Building a Mooring for 30' Mono

[pressuredrop]

hey guys i just bought this boat:
http://www.alliedseawindii.org/boats/SWI-025/SWI-025.html

untill i sell my other boat and can afford to haul it out, i will be doing as much work as i can with it on a mooring. got a quote of $1000 for a helix screw one, not happening. best case scenario, i can find an abandoned mooring (There are quite a few in the area) make sure its good and use that.
worse case scenario i will be building a concrete one. now the question is how heavy does this need to be?, and what is the ideal shape?, pyramid or cube/rectangle?
i dont expected this to hold to boat through a hurricane just something that will hold up to the 2-2.5knot tide current and winds up to 40 (not common). i will only be using this for 3 or 4 months tops, but want a good margin of saftey. so say yee?

[Randy]

The issue with a concrete mooring is you build it on shore and it's heavy. What do you have to take it out & place it with? I'd inspect the abandoned ones, maybe refit one.
Maybe 500lb & good scope or 3 anchors placed at 120 deg from each other w/ good scope.

[pressuredrop]

if 500lbs is all i need then i think its doable, i have a rhodes 19 that i made into a power skiff with a 15 horse, and 20' seacraft with a 140 (which i am selling) but could use either to get it out there, i would like to use the rhodes because it is a beater, what i might do i build it on the boat then just roll it off the deck...

the 3 anchors is another idea, what size anchors would i be looking at? i have a few old spares but they are pretty small.

i guess i will evaulate the cost v.s. ease for each way and find the happy medium

Thanks
Ben

[captjohn360]

i made ome in the bahamas.make a 4 sided form about 5ft square and line it with plastic sheet.10 bags of quickset and a piece of pvc running through the center.run chain through the pipe to moor to. make it at the edge of the water when its set push it tn the water.get a lift bag or a 55gal drum tie it off fill with air float to where you want sink it.10bags will equal about 700 lbs in salt water....jt

[feelsgood]

I have used a truck wheel with the shallow side filled with concrete and the deeper side settled on the bottom . not only will it sink in the sand or mud but you get terific suction. I had a 6 ton yacht on this for many years in the british channel islands where it was subjected to 5 knots of tide and several times force 8 winds and it never moved

[Hydra]

Pressuredrop
In my area, the authority in charge of moorings recommends that the weight in air of the concrete block be at least one third of the boat displacement, because the apparent weight in water is much less. In the mooring I bought for my own 30" boat, the concrete block is just under 4000lbs (1.8t). It held the boat in 40+kt gales with 4ft waves

I got it second-hand from another yachtsman who has a boat of about the same size. Since the bottom is hard sand with 2-3 kts tidal stream, the block hasn't settled in 6-7 years, it is still above ground. Then, only its weight holds the boat.

Alain

[GordMay]

Originally Posted by Hydra

In my area, the authority in charge of moorings recommends that the weight in air of the concrete block be at least one third of the boat displacement, because the apparent weight in water is much less...

A dead weight anchor (mooring block) may gain some small advantage from suction in a mud bottom, otherwise it’s holding power is completely dependent on its weight, or more exactly on it’s submerged weight, which in the case of a concrete anchor is almost half* its dry weight.

* The Material Displacement Factor for Concrete is 0.55

Hence:

Concrete Dry Weight Required = Submerged Weight Required ÷ 0.55
ie:
If an apparent submerged weight of 1000Lbs is required:
Then 1818# dry weight is required
(1818 D.W. = 1000# S.W. ÷ 0.55)

Since concrete typically weighs about* 150 Lbs/Cubic Foot, the 1818 Lb (dry) block would be (1818 ÷ 150) about 12.12 cubic ft (or about: 3.0' x 3.0' x 1.5')

* Concrete weight depends primarily on the density of the aggregate used. (Could vary between 75 Lb/CuFt to 160 Lb/CuFt)