THE PLACING OF ANODES
Based on the above, on narrowboats, it is recommended that the anodes protecting the side shell should be placed in a line at about half the mean draught above the bottom plate with one in the middle of each swim and the others spaced at about 14LA
apart centre to centre. The marine
surveyor should, after the anodes have been fitted, measure and report the breadth from the outer face of the anode to port to the outer face of the anode to starboard. That distance should not exceed 2096 mm. (See Figure 3). He/she should also bear in mind that the density of the anode’s protection falls off with the square of the distance from its centre. The writer further recommends that anodes should be fitted at similar distances along the centreline of the bottom plate which should also be blacked.
It is good practice that every vessel should be provided with a docking plan showing, inter alia, the position and size of the anodes.
THE TIME AN ANODE WILL LAST
The amount of electric current required per unit of a metal hull’s wetted surface area from anodes protecting it is primarily a function of the water's flow rate and the quality of the metal's protective paint coating. It can be estimated for either a mild steel or aluminium alloy hull from the design estimates for fresh water given in Table 1.
For steel and aluminium alloy boat hulls (or underwater structures) the following formula can be used to calculate the time in hours that the total weight of a sacrificial anode will last before renewal becomes a necessity. A two-year renewal period should be considered reasonable.
HI where
AWS EC HI IR
WA
= = -
= =
wetted surface area of boat’s hull m2 energy content of the anode immersion time
required current density anode weight
AH/kg H
mA/m2 kg
The energy content for zinc anodes is 810 Amp-hours per kilogramme (AH/kg), for aluminium alloy anodes 2438 AH/kg and for magnesium anodes 2196 AH/kg.
Table 1 Estimate of the Current Generated by an Anode Item
Speed knots
Well coated
Poor or Old Coating
Uncoated
Stationary 0-0.45
16.1 21.5 32.8
Mild Steel Hull mA/m2
0.45-1.8 21.5 43.0 53-160 1.8-4.5 32.3 53.8 160 -320 ≥4.5 53.8 110 270 -1080
Stationary 0-0.45
5.4 10.8 21.5
Aluminium Alloy Hull mA/m2
0.45 -1.8 10.8 21.5 44 - 86 1.8 -4.5 21.5 32.3 54 -130 ≥4.5 32.3 53.8 108 -270 The views and opinions expressed in this article are those of the author solely and are not necessarily shared by the International Institute of Marine Surveying. = EC
x WA AWS
x 10³ x IR Hours (5)
How long could the anode shown in Figure 1 above be expected to last. Assume that it is a magnesium anode attached to a mild steel hull and that its weight is 2.50 kg and that the area it covers is 10 square metres. If the vessel is stationary in a marina and has a reasonably well-coated hull which remains so, the current generated would be of the order of 16.1 mA/m2
1. Then for a magnesium anode: HI
= EC x WA AWS x IR
x 10³ x 168
N.B. There are 168 hours in a week and 8765 hours in a year.
For example:
from Table weeks
= 2196 x 2.50 x 10³ = 203 weeks 10 x 16.1 x 168
i.e. about 3.9 years
That figure makes a number of assumptions that, most probably, would not be borne out in practice such as the coating remaining in good condition for that period – a most unlikely event. As the coating deteriorates, the current generated increases and the life of the remaining anode reduces. It is also usually recommended to renew the anodes when they about 80% dissolved. Allowing for coating deterioration and the 80% figure, experience will show that the anode would probably be satisfactory for a period of about two and a half years at most. It is good practice, therefore, to assume that anodes require renewal every two years.
The Report • June 2021 • Issue 96 | 109
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