"crash back" - "all back emergency" & full astern trials

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Little h
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Re: "crash back" - "all back emergency" & full astern trials

Unread post by Little h »

Crash Astern = Royal Navy terminology

Andrew Watkis @AndrewWatkis
I've been at sea for a short while again. I thought I might explain what we've been up to in @HMSQNLZ
1/
7:31 am · 18 Sep 2020

Andrew Watkis @AndrewWatkis ·18 Sep
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@AndrewWatkis
Following summer leave and a bit of maintenance the ship has proceeded to sea for a RTP and SPA, or Recovery and Trials Period and Ship's Performance Assessment 2/

Andrew Watkis @AndrewWatkis·18 Sep
The RTP is a bit like taking your car out for a quick spin when you get it back from the garage, checking it all still works and has been put back together properly. It might include some specific trials following repairs or newly fitted equipment 3/

................... tweet No 5/reads:-

Andrew Watkis @AndrewWatkis · 18 Sep
Normally that would be it, and head back home or out on task, but we have also been undertaking a Ship's Performance Assessment. This is a much, much more intensive set of trials normally overseen by the RN's Machinery Trials and Commissioning Agency, MCTA 5/

.................. tweet No 7/ reads:-

Andrew Watkis @AndrewWatkis ·18 Sep
For Marine Engineers, the highlight is the PPA, Power Performance Assessment. This is a comprehensive test of the propulsion system, a thorough work out. Every class is different in detail, but it broadly looks the same 7/

Andrew Watkis @AndrewWatkis · 18 Sep
It includes timing from stationary to top speed, measuring handling at top speed, timing a crash astern (top speed to backwards) and an extended full power trial to thoroughly test the plant. Acres of readings are taken, but it is all digitally recorded now 8/

Thommo @Thommo_Says · 18 Sep
Replying to @AndrewWatkis and @HMSQNLZ
As a YO I stood on the flight deck of a 22 as we did a crash astern during BOST. Amazing sight!

And yes, for some of my followers that was a sea dit- I don’t have many but those I do are good!
Little h
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jbryce1437
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Re: "crash back" - "all back emergency" & full astern trials

Unread post by jbryce1437 »

We did a full ahead to full astern on HMS Exmouth to show off to some VIP's, they forgot to tell the stokers mess down aft to close the hatches. Puddles of water everywhere and the lifebuoy ghost had to jump up on top of the mortor handling room.

Jim
HMS Raleigh 1963 , HMS Collingwood 1963 & 67 , HMS Ark Royal 1964-7, HMS Undaunted 1968-71, HMS Victory (Fleet Maintenance Group) 1971-72, HMS Exmouth 1972-74
JEM, EM, OEM, LOEM, POOEL
Then 28 years in the Fire Brigade
Retired since 2002
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oldsalt
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Re: "crash back" - "all back emergency" & full astern trials

Unread post by oldsalt »

During full power trials extra temperature recordings are taken. High temp. recordings required lead to be placed in thermometer sockets for transferrence of heat, you had to remember to remove the high temp. thermometers before returning to normal steaming, otherwise the lead solidified & trapped/ broke the thermometer.
timlewin
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Re: "crash back" - "all back emergency" & full astern trials

Unread post by timlewin »

i believe, but stokers will correct me, that the trouble with sustained high-speed astern in the days of steam starved the condenser of in the inflow of cold water?
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oldsalt
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Re: "crash back" - "all back emergency" & full astern trials

Unread post by oldsalt »

The sea water inlet for destroyers were in the aft boiler room. The large diam pipes were arranged to act like scoops when going ahead, this enabled the steam usage for the main circulating pumps to be reduced. It was possible to cause damage by prolong full astern movements. Similarly going ahead at revs less than 40 rpm could also cause turbine damage because the velocity of the sream entering the LP turbine was slower than the turbine blade velocity being driven by the main gearing.
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oldsalt
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Re: "crash back" - "all back emergency" & full astern trials

Unread post by oldsalt »

After consulting my 1963 Marine Engineers BR, the problem with prolonged astern running is that the LP turbine casing can becomme distorted because of the high temperature at the the top caused by the superheated steam from the attached astern turbine & the lower part cooled by the condenser. The difference in temperature can cause the casing to "Hog", the top expanding more than the bottom. Here endeth the lesson!
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