sweyn avatar image
sweyn asked

Earthing on a boat.

I’m refurbishing the electrics on my boat with Solar plus some MPPTs, AC shoreline, a multiplus 12/800/35, a 200ah smart Victeon LifePo4, a DC/DC charger to charge it, Lynx Smart BMS and Distributon, CerboGX plus Touch 50, battery protect etc.

I think I’m pretty much understanding everything except one thing.

On a fibreglass boat, what do I consider ground?

I’ve done a lot of searching online, but the answers are very woolly.

It‘s a 50 year old boat, and nothing has ever been bonded to anything else, only has one sacrificial anode on the prop and it seems perfectly happy like that.

I’ve got a sintered bronze plate that is left over from an old SSB radio that I connect all the electronics RF grounds to, but that doesn’t seem suitable?

Do I just consider the negative terminal of the battery earth?

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2 Answers
Trevor Bird avatar image
Trevor Bird answered ·

Hello @Sweyn , if you want to keep it original that would depend upon the brand of boat. If the mains ground and DC are separated perhaps it is a Jeanneau or a Beneteau? A couple of philosophies exist for ground on fibreglass boats. Both are valid and both have their own drawbacks. It sounds like your boat uses the isolated thru hull method. That means all sea cocks are isolated and the prop shaft, propeller and P bracket are protected by the single anode on the prop shaft. The P bracket is connected to DC ground ( the engine) by a braid so it is bonded to the stainless prop shaft through the gearbox. All sea cocks are isolated and are not bonded to a common anode.

You may find it useful to keep mains isolated from DC earth if the boat was manufactured that way. Many will say that is not right but many European boats come out of the factory like that. It has many advantages.

The radio ground onto the earth can be replaced by several capacitors connected to DC ground. For the purpose of RF it is connected but is isolated from a DC perspective. That is a safe way of ensuring no DC leakage to ground.

This is a complex subject and a bit hard to simply give you a yes, no answer but if the boat originally had an isolated mains ground I would tend to stick with that philosophy. It is VERY easy to make an expensive error when dealing with yacht grounds.

I hope this helps a bit



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sweyn avatar image sweyn commented ·
Hi Trevor, thanks for the answer.

It’s a Contessa, and never originally had mains at all, and the through hulls are indeed isolated.

The prop anode protects the prop and shaft, but here is no P bracket. The shaft has a flexible coupling so no electrical connection to the engine.

Yes indeed it is very complicated. Easy to find diametrically opposed viewpoints. I’m not dogmatic about any option.

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Trevor Bird avatar image
Trevor Bird answered ·

@Sweyn That is good information. If the prop shaft and prop are not connected to DC ground that is nice as they are isolated but protected by the prop shaft anode.

You may consider simply keeping mains ground isolated and treating the boat like is an appliance plugging into the dock. Or … you may want to ground the mains to your earth plate. That would be more in keeping with some safety standards for mains wiring but many boats do not have that.

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