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How heavy should a boat mooring be ?

How heavy should a boat mooring be ?

(OP)
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

RE: How heavy should a boat mooring be ?

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.

RE: How heavy should a boat mooring be ?

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.)

RE: How heavy should a boat mooring be ?

Have you checked Chapmans?

Mike McCann, PE, SE (WA)


RE: How heavy should a boat mooring be ?

why wouldn't you use an anchor ?

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

RE: How heavy should a boat mooring be ?

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!!!

RE: How heavy should a boat mooring be ?

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.

RE: How heavy should a boat mooring be ?

(OP)
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

RE: How heavy should a boat mooring be ?

(OP)
Thanks Berkshire, do you have a link for Chapmans.

RE: How heavy should a boat mooring be ?

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.

RE: How heavy should a boat mooring be ?

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.

RE: How heavy should a boat mooring be ?

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

Mike McCann, PE, SE (WA)


RE: How heavy should a boat mooring be ?

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.)

RE: How heavy should a boat mooring be ?

(OP)
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.

RE: How heavy should a boat mooring be ?

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

Thanks boo1 and all.
Dave

RE: How heavy should a boat mooring be ?

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


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RE: How heavy should a boat mooring be ?

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.

RE: How heavy should a boat mooring be ?

(OP)
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.

RE: How heavy should a boat mooring be ?

who on earth would make a rule saying anchor line should not touch the sea floor ? not a sailer ??

so you're going to use a large float to suspend your anchor line so that it doesn't touch the blessed grass. how do you show that the line doesn't touch in the lowest trough and highest wind ?

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

RE: How heavy should a boat mooring be ?

rb1957,
He is in an area that has eelgrass conservation laws/rules. chains dragging around on the bottom destroy the eelgrass.
This is where the triple block anchors work better because the chains don't move around.
B.E.

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

RE: How heavy should a boat mooring be ?

ok, use block anchors. but keeping the chain (or rope) off the sea floor creates a quite difficult problem ... in any sea state. problem much simpler if it is just in the static (still water, still air) condition.

or do you meant three anchors (so the boat doesn't move) ?

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

RE: How heavy should a boat mooring be ?

The chain only acts as a shock absorber because of of the sag caused by its weight. This also greatly lessens the uplift on the mooring weight making it far more effective. Floating the chain is pointless.

RE: How heavy should a boat mooring be ?

rb1957,
No, a permanent mooring is made by putting 3 anchors ( Or blocks.) on the seafloor in a equilateral triangle with the anchor chains going to the centre of the triangle, at this point a swivel is attached, then a Nylon rope goes up to the boat at a length for the correct scope, as the boat moves around, the 3 anchors take the load one at a time according to the wind/tide/current. The chains only lift up and down with the load, and only disturb a small section of the seabed ,
this is the preferred method in an eelgrass sensitive area , compared to a single chain from a mooring which will sweep around in a circle on the seabed tearing up anything in its path.
B.E.

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

RE: How heavy should a boat mooring be ?

and a clever arrangement ... there would be some movement of the chain on the sea floor, but not much.

there's a world of difference between "none" and "some". there'd be some slack in the rope, hence some drag on the grass ... I was picturing buoys to hold the cable off the sea floor. and the blocks are permanent

I didn't think you'd want 3 different anchor points

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

RE: How heavy should a boat mooring be ?

Now you know why it's called the Devil's Triangle. Something had to anchor the story!

Mike McCann, PE, SE (WA)


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