Shaft Idle

This sea story requires more technical back story than usual.

Or just skip the background sections and get straight to the story below.

Background #1

The “ship spec” is the bible of new construction, tells what is expected from the contractors to build the ship, to which they are evaluated at acceptance and final contract trials.  As I recall, ours was about 10-12” thick.  Computers were just coming into general use, and we might have had some of it as a text files, but for the most part you had to know where to dig in and read.

One of the “interesting” requirements was that the machinery control system was explicitly prohibited from “relying on shaft speed measurements to keep the plant in a safe operating condition.”  This was further interpreted to mean that control system could not measure shaft speed to control shaft speed.  This is like saying you cannot use a speedometer to know how fast you are going in a car. You can know what day of the week it is, how fast someone next to you is driving, what your oil pressure is, but not speed.

Reportedly, the throttle control programmer at GE told NAVSEA this would not work.  He was told to read and follow the specs.  So he did some interesting calculations and predictions, such as where the throttle was 10 minutes ago, outside temp, etc, and did his best.  He did sneak in a low-authority loop that measured speed over minutes and had some authority to fine tune the shaft RPM.

The designer retired from GE, and left a phone number he could be reached when the Navy was ready to correct the throttles.

The apocryphal story was that some former ship class had a shaft overspeed situation, so NAVSEA did not want to rely on a single sensor for shaft speed.  (But we had two speed sensors per engine, and an additional two on the reduction gear.)

Background #2

The Burkes were designed as old-style sub killers.  The  propulsion controls were designed to make them much quieter than the 963s.  With a controllable-reversible pitch propellor, as soon as the propellor comes off 100% design pitch, it starts making a lot of noise (cavitation).  The 963/993/47 class maintained a minimum shaft speed of 55 RPM, which meant there was a lot of noise up to 100%.

The Burke design was to keep the engines at idle, not to add any power until the propellor reached 100% pitch.  This meant that the shaft RPM would drop as pitch increased, to a minimum RPM at 100%, at which point RPMs would start to increase.  This gets through the noisy slow speed areas as fast as possible, the shaft is turning slower while noisy, hence provides much quieter lower speed operation. 

Since the engines stayed at idle, transitions in this region were much slower.  Fundamentally, it was designed to be quiet, not optimized for slow speed formation steaming.

For further complication, two engines on a shaft had a completely different profile – there was more idle power, so the 100% pitch was reached at a higher speed. 

And, since shaft speed dropped from 0 to 100% pitch, then started rising, there were two spots on the curve that had the same RPM, but very different speeds.

The worst intersection of the two conditions was ahead flank to a backing bell less than max astern pitch – the system would not add throttle to maintain shaft speed (which is was not monitoring anyway) – so the propellor shaft could end up rolling backwards. (Although this is one of the areas the original designer may have cheated and added some unauthorized boost.)


So we have met the two culprits – the nonsensical ship spec shaft feedback requirement, and the very intelligent quiet throttle control design.

We did not notice any issue at LBES driving into the water-brake, nor during sea trials, or the transit via Earle* to Norfolk.  It was during local Norfolk ops that I started getting called to the bridge to examine throttle control problems.  We would be alongside another ship, doing small boat ops, but were going 12 knots instead of the ordered 3 knots.  Or vice versa. Or on a cold or hot day, we would be making nothing like the ordered speed.  Some problems resulted from putting the throttle at the wrong position (i.e., picking the wrong 30 RPM position), some was from the loosey-goosey shaft control, and there was no way to know which was which cat.

* hmm, Earle reminds me of another sea story someone needs to put together

Well, background #1 and #2 may have been too subtle for some flags, one of which, after a few reports of uncertainty, ordered us tied to the pier until the system was “fixed.”  NAVSSES was able to do a quick fix, called “shaft idle” which imposed a floor of 55 RPM, like the 963s, etc.  The original designer was found on the beach in Daytona, said funny, he had been thinking about this, and redesigned the controls to monitor and maintain shaft RPM, still keeping the ASW quiet operation.

Rest of the story

We had two temporary propellor viewing ports, in an aft port side storeroom.  You could lay on your belly to look out the windows at the port propellor.  Most effective in clean Caribbean waters.  The design intent was easily visible – at the slowest 100% pitch position, the propellor was clean.  Drop pitch slightly, and the cavitation was instantly visible and felt through your belly.  Same flag officer that tied us up laid on his floor in SR#3 for several hours, relaying orders to the bridge and watching the results.

The viewing ports were removed in PSA.

I always thought if we really needed to hunt subs, we would have had an ASW mode that locked pitch at 100% and avoid putting the noise knuckle in the water.  Problem was that sub hunting actually was a minimal part of ship employment, sometimes controllability was more important.