How heavy should a boat mooring be ?

[TMcRally]

Hi all

It's way beyond me to figure a way of calculating the weight of the block that I should drop to secure the boat to or even an estimate. I was just hoping someone may be able to help.

The boat = 50' catamaran, small windage possibly 35m^2 sideways and 18m^2 head on.
Weight = 8t
The mooring will be approx. 7300kg/m^2
Sea floor = sand
Depth = 6m to 8m
Line from weight to buoy length = 24m
Expected max wind = 60kn / 111klm / 69mph

Anyone care to have a go ?

Thanks
Dave

 

[geesamand]

We've studied this problem for mooring industrial equipment in ponds. The huge question is how much friction exists between the sea floor and your weight block.

With sand you might enjoy the mooring sinking into the sand, forming a bit of suction. That would be beneficial.

In any case I think you'll need a sensible way to account for this. Perhaps someone with maritime experience can answer.

 

[Artisi]

I would agree, the block will settle into the sand and the line from the block should start out as heavy link chain prior to joining the "rope" mooring line, the chain besides being less likely to be abraided away adds massively to the mooring capability. An 8ton block and chain correctly installed would probably hold the USS Enterprise.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[msquared48]

Have you checked Chapmans?

Mike McCann, PE, SE (WA)

 

[rb1957]

why wouldn't you use an anchor ?

another day in paradise, or is paradise one day closer ?

 

[DSB123]

Artisi,
Hope you were on about the naval aircraft carrier USS Enterprise rather than Starship USS Enterprise, serial number NCC-1701, of the United Federation of Planets as I have never seen the later in water!!!

 

[berkshire]

TMcRally and msquared 48,
I just checked Chapmans I have copies of the 52nd and 53rd editions , Their recommendation is a star mooring with three mushroom anchors, there is also a size and rope diameter table there . This occurs on pages 114 and 115 of the 52nd edition.

B.E.

You are judged not by what you know, but by what you can do.

 

[TMcRally]

Thanks all, best guess ?

I have talked to a number of companies that drop moorings and have had 4 replies. 800kg, 1400kg, 2000kg, and 5000kg and breaking strain line from weight to buoy of 12000kg to 30000kg.

It has to be rope for Eco reasons to stop the surrounding damage to sea grass. I take your point on the chain and will run a metre of chain so it can't damage the grass and hopefully reduces the chafing on the rope.

The weight is made from old worn out train wheels so I do expect them to settle in the sand to an extent.

Chapmans ?

Can't use an anchor in a storm over 30kn for insurance.

Regards
Dave

 

[TMcRally]

Thanks Berkshire, do you have a link for Chapmans.

 

[berkshire]

TMcRally ,
I just have the printed books here, the book is published by Hearst corp you could do a Google search.
B.E.

You are judged not by what you know, but by what you can do.

 

[berkshire]

TMcRally,
Amazon has some copies new and used starting at around $30.00 .
B.E.

You are judged not by what you know, but by what you can do.

 

[boo1]

check ABS mooring part3

 

[msquared48]

Chapmans is the nautical bible for boaters. Every boater should have one.

Mike McCann, PE, SE (WA)

 

[GregLocock]

A metre of chain is far too little. We use about 4m on a 21 footer. It is your main shock absorber.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Artisi]

I would opt for 4 - 6 metre of chain minimum and can't see chain doing any more damage to the sea grass than rope.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[TMcRally]

The chain isn't allowed to touch the ground. The rope has to be one that floats.

I'm going there this weekend so I'll dive on others and see what they have, they are rentals though and for much smaller boats in calm weather only.

 

[TMcRally]

I'll go through boo1's ABS work through and see where I end up.

Thanks boo1 and all.
Dave

 

[GregLocock]

I suggest you use two buoys and put a loop of chain between them, short enough that it doesn't touch the seagrass. I cannot over emphasise the huge benefit of a decent length of chain, especially if this is a permanent mooring.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Denial]

Take heed of GregLocock's comment on the shock-absorber effect of a length of chain.[ ] Particularly if your line is made of something that has close to neutral buoyancy.

 

[TMcRally]

I take your point with the chain, thanks for the tip. I could fit a max of 10m of very heavy chain between two buoys, they also make snubbers a Bungy cord you attach across a loop in the chain.

I was thinking's of a rope with lots of stretch like nylon, but not nylon.