Mooring installation - 1 big or 3-4 smaller helical screws?

[Davidhoy]

I'm doing some research in installing a semi-permanent mooring for a 47ft boat, and looking at anchoring options. I could go with a single big 1.5" square-shaft helical screw going down 10ft into the bottom, or 3 or 4 smaller 3/4" round shaft screws going about 5ft down with a bridle and swivel connecting to the down chain. Question is, which would be better/stronger?

-David

[Shrew]

There is no guarantee you've put pulling force on multiples equally, unless you're lifting straight up. I don't see how you'd distribute weight across all of them. That means you'll be loading a subset of the group of moorings. Can the boat be held with just two of the smaller moorings?

[Manateeman]

Hi. I see you are in Florida. I might be very wrong on this but my understanding is that the state of Florida owns almost all the submerged lands.
I think you have to obtain permits from the US Army Corp of Engineers in some places. I’ve been asked if my mooring ball was a mooring as it does not have any official numbers on it and I told the Marine Patrol Officer it was a big anchor...which it is...and that my 2500 # windlass could bring it up if he wished.
He didn’t ask anything else and was polite and left.
Official mooring fields are suppose to have buoys with lights to show where they are but a lot do not. I anchored “ inside” one such field with Helix moorings but lost balls. No one said anything in the city which managed it but you would have an interesting case in Admirality if you hooked a chain in an improperly marked mooring field or aquaculture area. I’ve seen Helix moorings fail because people just don’t pay a diver to inspect them and change shackles. Give me a big dumb Dor-Mor or an anchor I can pull up and look at anytime.
Helix screws and bungees work well in very tight harbors where they get inspected every year. I’m not sure why it’s your choice so please add a bit more about your thinking and what’s the bottom, wind , etc in where you intend to put your boat. The USACE knows a lot about this subject...I’d check it out first.
Happy trails to you.
Captain Mark and his very sad manatee crew mourning the death of yet another Right Whale killed by entanglement.

[StuM]

Originally Posted by Shrew

There is no guarantee you've put pulling force on multiples equally, unless you're lifting straight up. I don't see how you'd distribute weight across all of them. That means you'll be loading a subset of the group of moorings. Can the boat be held with just two of the smaller moorings?

Or even one in the worst case. I can easily visualise a situation where the the small screws get loaded and pulled out one by one as the vessel is moved around by strong winds.

[vpbarkley]

If the load can be distributed among them, multiple screws are superior to a single screw. If a single screw fails for any reason the mooring is failed. In terms of failure tolerance, multiple screws with multiple chains makes sense. The loss of a single screw/chain doesn't disable the mooring, assuming there is adequate capacity in the remaining screws/chains. Periodic inspection is necessary to assess the status of the mooring.

[MJH]

Originally Posted by Davidhoy

I'm doing some research in installing a semi-permanent mooring for a 47ft boat, and looking at anchoring options. I could go with a single big 1.5" square-shaft helical screw going down 10ft into the bottom, or 3 or 4 smaller 3/4" round shaft screws going about 5ft down with a bridle and swivel connecting to the down chain. Question is, which would be better/stronger?

-David

I have no experience with helical screw moorings as I have used a three legged mooring system since 1995 using Danforth anchors for the mud bottom. I would contact the screw manufacturer or posing a question to a commercial installer for some incite.

I assume the 3-4 screws would be in the same immediate area. That said, I would think bigger and deeper is better than smaller and shallower BUT there is no redundancy while multiple screws in the same bed area doesn't sound secure to me.

However, if you spread the screws out over a larger area, resulting in longer rodes and redundancy, the affect may be the same as what I am getting; each leg of my system is 3/4" x 300' nylon three-strand leading to a center chain bridle and angled to prevailing North, West, and East winds. It is a permanent 24/7/365 system. At the boat's last haul-out it weighed 32,545 lbs.

Good luck.

~ ~ _/) ~ ~ MJH

[skipmac]

I would concur with the previous comments. On the helical mooring I think it highly likely that you would pull out the smaller one at a time.

I also checked on moorings in FL and ran into the same issue with permits required by Corp of Engineers, etc. I eventually found a cheap dock so didn't follow through but I was going to do like Manateeman and just get a great big anchor and drop it where I wanted to moor.

[David B]

Originally Posted by Davidhoy

I'm doing some research in installing a semi-permanent mooring for a 47ft boat, and looking at anchoring options. I could go with a single big 1.5" square-shaft helical screw going down 10ft into the bottom, or 3 or 4 smaller 3/4" round shaft screws going about 5ft down with a bridle and swivel connecting to the down chain. Question is, which would be better/stronger?

-David

A single big one.
The smaller ones will at some stage each take the full load, depending on wind and tide, and in extremes could slowly work out (I doubt it, but ...)

Also, you have the potential for chafe or wear with a bridle between them all.

[Jim Cate]

Another point favoring the big one: it goes much further into the sea bed than the smaller ones, and thus is likely to get into much firmer soil with more reliable holding power.

The redundancy issue is a red herring IMO. Any mooring must be serviced regularly, checking for wastage of chain or shackles or swivels and condition of the riser. If maintained, breakage is quite unlikely. And likely it will be less expensive to do one big one than several smaller ones... just a matter of diver time, for the parts are pretty inexpensive.

Opinion only... no expertise or experience.

Jim

[MJH]

Originally Posted by Jim Cate

Another point favoring the big one: it goes much further into the sea bed than the smaller ones, and thus is likely to get into much firmer soil with more reliable holding power.

The redundancy issue is a red herring IMO. Any mooring must be serviced regularly, checking for wastage of chain or shackles or swivels and condition of the riser. If maintained, breakage is quite unlikely. And likely it will be less expensive to do one big one than several smaller ones... just a matter of diver time, for the parts are pretty inexpensive.

Opinion only... no expertise or experience. Jim

Disagree. Many spend thousands, perhaps hundreds of thousands, and try to get by cheap with their mooring system...not a wise exchange. " A chain is only as good as the weakest link" goes the saying. A single attachment puts all your eggs in one basket so you better protect that attachment. With a three legged system you have more support and should one leg fail the other two will save the boat...I sleep well at night.

After 25 years mooring my boats I learned quickly that 3/8" galvanized shackles threads will rust well within three years so upgraded to 7/16" and religiously service the bridle system every three years...I don't use a diver or dive myself...everything is done on the surface and used parts are evaluated for wear with changes made where appropriate. Once I upgraded to a larger boat all items of the system were upgraded as well. A system may work well during normal weather cycles but it must be ready for that that exceptional storm. I learn something new with every maintenance completion.

Lastly, while the cost to install a three-legged system may appear high to some, once divided by time the life cycle cost becomes very, very affordable.

~ ~ _/) ~ ~ MJH

[Jim Cate]

Originally Posted by MJH

Disagree. Many spend thousands, perhaps hundreds of thousands, and try to get by cheap with their mooring system...not a wise exchange. " A chain is only as good as the weakest link" goes the saying. A single attachment puts all your eggs in one basket so you better protect that attachment. With a three legged system you have more support and should one leg fail the other two will save the boat...I sleep well at night.

After 25 years mooring my boats I learned quickly that 3/8" galvanized shackles threads will rust well within three years so upgraded to 7/16" and religiously service the bridle system every three years...I don't use a diver or dive myself...everything is done on the surface and used parts are evaluated for wear with changes made where appropriate. Once I upgraded to a larger boat all items of the system were upgraded as well. A system may work well during normal weather cycles but it must be ready for that that exceptional storm. I learn something new with every maintenance completion.

Lastly, while the cost to install a three-legged system may appear high to some, once divided by time the life cycle cost becomes very, very affordable.

~ ~ _/) ~ ~ MJH

Redundancy: even a three legged mooring will have but one riser... and some form of joiner for the three legs... an additional failure point.

And surface inspection of the mooring without a diver is kinda difficult with a screw mooring, and that is what the OP was asking about.

And lastly, your cost analysis, where you "divide by time" is true no matter how many legs the mooring may have. And a three year maintenance schedule is pretty slack. Annual inspection is recommended, and down here in Tasmania, I believe bi-annual is mandated.

Jim

[Benz]

I prefer one big anchor point to multiple small ones. If it's big enough to not need a backup (that is, has a big safety margin built in), and it's maintained correctly, it's what I prefer. I'll go to a single helix next time I get a mooring.
Multiple artificial anchor points, even when equalized (rockclimbers are obsessive about this) do not equal the security of a single solid tree or rock pillar. Few will belay from a single piece of hand-placed gear; but all will belay from a single tree.

[MJH]

Originally Posted by Jim Cate

Redundancy: even a three legged mooring will have but one riser... and some form of joiner for the three legs... an additional failure point.

And surface inspection of the mooring without a diver is kinda difficult with a screw mooring, and that is what the OP was asking about.

And lastly, your cost analysis, where you "divide by time" is true no matter how many legs the mooring may have. And a three year maintenance schedule is pretty slack. Annual inspection is recommended, and down here in Tasmania, I believe bi-annual is mandated. Jim

Yes, there is one riser (bridle) but that bridle is 1/2" galvanized long link chain and the three legs are connected via 1/2" galvanized shackles.

Yes the OP was talking about a possible screw mooring; I was giving my opinion based on my 25 years experience using off the shelf materials on a multiple-leg system which he is considering.

My three year maintenance schedule is based on my personal annual inspections and modifications over the first three years. After that each inspection added to my knowledge base and upgraded where needed. It was not based on some bureaucratic reasoning.

Lastly, mine is a permanent system used 24/7/365. I keep very detailed records, note all modifications, and expenses. My average cost over 25.6 years has been less than $15 per month which beats the hell out of anything else except perhaps keeping a boat on a trailer.

~ ~ _/) ~ ~ MJH

[Jim Cate]

Lastly, mine is a permanent system used 24/7/365. I keep very detailed records, note all modifications, and expenses. My average cost over 25.6 years has been less than $15 per month which beats the hell out of anything else except perhaps keeping a boat on a trailer.

Exemplary usage, but your costs for your system are not related to the OP's proposal for screw based moorings. There are many reasons he might need to use such rather than mooring blocks, including bottom type, grass issues and local mandates, or even the availability of a mooring tender capable of servicing a conventional mooring.

Finally, the long term success of your mooring is great, and I'm glad that you are so happy with it. However, that record does not mean that his proposed single point screw mooring would not be equally good (or his three point screw mooring, for that matter).

I'll continue to believe that the single deep mooring screw, if well designed and maintained, is a good solution for the OP.

Jim