Describing a mooring & a bit re anchoring

[asteven221]

This might appear a daft question but here goes. What is actually meant when a mooring is described as a "15 tonne" moooring?

1. Max load on the mooring from the boat pulling on the mooring?
2. The dry weight of the boat?
3. The displacement of the boat?

And how do they test it to come up with these figures?

Seems a bit of a random description to me. If it referred to "1" then that makes sense - to me at least!

Anchoring. No nothing to do with Delts vs Rocna vs Manson etc...etc... but I am confused about something. After ages on the internet looking at all things anchoring, why do so many of the experts give very different recommendations for anchor rode and scope for what seems the same application?

I am not talking anchor types, just basic techniques? Even the manufacturers of anchoring gear make recommendations that are totally different. So do books. I thought I would try and apply some science to it and get some figures. It struck me immediately that the maximum loadings caused by the pull of the boat on the anchor gear are relatively low. A sweeping generalisation I know but I am thinking GRP boats sub 50ft with wind up to F7 or more likely up to F6. None of the loads appear to get anywhere close to the breaking strain of any component commonly used, assuming reasonable anchoring gear.

For example if my memory serves me right, MBM (or maybe YM) said that they gave up at 1200kg when trying to break out a Delta anchor for fear of causing damage to the test boats test gear. A test in a US mag said that their 45 foot yacht placed a load of 635kg on the rode at 23 knots of wind periodically during surges. These sorts of figures are easily accomodated by ropes that are much thinner then any of us would use, so why do we (me actually) worry things are going to break? Is it an irrational fear?

Maybe I should cut to the chase. What max wind/depth combination would you be happy to anchor in if you had my 44ft Sealine with 40m of 10mm chain with an additional 30m of 18mm polyester rope? I guess that's really what I need to know!

 

[jimg]

Ah, but you forgot to tell what was attached to the chain and rope!

Is it a fishing hook or a redundant anchor off of a cruise liner or something in between?

 

[Richard10002]

70m rode = 10m depth - less distance from roller to water.

Beyond 7:1 the angle between the rode and the bottom changes little.

 

[benjenbav]

Oddly or maybe not, before I read Richard's answer I was about to say 8-9m depth (because I'd already made an allowance for the distance between roller and water).

 

[asteven221]

Ah, but you forgot to tell what was attached to the chain and rope!

Is it a fishing hook or a redundant anchor off of a cruise liner or something in between?

Thanks for the response. Does it matter? When someone says as a statement of fact that the mooring is a 15 tonne mooring, what do they actually mean? 15 tonnes displacement, dry weight, load on mooring? Thinking about it, I guess it's not the load as 15 tonnes would be enormous. My hunch is displacement, but I would be interested to know.

 

[asteven221]

Oops jimg, I thought you were referring to my mooring query. Maybe you were referring to my anchor query. I have a 10kg Delta, with not chance of changing to something heavier due to Sealines bow design. Sorry for my confusion.

 

[jimg]

Oops jimg, I thought you were referring to my mooring query. Maybe you were referring to my anchor query. I have a 10kg Delta, with not chance of changing to something heavier due to Sealines bow design. Sorry for my confusion.

Indeed I was, I only asked because as you are obviously aware anyway that the supplied anchors are often under spec. for anchoring in poor conditions.

 

[prv]

This might appear a daft question but here goes. What is actually meant when a mooring is described as a "15 tonne" moooring?

1. Max load on the mooring from the boat pulling on the mooring?
2. The dry weight of the boat?
3. The displacement of the boat?

Something in the vicinity of 2 and 3. It's not an exact science.

Obviously a lightly built towering motorboat with balcony on top of patio on top of conservatory will have far greater windage than a chunky old gaffer of the same weight that's mostly below the waterline. But what else are you going to paint on the buoy? No good saying "2 tonnes max pull", as nobody will know how much load in figures their boat applies, never mind the variation of the coming weather. So a limit on size of boat gives a rough guide.

Pete