Perkins M50 Prima engine - to ground or not?

[Gerry]

I am confused by conflicting advice. This engine has no integral anode and has a negative earth relay, so I suppose it should be isolated from the earth/grounding anode. Yet mine is grounded to the negative anode. Is this right or wrong?

Other than the engine wiring harness, what other connections should there be to the engine (alternator, starting motor, etc)?

What will happern if the negative earth remains?

 

[Ships_Cat]

I think the M50 is the same basic engine as the Volvo MD22, but how similar they are electrically I don't know (the MD22 also has no anodes so that indicates some similarity there). If the earth relay you refer to is the one that temporarily bonds the heater negatives to the block during heating (they being the only things with DC negative connected to the engine, albeit only when heaters turned on) then it would seem the two engines are similar electrically - maybe someone else knows?

Without inspecting the boat it is difficult to be categoric, but from the engines own point of view, if it is not grounded then it should not be a problem for it. ABYC requires the DC negative to be bonded to the engine, but I think this is a hangover from the past and I don't think there is any really good reason for it (the common given reason is for protection in the case of a negative to AC line short, which is extremely unlikely in a well built boat and not possible at all if no AC is fitted).

That extends to the bonding of any metal fittings in contact with the sea eg thru' hulls which also may be bonded to the DC negative - in my view that is unecessary with the DC systems being allowed to float above (unconnected to) the boats metal fittings too.

There has been some dispute on these forums with claims that this is not allowed, in particular has been claimed that the classification societies even being mentioned - however, this is not so, the classification societies do allow it and many (especially metal) boats are now built this way to class rules (probably the majority of non ship sized metal boats, at least). Generally, two pole breakers are required (by class) and should be fitted on a metal boat, which means a larger electrical pannel as each breaker is roughly twice the size of a single pole one, but if not existing on a plastic or wooden boat I would not think it would matter.

If one has AC on board, there must be an earth using the sea for that, and on some boats eg plastic with plastic thru' hulls, the engine/shaft may be the only suitable electrical contact with the water for the AC earth.

As I started out by saying, it is difficult to be categoric without inspecting the boat. Unfortunately, this is an area where many boatyards building smaller boats are very weak in - if possible, if you can get a qualified electrical engineer from a yard building bigger high quality vessels, they should be able to give you authorative advice specific to your boat. Matters typical of what he would be interested in could be the quality of the materials in shaft/propeller, whether there is an insulated shaft coupling or not, whether AC on board and the existance or not of a galvanic isolator or transformer if so, other sea water contact equipment such as water cooler refrig condensers and the nature of the DC wiring to such things, etc.

John

 

[Gerry]

Many thanks for the reply. The M50 has an overhead camshaft driven by a timing belt and was encouragingly described by one maintenance engineer as the worst engine ever put on a boat.

I have also been told it is a Leyland van engine - and their parts are much cheaper - and that it is actually a Mitsubishi engine rebranded by Perkins, Detroit Diesel, Northern Lights, etc.

Does anyone know?

 

[ElieH]

This engine as the Volvo MD22L is a strange beast. Originaly a petrol engine, is it a conversion to diesel made for Montego van. The ground is insulated and no anode is fitted into the block. I did however fit an anode by drilling and tapping one heat exchanger inspection bolt. It is a good engine, but a complicated one with its ovh camshaft belt. The beauty is that all parts between Volvo MD22L and Prima are identical.

 

[Ships_Cat]

That is what I understood too, regarding the history of the M50 and MD22, ElieH but wasn't sure.

Just a small point on parts interchangability with the Volvo. There may be some of the routine replacement parts that are not compatible, for example, the MD22 had at least 2 different oil filters during its lifetime. With respect to the interchangeability of M50 parts with the road engine version I could not comment.

Many marine diesels are insolated from DC now - also it seems that the Volvo version at least must have a good service record because, despite the fact they were a common fit to many production boats one rarely finds any trouble type posts in forums about them. I assume the M50 is similar in that regard.

John