Critical RPM and manoeuvring - Opinions sought

[YoungMariner]

I spent the entirety of my seagoing career onboard ships with controllable pitch propellers and now in my second year of piloting the majority of ships I handle are fixed pitch.

One area of concern is the critical revs and the overall advice is to be cautious passing through the critical range as there is a higher risk of blackout. I notice that some ships have a form of computer load program that seems to push up through the critical range when its ready whereas others that sometimes dither in the critical range for a while until they eventually power through.

As a pilot on a loaded ship outbound it is important to get up to passage speed as quickly as possible so as to overcome the effect of the current/wind quicker and get the ship underway. There is nothing worse then a ship at slow ahead dithering at 1 kn with no appreciable increase in speed and having to push through to half and than full ahead wondering if this is good for the engines and the ship. Where possible I will utilise a headline pulling ahead to assist the ship.

Now most of the guidance from fellow pilots about the critical revs and the pros and cons of pushing up the engine settings quickly are obviously from those with experience up top and I am interested to know what the opinions are from Engineers. How do they feel about the way the engines are utilised during manoeuvring, how do they feel about the critical range, do you operate ships that have load programs operating in the lower power settings?

[Silvertop]

Do the Captains on these ships not give you an insight into how the engines will react? I always try and give the pilot a short brief on what to expect from the engines, some listen some don?t. But with an electric engine it can be very slow and if I don?t tell them they start asking if something is wrong.

But to your question, I have been on motor and steam ship which have critical speed ranges and have never had any issues with blackouts as you mention. If on a load program it pushes through as quickly as it can, if not it would normally be between manoeuvring positions, say half and full ahead. It is due to high vibration and advise would normally be to avoid prolonged running in these ranges, so if necessary you could stay at critical revs, it would just be very uncomfortable.

I imagine you are seeing more ships than my life on oil and gas ships so there may be additional cause and effects on other ship types which make critical revs more critical. I will see what others have to say of their experiences.


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[YoungMariner]

Do the Captains on these ships not give you an insight into how the engines will react?

Thanks for your reply. Very soon after transitioning from seafarer to pilot I realised that the vast majority of Captains are completely passive during pilotage and provide very little input even though we have a very proactive BRM process in our port (continuous commentary of our progress and updates, offering the Captain the use of the PPU screen to monitor the passage etc). I abandoned asking the 'let me know when I can push her up to half ahead or full ahead' questions, since it was generally met with a blank look and a shrug. I'm certain if I asked for full ahead straight after lifting off the berth on a cold engine they would probably try and provide it without question...

[agibbs98]

Why, during critical manoeuvring, would your control of the main propulsion plant put the ship at risk of blackout? I was taught that during standby periods the electrical load is to always be generated by the diesel alternators as the use of main engine driven shaft alternators during high engine load changes isn’t entirely reliable. Even on ships which are fixed RPM with CPP the sudden change of pitch whilst running a shaft generator is enough to potentially blackout the ship because of electrical frequency trips when the engine can’t get back to the fixed RPM quick enough.

As for answer to the actual question regarding critical range, sadly I’m not sure as I’ve only sailed with fixed RPM. Hopefully someone else can shine some light.

[Pilot Chris]

Due to the criticality of our port we have had engine manufacturers add a button to whatever load programme buttons they already had installed, all companies are aware of what’s required here and any that don’t have the ability to immediately bypass this are not allowed in. One engine manufacturer called their button the “name of our port” button as previously they had no load programme bypass and engine was one of those aweful eco-engines.

We also in some states of tides have a tug push and tug lead ahead to speed the process up. In general we try to be doing at least a couple of knots when going through the critical but you can only leave it for so long, we get the odd main engine slowdown but this is often due to masters and chief engineers forgetting to disengage all the limits.

I know you asked from an engineer point of view but the above relates how we are going through to not only the company owners but also the manufacturers about the issue.

[YoungMariner]

Due to the criticality of our port we have had engine manufacturers add a button to whatever load programme buttons they already had installed, all companies are aware of what’s required here and any that don’t have the ability to immediately bypass this are not allowed in. One engine manufacturer called their button the “name of our port” button as previously they had no load programme bypass and engine was one of those aweful eco-engines.

We also in some states of tides have a tug push and tug lead ahead to speed the process up. In general we try to be doing at least a couple of knots when going through the critical but you can only leave it for so long, we get the odd main engine slowdown but this is often due to masters and chief engineers forgetting to disengage all the limits.

I know you asked from an engineer point of view but the above relates how we are going through to not only the company owners but also the manufacturers about the issue.

Thanks Chris. It's useful to hear of your experiences. I might trying asking for your override at some point in the near future.

[Chiefy]

Over the years I've seen various barred speed ranges, and similarly various mitigations, however they usually amount to the same thing.....don't do it

The old Far Easters had a bared speed range in the 70-80 rpm area (if memory serves)thus for manoeuvring it wasn't an issue as full ahead was mid 60's (all numbers could be 10 lower, it's been a while)

More recently I have seen barred speed ranges in the 24-27 rpm range, for this there is an alarm, but dead slow is 21, and slow is 30, thus the only time we really have an issue is those few second on starting when it over speeds and hovers briefly in the range while the computer has a think, occasionally there is a sudden slow down we might get the alarm while the way comes off the vessel, though I belive as we aren't actually driving the engine (not adding any exciting force...engine not firing) the effect is less dangerous .......then there is Suez where by a dint of being sues they always want 28 rpm +/- 2 .......gaaaaa that said most captains cheat and get the speed by going high/low on opposite shafts and swapping them over depending on bank effect and steering etc.

We do have and have had on pretty much every ship, a program override button, which will do just what it says on the tin, I don't like it as some people use it routinely where as other use it as intended. ........a kind of of **** button.

The builders really should be buildong ships with the range either suitably low or suitably high so that critical operations arent interfered with.