Mooring capacity

[Gordonmc]

I may be missing something, but...

A yard neighbour has negotiated a swinging mooring for a Colvic of about 6 tons. In conversation he mentioned it was a shame my boat at 8 tons displacement was too heavy for the tackle.

It started me thinking, but not wanting to appear that I was starting an argument I let it go. Perhaps RTR can help: why is a boat's weight relevent to the capacity of the ground tackle?

I can understand that windage would be important in effecting forces acting on the gear and so, in current, would the underwater profile of the vessel . But in dead still water on a windless day all vessels will exert the same lateral forces on the mooring.

Conversely with a 2 knot tide on a windless day a 15 ton catamaran will exert less force than a deep keel 6 ton monohull.

Someone put me out of my misery, please.



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[jimi]

There was an earlier post which raised exactly the same question in my mind, with windage and wetted surface area being the two factors I considered. The thing that did occur to me was that the heavier the boat the more windage and wetted area she'll have.

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[poggy]

It is probably just an easy way of applying a rule. They use a weight cut off as the simplest way to calculate the stress on the ground tackle for the reason Jimi mentioned.

I think if you discussed it with them, you may be able to get round it, that is of course if their insurance doesn't have the clause in.


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[mainshiptom]

Bigger boats have bigger anchors !

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[mainshiptom]

Bigger boats have bigger anchors !

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[snowleopard]

snubbing...

is when a mooring will break. the heavier the boat, the harder the pull when it's brought up with a jerk.

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[colvic]

Our Colvic 26 suffers far more from windage than many 33ft. boats we know because of the height above the water line. If the Colvic in question is a 26ft Watson which comes in at about the 6 tonnes, then the ground tackle must be quite light if it will not go beyond 8 tonnes as we have sat-out force 9's in the Med on 60 meters of 5/16 chain BUT with a snubbing line...makes an enormus.


Phil

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[SailingDoc]

Clearly, on a windless, currentless mooring, there is no strain on a mooring line - in theory you could 'moor' a supertanker by a piece of thread, just so long as the ship does not try to move.
If there are forces, the mooring chain must be up to the job; the forces, I think I'm right in saying, are proportional to the surface area facing the force (i.e. the frontal area) and the square of the velocity of wind or current. Also, a heavier ship has more inertia (reluctance to move) but also momentum (force required to stop it moving) and these must be factors in choosing a suitable mooring chain.
I wonder, in addition, if one should take into account the effect of prolonged immersion on the chain - so you can't compare it with anchor chain, size for size.

Brian

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