Yacht adrift from failed Studland eco mooring last night.

[Mark L]

But just as with anchor threads, boat weight is a stupid measure in the first place. The buoy isn't used to lift the boat. A raft with a 5 ton weight on it will have far less drag than a tall ship built from balsa.

I don't know for certain, but I would strongly suspect that dynamic loading is more significant to the ultimate strength (and therefore rating) of a mooring than static loads. In which case, weight (or more strictly mass) will have a major impact along with the other factors suggested.

 

[oldharry]

I don't know for certain, but I would strongly suspect that dynamic loading is more significant to the ultimate strength (and therefore rating) of a mooring than static loads. In which case, weight (or more strictly mass) will have a major impact along with the other factors suggested.

The windage on a mobo witha high flying bridge can be quite significant, and not in proportion at all to displacement and size. So the weight limit is meangless really. Would these moorings hold even a 5 tonner in a F10/11 squall? Specially if there is much of a sea running.

 

[ylop]

The windage on a mobo witha high flying bridge can be quite significant, and notn in proprotion at all to displacement and size. So the weight limit is meangless really. Would these moorings hold even a 5 tonner in a F10/11 squall? Specially if there is much of a sea running.

They have instructions not to be used in >1m waves. I think mass of the boat on a mooring IS potentially relevant, as the boat gets thrown around in wind and waves it accelerates and then jerks to a stop as if loads the mooring. The force required to stop a big mass of boat is greater than that required to stop a little one. Of course the cross sectional area of vessels is different and thus the load applied from constant wind speed is different, perhaps weight is a reasonable rule of thumb proxy because most people know their displacement but few know their cross section.

 

[oldharry]

All these factors add up, but until you have tried to pull your boat up to a mooring strop in even a fresh breeeze, its easy to forget how much added load wind applies. You know, that situation withthe strop in the bow roller and needing just another foot forward movement to slip it over the mooring cleat.... a gust comes along and gently but irrresistibly the boat pulls backwards and you have to let go before your hand gets trapped or damaged.

 

[Mark L]

The windage on a mobo witha high flying bridge can be quite significant, and not in proportion at all to displacement and size. So the weight limit is meangless really. Would these moorings hold even a 5 tonner in a F10/11 squall? Specially if there is much of a sea running.

I agree that mooring loads will be heavily dependent on air resistance and underwater resistance. My point was more that the peak loads (i.e. the loads most likely to snap or drag the mooring) will be dynamic. Lots of factors will influence this, but things like mass, moments of inertia and pitching moments will undoubtedly play a big role along with elements of the mooring design. There are lots of formal dynamic load models that will be applied by engineers for specific mooring applications but these are obviously impossible for a given yacht on a recreational mooring. The weight limits will either be empirical or based on a highly generalised load model. In either case, huge margins will be built in to account for the approximations and assumptions so I would be pretty confident that any yacht less than the rated weight will be safe on a well maintained mooring.

 

[dunedin]

Has there been any further objective / factual information on this reported incident?

Normally when a mooring fails there are two bits of key evidence remaining - the end still attached to the boat and the bit attached to the mooring. Examining these ends will normally help understand the cause of failure - eg.
- a shackle or attachment breaking or coming free
- a rope being damaged by a boat propellor or similar - perhaps before use by the boat that reportedly came free
- rope/ strop breaking due to pure load.

Would be good to have some factual data before the speculation goes too far.