Single pole and 2 pole conflict

I can only speak of UK narrow boats which are steel and dont sit in salt water but the norm is the hull is neg 12v for the engine and house batteries whilst the 240v ac is isolated from the hull apart from the earth which connects to the hull. The 240v feed for shore power goes via an RCD and an isolator is fitted in the earth consisting of two pairs of diodes back to back giving a drop of 1.4v and stopping hull corrosion.

 

Thanks guys for your suggestions, much appreciated. It seems cure all with electrical systems and metal hulls and the many variables that apply will be more of art than an absolute science.

The method I propose is to ground the single pole starters to each fully isolated engine block from the hull, the same isolation for vee drive/gearbox. The rest of the vessels DC/AC system installed as usual with galvanic isolator on shore power and AC earth leakage breaker protection.

For future projects I'm also thinking of deleting the practice of common bonding, instead isolating all power equipment from the hull - our Pilot boats with DC winterized systems like heated windows, split system DC aircons etc are full of potential stray current opportunities.

 

You could also use a 2 pole solenoid switch to break plus and minus power feed to the starter motor.
You would naturally have negative on the hull for the short start cycle , but not for the life of the boat.

http://www.albrightinternational.com/files/downloads/catalogues/SU190_Catalogue_Sheet.pdf

Ive got these Albright two pole ralays all over the boat so that equipment is isolated when no in use. No earth leak possible.

 

I would like to see a diagram of what Watchkeeper calls a single and a double pole circuit. A solenoid can only operate with two poles. One pole may be the housing. Unless you use correct terminology, this discussion makes no sense. A diagram would give us a concrete thing to discuss about.

 

Gonzo - You're becoming a wee bit tedious - use a coin, call Yanmar for an explaination then read ARANDA1984 and Landlubber's posts, they both explained the difference very concisely...even I understood it

 

If you are satisfied with the answers, then that's fine. I still don't understand what you mean by a single pole diesel.

 

Gonzo, I wouldn't describe it as a single pole diesel, it's a diesel that for some unexplained reason Yanmar fitted with a single pole starter that has the potential to really complicate stray DC current protection.
I can map out a basic practical marine electrical systems but I'm way out of my depth on the finer details of designing control/switching equipment.

Our electrical designer with 30yrs big ship systems experience is nervous with DC gear especially when protection becomes an issue - bad electrics are death to an aluminium hull operating 12hr/day shifts 52weeks a yr.

 

The "dissimilar metals" issue must not be exaggerated. You do not generate damaging currents by connecting steel to aluminum or copper as long as the contact area is sealed or otherwise kept dry. Only when an electrolyte like seawater is allowed to fill the gap around the contact area there will be corrosion.

In electrical installations there are always dissimilar metals. The copper coil in the starter solenoid is spot welded to the steel casing, the brass negative brush holders are riveted to the steel end plate of the motor. The negative cable has a crimped on copper lug, tinned or zinc coated and connected to the cast iron or alloy engine block.

All the many millions of car and boat engines, outboards included, use such constructions without any problems. The only visual difference between these applications is the use of shrink sleeves, flexible coatings and rubber caps in marine installations.

It is possible, but expensive, to fully isolate all electric circuitry from an engine block, but why should you?

The carbon/copper brushes in the starter motor wear pretty fast and deposit a conductive dust layer in the housing. Near the positive brushes the supply current instantly burns the deposit, but in a fully isolated motor it may be the negative side where the dust layer gets broken, leaving you with the whole engine, gearbox and shaft connected to + 12V.

This is just one example why complete isolation may work out counterproductive. Others are the alternator, the flimsy isolation in sensors, relays and electronics like a MMU.

 

Ya, I guess. I can only pass on my observation of how the top class yacht builders treat metal and particularly aluminium boats. With aluminium its great attention to detail. Dissimilar metal are ok, if the fitter uses proper technique to fully waterproof the joint and control stray electricity.. I just removed a bilge pump sensor aluminum bracket that was ss fastened, drilled and tapped, into an aluminum frame, from a wet bilge area that has been stationary, untouched , for 15 years. . The 6mm ss fasteners came out without fuss because the bracket to frame joint was waterproof bedded in a sikaflex like compound , the 6mm fastenings were bedded in waterproofed assembly paste and with the correct plastic flat washer. . Very good Attention to detail by the constructor.. Just like an isolated electrcal system.

Hey CDK, do you know anything about the underwater anode field? Its always a mystery to me. Particularly as a boat ages. When Im in northern Europe around the big shipyards there seems to be plenty of specialists who look at these details. Once Im on the road I never seem to be able to recieve good advice. Is their a good thread on Boatdesign net ?

 

Aluminum hulls should have anodes in the bilges too.

 

How many have you designed that have this arrangement, and for what reason?

In over 20 years of designing ally hulls, i've never added anodes inside the hull. Where there is cause for concern, or a belt and braces approach, just paint the bilges with white epoxy primer. Job done!

 

The reason is the same as for having anodes on the outside: to prevent electrolisis caused by dissimilar metals. Painting only works when there are no scratches or pinholes. Any scratch will make all the corrosion happen in a tiny area and make a through hole. We had that problem with painted fuel tanks.

 

More on galvanic corrosion

Ad Hoc has shown an example that is quite common and the mistake of putting two slightly different materials in a tank can be made even by experienced engineers. It was an honest oversite, that anybody can make.

Putting the stiffeners outside was the solution.

Sometimes just by having a slightly different alloy with some more anodic elements in the metal can start the galvanic corrosion process.
Paint, plastic covers, proper grounding, insulators are the answer.
Tanks can be covered on the inside with plastic by rotational molding process.

That would have solved the problem also.

Stephen I. M.

 

Before you drivel further with your anecdotal sailing experience, you would do better to understand some basics. Read the articles I posted above. And listen to those who know the business, like CDK.

 

Never seen an anode on the inside of a metal boat. Inside tanks yes, inside the hull ? How would they work ?