Assuming that the anode in question is of the thin oblong type with rounded corners, its dimensions are usually of the order of:


Anode Length Anode Breadth Anode Thickness


LA BA TA


35.6 15.2 3.2


cm cm cm


and the anode’s effective wetted surface area (AWSA) is length plus twice the thickness times breadth plus twice the thickness minus the corner excess areas, i.e.,


AWSA = (LA + 2TA )(BA +2TA ) – [(2TA )2 - πTA2 )] = wetted surface area of the anode


42.0 x 21.6 – [40.96 – 32.17] = 907.20 – 8.78 = 898.42 cm2 The area on a flat plate (CAA


CAA = 14(LA ) – 4 x 2/3 x (14 – 1)(LA -BA ) cm2 cm2


) covered by a single anode is, therefore, x BA


= area of steel covered by the anode = 142


= 106059.52 – 707.20 = catalyst.


and the area ratio per anode is = 105352.32/898.42 =


See the Figure 2.


It is good practice for the marine surveyor to measure a number of popular anode types, calculate their wetted surface area and make a note of those figures in his/her private notebook.


Load Water Line Anode


As the particles that form the current possess mass, they are also governed by Newton’s first law of motion which means that it does not pass obstacles such as chines or bilge keels, nor travel round bends in the plating such as the luff of her bow (her shoulders), the loom of her buttocks or around corners. It also means that anodes fitted to the swims of narrowboats, for example, do not protect the parallel mid body or the boat’s bottom plate. Forward, they protect the swim as far aft as the luff of the bow (or her shoulder) and aft, they protect the underside of the uxter plate and the swim as far forward as the loom of the boat’s buttocks. The marine surveyor’s experience will show that, even when anodes are fitted to the swims, those areas still often show some galvanic pitting. Oxygen is not absolutely necessary for the process of galvanism but may/will act as a catalyst.


(35.6 x 15.2) – 4 x 2/3 x 13(35.6 - 15.2) 105352.32 cm2


117.26


The black area in Figure 2 is above the waterline and the uncoloured area below the underside of the keel. Obviously, neither would be covered by the anode. The analysis above assumes that the boat is stationary and, therefore, the covered area would be similar to the shape of the anode. If, however, the boat was in motion, there would be a force from the relative motion of the water past the hull acting on the particles so that the covered area would be moved slightly aft depending upon the boat’s speed.


Load Water Line Protected area


BA 14WA 14BA LA 14LA14LA USK Figure 2 Sketch of One Anode on a Plain Mild Steel Side Shell Plate


Figure 2 - Sketch of One Anode on a Plain Mild Steel Side Shell Plate The sketch shows the area ratio calculated above


The Report • June 2021 • Issue 96 | 107


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