Model Boat Mayhem

Mess Deck: General Section => Tugs and Towing => Topic started by: Tug-Kenny on January 10, 2012, 08:46:56 PM

Title: Further question on Rudders
Post by: Tug-Kenny on January 10, 2012, 08:46:56 PM


How can a tug with twin propellers and One centre rudder be made to steer better.

This is an effect which I get with my Lady Wooes tug.  It's verging on impossible to steer around the lake. I've found it will not go in a circle less than 20 foot diameter and I have ACTion's variable speed control to the props set for slow down one side when turning.  I can change this to Tank steering but it creates a lot of wash and tends to look gawky on the water.

Below is the only picture I can come up with at the moment. This shows the fitting out and I was committed to the positioning of the props height by the original moulded prop tube outlets

Any idea's  ??

Ken
Title: Re: Further question on Rudders
Post by: Colin Bishop on January 10, 2012, 09:13:03 PM
Ken, I had exactly the same problem with the original Imara. The problem is that the propstream doesn't impact directly on the rudder to help 'kick' the stern round so until the boat is moving, the rudder does nothing at all and then not very much after that. I forget just what options the ACTion mixer has but I think you need a setting that actually reverses one prop to some extent when the rudder is hard over. If you get a lot of thrashing from 'tank steering' then maybe smaller props which will not be so near the surface may help.

In my long running Fishery Cruiser project, which has still to see the water outside of the bath and is not fitted with a mixer, I have rigged the radio to use the TX 'undercarriage retract switch' to swap between grouping the motors under the left hand stick for general running and independent sticks for 'tank' steering for slow speed manoeuvring.

Another possibility you might like to consider is an underwater temporary extension to the rudder to bring it into the propstream. Not authentic perhaps, but needs must...

Colin
Title: Re: Further question on Rudders
Post by: Shipmate60 on January 10, 2012, 09:17:16 PM
Ken,
To get her turning sharper you will need to have more propwash going over the rudder.
All I can suggest is larger, coarse pitch props as close to the hull as possible inward turning.
Most tugs have large slow spinning props to "bite" the water and give a good bollard pull at the whole rev range.

Bob
Title: Re: Further question on Rudders
Post by: poll on January 10, 2012, 09:27:40 PM
Hi Ken.
Have you thought of making a Becker type rudder so it will pick up more wash.

John
Title: Re: Further question on Rudders
Post by: Tug-Kenny on January 10, 2012, 09:30:50 PM

Thank you all for the ideas.  I've sketched this idea of Colin's.  Perhaps a matched pair which clip on each side ? ( when going in the water)



 
Title: Re: Further question on Rudders
Post by: farrow on January 10, 2012, 09:38:53 PM
Kenny,
If from the photo that is the final postion of the props, then as mentioned earlier, the prop wash probably like the original never hit the rudder, the rudder works only when the hull moves through the water causing a flow across the rudder. Tugs tend to operate at close quarters, so when working the master uses his props, so you need the props set to turn outwards like the original boat, that way you will get the full paddle wheel effect. I have known of older masters in the P.A.S. on old twin screw tugs take there tugs all the way down and out of the Plymouth Sound from the top of Devonport harbour just using the screws and the Helms man standing at the wheel like a lemon when they have point to make. Close quarters you use tank operation and going light speed through the water slowing one engine or stopping it to help a tight turn. Also how is the trim of your tug, it should be deep at the stern and the bow a lot shallower, a level trim will make it handle awkwardly.
Title: Re: Further question on Rudders
Post by: Colin Bishop on January 10, 2012, 09:47:46 PM
Not quite what I had in mind Ken, but an interesting idea all the same. Von Richthofen would be proud of you! What I thought was to extend the rudder at the bottom so it sticks out further. Some of the Thames barge modellers do this.

Going on from Bob's post, I see that your props are outward turning. As he suggests, swap them over which will help direct the propwash over the rudder. Certainly that is the first thing to try. However, Dodes also has a point if you accept that the rudder isn't going to be very much use anyway in which case you are back to tank steering or a mixer which actually reverses the inner screw on the turn. The ACTion P94 in Mode 3 might be the answer. I think Dave is away at the moment though.

Colin
Title: Re: Further question on Rudders
Post by: Tug-Kenny on January 10, 2012, 09:51:41 PM

Thanks for the input Dodes.  I think you are on to something here. I seem to recall having the bows higher was a feature of this crafts sailing ability.  After all, the water line does seem high as per drawings.  I was sailing her flat level.  %)

This would lower the stern deeper and maybe affect the turning ability.  I would have liked the props up higher but as said previously, I was governed by the moulded tunnels.

Colin, I'm away from the boat at the minute but looking at the picture, maybe I could raise their diameters and change them over.  I'm all right with the ACTion unit as it's working well.
 
Ken
Title: Re: Further question on Rudders
Post by: Colin Bishop on January 10, 2012, 09:56:37 PM
Well, you have got a few ideas to try! As Dodes says, get the stern down as far as you can which will reduce or eliminate thrashing. If you can get the model to swing in the bath then you are on the right track and she will perform better on the pond.

Colin
Title: Re: Further question on Rudders
Post by: grasshopper on January 10, 2012, 10:27:08 PM
Is it an optical illusion that the visible prop shaft looks like it is not parallel to the keel if viewed from above? - if that was  the case then the wash from the prop would miss the rudder all together - if it they were angled inwards there would be greater force impinging on the rudder and increase turning force.

Rather than add extra foils - could you not increase the width of the rudder blade?

I'd also heard a rumour that sometimes the inner engine revs were increased to pile water onto the rudder blade to power the stern round and t he outer engine reversed to hold forward speed
Title: Re: Further question on Rudders
Post by: longshanks on January 10, 2012, 11:08:09 PM
Colin,

I was interested to read your idea about switching between group and tank motors.

Being a bit suspect with electrics any chance you could put a wiring diagram up

Cheers

longshanks
Title: Re: Further question on Rudders
Post by: Colin Bishop on January 10, 2012, 11:28:05 PM
Quote
I was interested to read your idea about switching between group and tank motors.

Being a bit suspect with electrics any chance you could put a wiring diagram up

I did this a while back so I'm not sure if I still have the full details. Basically, what I did was to connect the right stick output from the RX to the two speed controllers with a Y lead to group them together. I then  had a connection from the right hand stick to the right hand speed controller to give tank control. The 'clever' bit was to route these two sets of leads through a switch controlled by the undercarriage channel on the RX which enabled me to select grouped or separate control.

Quite honestly, if I was doing this today I would use a mixer such as the ACTion P94 which gives several alternative setups to suit a particular model.

Colin
Title: Re: Further question on Rudders
Post by: oldflyer2 on January 10, 2012, 11:49:06 PM
Just an observation from a novice ... that looks a pretty dinky prop for that size of boat. A tug needs pretty big props to get their power. Also, it looks from the photo that the shaft angle is too high. Hoe high in the hull are the motors mounted?  I know that the prop shaft tunnel was already molded, but it wouldn't be the first kit to have it slightly out of alignment and in need of redrilling.  A new hole could be made and a new tunnel molded with putty.  The prop just looks to be too low down and directing the water flow beneath the rudder. When you look at pics of most boats when they are out of the water, the props are closer up to the hull.

The rudder looks to be quite large, but it seems to stop short of prop wash.

Just some observations ... I will watch to see what the fix is.

Cheers

Tom
Title: Re: Further question on Rudders
Post by: farrow on January 11, 2012, 09:34:21 AM
As I mentioned in another thread on this site regarding the problem of twin screw single rudder, ships fitted with this system usually have no rudder effect until the ship has sufficient way on to cause a strong stream of water over the rudder. Also a twin screw vessel would have substantially smaller props than a single screw boat, after all the engine power is split in half and the MoD dog class boats had quite small props. If the screws are outward turning as per the original vessel, then you get paddle wheel effect which helps the turns when using one engine or tank driving as some people refer to when using the engines opposite each other to turn as the real boat would when at rest or at slow speed. With a dog class tug twin screw and twin rudders, I frequently used a combination of different engine moves to turn in basins and close quarter situations, as the rudders are only effective up to a point and on real vessels 35 degrees is usually the max rudder angle anyway.
Title: Re: Further question on Rudders
Post by: GG on January 11, 2012, 09:50:52 AM
The rudder will be fully immersed in the slow moving boundary layer around the hull.  Try extending it downwards into the faster flowing water stream.  Extension could be a "clip on" type so as not to spoil the appearance out of the water.
Glynn Guest
Title: Re: Further question on Rudders
Post by: derekwarner on January 11, 2012, 10:43:35 AM
Hullo Ken.....without going back to your PS Waverley thread  :o ....did you increase the rudder surface area to attain acceptable steerage ....ie., with the paddler rudder in relatively still water? ....Derek
Title: Re: Further question on Rudders
Post by: Tug-Kenny on January 11, 2012, 11:19:33 AM
I've just been out to photograph her Props and the line up.

They do look a bit too low, don't they. Also the angle seems a bit steep.  I seem to remember when I fitted the motors that I went for the lowest position I could.  The size is now 60mm and as this takes them slightly below the Hull.  

The  'A'  frames also govern the size as the prop tubes as they have to be parallel. This has brought the sides of the props near to the hull, so I don't think any small changes here will affect the outcome.  To alter anything now would be a major refit, so it's onward to invention of a different rudder.

Derek.  On the Waverley I increased the rudder's front to back width during construction and it works reasonably well. I think the turning circle is modest but not impressive.  %)

Glyn & Colin. I think your ideas are rather good and appears to the easiest way to overcome it. Thanks.  :-))

Ken

ps

Forgot to mention the magazine picture I also followed


Title: Re: Further question on Rudders
Post by: Colin Bishop on January 11, 2012, 12:43:39 PM
Ken, Glynn is quite right when he says that a rudder extension that projects below the hull will be more effective so if you make a clip on extension which both extends and lowers the rudder you will hopefully see a big improvement. The comments made earlier about tank steering on the full size ships are very interesting, you learn something new every day! I knew that the old RN destroyers used to use this method when coming alongside or casting off and their props usually turned outwards I believe. You may still need to have the props inward turning to get the most out of a rudder extension but this could be at the expense of tank steering response to some extent but it's easy to just swap them over and see what happens. Think of it as fine tuning! It wouldn't be fun otherwise, would it?

Colin
Title: Re: Further question on Rudders
Post by: Tug-Kenny on January 11, 2012, 12:54:13 PM

I'll certainly be having a go at that idea.  Thanks guys.   :-))


ken

Title: Re: Further question on Rudders
Post by: Xtian29 on January 11, 2012, 04:02:56 PM
Hello

I fully agreed with Dodes
Quote
twin screw single rudder, ships fitted with this system usually have no rudder effect until the ship has sufficient way on to cause a strong stream of water over the rudder
Then when working at slow speed you have to move one engine on different way than the other and it works far better than enlarging the rudder or adding flank plates on it

It's a pitty to have this propellers so low, it's hurt the eyes of mariner like me. Is it not the normal position of the shaft support ?

(http://nsa22.casimages.com/img/2012/01/11/120111051151207058.gif) (http://www.casimages.com)

Xtian
Title: Re: Further question on Rudders
Post by: RaaArtyGunner on January 11, 2012, 08:57:42 PM
Hello

I fully agreed with Dodes  Then when working at slow speed you have to move one engine on different way than the other and it works far better than enlarging the rudder or adding flank plates on it

It's a pitty to have this propellers so low, it's hurt the eyes of mariner like me. Is it not the normal position of the shaft support ?

(http://nsa22.casimages.com/img/2012/01/11/120111051151207058.gif) (http://www.casimages.com)

Xtian

Now that is food for thought even to a novice like me?
Title: Re: Further question on Rudders
Post by: oldflyer2 on January 11, 2012, 09:18:05 PM
I didn't see those molded pads at first, but that kinda looks like they were there for a purpose, and that would put the props in a much better location to throw water over the rudder.

Cheers

Tom
Title: Re: Further question on Rudders
Post by: Talisman on January 11, 2012, 10:03:31 PM
Hi Ken,
I'm not going to say much on the subject as i see there are allready been good replies.

What i would say, being slightly bias as a Powerboat instructor is this -
Go do a powerboat course in a twin prop boat (with an instructor that knows what there doing) and learn to mix in the real world then take it back to model boats and you'll be a master mariner 'pond style' ...
It works the other way to as i have often used my models on Powerboat courses .. but with three independent props, certainly hones the understanding & skill. As with most things in life difficult at first but opens up a whole new range of skills and enjoyment.

As for what you'll learn about the IRPCS ... well sometimes i think that should be a mandatory course on Joining a club  {-)

Regards,
Kim
Title: Re: Further question on Rudders
Post by: Tug-Kenny on January 12, 2012, 11:08:57 AM

Thanks for the info Kim. Unfortunately I suffer from sea sickness so am unable to go sea any more. It all started when I was taking my Yachtmasters in the Irish sea , during a force 9 gale many years ago.   I just wanted to die. {:-{

I agree with the comments so far on the location of these prop tubes and would have altered them earlier. It might be worth re-thinking dropping the height inside the boat to effectively raise propellers. Perhaps I might be able to belt drive the shafts from above.

An interesting subject though.

cheers

Ken
Title: Re: Further question on Rudders
Post by: 787Eng on January 12, 2012, 06:51:57 PM
Ah,
You mean something like this installation in my Lady Wooes :}
I can spin her on the spot round her funnel with my setup.
Two MMM T12's turning Prop Shop 75mm 4 Blade Props

(http://s15.postimage.org/pzv3qv2dz/IMG_20120112_00002.jpg) (http://postimage.org/image/pzv3qv2dz/)

(http://s18.postimage.org/dhqygx791/IMG_20120112_00001.jpg) (http://postimage.org/image/dhqygx791/)

(http://s18.postimage.org/4hmcwqb5h/IMG_20120112_00004.jpg) (http://postimage.org/image/4hmcwqb5h/)

Respectfully
Mark
Title: Re: Further question on Rudders
Post by: tugmad on January 12, 2012, 08:18:20 PM
That is a nice installation you have there. :-)) :-))
Title: Re: Further question on Rudders
Post by: Tug-Kenny on January 12, 2012, 08:57:44 PM

I agree.  Quite beautiful.  Should be easy to re make my drive system as this is obviously the way to go. Mind you,  moving the tubes down into line could be fun.  %)

75 mm props at 4 blades as well. I am impressed. You've re-rejuvenated an old man.  :-)) :-))

Thank you for showing us.

Ken

 
Title: Re: Further question on Rudders
Post by: 787Eng on January 12, 2012, 10:31:22 PM
Thanks Both for the kind words....

Kenny, how Ive removed epoxied shafts before was to cut the tube almost flush with the hull on the outside and then heat up the remaining tube with a solering iron shoved up the tube. Then with a big pair of pliers on the inside, start by gently twisting, eventually the heat will soften the epoxy and out the shaft pops....

The shafts I used were the MMM 10" x  5mm ones, with the matching heavy duty A frames in the positions marked on the hull...
Brive belts were from http://www.motionco.co.uk/timing-pulleys-timing-pulleys-aluminium-c-25_35_50.html (http://www.motionco.co.uk/timing-pulleys-timing-pulleys-aluminium-c-25_35_50.html) but were modified to remove the mounting hub.

Mark
Title: Re: Further question on Rudders
Post by: Tug-Kenny on January 12, 2012, 10:38:02 PM

Once again, brilliant information. I shall get on it when the  "Yard" is clear.

Thanks,

Ken
Title: Re: Further question on Rudders
Post by: 787Eng on January 12, 2012, 10:44:18 PM
Kenny the belts are motionco P/N B3M065.

They also have a very nice belt length calculator if you wish to play with the ratio's...

Mark :-))
Title: Re: Further question on Rudders
Post by: farrow on January 14, 2012, 03:28:19 PM
Hi Kenny, seems 787Eng has solved your problem, looks a nice system.