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

[JoeQ]

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

OK, I know all about Archimedes' Principle but the actual displacement of a vessel has nothing to do with the "Maximum weight" of a mooring. What is important is the vessel length and windage.

So, can somebody provide a detailed explanation. (I've already googled)

/forums/images/graemlins/ooo.gif

 

[fireball]

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

Max weight 15 tonnes means that it is certainly no problem for my little boat!! /forums/images/graemlins/laugh.gif

 

[VicS]

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

I would assume it meant the maximum weight or displacement of vessel it was intended to accommodate. However these limits are presumably set by some mooring authority or by the commercial enterprise operating them so these are the people to ask.

Maybe registered tonnage of course.

 

[Billjratt]

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

Weight (mass) is quite pertinant, in spite of a floating body weighing nothing. Imagine that the boat is being pushed around by the weather and brought to a halt by the mooring, weight times speed is what the mooring has to stop. And on top of that, wave-chug, once you get a few tons moving, they can take a lot to stop.....

 

[capnsensible]

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

Oi reckon this.. If your boat is !5T or less, this mooring is OK. If more, then its not. Or am I being obvious....

 

[john_morris_uk]

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

[ QUOTE ]
OK, I know all about Archimedes' Principle but the actual displacement of a vessel has nothing to do with the "Maximum weight" of a mooring. What is important is the vessel length and windage.

So, can somebody provide a detailed explanation. (I've already googled)

/forums/images/graemlins/ooo.gif

[/ QUOTE ]
The other responses have more or less explained all you need to know.

Although what you say is true - ie windage would appear to be the biggest issue as it is wind that produces a lot of the force the boat applies to the mooring, the reality is that the forces on a mooring are a complex mix of factors.

Tonnage crudely relates to windage, but that is almost irrelevant.

In real life, the major forces that are likely to damage/break out the mooring are snatch loads from the boat snubbing against the riser from wind and wave action. For these these loads tonnage of the boat is a critical component. Nuff said?

Perhaps this is a good example of why that Professor on the news recently was worried about the alleged power of 'Google'? One doesn't always find the answer one wants because one doesn't always ask the right question? Its also a good reason why a bit of elasticity in your mooring strop is a good idea.

 

[wotayottie]

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

There is the further factor of lift. Imagine the mooring at full stretch in a blow and along comes the harbour tug at 15 knots with a bow wave the average stinkie would die for. As the wave moves through your boat lifts, and being a 15 tonner it lifts with way more force than a 5 tonner. So it lifts the sinker and the mooring moves ........ That scenario depends on a lot of things, but its one way we lift loose mooring anchors at the club.

Anyway, all other things being equal, windage and length and beam go up with weight so placing a weight limit on it is a quick and dirty solution to saying what the limit is. They couldnt really put a sign on a mooring saying so many square metres of windage max and so many metres loa etc.

 

[Swampyhotdog]

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

My immediate thought on that question is this - it's really a rough guide to stop huge boats tying up to a mooring that isn't suitable.

 

[JoeQ]

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

A lot of guessing and no hard facts, just what I thought!

 

[chrishelen]

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

Yes but does he mean Thames tonnage when he mentions boat weight?

 

[jsl]

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

And another soft fact: '15 tonnes', spelt thus, ought to refer to displacement, but might be an error for 15 tons Thames Measurement. A 15-tonner (TM) might weigh over twenty tonnes, but is likely to weigh around 12. So now we are all confused.

 

[Aja]

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

[ QUOTE ]
My immediate thought on that question is this - it's really a rough guide to stop huge boats tying up to a mooring that isn't suitable.

[/ QUOTE ]

Ha Ha /forums/images/graemlins/wink.gif

Brightened my whole day up!

Donald

 

[john_morris_uk]

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

What exactly did you expect? You can come to some sensible decisions about the size of a mooring and forces on it - but its not an exact science as there are too many variables.

 

[JoeQ]

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

[ QUOTE ]
What exactly did you expect?

[/ QUOTE ]
I expected (and hoped) that someone would post a factual explanation. So far we still haven't had this.

Does the 15T rating mean the SWL breaking strain on the tackle? As prudent and safety conscious mariners we should be aware of the type and size of mooring we can attach to whether it's for a lunch stop or overnight. We should have a rough idea of the windage of our boats and the expected loading on our ground tackle or mooring in various rates of blow. If conditions were less than ideal I would use my own ground tackle but in light winds if I wanted a lunch stop somewhere I need to know the limits.

What is evident so far is that no one appears to know and some are taking unreasonable and possibly unsafe decisions on moorings.

[ QUOTE ]
but its not an exact science

[/ QUOTE ] Why? Mooring contractors charge for laying moorings. If one failed in catastrophic circumstances they would have to show how they calculated the rating. There are Council owned moorings in various areas of the West coast of Scotland, with "no win no fee" litigation spreading those responsible for moorings must be careful. Do you think they just pull arbitrary figures out of thin air?

 

[Billjratt]

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

[ QUOTE ]
A lot of guessing and no hard facts, just what I thought!

[/ QUOTE ]

Just how hard do the facts have to be? The moorings authority/owner is trying to indicate in a simple way that he doesn't guarantee the safety of the mooring for boats heavier than 15 tonnes. It's all to do with the chug/pull/snatch/lift function during a choppy blow, where the force to decelerate a mass of up to 15tonnes from a speed of say, 2 metres/sec vertical plus constant horizontal wind pressure of (speed X area-squared) to zero, requires an energy absorbing force created by a catenery of ground tackle plus whe weight of the riser both of which must have a breaking strain commensurate with the snatch loads. Now that is a load of waffle, but it's enough to let you know what's going on below you next time you "borrow" an unmarked mooring because you can't get the anchor to hold...
If you still don't understand,...... Get someone to push your car along a level driveway, with a tow rope attached. Let it coast, then grab the rope and try to stop it without moving your feet. /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif

 

[JoeQ]

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

Sorry, but I think you're guessing................

 

[Billjratt]

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

I'm taking this big key out of my back now.

 

[Bilgediver]

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

What is important is the vessel length and windage.
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I don't think so!!!!!!!!!

I'd be more interested in the displacement weight of the boat,ship. This is what the mooring has to cope with under surge conditions and as this also relates to the underwater wetted area then it will also be a good indicator for tidal effect.

A submarine has small windage but you wouldn t get away with a small mooring!!!!

 

[Bilgediver]

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

OK, I know all about Archimedes' Principle but the actual displacement of a vessel has nothing to do with the "Maximum weight" of a mooring. What is important is the vessel length and windage.

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You can consider the forces on a mooring being produced from a combination of three basic sources...

there are tidal forces on the hull.....Wind forces on the above waterline.......Surge forces generated by movement of the boat due to wave action and surging around.

The mooring has to be designed to take all of these into account and is not just a matter of picking the heaviest weight as the surge reaction can be accomodated by the catenery effect of the chain which is also designed for the specific water depth where the mooring is intended to be used.

In shallow water ther will be little or no catenery effect and so a heavier mooring will be necessary than in deeper water where thaer is a much larger catenery effect.

In you question you were very short of facts and didn t give the context in which the figures were discussed...Ie did you say you had a 15 ton boat......or did some well intending mooring supplier se a picture of your boat,estimate its displacement and tell you that you needed a 15 ton mooring!!!!! Two totally different figures!!!
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Google will usually come up with a good answer but you do have to ask the right question.

This is how CAnuks do it!!!

 

[VicS]

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

[ QUOTE ]
A lot of guessing and no hard facts, just what I thought!

[/ QUOTE ] Then do as I suggested (2nd reply to your original question) ask the people responsible for the mooring.

They are the people who will know. If they dont then nobody will!

I f you thought you'd get loads of guessing and no hard facts why ask on here in the first place. /forums/images/graemlins/confused.gif