Why a 24V system?

[MissFitz]

Our new (to us) boat has a 24V system, which seems to be pretty unusual. Can anyone tell me why it might have this? Is it something to do with the engine (an old Watermota Sea Panther)? And what are the implications in terms of equipment - can you use 12V stuff, & does it work out a lot more expensive in batteries?

 

[VicS]

The main advantage of 24 volts is that for any given power consumption (Watts) by equipment 24 volt versions will only draw half the amps

Watts = amps X volts so if the volts are doubled the amps are halved if the watts remain the same.

This means that wiring can be lighter and volts drop on long runs is not quite such a problem etc.

However I think you will find that the Watermotor engine has 24 volts feeding a 12 volt starter motor in order to make starting easy. This means that you must be very careful to limit the time of cranking to avoid burning out the motor!

Some boats actually have an all 12 volt system but two batteries connected in series for starting. Charging the second battery then becomes a bit of a problem.

It has been discussed on here before and also IIRC on the Westerly owners association forum.

EDIT:

Just checked and found that you do have a Westerly. It would be sensible, if you have not already do so, to check to see if that the above comments about 12 volt systems but 24 volts for starting apply to you!

 

[JerryHawkins]

As VicS has said, but you will always end up with a mix of 12v and 24v systems. Any 12v kit will need to be supplied via a 24v to 12v DC-DC converter. These are available in various power-handling capacities from various manufacturers including the likes of Victron. Don't be tempted to take a 12v "tap" between the 24v banks.

 

[TrueBlue]

FWIW - and why I built my boat with 24v electrics, is that for a given physical size a 24V alternator will give more power than its 12v equivalent.

For example a 12V 100 amp alternator model (1200W nominal) would also be available as a 70 amp 24V unit = 1680W, and often for the same money.

In modern boats there is often restricted engine mounting space for large sized units; which is of course where folks like Merlin find a market for their small sized but high output (And priced) alternators.

edit: Oh, and yes, I do have a 12v tap on the batteries, but I only run things like inspection lamps and mobile phone chargers at 12v

 

[halcyon]

Are you sure it is 24 volt ? the White Water Wolf was the only 24 volt Westerly made, from memory ?

Brian

 

[Bodach na mara]

I also have the Watermota engine and as VicS states, that engine uses a standard 12 volt starter but hits it with 24 volts. This is the "cold start" system as devised by Watermota when they designed this engine by "dieselising" a Ford 1600 cc petrol engine and finding that there was no room in the cylinder head for heater plugs.

It is a great engine however and I have progressively moved to make more use of the 24 volts available where possible by installing heating system, waterpump and refrigerator that all operate from 24 volts taking advantage of the lower current requirement. I also have an Alphatronic 24 to 12 volt convertor for all the other requirements. Strangely enough the binacle and instrument lights were 24 volt, but the rest of the lighting is the usual 12 volt.

Just seen Halcyon's post and I think there are quite a lot of Westerlies with the 24 volt alternator. All the ones with the Sea Panther from the 70's and early 80's anyway. That incorporates quite a number of 31's (Berwick, Renown etc) and a few 26's (Centaur et al.)

 

[halcyon]

They were all 12 volt when I started supplying Westerly in about 83, so we must have been at the end of the change-over. But I never have came across a 24 volt, live and learn.

Brian

 

[saltwater_gypsy]

We have mainly 24V and it means lighter wiring for the same wattage. Particularly good for the windlass etc.
Most modern instruments are rated for 10V to 36V, HOWEVER, ST60 are 12V as is some NASA kit so we also have a 12V system as well. If I was designing it I would have 24V and a convertor for 12V for those instruments which required it.

I found out the hard way that our starter was 12V being run off 24V. I used to boast how smartly the engine started until the starter motor went up in a cloud of smoke. It is now a 24V job which is in any event quite common in lorries.

 

[MissFitz]

That's very interesting about starter motors, 'cos we had two blow in rapid succession at the end of the summer. Previous owner claimed he'd never had starter motor trouble in the five years he had the boat - makes you wonder....

 

[tcm]

yes, Vic alludes to (but didn't actually say) that the advantage of a higher voltage is that the energy/ power loss is lower. It's for this reason that we have 240 V in homes, and much higher voltages on pylons.