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

 

[rb1957]

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 ?

 

[berkshire]

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.

 

[rb1957]

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 ?

 

[Compositepro]

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.

 

[berkshire]

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.

 

[rb1957]

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 ?

 

[msquared48]

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

Mike McCann, PE, SE (WA)