does any one know how to work out the wieght for a mooring for a 13 tonne boat?

Question

what should the mooring block wieght should be???

Answer 1

There are several factors that come into play on the weight of a mooring block.

There is a pretty good article in the march/April issue of Good Old Boat magazine (www.goodoldboat.com)

The bottom type, water depth, shape of block, material of block, exposure and boat comes into play. The size, shape and type of boat makes a huge difference too. sailboats typically have bigger anchors and tackle on them than cruisers.

A 55' cruising sailboat in 20' feet of water, mud bottom with moderate exposure. With a 55 foot heavy chain would need approximately 550 mushroom anchor or approximately a 3000 pound concrete block.

this is making alot of assumptions too, would ask what locals use first.

Answer 2

In researching my book " A Cruisers' Guide to Hurricane Survival I found a bunch of information on forces to expect and loads on deck hardware.....

LOA Beam Wind 15k 30k 42k

40 14 300 1200 2400 pounds of pull

50 16 400 1600 3200 pounds of pull

notice force goes up as the square of the wind speed; 30 knots is 4 times 15 knots; 42 knots is nearly NINE times 15...so if you want to put in a mooring that will withstand an autumnal gale of 60 knots, multiply the 15 knot numbers by 16 ( yes SIXTEEN)

In the book I also was allowed to use some tests from BOATUS:


"While the folks at BOATUS were mangling anchors, they also decided to look at moorings. What they found was rather shocking to those of us that tend to apply some sort of concept of strength and permanence to the word mooring:



Table 4

Mooring HelicalScrew Manta Mushroom Concrete

2000lbs 8000lbs
Pounds 12,000 12,000 1,200 800 4,000


The tests were done by having a tug attached to the mooring; the line was on a strain gauge; the tug was put into gear and pulled on the mooring set up till they hit 12,000 pounds or the mooring pulled free. Notice how shockingly poor concrete did...the moral is weight means nothing; its how well a mooring is engineered to withstand pull out......

yes thats right it only took 4,000 pounds of pull to drag out a 8,000 pound block of concrete, which is, after all ., mostly sand and air....

So, a the other guy said whats the bottom: mud for an anchor to bury in; hard sand, loose ooze like in a river? a 500 pound mushroom if its buried would be a good start; two or three 60-70 pound Danforths or CQRs set in a triangle would be better; all three say with 3/4 inch chain lead to a central VERY BIG shackle; then 1.5 to 2 x the water depth in 3/4 inch chain attached to 3 x the water depth in say, 1/2" chain, which leads up to the mooring ball and 3/4"nylon pennants from there to the deck cleats would be a pretty adequate set up