Mushroom or Pyramid:
A few questions:
What is the proper set up for mooring the boat?
If a mud bottom a 400 pound mushroom, min, or bigger. Many towns use the formula; length X beam = X multiplied by 1.5 for mushroom or pyramid moorings.
This means a 30 footer needs:
30' X 11' = 330 X 1.5 = 495 or a 500 pound mushroom
Dead Weight Moorings:
Deadweight anchors are widely used New England. They are most often blocks of granite which settle into the seabed and are too heavy to "drag".
The holding power of a deadweight mooring is mostly driven by its sheer weight and not by its design shape. While a good deal of its "set" performance can be derived from "suction" to the bottom when a deadweight is pulled or dragged out of its "set" location, itcan have great difficulty re-setting or sticking into the bottom.. Its weight provides constant resistance to drag, by design, but may not be enough if these are not sized very LARGE.
Blocks do become embedded in the harbor bottoms over the years, providing a degree of suction resistance in bottom material which certainly has strong cohesive properties. A deadweight anchor is not likely to break free from its set, but it can and if it does it needs to be heavy enough to limit dragging.. An mooring would be considered "set" when it becomes buried in the harbor bottom over time.
The weight of any "dead weight" mooring should ALWAYS be the submerged weight of the mooring and NOT the dry land weigh.
Minimum deadweight submerged mooring weight:
(this is an actual towns minimum requirement)
10 - 17 1000 lbs.
18 - 26 1500 lbs.
27 - 35 3000 lbs.
36 - 45 4000 lbs.
45 - 55 5000 lbs.
Submerged Weight Calculation Factors
Concrete = required weight /.55 (example 1000 lbs./.55 = 1818 lbs.)
Granite = required weight /.64
Even for a small 30 footer using cement you need roughly a 4700 pound dead weight granite mooring to attain a 3000 pound submerged weight! Of course many towns simply ban the use of concrete as moorings and this is why I reference the granite.
With granite or dead weight moorings lower and longer perform better and limit "chain wrap" Try to keep a dead weight to 6-9" tall and it will usually settle flush with the bottom. Round granite moorings deal better with chain wrap but low rectangular or square moorings work well too. Tall dead weights create problems and un-set easier.
This is what a rectangular dead weight granite mooring should look like:
Deadweight moorings are easily dragged if not the proper shape or weight and thus require significantly more weight than would a mooring that "sets".
Moorings that set should be "set" in a settable bottom or be sized large enough to not drag on a hard bottom.. The proper orientation for a mushroom is totally buried standing vertically. The entire bell must be able to sink into the bottom and disappear other wise it is basically a deadweight mooring, and dead weight mooring weights should be used not mushroom mooring weights. NO PART OF THE BELL ON A MUSHROOM SHOULD EVER BE VISIBLE, ONCE SET.
Any mooring should be set for the bottom conditions in your area.
The quote below comes directly from Inamar Insurance, one of the largest insurers of pleasure vessels. They know the claims and the failures and why..
INAMAR Insurance Co.
In water, concrete loses almost one-half its weight; granite loses almost one-third, and iron loses only an eighth. This is significant. If a mooring is designed to withstand a 4,000-lb. pull, one needs 8,000 lbs. of concrete, 6,000 lbs. of granite, or 4,500 lbs. of iron. At a minimum, over one ton is needed for even a small, 25’ yacht.
To handle weights of this magnitude, a barge crane is needed. As long as this equipment is used to place the anchor, one might as well err on the side of excessive weight when placing it.
I've looked at a few pendants but am unsure what size/tensile strength I'll need.
It has nothing to do with tensile strength, even a small pendant can handle the loads. It has to do with enough thickness to make it through a storm before chafe wins the battle. By far the most robust mooring pendants made are the Yale Polydyne 2 pendants. There is no other mooring pendant that has been proven or tested as much as this product. They deal with heat far better than typical three strand or nylon pendants.
Also, should I double up? If so, what's the best way to do so?
Yes, definitely. Using two unequal length pendants will survive storms far better than two equal length pendants. It means the lazy pendant is always brand new, ready and waiting for the first one to chafe through.
I've never left a boat on a mooring before so I'm looking for a set up that will give me peace of mind.
Starting at the bottom:
Heavy bottom chain to 1.5 X depth = 1" to 1.5" chain
Shackle between top & bottom chain = As large as you can fit
Top chain 1X depth = 5/8" to 3/4" min
Shackle = As large as you can fit
Swivel = 1" minimum (this is the highest wear item)
Shackles to ball and pendants
I prefer bottom connection "soft balls" such as those by Polyform.
ALWAYS use high quality load rated FORGED shackles like those by Crosby. I like the Crosby G-209A shackles. NEVER use cheap Chinese made CAST shackles. They may look the same but they are FAR from the same...
Looks like this: