Author Topic: QM2 - still speedy!  (Read 1630 times)

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Online Thomas Hypher

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Re: QM2 - still speedy!
« Reply #20 on: Feb 01, 2018, 08:53 PM »
Back in the 'good old days' when (Carpathia-type) steam reigned supreme, it wasn't too hard to find an extra knot or three, by the simple process of firing the boiler enough to go past the red lines on the steam pressure and temperature gauges, and 'pulling the foot' a little more than the designers intended. Even screwing down the boiler safety valves was not unheard of when being chased by the enemy!

HMS 'Manxman', a fast minelayer of the Abdiel Class in WWII, managed to find an extra knot or two on top of the 40 she was designed for. But the 6 Abdiels had a 2-shaft propulsion system of 76,000 hp shoe-horned into what was essentially a large destroyer-sized hull. And in the 1950's, even after half Manxman's boilers were removed, she was still good for 26 knots! (Google some of the stories about her performance and how she made the US fleet in the Meddy look a bit silly one day - would loved to have been there!)
 
Simply put, motor ships and gas turbine power (diesel electric or otherwise) do not lend themselves as readily to exceeding their designed outputs as the old-design steam systems did. Being diesel-electric, the main switchboard breakers on QM2 have preset current limits, and when pushed beyond those limits, the breakers tend to operate the way they are meant to, and promptly open, thus immediately dropping the relevant GT or diesel generator off the board. This is done to protect both the generator and it's prime mover.

When the breaker trips at full power, the prime mover often trips it's overspeed trip when it exceeds max design rpm plus a preset percentage determined by the manufacturer, and then shuts itself down automatically - definitely not a good scenario when the machine has just been on maximum output, at the top of the heat cycle, and then the crankshaft or turbine suddenly stops. The immediate concern, if possible,  is to get the machine running again to prevent local overheating, then cool the machine down gradually. These sudden stops and/or big load changes are not good for the gear at all, so, when operating machinery these days, gentle reduction in loads, and avoidance of sudden load surges are much better for the longevity of the machinery. And, today's machinery is operated much closer to it's mechanical limits than the steam machinery of old ever was.

The other factor, especially on vessels with a common HV bus (11,000 VAC) that all consumers are fed from, is that the sudden loss of a generator off the board often leads to preference breakers and other load-shedding consumers being immediately tripped from their power supply as well. Usually leads to engineers racing around the MCR and ER frantically resetting trips again.

So, finding more speed in QM2 - very unlikely.

Skilly

I remember reading about the Abdiel Class including Manxman a while ago and being very surprised at quite how fast they were for their size but this could be a good example of a longer, narrower hull being faster for the same power than a shorter, wider hull - sometimes at the expense of manoeuvrability back in the day.

Anyway, back to topic! Thank you Skilly for your usual excellent insight on mechanical matters!  :)
First sailed on QE2 in August 2003 aged 6 years old. Last stepped foot and sailed on QE2 in July 2008. Last saw the seagoing QE2 in person from the decks of QM2, on QE2's last Transatlantic crossing (Eastbound tandem) in October 2008. Visited QE2, in her new life, in Dubai back in January 2020.

Offline skilly56

Re: QM2 - still speedy!
« Reply #21 on: Feb 02, 2018, 11:24 AM »
One major point that I should have emphasised in my post yesterday is that back in the steam days, the steam propulsion plant was usually independent of all electrical supply and auxiliary services, so if you had a propulsion system hiccup whilst pushing the machinery hard, with all readings up 'on the blood', a sudden failure didn't turn all the lights out/shut auxiliary systems down.

With modern diesel electric systems, the main generators (at up to 11,000 volts) supply power to both the propulsion bus, and the auxiliary switchboards through various step-down transformers. So, if you try to load the DGs/TGs beyond their designed loading, the first thing to suffer is the bus frequency, which gets unhappy as it slides below 60 Hz (50 Hz). However, the power management systems are designed to normally prevent these sort of events happening by load-shedding - slowing the shafts or reducing the propeller pitch, depending on the type of plant installed.   

The only real way to increase speed is to kick all the pax and hotel staff off, shut down all hotel services throughout the ship, and then the extra 1-2 MW available could be utilised by the propulsion system, if that machinery can handle it.

Cheers,
Skilly
« Last Edit: Feb 02, 2018, 11:48 PM by skilly56 »