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Author Topic: Tug power  (Read 3873 times)

sandy1000

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Tug power
« on: May 12, 2011, 11:52:18 AM »

I pass this on for what it is worth. In 1971 a model boat builder in Townsville, Australia, Joe Clark (Scottish born), built a large powerful model tugboat. He measured it's power by means of kitchen scales secured from the stern to the side of a test pool. He fashioned a rudder which had two horizontal wings on either side, staggered ie not directly opposite each other. They were as of aeroplane wings in profile. The power produced was dramatically increased. Government officials came to observe this experiment.  And I guess that was the end of it but I occcasionally wonder whether the experiment is worth repeating.
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Roadrunner

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Re: Tug power
« Reply #1 on: May 12, 2011, 01:19:27 PM »

I pass this on for what it is worth. In 1971 a model boat builder in Townsville, Australia, Joe Clark (Scottish born), built a large powerful model tugboat. He measured it's power by means of kitchen scales secured from the stern to the side of a test pool.

That's known as the bollard pull test to see the puling force a tug or what not can achieve.
http://www.tpub.com/content/boats/TB-55-1900-232-10/TB-55-1900-232-100279.htm

He fashioned a rudder which had two horizontal wings on either side, staggered ie not directly opposite each other. They were as of aeroplane wings in profile. The power produced was dramatically increased. Government officials came to observe this experiment.  And I guess that was the end of it but I occcasionally wonder whether the experiment is worth repeating.

I'm assuming that these wings were possible a variation of a standard rudder that is winged shaped which are quite common on most large fixed screw tugs and some other boats, remember how lift works? same principle, just using water and instead of lift its gives more force.
Either that or it was some kind of rudder that squeezed the water to produce more thrust ( similar to a modern korts nozzle) ? i have no real clue why, just throwing some possibility's out there on that one!  {:-{

Not sure if its worth re-doing an experiment most boats are at there power limits in regards to weight and size issues, possible that some of the material used may only be able to withstand particular forces as designed, (oh and cost!)
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sandy1000

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Re: Tug power
« Reply #2 on: May 13, 2011, 12:16:53 AM »

Thanks for the reply. The water was not squeezed but simply ran over or through the wings. I have one of his tugs, similar in size and I guess that I could fashion the wings from memory but it really needs an inventive approach and someone with an engineering frame of mind. But I do recall watching Joe switching rudders and the remakable difference that occured when he used the winged rudder. And in writing this the thought comes to me that the people who could be really interested is the model yachting boys, they are really innovative.
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Geoff Cropper

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Re: Tug power
« Reply #3 on: May 26, 2011, 03:27:32 PM »

Have you noticed how a tugs stern sinks lower in the water when power is applied to the prop.            This  happens because a tugs prop. is much lower than its midships lateral pitching point and the prop. is trying to push the bows up round a vertical circle centered somewhere midships at water level.           So adding aerofoil wings to the rudder would create lift at the stern to give a more horizontal thrust line rather than downwards thrust and wasted power.                Did you notice what happened to the model's stern when power was applied ?   did it dip down or not ?                    I had a Mystic yacht once that had aerofoil bilge keels with the concave side facing outward to drive the hull to windward to counteract leeway.         Maybe the wings were staggered simply to allow bolting through the rudder.          A very interesting subject.         Geoff.                          PS.    If you drive a displacement hull fast enough it will try to climb up its own bow wave.         The tug I was on used to travel head down at 10 knots max. running free, like driving downhill.     She was surfing down the back of her bow wave and the helm (steering stick) was very tender, difficult to steer in a straight line.       It had twin props in fixed kort nozzles with becker rudders behind each.        She was built and leased to us by Smits Rotterdam.
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Norseman

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Re: Tug power
« Reply #4 on: June 19, 2011, 11:20:11 PM »

Hi

Could anyone put the above arguments into a couple of simple diagrams please? Including if possible the optimum prop set up for a tugs pulling power (not its manoeuvrability). Thanks

Regards Norseman
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Roadrunner

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Re: Tug power
« Reply #5 on: June 20, 2011, 12:20:48 AM »

No need really, most of it is speculation as to different effects of rudders shapes, wings etc, thing to remember is when water rushes past a rudder either wing shaped, wedge or flat (commonly on fast electrics for the last two rudder types) the water rushing past the rear edge of the blade can cause a vacuum of air leading to loss of rudder or very reduced rudder function, not from lack of servo or movement, but that the rudder is moving in an air void rather then moving in water which is where you get your steering from. this only explained lack of rudder movement or steering within some types of vessels that move at speed, tugs are in this group due to there power not there speed. ( the ability to move large quantity's of water with less rpm)

You ever tried steering a model with one prop in reverse and it only turns one way? yup that's the props direction pulling it one way, there is a trick to steer in the opposite direction, which is when you reverse set the rudder hard over and get the boat going, it will pull the opposite way, let off the power for a few moment and let the boat under its own force drift, it will start to turn in the direction you want to go providing you keeping the rudder hard over, once the turn start in-cress the power and bingo you have steering in the direction you wanted to go. ( dunno why i typed that though it was relevant some how)

Normally all boats with submerged drives will sink the back slightly when going forwards this is the thrust created from the props to push the boat forward with a combination of bows shape. I say normally i have seen boats weighted down in the bow to counter this effect to they run 'flat' under way, and stationary they drop the bow (great braking for large vessels, not so good for fast boats aka submarine!)

Most tugs & other vessels have upward sweeps on the bow in some form which aids then 'lift the front while the rear 'digs' in, Springers for example and flat fronted vessels tend to push against the water making one hell of a bow wave, which in turn usually makes then dig in at the front bring the rear upwards, and dropping the nose under, simply as they are pushing large volumes of water rather then getting over the top of it, this does explain the bow shapes of many oil tankers and war ships with the 'egg or cone' shape on the tip of the bows keel, to aid then cut through the water, that's efficiency coming into play as the vessels are so huge pushing water is not practical nor efficiently (unlikely to actually move the ship in some cases)

Back on subject i tend to wander at time (mostly) if you want maximum pull from a tug, the rudder plays a tiny role which is tracking the vessel straight, to achieve maximum thrust running off a twin prop (korts or not) then its best to have both prop rotating inwards (to the centre) so looking from the rear the right hand prop rotating counter clock wise, and the left turning clock wise, (usually fitting the lh prop on the rh side and fitting the rh prop on the lh side) this maximises thrust as the water is being forced centrally rather then outwards if the props were rotating outwards, A type of ''jet effect'' which you already get with korts which is increased when the flow is then centralised.

So max thrust is obtained via prop rotating inwards, maximum maneuverability is props rotating outwards. This works for both fixed and rotating korts (not the full 360 ones)

Make sense no? me either anything after midnight i get confused too... j/k
 :-))

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tweety777

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Re: Tug power
« Reply #6 on: June 20, 2011, 09:14:26 AM »

This sounds pretty interesting to me.
I have heard about ships having horizontal plates at the lower and higher end of the rudder to make sure the water will flow over the rudder surface, and if those plates are large enough I can imagine that it also helps a little bit to reduce the sideways movement of water due to the rotation of the propeller, but I don't think that would make such a difference.
It really sounds like something to try again.

Greetings Josse
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Roadrunner

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Re: Tug power
« Reply #7 on: June 20, 2011, 09:38:38 AM »

To be honest i do think most of this to do with planes/wings or fins fitted was literally an experiment with very little results at the end to warrant them being put into effect on full size vessels.

A horizontal plane would give you the ability to sink or raise the back under way the same way a sub works (hopefully with out the sinking part), whether this gives you extra pull power is really an unknown ( but it may do as your digging the stern deeper, which most know the deeper the prop the more power/thrust it can achieve)  only way i would have though of using vertical and horizontal planes would be to channel water into a funnel to give extra thrust the same way the korts work, but being able to up the power/force without increasing the rpm of the engine/prop's which makes for a more fuel efficient pull. 

Thing to remember is that a tug or tugs only use full power for a very short period of time during a pull to get the tow moving, once under way its a matter of keeping the movement going using a combination of a taught line from the winch & the tugs movement which would be around 1/2 or 3/4 power generally.

Full power again is used only to brake, or if something goes wrong.

The bollard pull is there to establish how much the tug can pull from a standstill, i'm not sure of the maths involved to calculate how much they can pull on something not fixed, as tugs can pull great weights greater then there bollard pull at times, so i expect this is calculated in, but braking would be the show, as the tug is pulling back /stopping movement of the vessel it can only brake as its maximum bollard pull to counter the dead weight, this would explain why 2 or 3 tugs are used to pull in some ships as even with there great power it takes 2 or 3 to counter the effects, or the vessel is slowed over a period of time to aid in the final braking and that's with the help of a harbour pilot  controlling the main vessels moment with thrusters ,rudder etc.

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sandy1000

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Re: Tug power
« Reply #8 on: August 21, 2011, 08:41:10 AM »

Quote
the rudder plays a tiny role which is tracking the vessel straight, to achieve maximum thrust running off a twin prop (korts or not) then its best to have both prop rotating inwards (to the centre) so looking from the rear the right hand prop rotating counter clock wise, and the left turning clock wise, (usually fitting the lh prop on the rh side and fitting the rh prop on the lh side) this maximises thrust as the water is being forced centrally rather then outwards if the props were rotating outwards, A type of ''jet effect'' which you already get with korts which is increased when the flow is then centralised.
So max thrust is obtained via prop rotating inwards, maximum manoeuvrability is props rotating outwards. This works for both fixed and rotating korts (not the full 360 ones)
Make sense no? me either anything after midnight i get confused too... j/k  :-))

Is there some glimmer of hope here that I can retain my 2 props spinning the same way on my 1873 gunboat  in that in
having them rotate the same way I average out the major benefit of each so I get 50% max thrust effectiveness and 50%
max manoeuvrability effectiveness each part combining as a whole to be... no, no, I'm getting confused here - what I mean is
 that if both props rotate inwards you say that I would get max thrust but "xxxxx" all manoeuvrability and if both props rotated
outwards you say that I would get max manoeuvrability and b...  all thrust. Is that right? Is that what you are saying? If so I will simply say to critics that the props were
deliberately designed that way to ensure that as a river fighting boat (the Danube as it happens) it created max thrust or mam..
manoeuvrability either of which was highly desirable in that environment.






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Timo2

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Re: Tug power
« Reply #9 on: August 21, 2011, 10:01:15 AM »

Hi

  MY rule of thumb is 

                  Displacement hull pros go outwards ( that the top  )        :-)         Planning hull props go inwards ( that the top )

  Timo2
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nick_75au

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Re: Tug power
« Reply #10 on: August 21, 2011, 10:22:57 AM »

All the full scale twins I work on have port prop CCW and stb CW, these are fast planing leisure boats, the race boats Class One offshore have port CW stbd CCW. They don't need to manoeuvre at low speed ok2

Kort nozzle props should have nil difference as they have no paddle wheel (prop walk) effect. In saying that I have heard that they go with port CW stbd CCW to gain the small percentage of thrust.

The difference is minimal on the thrust factor, I was told it makes 4 MPH difference on the 160 MPH race boat from outboard to inboard rotating, you are not going to be able to see a visible difference in a model boat, you will see a difference in manoeuvrability at low speed,  go with port CCW Stbd CW.

Nick
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BarryM

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Re: Tug power
« Reply #11 on: August 21, 2011, 11:56:09 AM »

Racking my remaining brain cell, some memory from the past tells me that a rudder with fins normal to the rudder post and attached to the rudder sides has been trialled at full-size in the past but where and when is long gone.

The rudder mentioned with plates top and bottom would be a Schilling rudder which has a fish-like profile and is a high-lift type similar in its effect (but not in its construction or operation) to a Becker. Both are highly effective at angles beyond the standard 35 degrees which is the norm for 'ordinary' rudders. The end-plates are intended to eliminate certain unwanted aerofoil effects. 

The fins fitted as you describe, would be intended to have a 'wake-saving' effect converting the propeller vortex to more effective thrust.  There are/have been several devices of this kind.  Remember the Grimm Wheels originally fitted to the QE2 which unfortunately had self-dimantling properties? These were intended to straighten out the propeller vortex in a similar fashion to contra-rotating propellers.  Another system uses an asymmetric rudder, i.e. it maintains an aerofoil cross section but it has a twist in it from top to bottom counter to the propeller stream.

As has been noted before, the thrust generated by a propeller can be represented by both side and longitudinal vectors. The latter is what is wanted to push the vessel along. The side thrust is unwanted except perhaps as an aid to manoeuvring (in which it can also be an encumbrance) and anything that converts some of that whirly stuff generated by the propeller to a thrust in line with the vessel is desirable.

Of course the downside to any addition to the stern gear, apart from cost and durability, is the additional drag created. On relatively slow-speed vessels this can be accepted but increasing speeds whittle away the benefits.

Hope this helps,

Barry M
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sandy1000

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Re: Tug power
« Reply #12 on: August 21, 2011, 01:27:39 PM »

[but increasing speeds whittle away the benefits.

Hope this helps,

Barry M
[/quote]

Conclusive really. Thanks
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farrow

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Re: Tug power
« Reply #13 on: September 06, 2011, 09:04:02 PM »

Bollard pull on real tugs is the maximum pull a tug can achieve on its towing hook for a sustained period. When I carried out a bollard pull test on RMAS Bustler on completion of a refit, it was done in the traditional method. I.E a very large weight measuring device was attached to a sturdy bollard with the tugs towing hawser attached, I paid sufficient tow line so that the tugs wash would not hit the wall, then gently built up to full power and maintained it for 20 to 30 minutes so that the sustained bollard power was obtained which at the time was about 35 tons.
The best handling single screw boat I have worked on was the RMAS Newton, because she had a steering Kort nozzle. Because of this she steered stern first as well as ahead, much the same principle in monourving as handling a Kitchener fitted work boat.
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