THRUST MEASUREMENT

[DYV1973]

LS,

We have recently had a discussion about applying thrust measurement on the propellor shaft of a ship, but so far the most important things are not clear to me:

1. Why is thrust measured instead of the more common torque measurement?

2. What is the best way to measure it, and where is this kind of equipment available?

Thanks in advance,
DYV

 

[IRstuff]

Because a ship's designer is interested in how much thrust he can get to move his ship. Torque is a derived requirement, based on compromises made with the propeller.

TTFN

 

[notnats]

2. Strain gauges on the thrust bearing mounts?

Jeff

 

[IRstuff]

http://ittc.sname.org/2002_recomm_proc/7.5-02-03-02.1.pdf

TTFN

 

[notnats]

IRstuff,
That's an interesting link, but it seems to describe tank testing. I think DYV1973 was asking about taking thrust measurements from a ship at sea. I don't know if this is normally done, but it wouldn't be all that hard to do.

Jeff

 

[IRstuff]

I suppose that one could, but why would one do such a thing?

It's absurdly difficult to control the conditions for such a field test, which means that the results will always be questionable. Additionally, how are you even going to control or measure the characteristics of the ship itself?



TTFN

 

[GregLocock]

It was regularly done for warships in the early twentieth century. Froude and so on spent a long time measuring the actual thrust and the ihp of the engines so as to establish the real world efficiency of propellers as installed.

A very cheap and possibly effective way is to use a hydraulic thrust bearing and measure the pressure in the bearing.

Cheers

Greg Locock

 

[notnats]

I spoke to a friend who was a chief engineer on container ships, tankers etc. today. He said he had never seen thrust measurement on a ship and thought it would be hard to find a place for a strain gauge.

I suggested to him that it might be handy to monitor thrust to check for prop condition. He replied that fuel consumption and power settings soon tell you if the prop is fouled.

I like measuring bearing oil pressure, Greg, but won't that depend on where you read it. Bernoulli makes the pressure lower at larger diameters on the Michel bearing if I remember correctly.

Jeff

 

[DougMSOE]

About 1977 I did some testing on a USCG tug. This tug did have load cells for thrust measurement. They were located in the thrust bearing just forward of the propulsion motor, aka diesel/electric system.

 

[David95]

I have seen systems on a propeller shaft with strain gauges for both thrust and torque measurement. The thrust measurement was probably used for calculating propeller efficiency(?). When you measure thrust in the thrust bearing near motor or gearbox, isn´t it that you neglect friction in other bearings?

David

 

[GregLocock]

Yes, you are ignoring the axial thrust in the stern gland system and any radial bearings downstream of the thrust bearing.

But that seems to me to be a very small error, as there is no significant axial motion in the shaft.



Cheers

Greg Locock

 

[DYV1973]

Thanks for your input so far. But are there any known manufacturers of systems for thrust measurement?

I have recently heard of thrust measurement with load cells for POD propulsion systems. Why is thrust measured in a POD, but not on a "normal" (pitch) propeller shaft?

DYV

 

[pko]

You can get instrumented (load cell) shoes for Kingsbury type thrust bearings. Was involved in a recent (1995) contract which required demonstration of thrust for acceptance purposes. The load cell was left in place and the leads disconnected, since there was no requirement for continuous thrust measurement.

Don't overlook the fact that the foundations cannot be assumed infinitely stiff, so you're going to have some frequency dependence in your load measurement. We went to the expense of modelling the entire reduction gear and local structure and decided that the foundations were not much stiffer than the shaft itself.

I'm a proponent of shaft torque measurements to be used to optimize engine operations - at least in smaller high speed diesel plants. I haven't considered the benefits - if any - for large slow speed diesel installations.