Model Boat Mayhem
Technical, Techniques, Hints, and Tips => Other Technical Questions... => Topic started by: J.beazley on June 20, 2007, 10:05:42 pm
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Need help with this one.
Am i right in thinking that 1 prop with 2 rudders each either side of the prop set back a little would be good for turning ??? what im stuck on is would the wash from the prop deflect off the inside rudder forcing the power to left or right thus turning the boat at the same time.
Not sure if the picture helps
(http://img.photobucket.com/albums/v479/jayimageservices/boatwash.jpg)
Im in 2 minds on going twin screw/twin rudder or single screw/twin rudder.
Jay
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You'll have better steerage (cant spell manoverability!) with twin props provided you fit a mixer to slow the inner prop in a turn. This is more expensive as you need to buy two motors, two ESC's and a mixer.
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Need help with this one.
Am i right in thinking that 1 prop with 2 rudders each either side of the prop set back a little would be good for turning ??? what im stuck on is would the wash from the prop deflect off the inside rudder forcing the power to left or right thus turning the boat at the same time.
Jay
I hope it works Jay. Thats what the PT 15 has got.
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Hi JB
The biggest problem when the rudder is not directly in line with center of the prop will be at slow speeds, and will depend on how far off centre they are. The less flow you get from the prop over the rudder, the less steering, as it will rely on water speed over the rudder blades.
Bob
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Wouldn't you do better with a nozzle around the prop to direct the thrust, and then either swivel the whole thing, or fit a single rudder to the back of the nozzle? I know one of these is a Kort nozzle but I'm not sure which one. I think two rudders aft of the prop will just add drag - I reckon you'd be better off with one balanced rudder on the centre line.
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There are actually a couple of interesting effects at play here which you should consider when designing such an arrangement.
The first is that two rudders either side of a prop like this actually creates an effective nozzle, deflecting the vast majority of the wash exactly where you want it. Bear in mind though where the pivoting point for the rudders are hence are they a balanced, semi-balanced or unbalanced rudder. The ones you show above are unbalanced.
Then if you think about the cross sectional area of the space perpendicularly across the rudders as they turn it actually gets smaller. This can create an eductor effect and will lead to an accelleration of the water out of the rudders and therefore an effective turning force. The disadvantage is when turning high rudder angles at high speed as the reduction in cross sectional area can effectively throttle in the wash and stall the flow over the rudder. An advantage is that you get effective steering at slow speeds!!
What would be ideal with this arrangement would be a mixer that reduced the rudder angle depending on the power to the motors. The faster they go the more the maximum rudder angle is reduced, now there's something to think about!!
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Cheers for the help people, slow speed turning wouldnt be too much of a problem as a graupner bow thruster takes care of the front end when at slow speeds.
I think what im going to do is put both rudders in line of the wash say around 2-3mm in from the prop edge, while having them set back about 4-5mm
surely this would give a greater steering ability and keep the prop wash running over the rudders like you suggest.
I could go twin prop but the props wouldnt be any larger then 20mm as the space under the hull would be tight, depends on what kind of pitch i could get on these size props.
Jay
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Another point worth remembering is where the "pivoting" point of the ship is. If it is too far aft or too far forward (relating to the ships length) undue strain will be put on the rudder(s). The pivoting point should be somewhere around the ships longitudinal centre of gravity. Single screw, single rudder gives the best results.
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Why not try and fit a single 'Becker' type ballanced rudder they are easy to set up and give super turning control at any speed
Richard,
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Not seen a becker type rudder set-up, is it like a kort nozzle???
Jay
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Hi Jay,
A Becker rudder is like having two on the same pivot though one is a small blade at the rear of the main rudder which is turned at a greater angle to the first via a link at the top of the rudder.
There is some where on this site a full description and pictures (I think) of becker rudders
I fitted a pair to a Smit Nedland and wow what a difference
Richard,
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Oh no you didn't Chiefy, the Nederland comes with Beckers, you put them on the Portgarth for me. ;D
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Cheers for the info Richard, going to search the forum for information and details on these now ::) SWMBO isnt happy im going to be sitting here for a while (whats new there then ;)).
sounds like just what im after but how small do they make them???
Will research this more before i decide on what to do with the running gear but these becker rudders sound like the trick i need.
Jay
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Hi Jay,
I know Westbourne's have two sizes in stock one is about two and a bit inches high with about 1 1/4 inch main blade and a 5/8 inch thrower blade
Mmm dont quote me on the sizes as it's been a few months since I did that job and capt jack has no longer got the tug
Richard,
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Found all the info and more on these style rudders and from all the posts on the forum they seem to be pretty good.
becker style rudders its going to be i think.
Jay
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This is what they look like. The tail flap actually rotates through twice the angle of the main rudder giving you a near 90 degree thrust on full rotation.
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I think we could do a "Name the Ship" on the basis of that picture Bunkerbarge!!
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I think we could do a "Name the Ship" on the basis of that picture Bunkerbarge!!
Stop it now!
One's enough! ;) :P
-Rob
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I did some time on the "Wimpey Sealab" when it was first converted from one of the Bowater ships and it had a Becker Rudder fitted with a small pod with a small propeller on the 'flap'rudder. It was all disconnected when she started service with Wimpey so I never found out about any benefits from it. Anyone out there served on Bowaters ?
MikeK
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I think we could do a "Name the Ship" on the basis of that picture Bunkerbarge!!
That would keep them guessing for a bit!!!