earth in a steel yacht

Hi there,
Put a new engine in my steel yacht. It's a marinised nissan Td27.
everything is earthed through the block which is then taken back to the negative on the battery Just like a car. This also means the earth connects to the hull through the driveshaft. I'm just wondering if this is ok or shall I put some sort of insulator between the engine and shaft to break this connection.
Was just thinking of putting a thin bit of plastic between the drive plate at the back of the gearbox and the cv shaft. I'd also put plastic washers/ sleeves on the bolts.
Or shall I just leave it as it is. any help much appreciated

 

You need a good connection between the negative and the hull or grounding plate. That is, a thick wire.

 

HI cheers for reply. Could you explain why I need the hull connected to the batt neg? None of the electrics negative are connected to the hull. they all go direct back to the batt or through the engine block back to the batt. it is a steel hull so no grounding block. just wondering why I would want to connect the hull to anything electrical. could set up galvanic corrosion in marinas etc.
Is it to do with shore power ( AC) and safety? what if i never run off shore power?
[/I]

 

The sacrificial anodes need a good connection to work. Also, you will have a voltage difference between adjacent metal parts submerged in water which will increase electrolisis.

 

There are differing views on bonding.

One scenario is tht the hull is unbonded to the negative and a fitting or wire shorts the positive to the hull:
No fuse blows but the rubber mounted engine unless it has isolated starter and alternator is now massively negative relative to the hull as the prop and shaft can be as well.

The resulting corrosion would be woefull.
The anodes become low resistance immersed cathodes and will be very happily protected by all the other metals driven negative that are in contact with the water.

If the hull was bonded to the negative then no electrical fault will drive any part negative wrt the hull and anodes but the fuse will blow or the battery will flatten.

AC shore power is a different consideration and there are regulations that cover the requirements here, you can use an isolating transformer or a galvanic isolator to protect yourself in a 'hot' marina.

 

TD27 marinisation

Hi, sorry to jump off-topic, but I wondered if you might have some links to where you could get TD27 marinisation parts from?
I have a TD27 in a canal boat in England, I've connected the cooling to a keel cooler, and a bowman oil cooler for the gearbox, but otherwise it's just as it was from the car.

If you know where I could find a water cooled exhaust manifold from, I'd be really happy

Cheers,

Patrick

 

I suggest you go over to the Metal Boat Society at http://www.metalboatsociety.org/
Whether or not to electrically connect the hull to ground is a hugely (and I mean HUGE) controversial subject. It has been discussed at great length at the Metal Boat Society.

I am of the opinion that the hull should not be electrically connected. Connecting the hull creates the possibility of introducing stray current into the water with resulting stray current corrosion. If you have AC on board this also can result in the introduction of AC into the water which is extremely dangerous.

 

Your boat is a giant ground electrode. Connecting Batt - to it has no influence whatsoever on what happens in the water; it only ensures that the DC system stays at 0 potential. In a floating system leakage current or capacitive coupling in an inverter can create a considerable voltage differential between the hull and the wiring. Also shielding RF signal wiring (radio, radar) works better when the hull is bonded to DC-.

 

Like I said, this i s avery controversial subject

 

Here is a link to a thread on the metal Boat Society that discusses this. http://metalboatsociety.org/phpbb2/viewtopic.php?f=13&t=971 There are two very good links on that thread to info on Electrical Systems. I disagree with above statements about grounding to the hull. I believe that on a metal boat the hull should never be connected to the electrical system, whether it is DC only, or DC and AC. Doing so just leads to too many problems.

ABYC says that the hull should never be used as a conductor, that is, a return path for current. If you connect the hull to ground and get a fault in the ground inside the boat you now have the hull being used as a return. Worse, you won't know it until bad things start to happen.

 

Using the hull as a return path is quite another matter. It can be tempting because it reduces the amount of wiring by 50%, but there are distinct disadvantages.
The hull must be protected against corrosion both outside and inside, grinding to bare metal and drilling holes for electrical connections introduces starting points for corrosion. There will be dissimilar metals and capillary action, exactly what nobody wants in a boat.
If large currents flow through the poorly conducting steel, there are voltage differentials that lead to corrosion as well.

Car makers have used the metal body as a return path to save money. Most electrical problems in older vehicles originate from corroded ground contacts, sometimes ragged holes are left by a ground strap screwed to the chassis.
Fortunately the increased use of plastic body parts has forced them to use proper return wiring.

 

The correct way to wire a 12V system is the engine must be earthed and the negative gets earthed, as well as a negative wire or cable must be istalled that will carry the current as if the wiring was not earthed. The earth is a reference and can be done with a modest earth strap, and should be done in multiple locations. Vibration prone parts sre earthed with a fine stranded earth strap.

 

Lab studies , field studies or even sensible theoretical physics papers that support that position?
There's a lot of urban myth and misunderstanding in the field of marine corrosion. Opinions abound (as do links to articles that support opinions). That unfortunately is the level of knowledge in the industry. Imagined Electrolysis is also a handy whipping boy.

For example most of the steel boats in my region have negative bonded hulls, and we simply don't see any problems. Even the much more problematic 240v systems have a bonded earth and even the neutral is bonded to the hull, there is always some capacitive coupling and a few millivolts apparent using a sensitive meter yet there is never corrosion in parts of the hull away from local contacts and the earth leakage circuit breakers trip at a few 10’s of milliamps if there is any leakage.

I doubt corrosion potentials or stray currents can ever get high enough to cause problems to the hull. . The physics just doesn't stack up, nor does the practical observation.

To put it in perspective from experience I'd suggest that your paint system is many orders of magnitude more important in preventing corrosion than any theoretically possible effect of stray currents in your hull and I'd challenge anyone to show otherwise for small currents (except for small localised connection points). Yet small milliamp current paths are often conveniently and incorrectly blamed for external underwater problems.

Electrolysis simply gets blamed for a lot of poor paint application or a poorly implemented zinc anode protection system. It's easy to blame something controversial, poorly understood and abounding in a level of understanding which is based on hearsay.

In reality and for perspective, you could put thousands of amps through a small ships alloy hull, before you get anywhere near a corrosion potential for a bare immersed metal in seawater. That’s not even considering the epoxy barrier coat or the bare anodes, the corrosion for even the bare metal would be the usual localized galvanic cells.

 

Perhaps a simple way to explain why you don't use the body (or hull) to conduct current through. If you push the + and - leads of a battery in the sea water it begins to boil the water.

This is due to the salt water being conductive. If you pass current through the hull then the same effect may take place - even a very small voltage drop over two point will cause conduction.

The furter effect is that the area between the two points I-in and I-out will probably rust more likely due to the agrivation the current conduction could cause.

To make contact with the hull where the cables are connected to, there cannot be insulation.

While the above are only possibilities, it is just bad practice to conduct current throug the metal work in any case.

If you generate AC then both L and N should be insulated from earth. To prevent static buildup for whatever reason a 10K 5W resistor is usually connected between one AC wire (Neutral) and ground.

 

I know nothing about wiring on steel hulled boats, but I can tell you from extensive observations and rebuilding various old cars, that if you do not ground each component separately to the chassis, you will get extensive corrosion between the parts.

For example, the head on an engine gets its own ground (or earth) strap to the body, as does the block, and each part of the transmission assembly (for example on my 4 wheel drive Toyota, there is a ground strap on the bell housing, the main transmission housing, and on the transfer case of the complete transmission assembly). The heavy load items like the alternator, starter, headlights, etc. all get their own direct negative connection to the negative terminal of the battery. I have bought old cars that had one or more of these ground straps left off from previous self wrenchers, and corrosion at the point of contact (usually the bolts) always results. If the manufacturer installed them, than they are necessary I would say, otherwise they would save the cost of installing them.

So it seems to me that each part of the engine should have an earth strap, plus each component should run their own negative wire. Stray voltage build-up in the hull, even static charge from the motion of the sea water, could be very dangerous, so it must be grounded to a common contact point.