Miscellaneous Boating Stuff
Race Committee Boat Mooring
  Regerding the Race Committee Boat Mooring Ground Tackle
                                                        by Nick Kluznick

5-1-92

1.  FACTS:

1.1  Almost everyone is reluctant to believe the facts.

1.2  The West Marine catalog says that a 40' boat with an 11' beam in 42 knots of wind will
put a 2400# load on its anchor rode due to windage alone, i.e. it does not include the loading
effects of the waves.  This number, from the American Boat and Yacht Council, has no safety
factor in it.  Therefore, if a 50% safety factor is used to account for wave loading and margin,  
the design load will be 3600#.  Since we occasionally have a 40' RC boat and 42 knot winds,
and even though an 11' beam is a little narrow for a 40 footer, a design working load of 3600#
seems reasonable for our RC boat mooring buoy.

1.3  A standard Danforth of 103# is rated for 3500# of holding power.  However, this is the
holding power the anchor can generate in an ideal bottom, like clay.  Mud is a much worse
holding ground.  Therefore, a somewhat bigger anchor is needed to get to 3600# of holding
power in mud.

1.4  Mushroom anchors are better for moorings than are Danfoths because they do not break
free during reverse winds or currents.  However, they weigh more for the same "fluke" area
and, therefore, they cost more.  Moreover, there are no west coast sources so that the
addition of the shipping cost make them prohibitively expensive.

1.5  Pillar Point Harbor Mooring Specifications:
1.5.1  For up to a 30' boat - 150# Danforth with 3/4" chain of length 1.5 X the max depth plus
1/2" chain of length 1.0 (now 7.0 @ Nov '06 telecon)* X the max depth.
1.5.2  For up to a 45' boat - 250# Danforth with 3/4" chain of length 1.5 X the max depth, plus   
5/8" chain of length 1.0 (now 7.0 @ Nov '06 telecon)* X the max depth.
  •  It seems that they have gone from too short to too long.  I would think that 3 to 5 times  
    the max high tide depth would suffice for the length of the smaller chain.  Now that I think
    of it, I would make the big chain 2.5 X max depth and the small chain 1.5 X max depth
    including storm surge.

1.6  The Defender Industries catalog, a NY mail order hous, recommends a 300# Mushroom
anchor for a 30' boat in 20' of water, and 30' of 5/8" chain plus 35' of 1/2" chain.

1.7  Concrete - In air, concrete has a weight of 2.7 times that of an equal volume of water; in
water, it has a weight of 1.7 times water.  Sea water weighs 64 # / cubic foot.  Therefore,
a 1' x 1' x 1' block weighs  173# in air and  109# in water.
a 2' x 2' x 2'    "         "        1382#  "  "   "     870#  "   "
a 3' x 3' x 3'    "         "        4666#  "  "   "     2938#  "   "

1.8  Bill, of Coast Marine, said that he used a 3' x 3' x 3' block of concrete with rebar outriggers
to hold a 65' boat in Florida.

1.9  Just about every mark that we have ever put out has dragged and most were lost.


2.  OPINIONS:

2.1  Whatever we did in the past was wrong.  Either anchors were not big enough, or chains
were too small and rusted or abraded through too fast, or the scope was too short, or all of the
above.

2.2  To get 3600# of holding power from submerged concrete, one cubic yard, weighing
4666# on land, seems to be indicated.  This ignores the fact that there will be a side load
rather than a lifting load.  Generally a weight will slide with less load than it takes to lift it.  But
the mud may resist its sliding, especially after it settles into the mud and especially if it will
cause a suction on the side of the block.

Be that as it may, a one foot cube of concrete is grossly inadequate.  A two foot cube might be
adequate but it is too heavy to handle.  Can you imagine toting a 1382 pound block of
concrete from the back yard of the club to a boat at the guest dock?  And it is still probably
inadequate.  Handling a three foot cube is incomprehensible.

2.3  Based on the Pillar Point spec and the Defender Industries recommendation, and all the
other facts, I think that a 200# Danforth is called for.

2.4  Certainly the heavy chain is the right thing to do.  The heavier it is the longer it will last
before it rusts or abrades through.  (Abrasion is the dominant factor.)  3/4" chain resting in
near-oxygen-free mud should last forever.  The 1/2" chain should be the only thing that needs
replacing in the distant future.  Moreover, the weight of the chain and big anchor tend to make
it theft proof.

2.5  Since nearly everyone thinks that this is too much, maybe a compromise will be almost as
good.


3.  AVAILABLE ANCHORS:

3.1  115# Danforth, from Al's Marine for 388$.  66" long, 48" wide, 36" long flukes.

3.2  150# Danforth, from Coast Industrial Marine for about 350$.


4.  CONCLUSIONS:

4.1  Concrete is cheap but we can't handle it.

4.2  A 200# Danforth is best but even a 65# Danforth is probably better then what we have
done in the past.

4.3  The recommended anchor system is as follows:
a.  150# Danforth                    (though 200# would be better)
b.  30' of 5/8" chain (125#)         (though 3/4" would be better)  (50' would be better.)
c.  20' of 1/2" chain (56#)           (though 5/8" would be better)  (30' would be better.)
d.  A 5/8" swivel between the two chains.

4.4  We should assume that fishing boats will tie up to our turning marks and, therefore, we
should design the ground tackle accordingly, i.e. with an adequate anchor, chain, and scope.  
I would say a 22 to 35# Danforth with 30' of 5/8" chain plus 20' of 1/2" chain should suffice.


p.s. 1-9-07

The area of greatest wear is the smaller chain at a depth between the low water level and the
high water level.  For example, if the lowest water depth is 7 feet and the highest water depth
is 12 feet then the chain will be abraded the most between 7 and 12 feet below the buoy.  
This area must be inspected once a year!!!