Hi Rod -
Please tell us your thoughts about the dc generators in regard to Britannia.
Thanks !
June 
Would be interested to hear also Rod!! I only go with what I have seen and read etc - I'm a land lubbing Elec Engineer - you have the hands on!
DC generation on older boats is not that unusual and Britannia was a child of the 50s when DC (Direct Current) was still more prevalent in ship design.
Waverley had DC when she was built in '47 - generated via a steam driven dynamo. I think it was either 50 or 100VDC but not sure - a Lloyds Register between 1947 and 1974 would detail this I would think.
Through time as AC equipment on board became more widespread she had an 400VAC (Alternating Current) generator added. This meant that there was a mix of voltages on board so I would imagine you'd need to be very careful when working on any of the circuits.
Post rebuild she now has 2 x diesel driven generators which operate on a Duty / Standby basis and 1 x Emergency Generator (located port side aft on the Promenade Deck. All of these generators run at 400VAC, 50Hz, 3phase. The only DC on board is 12VDC on the bridge but I cant remember what thats for!! Nevertheless theres a 230VAC - 12VDC Power Supply up there anyway!
The vast majority of vessels built up to the 50s had DC electrical power (usually 220VDC I think) - the Titanic for instance had DC power supplied by 4 x 440kW steam driven dynamos which would give a total output of approx 17600A at 100VDC! These dynamos had their own steam engines and boilers to supply the steam to drive them.
I think as time wore on 220VDC was the common voltage used as DC systems suffered from excessive "volt drops" over long cable runs - this is where the resistive properties over the length of the cable cause a drop in voltage between the source to the destination which can lead to higher current in the circuit or overcurrent situations which can result in failure of the cable etc. This is reduced by using higher voltages and also in some instances smaller sized cables can be used as the current is lower on higher voltage systems.
This is less of a problem with AC systems as the voltage can be higher in general (eg 6600VAC) and transformers used to reduce this voltage where required - for example lighting circuits at 220VAC etc. I think most AC marine systems run at 60Hz. (Domestic power in the UK is 50Hz).
In the 60s when more and more modern electrical equipment was installed on board (radar etc) the DC systems were phased out.
I read somewhere that QM and QE were DC also but cant be sure of that!
On the safety side the effects of both on the human body are quite well explained here:
http://www.brighthubengineering.com/power-plants/89792-ac-and-dc-shock-comparison/Hopefully the above makes sense but I think Rod may be better able to explain!
Gav
