Moorings - What does "Maximum weight is 15 Tonnes" acually mean

Re: Moorings - What does \"Maximum weight is 15 Tonnes\" acually mean

Seems to me that he got a few sensible replies that tried to explain how things work, but didn't like them.

There's no need to be rude.

The bottom line is that 15 tonnes almost certainly refers to the maximum displacement weight of vessel that the mooring is 'designed to hold'. A slightly crude measure, but about as good as it gets for the various reasons suggested.
 
Re: Moorings - What does \"Maximum weight is 15 Tonnes\" acually mean

It wasn't that I didn't like the responses, I just couldn't find any evidence to support them.

I thought it might have referred to the maximum load on the mooring.

So I would need to have a rough idea of the wind loading. Take my boat for example with very rough figures;

Beam 20 ft Freeboard 12ft gives us approx 240 square ft
Add an arbritary figure for wind loading on the mast and rigging of, say, 100 square ft. I now have 340 square ft. Lets add say 20% for yawing brings us to approx 400 square ft wind loading.

Now if we factor in a wind loading at say 30 knots is 4.8 pounds per square foot there would be a force of 1920 pounds pushing my boat back (assuming it is bow to the wind)

If the wind picks up to 40 knots the wind loading becomes 8.5 pounds per square foot. The force on my boat would now be 3400 pounds.

All well within the rating of the mooring posed in my question. PERHAPS!!!

Problem is, my boat is 40 ton so............

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Re: Moorings - What does \"Maximum weight is 15 Tonnes\" acually mean

As already pointed out, windage is insignificant. Using a mooring marked "max 15 Tonnes" with a 40 ton vessel is just asking for trouble. If it breaks or lifts, owner will be asking you to pay for replacing it. I guess that's not what you want to hear, but it is common sense I'm afraid.

Morrings don't usually break in strong winds alone. They break or lift with short steep seas (typically wind over tide). That is why my mooring broke (with considerable damage to VERY chunky bow roller & fitting) only when the tide turned & the sea level fell, The seas got shorter & steeper. As already stated by others, it was then that the snatch loads on the strops & bow fitting became excessive, the anchors lifted, both strops eventually broke & the bow fitting was bent at right angles. Fortunately, no damage that couldn't be easily repaired.
 
Re: Moorings - What does \"Maximum weight is 15 Tonnes\" acually mean

Isn't it Occam's razor that says something like "the simplest solution is the best"?

Using that as a guide, when you float up to a mooring that says "maximum weight 15 tonnes" (your words), it is more likely that this refers to the weight (displacement) of the boat, rather than the maximum load the mooring can sustain. As you point out, calculating the maximum load is quite complex and requires an estimate of windage area, wind speed, force loadings resulting from those, current conditions, and snatch loads resulting from the boat sailing around the mooring and chop - to name a few. IMHO the simpler solution is the more sensible one.

You are correct that no-one has "hard facts" in that there is not (to my knowledge) any regulation or even widely recognised industry practice regarding the marking of moorings. Someone can, of course, correct me on this if I am wrong.

What people have provided you with is their best guess of what this means based on the extremely limited information you provided and a pretty healthy dose of common sense. Criticising the responses does you no credit.

As Vic says, if you really want to know what it means, ask the man that laid the mooring and put the sign on it!

If you want hard facts
 
Re: Moorings - What does \"Maximum weight is 15 Tonnes\" acually mean

Reminds me of a col regs problem and modern interpretation, what else but displacement would it mean, anything else is up to personal interpretation.

With a 20 foot beam and 12 foot freeboard, one must question the dispalcement, as well as windage on a 15 tonne ( or is that 15 ton ) mooring.

Brian
 
Re: Moorings - What does \"Maximum weight is 15 Tonnes\" acually mean

[ QUOTE ]
Criticising the responses does you no credit.

[/ QUOTE ]

I didn't critisise any of the responses. I had a valid question and opened it for debate. Just because I chose the figure of 15 tonnes doesn't mean that I had an actual mooring in mind or that it was even for my boat! We still haven't had somebody come on here and quote chapter and verse what the situation is.

When I said there had been a lot of guessing, that included me, I'm the first to admit that I don't know. or I wouldn't have asked the question!!!
 
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