Dynamic fluid pressure on a tug boat rudder

[RBPrice]

Could someone point me toward a source of info regarding the water pressure on a rudder used to steer a commercial tug boat.

The tug suffered a failure of the welded joint between the upper shaft and the pintle shaft which is the tubular pipe that runs down the axis of the rudder. I need some idea of how much pressure was on the rudder to calculate the torsional load. The link shows a photo of the rudder and the failure was at the joint above the plates at the top of the rudder. The rudder dimensions are approx. 65 in. wide and 115 in. tall and you are looking up at the stern of the tug.

Thanks a bunch

Bob Price
A*G*M[/b]

 

[chicopee]

Here is a report that you will be interested to read, particularly pages 26 and 27.
web.mit.edu/harishm/

http://www/papers/btech_thesis.pdf

· PDF file
Finite Element Analysis of a Rudder - MIT - Massachusetts ...
I found this site and I am fairly confident that more sites are available when you make an inquiry "US navy analysis of forces on ship rudders"

 

[MikeHalloran]

The MIT analysis appears to be on point.
However, it covers a cantilevered rudder.

Is Margot's rudder similarly cantilevered, or is that some sort of radial bearing adjacent its distal end, and apparently supported by a bar projecting aft from the skeg? If so, what condition is it in?





Mike Halloran
Pembroke Pines, FL, USA

 

[RBPrice]

Hi Guys and thanks for all the suggestions. The Margot rudder is NOT a cantilever since it is supported by the end bearing which is part of the massive skeg which I think you can see in one of my pictures. Secondly, the pintle shaft in NOT a tube but a 7 in. diam steel bar so it is stiff. Third I got some but not all of the data regarding the hydraulic system that drives the rudder, (the boat is busy pushing large tankers around in NY harbor).

Them I did some calculations based on 1800 psi hydraulic pressure, a 4.50 in diam. piston with a 2.00 in. diam. rod (the cylinder is double ended and turns the stub shaft with some 1.00 in. cable), an estimated 10 in. long lever arm from the ships First Mate who wasn't happy with my pestering him on his cell phone, and came up with an applied torque of ~261,000 lb-in. That compares with my guesstimate of 200,000 lb-in. load from the rudder with 2.5 psi acting on the centroid of the rudder x the radius from the centroid to the cntr line of the pintle shaft. I have been promised some more photos but from the one or two I have seen the lever arm is probably longer than 10 in. so the applied torque is probably larger which makes sense since they would want some extra torque available.

Here is a photo of the failed palm plate with more in the next couple of posts - what you see is the weld that joined the 7 in. diam. stub shaft to the top of the palm plate. The "ridge" is what is left of a fillet weld that might have been 0.50 high when it was made at least 6 years ago. There is no evidence of any weld prep prior to the fillet weld being applied.



Bob Price
A*G*M

 

[RBPrice]

Another picture - it shows a crack that started at the outside edge of the fillet weld and went out into the 1.50 in. thick palm plate. I was told the the steel conforms to American Bureau of Ships specification which calls for a minimum of 440N/mm^2 ~66,700 psi. I have no idea if the palm plates meet that spec but I'll bet one doesn't weld that just with any old stick or wire. Haven't had time to look that up yet.

The bright orange surface appears to a step machined into the plate to engage a step machined into the bottom edge of the stub shaft. There does not seem to be any weld material in the perimeter of the step near the bottom edge of the fillet weld. Since the plate is 1.50 in. thick, it seems to me that the shaft should have entered the plate to a depth of at least 0.75 in.

More tomorrow as SWMBO is suggesting I stop since I have to get up early and blow drifted snow out of our driveway.

Bob Price
A*G*M