>>>>> I have no AC or DC (not plugged in or bonded)
.<<<<<<< You have an engine
and therefore you must have a battery
to start the engine. Therefore you have a DC system. The battery negative terminal system is either connected to a grounding plate under the outside of the hull or to the engine which is grounded to seawater via the propshaft and propeller
. Normally any AC system also has its "ground wire (green)" connected to the ship's ground system or the engine.
- - In the USA boats are built to comply with ABYC standards which is then used by the insurance
industry to evaluate compliance with accepted norms for a "safe vessel construction". ABYC has historically required all vessels to have a bonding system which is a continuous loop of wire connecting all metal parts
of the boat to each other and then to a "ground." This is to prevent an electrical shock from injuring any people on the boat should they touch a lifeline stanchion or other metal part of the boat. The bonding system is also used to take all metal parts
that touch seawater to the same electrical potential and thirdly to help with dissipating lightning
- - It was found that bonding bronze thru-hulls/seacocks was causing accelerated electrolysis
in the bronze and even more importantly in a lightning
strike the bronze thru-hulls were passing too much of the lightning power causing them to heat sufficiently to allow them to melt their way out of the hull. Despite ABYC standards boaters were cutting the bonding wires to their thru-hulls for either or both reasons.
- - Then Marelon (reinforced fiberglass) thru-hulls/seacocks came on the market (They are sold in the USA by Forespar) and solved
that problem. Manufacturers flocked to Marelon as they were electrically inert (FRG) and could not corrode and were as strong as metal thru-hulls/seacocks.
- - The readings you posted are of no value. You cannot use a multimeter to measure electrolysis
- the reading you are getting are most likely residual stray or induced voltages from your body or the batteries inside the meter. You need an Electrolysis meter which has no batteries and two long cables
- one of which has a silver anode which is dropped over the side into the seawater. The other cable is attached to the part being tested. The millivolt meter will then read any voltage potential between the seawater and the metal part. The meters have scales on them for each type of metal and yellow and green zones to guide you to understand if you have too little, just right, or too much zinc protection.
- - Unbonded bronze thru-hulls/seacocks have a problem. Bronze is an alloy of tin and copper and some minor other stuff. Unbonded, which means not connected to a zinc, allows the tin to leach out of the bronze and the remaining copper is brittle and porous. The bronze will turn pink when sufficient tin has been leached out. That process is effectively stopped by a bonding system but then you have the possibility of a lightning strike melting the thru-hull out of the boat and 1.5" holes admitting seawater. Several boats have sunk that way. So it is 6 to1, half dozen to the other whether to bond or not to bond. Replacement with the Marelon system solves that problem. But sometimes that is neither cost-effective nor possible without major de-construction of the inside of the boat.
- - Bronze in the presence of water will corrode to a green "patina" - you will see this on those heroic statues in the city parks. The white stuff is from pigeons. Back to the boat - salt
deposits however, mean either salt water
is dripping onto the parts or there is a leak allowing salt water
to pass through the metal of the thru-hull/seacock. Determining which it is - is important as you may have severely thinned or weakened metal which can break easily or just a leaky thread joint. As others have suggested make sure you have soft wood cone plugs that fit, attached by a string to the thru-hulls/seacocks so if one breaks you can shove the "plug" into the hole and keep your boat from sinking.
- - Old fashioned "stuffing boxes" are usually attached to the boat by a rubber hose and the prop shaft is electrically isolated from the stuffing box by the packing material. So the stuffing box is not in the "bonding" system and will turn green by the normal reaction of bronze to water. Not a problem.