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S&S Swan Maintenance - 'Switchable' alternators
06 May 2020 - 15:05
Join Date: 20 July 2017
Posts: 102

'Switchable' alternators

Hi All,

hope you are well in these unprecedented times.

On Kairos, I believe we still have the original electric panel in the navigation area, see pictures below. Some parts are now obsolulete e.g. the engine operating hour counter is now in the cockpit as part of a newer multi-function display that came with the new engine we fitted last year. (Perkins M92B). The old panel works fine and we would like to keep it as original as possible, obviously we will have to maintain some 'servicing' on some of the wiring as we go along to keep it ship-shape.

As for alternators, we have an engine alternator and an additional 'service' alternator, both driven by the main engine. The engine alternator is now new as it came as part of the new Perkins but the service alternator was only overhauled and is the same as before.

As you can see from the close up below, we have separate old-fashioned amperemeter instruments for the engine alternator on the left, for the service alternator on the right, and a common Voltmeter in the middle.  (I know these are not very accurate at the best of times, we will be installing a modern battery management system and hide it somewhere soon :-)  but we like the 'original look' of these.)

There are also two switches "alternator ON-OFF", and with the old engine (and old wiring) these did exactly that - stopped the alternators from charging (or at least they stopped the Amps being shown on the instruments). I read the user manual of our Swan 57 and it states that this was to avoid interference by the alternators when using the radios.

Besides this, it would obviously free up more engine power for propulsion should you need it (and the batteries are empty/being charged). Especially for the service alternator, which I believe is around 150 A.

Unfortunately, what the manual doesn't say is how this ON-OFF switching was safely achieved. And even more unfortunately, the guys installing the new engine refused to wire anything via the existing panel ... so this functionality is currently lost.

I'm not an electrical engineer but I have heard that you should not disconnect the load (= the batteries) from the alternator. An alternative would be to operate a magnetic clutch to disengage the alternator's pulley but there seem to be none installed on the service alternator, which is original.

So I am assuming that these switches were affecting/switching the external excitation of the alternators? I.e. switching the excitation off = alternator cannot charge = idles and does not create interferences.

Any ideas would be most welcome - we'd like to get the system back to 'originality', or at least be able to still switch the service alternator - thank you!

All the best


PS: If you ever consider installing a Perkins M92B in a Swan 57 give me a shout first; it's a good engine but a very tight fit. Too tight I am actually thinking after the first season of sailing ...


Electric panel

Close up - alternator switches marked

06 May 2020 - 15:43
Join Date: 30 January 2007
Posts: 446


you are perfectly right, that switch was meant to cut the excitation to the field of the alternator.  Restoring it is pretty simple; you just need to use (or add) a two-core cable which supposedly runs from nearby the alternator to behind the switchboard.  Notice that in the original setup there is a 2,2 kOhm/10W resistor paralleled to the switch and it should be added if not present.  If the regulator is not an intergral part of the alternator, the excitation wire is easily found; if it is integrated, it is more tricky but it is still possible.

In most of the old Swans this facility was overridden because it has not such importance anymore; most of the good quality electronic apparatuses are provided with EM noise filters and there are smarter ways to avoid interference.

On the other hand, I believe that the need of more propulsion power from the engine is seldom necessary since most of the times the propeller configuration itself is not able to make use of it - but this is a matter on which I gladly let Lars Strom speak!

Daniel, 411/004 Luna Menguante

06 May 2020 - 16:02
Join Date: 29 January 2007
Posts: 928

Dear Stefan,

if you like the analogic instruments, as I do, you can still buy them and they are quite reliable nowdays. My electrician insisted I installed a PLC too, and I am now very happy now, as you can do a lot of things through the PLC (e.g. I can decide how many amps I want the alternator to produce, I can manage a lot of alarms, etc. etc).

The old switches are still available.

Please see my original switch panel and below the new one.

Fair winds!

matteo (47/069 Vanessa)

original switch panel

06 May 2020 - 16:33
Join Date: 20 July 2017
Posts: 102

Dear Daniel & Matteo,

thank you very much for your very fast advice, this is very helpful!

The service alternator has external excitation so I will get this reinstated; the new engine alternator I believe has got this built-in so we'll have a think.

Good point about the propellor, Daniel - I didn't think about that.

We do like the analogue instruments, luckily ours still work so no need to change at the moment but it is good to know that we can replace them if need be.

Matteo, your new panel does look very nice indeed - we are contemplating to get the Mastervol Easyview 5 installed but likely somewhere else as there won't be enough room in the panel.

Guess it is nice to be able to have the best of both worlds these days :-)


Thanks again,


06 May 2020 - 16:39
Join Date: 29 January 2007
Posts: 928

Dear Stefan,

I have the Mastervolt EasyView too (and a very good instrument), but no room enough in the panel.

Fair winds!



06 May 2020 - 18:21
Join Date: 02 January 2008
Posts: 1547

Dear all

Alternators that can be switched off are unknown today.

A big alternator that can be switched off provides an additional benefit when starting the engine. The starter motor has an easier task if the alternator does not charge when brought into rotation
Expanding on Stefan's thinking - if the engine has a hard time powering the boat against a strong headwind, with rpms dropping although full throttle is used - likely to be some sort of emergency - switching off the alternator will provide a few hp more going into the propeller shaft.
Daniel makes a good point, the propeller may not absorb all the engine power, and then the rpms will not drop in the situation above. Switching off the alternator will still make life easier for the engine.
If there is a refrigeration compressor on the engine it is also preferable to switch it off when starting, or when powering against the wind. There is a magnetic clutch for this purpose, operated by a panel switch.
Kind regards

07 May 2020 - 07:37
Join Date: 20 July 2017
Posts: 102

Dear Professor,

thank you very much for your thoughts, starting of the engine is a very good point indeed. Certainly worth getting the old set-up back at least for the service alternator - in addition to our more 'nostalgic reasons'.

We do have an engine-driven compressor for our fridge and freezer and it's pulley is indeed connected via a magnetic clutch. The normal position for the system is 'disengaged' for the reasons you mentioned and also because I believe the compressor likes steady rpm best. So we try not to burden it with the changing rpms of e.g. our eratic mooring manoevers.

Also, the compressor is so powerful and the whole fridge/freezer system well designed so that it is usually sufficient to run the compressor only for an hour or so every other day to recharge the holding plates in the fridge and freezer.

Believe this system is also originial and we are so happy with it that we had  the compressor replaced and the system refurbisehed last year by the orignial company, too - works a treat ans was well worth it.

All the best



Kairos (57/043)

07 May 2020 - 09:37
Join Date: 02 January 2008
Posts: 1547

Dear Stefan

Good to hear that the original system is used
The Iceberg hold-over refrigeration was designed for the tropics with 35 C watertemps, and one hour of compressor running time daily. In cooler conditions less running time is needed.
Assume you also have a smaller AC-driven compressor for harbour use.
The system efficiency is not quite the same today because the original refrigerant must not be used any more.
Kind regards

07 May 2020 - 10:05
Join Date: 20 July 2017
Posts: 102

Dear Lars,

yes there is also a AC-driven compressor, which we can run via shore or generator AC.

They are completely seperate system with independent refigerant loops, too. Both loops share the the holding plates, which saves a lot of space in the fridge and freezer. Some of our guests say this is so we have a redundant beer-cooling arrangement ... not sure what they are talking about ... but you can cool an enormous amount of food/drink in a very short time with the system.

Jonas Lindgren of Iceberg in Skaerbaek, Denmark is the man to speak to, really knowledgable and friendly chap. Believe he even remembered Kairos (then called 'Rampage' I think) when the system was installed almost 40 years ago.

The refrigerant is (or may become) a bit of a problem, believe we are currently on R404, which is due to be phased out I understand. Guess they all are not too friendly to the Ozone layer when released into the athmosphere.

Unfortunately, when we refurbished the system early last year there had no decision been made on what would be the replacement refrigerant so we had to use R404 again for now. It's also a question that the compressors are suitable to accept whatever new refrigerarant we will have to use in the future. Plus mixing with any old R404 that remains in the system after flushing also plays on my mind.

In the past, I believe the replacement refrigerants had similar properties to address these concerns so we keep our fingers crossed that this will be a relatively staright forward swap.




Kairos (57/043)

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