Estimated remaining life of existing anodes is calculated from L
L
%A k
= = = =
l, b, d = q
a
= = =
12 + 19.2k(1 - %A Life
Weight of one anode is calculated from wA
aqlbd/103 the dimensions of the anode
7.13 zinc 1.74 magnesium
0.38 for ovoid anodes 0.55 for long anodes 0.89 for flat anodes 0.59 for circular anodes
The Wetted Surface Area of the Hull is calculated from a modification of the well known Kirk’s analysis:
SA L
B d k
= = = = =
SA CD
= = =
k.L(B + 2d) Length Breadth
Mean draught Constant
Required current amperage is calculated from A
SA x CD /1000 Wetted Surface area
Current Density 10 – 30 generally about 20
Total weight of anodic material required to give this amperage is calculated from
WA
WA Y
= =
8760 = C
= =
= = =
A x Y x 8760 C
Weight of required anodic material Required life in years
Number of hours in one standard year Capacity of Anodic Material 780 for zinc
2700 for aluminium 2000 for magnesium
Recommended life of anodes for small craft is two years (3 years maximum).
kg Y h
ah/kg
m2 m m m -
Amps m2
mA/m2
It is generally recognised that it is good practice to place the anode(s) at a point where it or they can ‘see’ the item it is or they are protecting. For example, one can often inspect transom sterned motor boats where the anodes have been placed on the transom. While there is nothing intrinsically wrong with that, the anodes cannot ‘see’ the items they are protecting and, therefore, work less efficiently than those that would be placed under the flat of the bottom. The anodes are generally reckoned to have a working range of about 5 metres (15 feet) and should be so spaced to include that idea.
Again, for example, one often inspects narrow boats of, say, fifty feet length with an anode fitted each side on the forward swim and another on the after swim. Careful inspection of the hull will show that the middle section out of the 5 metre range will be badly pitted. Such boats should have an anode fitted at mid length each side taking care not to thereby put the boat ‘out of gauge’.
It has become common practice on such canal boats to place anodes inside small boxes inset into the hull. Experience will show that the steel forming the interior of the box will be in pristine condition while that plating surfaces even quite nearby will be badly pitted. The anode cannot see the steel it is supposedly protecting.
The Report • March 2017 • Issue 79 | 41 )
months months
percent loss of anodic material per anode 1.00 for zinc, 0.90 for magnesium
kg cm
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