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Sea Otters


blackrose

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Are (or should) the AC systems on Sea Otters be earth bonded to the hull like steel boats, and if so were they fitted with GIs or ITs as standard as I'd have thought isolation is quite important for an aluminum boat?

As far as hull-earth bonding of the mains system, I'm not sure if aluminum is quite as conductive as steel, but presumably it can still go live?

Edited by blackrose
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I don't know what was actually standard on the Sea Otters, but the reasons for earth bonding the AC system and fitting a GI or IT are just as valid for aluminium as for steel.

Earth bonding is for safety - if there is no earth bond, and something shorts to the hull, then the hull is live, and nothing will detect this fault. With an earth bond, a short to the hull will return a current to earth and trip an RCD. The same considerations would apply to any conductive hull material.

A GI or IT is to protect the hull against galvanic corrosion when connected to shore power. The galvanic voltages are slightly different for aluminium (-0.8 to -1.0V) versus mild steel (-0.6 to -0.7V) but the issues are the same.

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Grebe had an AC system fitted by Sea Otter.  Consumer unit, caravan type shore-power unit, immersion heater to calorifier and a single socket.  I'm sure the AC was bonded to the hull, but there certainly was no GI or IT fitted.

 

 

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10 minutes ago, Giant said:

 

A GI or IT is to protect the hull against galvanic corrosion when connected to shore power. The galvanic voltages are slightly different for aluminium (-0.8 to -1.0V) versus mild steel (-0.6 to -0.7V) but the issues are the same.

I thought the issues were actually more critical for an aluminum hull due to it being a less noble metal than steel?

 

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2 minutes ago, blackrose said:

I thought the issues were actually more critical for an aluminum hull due to it being a less noble metal than steel?

 

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I think aluminium is more reactive, but behaves differently to steel as it rapidly forms a protective thin oxide layer that does not fall off as rust does from steel.

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49 minutes ago, peter n said:

I think aluminium is more reactive, but behaves differently to steel as it rapidly forms a protective thin oxide layer that does not fall off as rust does from steel.

As far as normal corrosion in air then yes I'd agree, but when it comes to galvanic corrosion my understanding is that aluminum can corrode right through relatively quickly. After all, aluminum is used as a sacrificial anode material in some circumstances (brackish waters).

Edited by blackrose
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3 hours ago, blackrose said:

I'm not sure if aluminum is quite as conductive as steel

I would have thought Aluminium was a far better conductor than steel which is why it is used for cables - particularly HV Overhead lines. As electricity is actually 'conducted' using the surface of the cable conductors, then aluminium offers a lower weight for a higher cross sectional area - and in fact many power cables (aluminium) have sectorial - or 'shaped' conductors to maximise the surface area.

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A company I had dealings with in the '80s (called AWCO - Aluminium Wire & Cable Company) were leaders in this field.

 

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3 hours ago, Grebe said:

Grebe had an AC system fitted by Sea Otter.  Consumer unit, caravan type shore-power unit, immersion heater to calorifier and a single socket.  I'm sure the AC was bonded to the hull, but there certainly was no GI or IT fitted.

 

 

Well if I had a sea otter or any other aluminum boat with an AC system and I was plugged into the mains, I'd certainty want some form of isolation fitted pronto!

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1 hour ago, Alan de Enfield said:

I would have thought Aluminium was a far better conductor than steel which is why it is used for cables - particularly HV Overhead lines. As electricity is actually 'conducted' using the surface of the cable conductors, then aluminium offers a lower weight for a higher cross sectional area - and in fact many power cables (aluminium) have sectorial - or 'shaped' conductors to maximise the surface area.

1.jpg

A company I had dealings with in the '80s (called AWCO - Aluminium Wire & Cable Company) were leaders in this field.

 

Aluminium power cables where used in  late sixties when copper prices went sky high. Very iffy soldering lugs onto due to melting temperature then crimp lugs became the norm.

 

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