Getting a new mooring put down, 3000lb cast concrete block, 1" chain and 1.25" polysteel. I'm thinking the above will be a zero-chafe solution for the two side lines.
The problem is that it won't weight anywhere close to 3000 pounds under water and this is why many municipalities have made concrete moorings non-approved... I am really amazed municipalities even allow them these days..
For example if you were using concrete for a 30' boat in Rockport, MA, a town with quite well thought out guidelines, based on evidence, the dry weight of the mooring would need to be almost 5500 pounds for a 30' boat.... In a protected anchorage a 4500 pound dry weight mooring might do fine for a 30 some odd footer but this needs to be weighed against many factors such as exposure, propensity to storms, fetch etc...
Any dead-weight mooring is best to be long, low and flat so it can "work in" to the bottom and create "suction" to the bottom which adds to the performance of its "dead weight".. Ideally after a few years it would be flush with the surface of the bottom, if the bottom is soft enough. Long low and flat also limits the propensity for chain wrap.
This is what a typical dead-weight Maine granite mooring looks like, low and flat and and this is a small one....
Below are some suggested weights for deadweight moorings by towns that have pent extensive time studying this.. The guidelines below are from Rockport, MA, one town that has studied this stuff rather extensively. They have come up with their own mooring standards after losing boats in storms..
Deadweight anchors are commonly used in New England. They are usually blocks of granite which hold vessels in place because they are too heavy to be dragged or lifted by the vessel.
The holding power of the deadweight anchor is derived solely from it's weight and not by its design. When a deadweight is pulled out of its current resting place, it may settle somewhere else, because its weight provides constant resistance.
Blocks become embedded in the harbor bottoms over the years, providing a degree of suction resistance in bottom material which has strong cohesive properties. Thus, a deadweight anchor is not likely to break free from its set. (An anchor is "set" when it becomes buried in the harbor bottom over time.)
It is recommended that the weights indicated be submerged weight of the deadweight anchor. Use the following table to calculate required weight of material submerged. Consult with the Harbormaster before purchasing gear.
Minimum deadweight mooring weight:
Vessel length: (SUBMERGED WEIGHT)
10 - 17 1000 lbs.
18 - 26 1500 lbs.
27 - 35 3000 lbs.
36 - 45 4000 lbs.
45 - 55 5000 lbs.
Submerged Weight Calculations
Concrete = required weight /.55 (example 1000 lbs./.55 = 1818 lbs.)
Granite = required weight /.64
Example 20' Boat:
For a 20 footer using cement you need roughly a 2730 pound dry weight mooring to attain a 1500 pound submerged weight.
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. 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. This is where many municipalities who don't study storm damage, and asses what happened, can get into trouble.
Any mooring should be set for the bottom conditions in your area.
Here is one of the most common and accepted mooring diagrams out there. Image courtesy of Hamilton Marine.
We know what the town or Rockport, MA recommends for deadweight moorings and that is about 3000 pounds (submerged weight) for a 30 footer..
Scituate, MA requires even more min weight than Rockport at 2000 pounds for a 20 footer and for this harbor the only approved mooring is a granite block, because the bottom is not suitable for setting type moorings. No cement or "home made" moorings allowed unless specifically approved by the harbor master.
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. said:
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.
Ultimately a helix style mooring would be best but you would need a professional installer, and this gets expensive.
Here in Falmouth, ME where there are more than 1200 boats in the anchorage we have very, very strict rules as to how moorings will be installed, serviced etc... Start with the town first and if they don't have any advice go with the advice of a town like Scituate, MA, Rockport, MA etc.
This is but a sampling of our towns rules:
"All new or replaced permanent moorings shall comply with the following minimum specifications:
*Each permanent mooring shall consist of a mushroom, granite block or helix, a minimum one-half-inch heavy steel bottom chain attached to a minimum one-half-inch top chain (a single chain is acceptable), mooring buoy and a pennant: polypropelene use is not acceptable. Each mooring must have one (1) top and swivel; all swivels and shackles must be to the appropriate size diameter.
*All mooring blocks shall be constructed of solid granite with steel staples or eyebolt extending completely through the block. Cement blocks, old engines and other miscellaneous weighted objects are unacceptable as mooring anchors in the harbor.
Recommended mooring guidelines:
Under 15' power/sail 150 lb mushroom
15'--19' power/sail 200 lb mushroom
20'--27' power/sail 250 lb mushroom
28'--30' power/sail 300 lb mushroom
31'--33' power/sail 400 lb mushroom
34'--38' power/sail 500 lb mushroom
39'--45' power/sail 600 lb mushroom
46'--50' power/sail 1000 lb mushroom
51'--56' power/sail 1200 lb mushroom
56'--65' power/sail 1600 lb mushroom
*Winter spars must be used to mark moorings and such spars shall be connected to their moorings with non-floating rope (no wire cable permitted). Winter spars may not be set before September 1 and must be installed by December 31 and removed by June 1 each year. By January 1 all moorings shall have mooring balls exchanged for a winter spar. Winter spars remaining in the water after June 1 shall have their mooring removed by the harbormaster at the expense of the owner and be subject to a fine as specified in the town harbor fee schedule as adopted by the town council and in accordance with subsection 9-105(c) of this article.
*Moorings must be inspected every two (2) years by a qualified mooring inspector. A list of qualified mooring inspectors may be obtained by contacting the harbormaster. The harbormaster has the authority to require any necessary maintenance or replacement of parts or the whole mooring, tackle and/or gear. Inspection forms shall be complete and legible.
I would focus on a larger dead wight mooring and your pendants. Dual unequal length pendants survive better in storms...