Page 20 - WestlawnMasthead17_Mar.11.pub

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freely corroding potential in this
condition, but—if they're proper
marine alloys—will last many,
many years.
On larger craft, the isolated fittings
should be connected to their own
completely separate and com-
pletely isolated
secondary internal
bonding system—just as on a wood
or FRP craft. (Bonding conductors
on these secondary systems
must
be insulated to avoid contact with
the hull.) Even the zinc anode(s) for
this separate system must be com-
pletely isolated from the metal
hull. Dual-metal corrosion control-
lers are available to monitor the
hull anodes, as well as to monitor
and control the isolated secondary
bonding system.
Estimating Hull Surface
The best method for determining the wetted surface of your hull is to consult
boat's designer. Short of that, however, an estimate sufficiently accurate for siz
zincs can be made from the following formula used by the British corrosion comp
M.G. Duff:
Hull Wetted Surface (sq.ft.) = LWL (ft.) x (Beam WL (ft.) + Draft (ft.))
Multiply the answer by the following factors for different types of craft:
1.2
cargo vessels, tankers, dredges, and very heavy full-bodied craft
1.0
tugs, trawlers, ferries, heavy motor cruisers, and full-bodied sailing cr
0.75
for medium displacement craft, medium and light lobster boats, a
age sailing craft
0.5
for very light craft without external keels.
The zincs must be located so they are immersed all or nearly all the time. Usu
they're located on the hull underbody about halfway out between the keel and
turn of the bilge or chine. They must be able to "see" the metal fittings they're
tecting.