Electrical Isolation of Propshaft/Prop

[Skoal]

I am rebuilding the propulsion system on my '77 Pearson 30 sailboat - basically replacing everything from the transmission flange back (flexible coupling, split coupling, S/S prop shaft, Martek bronze prop, and new Cutlass bearing). I have all the parts and pieces, and I'm starting to put everything back together.

Because I'm using a flex coupling (R&D Marine), without any special connection (silver-coated rubber film, bonding strap, spring, etc.) the prop shaft (and anything connected to it - prop) will be electrically isolated from the engine.

My initial feeling is that this [isolation] is a good thing, another 'side-benefit' of using a flexible coupling between the transmission and the prop shaft. However, I want to be certain I'm not overlooking something fundamental, and looking for some advise from anyone out there who has experience in this area.

My reasoning for keeping the prop shaft isolated are the following:

1. The original drive had a flexible coupling (similar to a Vetus type) and there was no electrical connection between the engine and shaft. That what I found when I took it apart, and the limited documentation I have suggests that's the way it was when it was built in 1977.
2. If a connection was typically required, I would assume that the manufacturer of the flexible coupling would include a connection method. R&D Marine offer a connection as an accessory part if required/desired.
3. Other manufacturers of flexible couplings (e.g. Global and IsoFlex) treat the electrical connection as 'optional' ,
4. From a lightning protection perspective, I would prefer to have the energy dissipated through the keel not that transmission,
5. From a galvanic corrosion perspective, I plan on installing a sacrificial zinc anode on the prop shaft, and the engine has a sacrificial zinc anode plug that get replaced every season,
6. From a stray current perspective, in theory, the prop shaft *should* be at the same electrical potential as the engine because the engine is bonded metal components (e.g. prop strut, keel, through hole fittings, etc.) in contact with the same water that the prop and prop shaft are in.

All that being said, when I search the forums and Google this topic, it seems like most people feel that the connection should be made to bypass the electrical isolation provided by the flexible coupling, and electrical connect both sides of the coupling. My question is simply 'Why?' And I haven't found a compelling reason. Hence the post.

I'm interested in hearing how other folks have handled this situation.
Thanks!

 

[Charles Erwin]

Of itself the isolation is just fine. The problem comes if you have shore power. Shore power safety green (called the grounding feed) is run to 110 appliances, outlets, and the motor block too. If there is an AC short - safety green provides a path back to the shore power transformer and an onboard breaker trips.

Now if the safety green lead is broken - maybe because the shore power wiring is not done right, your shore power cord or plugs are broken or maybe you have a wiring disconnect in your own onboard wiring - there will be no path back to the shore power source. This is dangerous.

But, since safety green is also connected to the motor block, there is a second path through the motor block, the shaft, through the water and back to shore power source. Breaker trips safe even though your primary safety green return path is broken.

If you isolate the shaft there is no secondary path and you have defeated an important and useful safety feature.

Concerns over lightning, galvanic activity and possible stray current are a distant second compared to human safety - and by the way apart from lightning - the stray current and adequate anodic protection issues are easily identified by a simple hull potential measurement.

Charles

 

[Skoal]

Charles -
Thank you for that information.
Even from the shore power scenario you described, I think I may still be okay because there are other metallic structures besides the prop shaft that the engine is bonded to that are in direct contact with the water - the keel and the prop strut.
Regards,

 

[Charles Erwin]

You sound confident.

If you are well bonded and the bonding contacts are so reliable as to provide the safety valve I describe then adding a jumper makes no difference and such an addition will have no effect whatsoever on protection from galvanics, stray current, or lightning either. Right?

Furthermore - as you must know by now bonding integrity verification is typically ignored and bonding system corrosion failures are one of the most common systems failures we see - it is surely the most common system failure I see.

Incidentally - ABYC and the USCG consider this arrangement mandatory - which means it is a commandment - not an option. So why not jumper the the shaft and have a bomb proof reliable safety path?

It is up to you but if you do not choose this simple $1 bomb proof fix, then at least consider installing a 100% GFI protected on board shore power system.

Charles

 

[Skoal]

Charles -

I need to check and see if my shore power ground is connected to the boat ground that is connected to the engine. I'm not 100 percent sure that it is. I agree it would make sense if it was - but I'm not sure that's how they did it 1977.

Incidentally, I don't think I've ever used 'shore power'. That's not to say that I wouldn't in the future. It just that when I have used power to operated a tool or something on the boat, it's usually through a grounded extension cord. My boats AC electrical system is fairly primitive - a weatherproof twist-lock L5-20 and a single 110 receptacle in the cabin (not GFCI). I should replace it with a GFCI as you suggested (duly noted and added to my 'To do:' list).

So if the shore power safety turns out to be the only reason for connecting both sides of the flexible coupling together electrically, in my situation, it may be better to leave the prop shaft electrically isolated.

I'm thinking there may be benefits to an isolated prop shaft that we are not considering - trade-offs.

Question: If it was so important to ground the engine and transmission through the prop shaft, why would they [manufacturers] make the connection optional and why wouldn't they supply the parts (jumper, spring, foil, etc) or integrate the connection mechanism with the coupling?

Thanks again for your input.

Regards,

 

[Charles Erwin]

As to the proposition 'if it were so important why wouldn't the manufacturer provide a means to jumper the isolation' I suggest you call and ask them. I would be hugely interested in what the 'manufacturer' has to say about the matter of compliance with USCG and ABYC standards. I can say if the manufacturer is euro then they will tell you the US standards are different from the euro standards.

In any case, I can offer no more than what I have said. Be safe my friend.

Charles

 

[Skoal]

I found this grounding schematic among my documentation. It shows the boat's electrical grounded through the keel. That *may* explain why the electrical isolation provided by the original Vetus-type flex coupling was not bypassed with a jumper - there was already a ground present through the keel. That was probably how they did it back then. Fast forward to today, is that still a good way to do it? Or, is it preferred to ground through the prop shaft? What have we learned about marine electrical systems in the last 40 years?

 

[Maine Sail]

That image is for a Pearson 26 which had an external iron bolt on keel. The Pearson 30 keel was built with an encapsulated lead keel. This makes the keel unsuitable as a grounding point.

ABYC safety standards prohibit isolation of the prop shaft on a boat like yours..

 

[Gunni]

Shoal;
The schematic depicts lightning protection grounding.

 

[Ron20324]

[QUOTE... there are other metallic structures besides the prop shaft that the engine is bonded to that are in direct contact with the water - the keel and the prop strut....[/QUOTE]

Except that the keel and strut are encapsulated by how many cots of paint? And, does the strut have a rubber bushing between it and the shaft. Get strapped.

 

[Maine Sail]

... there are other metallic structures besides the prop shaft that the engine is bonded to that are in direct contact with the water - the keel and the prop strut....

Except that the keel and strut are encapsulated by how many cots of paint? And, does the strut have a rubber bushing between it and the shaft. Get strapped.

The Pearson 30 keel has a fully fiberglass encapsulated ballast thus it can not be, and should not be used as any form of Earthing on board the vessel. Bond a lightning down conductor to encapsulated ballast and you risk blowing a hole in the keel and sinking the boat. The lead ballast was dropped into the encapsulation it is not external lead...

 

[JamesG161]

I rarely comment on posts that experts like Charles and Maine have commented on.
But...
You can check your actual engine block - ocean ground with a DVM and a piece of say copper in the ocean.
The path and wire size is another issue.
Jim...