Monday, 2 April 2012

Rudder balance and galvanic isolation

Part one of the boat survey has been completed. On Saturday the hull was inspected and tested for thickness. Some pitting was found to a depth of 1.5mm in the hull sides (nominally 6mm). The surveyor thinks this is galvanic corrosion caused by being hooked up to mains electricity for long periods. The boat's current owner (no pun intended) was there welding on new anodes, so he filled some of the pits with weld (so the surveyor reports). I hope this is an acceptable treatment!

I had been particularly concerned to get the surveyor's opinion of the rudder. When we road tested the boat I found the steering heavy, and I wondered if the rudder balance was out, i.e. whether there was not enough of the rudder forward of the pivot point. The surveyor took a photo of the rudder for me, from which I estimate that the ratio is about 15% forward and 85% aft of the pivot point. This seems to be close to what is usually the case, and the surveyor suggested that experimentation with a longer tiller arm should be done before thinking about adding more steel to the leading edge of the rudder.

So that's reassuring!

As far as the corrosion is concerned, I'll make sure that galvanic isolation is in place before the boat is installed in the marina for Ally and Ben.

I've been investigating so-called galvanic isolation. This is the means of interrupting the mains earth as it tries to come on the boat via the shore line. Obviously an earth path is still required for mains fault conditions, but a continuous earth bond between boat and land can set up an earth loop, and small but corrosive currents can flow in the water, eating away at exposed metal. Sacrificial anodes are there to take this corrosion current on the chin, as it were, and prevent the rest of the boat corroding.

There are two common ways of providing this break in the earth line and still keeping things safe. One is by using an isolation transformer where mains current is allowed through, but the earth link is broken. The other is by using a "galvanic isolator", which is, I believe, the more common method on narrowboats. A galvanic isolator is a very simple piece of electronics. It usually comprises two diodes in series, in parallel with two more diodes in series but "facing" the other direction. The forward voltage drop across a conducting silicon diode is about 0.7V, so two in series gives about 1.4V. If a fault condition occurs, if, say, the live wire on board touches the metalwork of the boat, the conducting diodes will provide little resistance and the mains earth will be restored. But at voltages below about 1.4V the diodes will present a very high resistance. And the damaging "galvanic" currents are, I'm told, a result of voltages below this figure.

Now, these diodes have to be able to handle the maximum current which the shore line can supply. If they blew (went open circuit) then no earth protection would remain - obviously dangerous. So commercially available galvanic isolators are rated at 70A and above. Hence the heatsink, but I think £70 is a lot for four diodes!

An isolation transformer will provide complete earth isolation, but I need to do more thinking about the earth protection side of things. I've just spent an hour searching for isolation transformers on the internet. The best I could find was a 1kVA transformer from CPC at £150. But this wouldn't be big enough. No wonder the diode devices are more popular!


Bruce in Sanity said...

Hi 3.6 kVa do you for £250? Take a look here - it's a plug in beast.



Anonymous said...

Richard's done a lot of research into this for Indigo Dream - he put some of his thoughts and some useful links here -

Get in touch if you want to email/chat over it.


Sue, nb Indigo Dream

Bruce in Sanity said...

Sue: the link in your blog post don't not work not never no more; they've changed the URL. The correct one is the one my comment links to, same beast though.

Halfie: you've got spam, looks like.



Neil Corbett said...


I have no strong opinions about isolation transformers vs Galvanic Isolators, but there are those (Tony Brooks for one) who doubt the efficacy of GI's on a boat if you are using much electronic equipment on board. As far as I recall the problem is to do with computers, TVs etc dumping currents which may cause interference down the earth wire. If these set up an earth voltage above the 1.4v the diode takes, then the isolator has no effect. You might like to search through canal world forums to get chapter and verse on this as I didn't fully understand it and it was some time ago.

BTW I understand that welding up pits is standard practice and OK.

Anonymous said...

Thanks Bruce - the link is fixed!

Sue, nb Indigo Dream

Halfie said...

Bruce, thanks for the link. Now THATS more like an isolation transformer for the job! The £250 quickly becomes £321.60 with shipping and VAT, but, knowing about all the RFI and spikes from switched mode power supplies causing the diodes in a galvanic isolator to conduct, it seems the only sensible option. (Spam dealt with.)

Sue, thanks for that. I read that post at the time but didn't really take it on board (as it were). It's funny how owning one's own boat (almost) concentrates the mind!

Neil, thank you. As in my reply to Bruce, I agree with Tony.