

Eric Klem

Hi Neil,

If I understand where you are well enough, that is a pretty exposed location assuming you are not tucked up in one of those little coves. My guess is that you would be subject to significant chop from N to W and then you could get real surge in specific circumstances coming in from the ocean. I have a distinct memory of picking out Schooner Cove to the north of you on the chart many years ago as a place to ride out a storm and arriving and feeling like I was anchoring in front of lake houses and not ocean ones and being very aware of not wanting to drag onto someone’s lawn.

My general guess on this is similar to John’s in that a storm would be touch and go on a 4500lb concrete block and a direct hurricane hit would likely end badly even if the wind stayed in the right quadrants. For reference, around here I have seen a few instances of sailboats <50′ drag 4000lb granite blocks (the local regulation for this size in one of our biggest harbors with a decent mud bottom) in less than 50 knots steady and chop <2′ but also many instances of them holding the key being the overall setup including boat and conditions. Unfortunately, true hurricane moorings are hard to set up but it doesn’t mean we shouldn’t try. This is all a game of statistics and I think it is possible to make your chances of ever losing the boat low provided you are in a protected enough location.

To throw a few quick and highly general numbers at it, I think you reach a similar conclusion. On the loads side, the ABYC has published design loads which include a safety factor and for your boat they would give a number of 6400lbs (there is some debate on this as their tables are not quite clear and some would argue they say 4800 but that suggests a very strange relationship of windspeed and force so I use the 6400lb number which assumes that the relationship stays as the square of the windspeed). Various people have done work on what the real loads are and if you want to dig in, I would suggest checking out Drew Frye’s book and website. For reasonably well protected areas with a mooring setup to eliminate shock loading and a well behaved boat, the loads are more like 30% of the ABYC number so ~1900lbs. Note, even for these protected waters, Drew predicts a range of 1400-5500 lbs ( depending on your setup so a long length of really heavy chain (even for protected waters, I would be looking at like 30’+ of 1-1/4″+ bottom chain plus as heavy and long as practical top chain) and good pendants are key. Add in big waves and the numbers can climb way higher (stay out of the surf zone at all costs) and I am not aware of good data on it but I could see your numbers easily being double the 1900lbs for 60 knots in your location. And if you truly want a hurricane mooring, keep in mind that the loads will approximately double by the time you hit 82 knots steady compared to the 60 knot ones.

Going to your proposed block, its underwater weight is probably between 2300 and 2700 lbs depending on exact composition. The limited testing out there on deadweight anchors suggests that their holding power is on the order of their submerged weight and can vary a good bit (a major reason to add a safety factor unless you are very confident it will bury properly in mud). So in a protected location, I would calculate a safety factor for 60 knots for your boat with the proposed block of 1.2 which would be unacceptably low for me and it could well be like 0.6 in your more exposed location which means you are dragging. Adding more blocks could help but as John says, finding a big granite block is better. Around here in New England, there is lots of granite around and the mooring guys all know who has a rock drill and will put in 1″ or 1-1/4″ staples and finding blocks up to about 5 tons is usually pretty easy. There are also several local concrete places that pour blocks, typically up to around 8000lbs although in Maine you can get the Habitat Moorings up to 12,000lbs, I prefer granite so haven’t researched it much. And then there are the anchor types, most notably Dor-Mor in NH. What I found about all of these is that they have little to no web-presence and the key is getting on the phone and calling and when someone can’t do what you want, asking who can. Around here, the scrap yard guys often know where to go for stuff and will sometimes even get it for you (especially useful for mooring chain).


Neil Clifford

Hi John and Eric,

Thanks to you both. I have much to digest here and websites to explore. The local mooring outfits have undersized the whole operation considerably. None have suggested weights above 6000lbs of concrete in total. One outfit connected a 4000 lb block to a 2000 lb block using galv 3/4″ grade 30 chain, and then up with galv 5/8″ grade 30 chain.

In my thinking, that 2000 lbs is going to move easily in a big blow, and then the other will follow shortly after.

I am surrounded down here with massive boulders. So finding something should not be an issue. Eric, I am curious about staples. How deep are they, and what holds the loop in, and what is the thickness of the steel loop?

I imagined a full bore through the stone with a 1 1/2″ pin and then 1 1/4″ ground chain and a 7/8 or 1″ up chain, with appropriately sized shackles and swivels.

There’s a chain supplier in Dartmouth that carries good used heavy chain they get from the Coast Guard, at a great price. Will be sure to have someone with me who knows their stuff to look at it.

I like the idea of a shackle buoy. Heavy pendants to the fixed cleats port and starboard to finish.

Need to find people who’ll do the install and don’t think my idea for such a heavy setup is overkill.

Eric Klem

Hi Neil,

I have had very similar experiences to you in terms of the recommendations I get until our latest mooring guy who agrees we should be going heavier although he still installs the required size by the town for most customers so he doesn’t have to fight customers over the small increase in cost. These same mooring people all seem to claim that there was an extraordinary storm every 2 years when there is a big enough storm that many boats end up on the beach. Instead of this defeatist view, I prefer to take the view that I want the chances of having a boat go on the beach in my lifetime to be less than like 5% not like 50%. It seems that all the old timers around here have the story or stories of the time their boat ended up on the beach, the percentage seems way too high to me for a modest increase in up front costs. I have also had mooring guys try to really stretch the useful life of chain and until our current one, I had to keep careful track and inspect myself as they would let it go 1 or 2 more years than I would.

Unless you are going to have like 200′ of chain, 5/8″ chain is pretty light and will not provide much catenary if you have a big storm. Our 36′ boat has just shy of 600 lbs of chain for a location that I believe is somewhat more protected than yours and my design point is only 60 knots but I was pretty conservative in all the calculations. If we used 5/8″ chain, we would only have about 230 lbs of it which happens to be pretty similar to our regular anchor rode if we put it all out and I can say from experience that without a snubber, you can really feel it jerking you when it gets wild. Like you are proposing, we use ex Coast Guard chain for our mooring bottom chain.

One limitation is often what the local mooring guy can handle in terms of weight. In a place like Maine where the smaller fishing boats like lobster boats sit on moorings a lot, most can handle 5 ton or bigger blocks but in a place like Massachusetts where it is mostly pleasure, they often max at like half of that. I wanted to put down a ~7000lb piece of granite for our mooring and the mooring guy was not keen on it but willing to try, the max he had lifted was 6000lbs and the barge deck was nearly awash. Since I wanted to see the thing every few years and mooring guys change from time to time, I decided it was better to spend a bit more and get a 1000lb Dor-Mor which has a much higher holding power to weight ratio allowing me to use something that actually I can move with our boat in a pinch. Most of the small construction barges can easily handle bigger blocks but this is a small job for them and it will be expensive and you will be low on the priority list but it may be the best option.

With regards to staples, for cruising boats I think most people here are using staples made out of 1″, 1-1/4″ or 1-1/2″ carbon steel solid rod stock. With careful use of a torch and heavy machinery, it is pretty easy to bend around into a U. Then a rock drill, typically attached to an excavator, drills 2 holes at the right spacing deep (18″+?) into the granite. The really secure ones send the staple all the way through and then bend over the ends on the back side. All of them use some form of adhesive poured into the holes but I am not totally sure what is being used by different vendors. The only staple I have ever seen pull out was a 2″ one on a block that must have been 10 tons with a 250 ton displacement vessel on it and rumor was that the block was not well maintained (we saw it first because the boat’s bobstay and forestay sawed one of our buildings on a dock in half when the boat came in). Sending the chain through a hole would be fine too but that is a big hole and I think you want to make sure the pull is from the center so it can’t flip or spin the block so you probably need 2 giant holes so the chain can loop through. By the way, it is generally accepted around here that you want a low and flat block rather than a more ball shaped one.

Sorry for writing so much on this. It strikes me as an area that is neglected by much of the boating community and most of the articles published are unactionably generic or fall back on insurance. John sets a good example by talking about it and making sure his mooring is heavy with stuff like 1-1/2″ chain. Our collective insurance rates keep getting jacked up after storms and many of them should be survivable (like Hurricane Sandy with 65,000 boats damaged many on moorings, it was huge but the boats in those locations should be able to handle that strength of storm unless you take the view that any direct storm hit can’t be mitigated which I disagree with). I keep hoping that someone like Drew Frye will pick this up and create a calculator to spit out minimum requirements based on good science or at least a sizing framework that gets more adoption and would include stuff like frequency of storms and water depth which are ignored in most regulations. The calculator would actually be quite easy, it is knowing the constants and assumptions to use it in that is hard. I did enough research to satisfy myself that I was being conservative but I don’t claim to be an expert in this and would be totally uncomfortable if I had to size moorings for something like the BVI.


Neil Clifford

Hi Eric, I’m going to check out the Dor-Mor weight and see if it will work in the channel here. I think it’s a muddy bottom. Don’t know if anyone local has experience with them.

Thanks for the staple info. Makes good sense and running through the whole boulder is best in my thinking.

I’ve been welding for 30+ years, not industrially but as a sculptor working in bronze and steel, so it’s possible to fabricate a Dor-Mor type block, I’m sure. Could price one against the cost of steel and the cutting charges.

I want to understand better how increasing wind velocity pulls on the upchain and then with more force pulls tight the bottom chain. And then with further force starts trying to uproot the block. Once winds settle I suppose the actual position of the Mooring Ball could be in a different spot because of the movement of the chains on the sea bed. It can be shape-shifting even when you’re dealing with really heavy chain. The only part that you don’t want moving of course is the block itself. I’ll have until end of March to come up with a solid system to put the boat when it comes off the hard and back into the water.

This has been most helpful. Cheers.

Eric Klem

Hi Neil,

Dor-mor’s are definitely interesting. My biggest issue is that they base their sizing off of a single pull test as best I can figure out. I looked at local regulations for them and ended up only assuming they generate half the holding power they claim for our good medium/hard mud bottom but that is likely wildly conservative. It may be worth asking them what shipping would be to you, I was going to do the 4 hour round trip drive to pick mine up and discovered that the shipping cost was basically the gas money for that so had them ship it. If you want an example of one made of welded plates, check out what this outfit in South Dartmouth, Massachusetts is doing: I spoke with the owner years ago and if I remember right, they try to build up 3 weld beads thick between each plate. Still, that would make me nervous from a corrosion perspective as that is a relatively thin and corrosion prone area that could rust through. I am also not convinced by their explanations on the stepped shape not mattering, just look at Steve Goodwin’s testing to see how details really do matter with anchors. Our local mooring guy says that the knock-off pyramids from Seachoice and similar brands don’t last very long as the eye rusts out. Our family has 130, 500 and 1000lb Dor-Mors and none show any noticeable loss of material and the area around the eye is massively thick. All go completely under the mud line in under a year and the 500 is sitting in my parent’s yard now after being on the bottom for over 10 years and it looks basically new if you chip the marine life off it, the reason I pulled it is that the boat that was on it was sold.

Regarding chain weight, my method was an energy based method which involves a small amount of modeling and knowing the catenary equations. I believe this is closer to the ideal method when dealing with waves but I don’t think we properly know what constants to use. If you want to understand this further, you can play around with some of the online catenary calculators but keep in mind that you need to convert the output to energy such as in this post: You could also experiment with some of the apps that have recently come out specific to anchoring that use energy method, I haven’t played with them and can’t say if I agree with the methodology but I know at least a few now exist. Because we have a location with some exposure, I ended up making it so that the force in the chain would be at my expected max load when it had absorbed the energy of the boat moving backwards at 2 knots with a horizontal pull (overly conservative per usual). Alternatively, some people try to have enough chain weight that the pull on the anchor is always horizontal and the math for that is easier although in some cases you end up with much heavier chain (for reference, with our situation this method would about double the chain weight and what we actually did is already more than double what the town requires). To do this, you take the expected max rode load (from above something like 1900lbs in protected water at 60 knots steady for your boat, scale it to what you really want) and divide by the scope to get chain weight. So if you have room for 3:1 scope, you are looking at 633lbs of chain minimum and then if you design for more exposure or more wind, it can climb a lot higher. Actually having only a horizontal pull seems a little overkill to me, if it pulls up on a 4,000 lb submerged weight block with 100-200lbs, who cares.

One random thought: who sets up the moorings for all of the fish farms in your area? They may be doing something that is applicable to you and some are in pretty exposed locations although they may also be just simply using Danforths or something with a single direction of pull. I was on Hamilton Marine’s website earlier today and by chance noticed that Peerless (Acco) sells mooring staples:–x-36–9–ctr-56689.html.

I see all of this as a probability game. Zero probability of loss is unachievable but a very low probability of loss over the lifetime of the vessel should be achievable, especially for those outside of the area where major hurricanes roam. I think it makes sense to pick a design point that represents something like a 1% chance per year of being encountered and then have some safety factor. It is easy to start stacking worst case on worst case and getting into implausible scenarios. At our mooring for example, our worst exposure direction is a direction that implies we are on the weak side of the storm so I looked at both that scenario with lower winds and bigger waves and a scenario of max wind with very small waves and in our case, I thought the worst was the lower wind but bigger waves. This doesn’t mean we should be engaging in wishful thinking such as not including storm surge in our scope calculations but we shouldn’t get too far out in conservatism or we get something that isn’t practical.


Arne Mogstad

So, this may be a stupid question, but why could one not leave a yacht on anchor on a semi-permanent basis, and skip moorings altogether? Some people I speak to say they are worried to even go scuba diving while the boat is on anchor, much less to leave the boat unattended for a day or two. Having now ridden out a few storms (60+ knots sustained) and well in excess of 120 anchor-days since I bought my boat this summer, I feel pretty confident to leave the boat on anchor provided the forecast is not too bad. Is this a false confidence?

(This question is copied from a different thread as I think this is the correct place for it, sorry for the double posting John, feel free to remove one of them, I could not delete it myself.)

Kindly, Arne 🙂

Rob Gill

Hi Arne,

Interesting idea!

The biggest issue may be convincing your insurance company that you have safely “moored” your vessel. As I understand it, to be moored in NZ for insurance purposes, moorings have to be registered with the local council and have a current inspection certificate and mooring number. If anything happened to your boat left unattended at anchor, then you may be uninsured (even if a vessel collided with you while at anchor). Even so, boats on mornings here pay a larger premium than those in marinas.

Our insurance policy for example specifically allows 36 hours unattended anchoring in settled weather, so leaving our boat in a gale would not be accepted, unless there were reasonable mitigating circumstances. So every 36 hours, we would need to return to our vessel and likely log our checks for things like battery / bilge / anchor system. Presumably we could then leave our boat for another 36 hours.

Last thought, modern mobile Apps like Anchor Pro have the ability to message you if your boat position moves outside a pre-set circle (even with different arc sector lengths). So if you have coverage, you could leave a spare mobile plugged in running a pre-paid SIM and App to alert you if the anchor is dragging, although there may be little you could do in really rough conditions.

Eric Klem

Hi Arne,

To me, while many of the requirements remain the same, there are some very important fundamentally different ones which mean that the solutions end up being quite different as everything is a compromise. On the anchor side of things, the most important requirement that a mooring does not share is weight. A 40′ boat with a 6,000lb main anchor just wouldn’t work unless the anchor was magically part of the keel. As a result, when we look at holding power, a typical measure is holding power per unit of weight. Chain is similar in that we try to run the lightest chain that is strong enough and use a snubber to help limit peak loading as that same 40′ boat probably couldn’t cope with 500+ lbs of chain with individual links on the order of a foot long.

On the mooring side, probably the most important requirement is for high reliability while unattended. Even a 1 in 1000 failure rate in a veering situation would be completely unacceptable, it would need to be at least another few orders of magnitude and I doubt that any anchor would really meet this as the bottom conditions will limit you. This has a lot of derived requirements like not being self-fouling, not mud fouling, not moving during the veer, etc. In the end, the easiest way to meet this requirement is to have something that is axially symmetric so the angle of pull simply doesn’t matter. As John mentioned, scope is also very important and you would be right at the minimum scope where a lot of anchors won’t set right and have very limited holding power. There are also things like longevity where an anchor will rust through very quickly and become expensive as a result.

To me it comes down to the fact that until you get near the holding power limit of a deadweight anchor, you just don’t have to worry about whether it will work.


Wilson Fitt


This is a late addition to the conversation. I have seen many mooring chains that started in the ½” to 5/8” range but had corroded down to doggie chain diameter in the middle section. The bottom part that usually lay in the mud and the top part that was easy to see from the surface always seemed relatively OK. The worst corrosion was in the middle portion, often obscured by copious marine growth.

A very smart person, well versed in such things, told me that the corrosion was caused by a difference in salinity of the water column (fresher on top from land runoff) that set up a very slight difference in voltage potential along the length of the chain. Whether this is correct in theory or practice I cannot say, but the phenomenon is real.

My solution for the last mooring I set up was to have a very heavy ground chain (1” diameter) and a heavy nylon riser, slightly shorter than minimum low water depth to avoid chafing on the rough corners of the mooring block.  Next time, instead of nylon, I will probably use dyneema or equivalent for the riser.


Neil Clifford

So I have been at it with Atlantic Wharf Builders at the head of St. Margarets Bay and decided to drop 4- 3200lb concrete blocks with 1″ ss loops in a tight line running up the channel I am at the bottom of. I have shackled them together at the loops with 1″ galvanized chain, which in turn shackles to 37′ of 1 1/4″ ground chain and shackle swivel shackle to 30′ of 5/8″ galv long link upchain through a T3C ball. From there I have two 1″ poly mooring bridles to the boat.

I asked those pouring the concrete to place a 2″i.d. ABS pipe at the centre of each form, so there is a hole running through the concrete. The blocks were lowered to the seabed such that the holes are open on the sides of the row. This allows me to chain the four blocks together so none can move alone. The holes will also be available to slide solid steel pins through or 5/8″ chain when, down the road, the top links have lost their integrity.

I don’t have a lot of swing room, given the proximity to shore, shallows and other moorings. But I think my math is good, and I should have a bit of space between the stern and anything else, in any sort of blow.

(latest news is my diver went down and noticed the heavy ground chain snagged against a very heavy non-descript piece of rusty steel, protruding a foot out of the seabed. Im imagining it to be a mooring block from days gone by, and will need to rememdy the situation soon, as the fall winds are approaching!

Neil Clifford

John, to be clearer.. The loops embedded in the top of the four blocks are SS. I assumed they knew what best to use. Between the blocks, and through the four loops run a 1″ galv chain, shackled as tight as possible. At the end is the connection to the larger ground chain. I asked them what happens to the blocks when the SS loops wear out (which they will over time) and at that point I figured that to continue to use them I would need to have the abs pipes running through them to insert new chain (or heavy steel bars with eyes) that the ground chain would be shackled to. I took my cue from your use of bored out granite with steel bars! What Im not crazy about is the idea of the ground chain being pulled around and over the square blocks. The only way to deal with that is to dive down prior to a blow on its way and make sure the chains aren’t wrapped around anything. Seems labour intensive, but not a huge deal in max 30′ of water. Then, there’s the big blows which will move the boat into whatever position along with the chains. So, no perfect solution as far as I can figure out, other than to monitor the movement and position of the chains..

Neil Clifford

Ya, it was the same peeps that both poured the blocks and dropped them. I can attach the chain running through the centre of the blocks directly to the ground chain. Good call. Thanks.