Mooring block size

[Windwalker44]

I realize a lot has to be considered with tides and current but is there a formula that relates to mooring block size (weight) and boat displacement.
My Gulfstar is 26k displacement, should I worry about a 2-3k lbs. piece of granite ?

[jtsailjt]

Originally Posted by Windwalker44

I realize a lot has to be considered with tides and current but is there a formula that relates to mooring block size (weight) and boat displacement.
My Gulfstar is 26k displacement, should I worry about a 2-3k lbs. piece of granite ?

If that's the rock you currently have, it would take quite a stiff breeze for your boat to drag it, especially if it's located in a fairly protected harbor and is fairly flat in shape (large surface area) and sunk into the mud as opposed to sitting on top of a gravel or rocky bottom so there's no big rush to get rid of it. I don't know what the official guidance is but I told my mooring guy I wanted one size up from what he'd normally recommend for my boat in our harbor, and as a result I have a 5000# rock in a pretty protected location on a muddy bottom, and my boats displacement is about 23k, but I've always considered a big rock and heavier than recommended mooring chain and pennant to be cheap insurance. It's only a couple of hundred dollars more to upgrade by 1000# or so and granite doesn't wear out so it will last about forever if the staple is also sized large. If I were in your shoes, and owned a mooring like you mention, no big hurry, but I'd try to sell it to someone with a smaller boat, and when you eventually do, have a bigger stone put down there. Or, whoever takes care of moorings in your harbor may take your 2-3k rock in trade for a bigger one, especially if he knows someone who will want to buy it from him this spring. You may never need it but if you do...

[Hydra]

In my area, the administration in charge of moorings recommended a concrete block half the boat displacement (hard gravel bottom in the area). I had 1800kg for a 3.5 t boat.

Alain

[Blue Stocking]

It comes down, weight-wise, to the specific density of the object. Water being the standard 1, the efficiency of all other material will relate to this. Concrete, for instance, loses 20 -30% efficiency when submerged.