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[Rex Hunt]

After seeing all the discussion on here about prop rotation.....and reading some interesting theories.......I have a related (sort of) question for 'The Panel'!



I am currently working on a large twin prop vessel........and the plans show that the rudders are set inboard of the lines of the two props.

What is the thinking behind this as I would have thought that more control would have resulted from mounting the rudders 'in line' with the propshafts so that the thrust from the props would act directly on the surfaces of them?


Can someone enlighten me 'BEFORE' I drill holes in the hull for the steering gear?

 

Thanks

Rex

[Colin Bishop]

It is quite possibly to facilitate withdrawal of the tailshafts for inspection/servicing without having to dismantle the rudder. Good idea on models too!

[Roger in France]

Yes, I wondered if it was about servicing the prop. shafts.

I also wondered if it was anything to do with gaining an effect from the props. rotating in opposite directions and then washing across the rudder blades?

Roger in France.

[Bunkerbarge]

Depending on the type of vessel in question there may be an argument for sacrificing manoeuvrability for maintenance if say the vessel was to operate in very adverse water conditions such as a lot of sediment etc, which may require the shafts to be pulled more than normal.

There is definately a loss of rudder effect though so there would have to be a good reason for wanting to compromise this.

[John W E]

hi there all

I will put this theory into the equation as well; if we take the standard propeller - the thrust which actually comes from the propeller does not come from the total area of the diameter of the propeller.  It comes from one side of the propeller - well at least 75% of the thrust does    to see this effect - next time the model is in the bath - and the motor is running - gently begin to lift the stern out of the water and watch where the flow of water comes from on the propeller blade.   That is just before we lift it too far out and we get soaking wet  .   

Cos this is the other part of the equation which comes into play - central fugal force - the tendency for the water to spin out over off the propeller - this is what gives us the 'paddle wheel' effect when the boat tries to move sideways.   If we have a look at some high speed boats or models, you will see that their rudder has been placed to one side of the propeller - directly in the main thrust flow of the propeller.

The other thing to think off or the other way to look at it - those who are lucky enough to have been to sea, on a single-screw vessel, and have had a look over the stern at the wake from the propeller, you will see that it curves in the same direction as the rotation of the propeller.

As for ease of propeller shaft removal; it is a feasibility on smaller vessels, where the shaft is in one length.  On the majority of large vessels, the shaft is in several lengths and normally, if needs be, the centre section of the shaft can be lifted out of place and the stern section pulled inboard, obviously after the removal of the propeller then the shaft will be winched inboard.   Obviously, this is done in dry dock.

We must remember that some of these propeller shafts are 2-3 feet in diameter; and, sometimes 20-30 foot in length.    Believe it or not, these shafts do bend and flex, (when they are at sea) - so we can ponder over these thoughts as well 

aye
John e
bluebird

[Admhawk]

Some examples I posted on ModelwarshipsUk. Rudder alignment with prop wash seems to be a common expectation. I was asked about the Canadian Frigates a while back because they only have a single rudder between the two screws. I'm not a hydro engineer, but I can tell you that the one large rudder turns those babies extremely well!!

Another example is the USN Perry class frigate, single prop, single rudder, but the rudder is offset a couple of feet from center.

USS Enterprise, CVN-65 has angled rudders, while the rest of the Nuke carriers are straight down.

A lot of battleships had two rudders and three to four props.

I guess it's all a function of vessel size vs rudder shape and size and water flow under the stern. And probably whether the twin props have variable pitch which can assist with turning.

A comment about John's bending and flexing shafts, The Turbines and Generators at the electricity plant I work at have lube oil supports on the shafts and the shaft has to be kept turning at a slow speed even when shut down to prevent sagging. The shaft is made up of several lengths and can be disassembled and removed for mtce. It's probably a similar size as some of the larger ships, but I suspect the ships have more supports along the length.

[Martin (Admin)]

While we're talking about hydrodynamic theory, I've always wondered what effect the swirling motion of the prop wash
has in a large flat surface just behind it.... say like a huge rudder? Is there a lot of wasted energy from the engines trying
to twist the rudder into a nice coil?

..... just wondering.... 

[boatmadman]

The power station I work at also has oil support for the shafts. We call it jacking oil, it lifts the shaft off the bearing surface just enough to establish an oil film between the bearing and shaft to minimise bearing wear on startup/shutdown.

The bigger of the two sets has a rotating mass of 120 tonnes and sits on four bearings.

martin, did you mean this effect on rudders?

Ian

[Colin Bishop]

The liner SS United States also had to keep the shafts slowly turning over while in dock for presumably the same reasons. I wonder what would happen if they tried to start her up now!

[John W E]

Hi there

There are several reasons for keeping very heavy shafts rotating - the hull flexes as well as you will know - and all the bearings which support the main shafts are located and fastened to the hull at some point - and they move with the hull.

There is another reason, if a very heavy shaft settles in a white metal bearing (over a period of time) it actually begins to impress into the white metal, mis-shaping the white metal bearing.

aye
John e
bluebird

[boatmadman]

This thing about turning shafts: any steam turbine has to keep the shaft turning whilst it is hot, or else it will bend and at least cause excessive vibration when runs up to speed and at worst wipe out bearings.

Interestingly, a shaft that gets bent can be straightened simply by keeping it turning for a couple of days (length of turning actually depends upon the size of the machine).

Large gas turbines are more forgiving than steam turbines, they can withstand rapid temperature changes much more easily. In the early days at my work (which was the first large gas fired power station in the uk) we force cooled the gas turbine once to get access quicker for maintenance. We soon learnt thats not the best way, the rotor seized solid due to unequal cooling.

The fix? Walk away and leave it for 48hrs, come back, get it turning slowly for a few days and run it up, no problem.

Ian

[John W E]

... Hi ya there Boatmadman

A slightly different ballgame when dealing with gas turbines.   This is due to the fact that they are operating at a lot higher temperature.  They have to disburse the heat rapidly and not have any areas which can store the heat.   

A lot of gas turbines have a hollow outer shaft which holds the majority of the turbine rotors on.

With the shafts being hollow it disburses the heat quickly., but, like the steam turbines - yes they do have to be kept rotating when not in use, due to the fact of the total weight in the makeup of the rotors.   

The majority of the shafts and rotors in a gas turbine, are made from a material called Wasperloy.  This material is similar to stainless steel to look at - but - it is extremely tough.   It can withstand a lot of heat without discolouring; it is a lot like aluminium - it can get red-hot but still looks cold and silverish in colour.   It's expansion rate, when it is heated up, is very small.  It disburses heat very rapidly as aluminium does - when it is in a thinned walled section.  However in mass - it will hold heat.

Ponder............................ 

hey I can tell you a little story from Rolls Royce - about the whizz kids from a University coming up with lab techs to check out a new CNC machine - the machine was 'Morraseetie' (I may have that spelling wrong!) vertical boring machine - and it was equipt with a 110 chain tool belt, in other words it could select 110 tools to machine with.   

When these whizz kids came round they had a set of tools which were of a ceramic material, and, the idea was to rough out a centre rotor for an RB211 engine from a cast ingot of Wasperloy material; when we machined these manually from ingot - there was roughly about 48 hours of pure machining time went into them and that was only a semi-finish job.   

These whizz kids thought they could knock it down to about 18 hours, by increasing the feed & the speeds with the use of these ceramic tipped tools; the end of the story was... they couldn't keep the coolant directed onto the job -  because it was turning to steam as soon as it was hitting the job as it was being machined. So................. 

The whizzkids thought that they could run these ceramic tools without coolant, (because ceramics can operate at extremely high temperatures)  fine until mechanics fail........................

 until, one of the hydraulic dogs which holds the jobs into place on the fixture decided to give up - due to the immense stress which was being put onto it.    The job promptly came out of the machine - destroying half of the tool rack, the turbine disc....and cleared the area.

It took us half an hour, with fire extinguishers, to cool the turbine disc down.

Funny, but, true - we never saw the whizz kids again........

The machine was out of action for about 4 months...

slightly off topic I know, but, when we talk about bending shafts etc., it does bring back some funny happy memories.

aye

John e
bluebird

[Martin (Admin)]

The liner SS United States also had to keep the shafts slowly turning over while in dock for presumably the same reasons. I wonder what would happen if they tried to start her up now!

Do ant machine suffer that problem today?