NOT its mass. The mass or weight of the material affects only the length of time that the anode will last before it requires replacement.
Newton’s law, similarly, implies that the path of the particles is a straight line and that such particles will not go round bends in the plating, pass over such items as bilge keels, wearing chines or round corners in the structure etc. Anodes fitted to the swims of a narrowboat, for example, will NOT protect her parallel midbody or the underside of her bottom plate but will protect only the swim areas. (See Figures 3 and 4). The rather expensive practice of letting anode boxes into the side and bottom plating of narrowboats, which is still occasionally seen, was and is pointless as the Newtonian law effectively confines the protective current from the anode to the inside of the box and so provides no protection to the shell plating elsewhere.
Experience will show that the best anode for such a boat is thin, flat and of approximately oblong shape. Such an anode will only protect an area of approximately the same general shape of the anode which has a total principal length of roughly fourteen times the length of the anode and a principal width of about fourteen times its width.
ESTIMATING THE WETTED SURFACE AREA OF A BOAT’S HULL
The first thing for the marine surveyor to do when designing a cathodic protection scheme is to make a reasonably accurate estimate of the subject hull’s wetted surface area i.e., the area of her shell in contact with the water in which she floats. That is usually carried out by means of a simple formula using the vessel’s principal dimensions i.e., her waterline length, her waterline breadth and her mean draught. Those dimensions should be measured when the boat is undergoing an out of the water survey. For good, legal reasons, the marine surveyor should never rely on a broker’s or even an owner’s dimensions as, far too often, they are incorrect or incorrectly defined. BEWARE!
He/she also will often come across a standard formula published by a well- known supplier of anodes which states that:
AWS where
AWS B
LWL TM
= = = =
area of wetted surface waterline breadth waterline length mean draught
m2 m m m
That formula, which appears to be based on Kirk’s analysis the details of which may be found in any good standard work on naval architecture, is manifestly incorrect, will give wrong answers and should NOT be used.
A correct version of the formula is: AWS
= where B
LWL TM k
= = = =
waterline breadth waterline length mean draught a constant
for full displacement motor yachts, narrowboats and long keel sailing yachts for medium displacement vessels for fin keeled sailing yachts
m m m -
k = 1.00 k = 0.75 k = 0.50
Even so, the formula is not strictly accurate, but the answers given lie well within the limits of experimental error.
k x LWL x (B + 2 x TM ) m2 (4) = LWL (B + TM ) m2 (3)
The reader should note that, in the case of narrowboats, Dutch barges and similar vessels, the formula gives the total wetted surface area of the bottom plating as well as the side shell plating. That fact is important and should be taken into account when calculating the number of anodes to be fitted to a given boat.
THE EFFECTIVE AREA COVERED BY A SINGLE ANODE
The fundamental principle underlying this calculation is that the area actively covered by the anode is a direct function of that anode’s wetted surface area and independent of the weight of anodic material. The weight of anodic material controls only the length of time that the anode will be effective. As the electric current involved is in a simple DC circuit, it is governed by Ohm’s law and the specific resistance of the electrolyte which facts necessarily limits the range over which the current is effective. The voltage in the circuit is the difference between the static potential of the anode and that of the shell. The resistance will be that obtained in fresh(?) water and increases with distance from the anode. The covered area, from the analysis of survey experience, as stated above, is of the same general shape as the anode with a major axis about fourteen times the length of the anode and a minor axis about fourteen times its breadth.
106 | The Report • June 2021 • Issue 96
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136
