REQUIRED NUMBER OF ANODES
If, as is usually recommended, the flat, oblong type of anode defined above is chosen, it may be assumed that each will protect about 9 to 10 square metres of shell plate wetted surface area. On a boat, say, 15 m x 2 m x 0.7 m mean draught, the wetted hull surface area (AWS) would be about
AWS = 15(2 + 2 x 0.7) = 51 m2
That, however, does not tell the whole story as that 51 square metres would be divided unequally between the bottom plate and the two sides which have to be dealt with separately. The bottom plate would account for about
ABP
and each side ASP where
For canal boats, the anodes should be of magnesium or aluminium. In fresh water, zinc gets coated with an impervious layer of zinc oxide and the anode ceases to work.
ABP ASP 0.8 12
= =
= W
Anode W
Anode Unprotected area Unprotected area
area of bottom plate area of side plate
coefficient of baseplate area length of bottom plate
C. L. BWL
m2 m2 -
m
C. L. BWL
The sides ARE protected Anode The sides ARE protectedBottom Plate Maximum 2096 mm Maximum 2096 mm
Figure 3 Maximum Allowable Distance over the Anodes Protected Area
Figure 3 - Maximum Allowable Distance over the Anodes
Figure 3 Maximum Allowable Distance over the Anodes Protected Area
Anode Anode Shell Stem Stem Shell Forward Swim Forward Swim 108 | The Report • June 2021 • Issue 96 Figure 4 Area of Protection afforded by a Swim Anode Unprotected Area Unprotected Area
Shoulder or the Luff of the Bow Parallel Mid-Body
Anode Bottom Plate T T
For canal boats, the anodes should be of magnesium or aluminium. In fresh water, zinc gets coated with an impervious layer of zinc oxide and the anode ceases to work.
L L
For canal boats, the anodes should be of magnesium or aluminium. In fresh water, zinc gets coated with an impervious layer of zinc oxide and the anode ceases to work.
= =
12 x 2 x 0.8 (51 – 20)/2
= =
15.5 m2 20 m2
To allow for some overlap of cover along the sides, each anode should be assumed to cover a length of hull of about 12 times its own length i.e., about 12 x 0.356 = 4.27 metres in length, that would imply, for the example, at least (and not more than) four anodes along each side (i.e., one on each swim and two equally spaced along the parallel mid-body each side) and four along the centreline of the bottom plate. Experience shows that to be both practical and sufficient.
It should also be remembered that, just as it is possible to fit too few anodes, it is also possible to fit too many.
Shoulder or the Luff of the Bow Parallel Mid-Body
C.L.
Figure 4 Area of Protection afforded by a Swim Anode Figure 4 - Area of Protection afforded by a Swim Anode
C.L.
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