Kitchen rudder

Discussion in 'Boat Design' started by duluthboats, Mar 8, 2002.

  1. timswait
    Joined: Jul 2008
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    timswait Junior Member

    More musings

    Surenw, yes I can email you my drawings, they're not great though as I was in a hurry making them and they were only for my own reference. I intend to make better drawings for my MkII version.
    Been out for a bit longer cruise with the boat, and it's now ballasted correctly so the prop's no longer surface piercing ;) The concept's great, but there are some design issues I need to deal with, I'm thinking of fitting a MkII redesign over the summer. In forwards (with the rudder fully open-more of that later), it's great. I can believe it improves efficiency, the wake is very flat with no white water and the boat feels pretty quick by narrowboat standards (which are probably about the slowest things on any water anywhere in the world). Steering is very responsive, and about right in terms of force required. The tiller stays pretty much where you put it, but does slowly migrate away from the centre, and I'd rather it did the opposite. Reversing is good. Quite slow but extremely controlled, in fact I've actually got more turning power going backwards than forwards. Stopping power is very good, and having control while stopping is brilliant, I just need to get my head around moving the tiller the 'wrong' way to steer when the rudder's in reverse:p . The biggest problem is the amount of force required on the tiller in reverse. I'm young and fairly fit and can only just hold it when the engine's at tickover (actually due to gearing and prop-sizing tickover is rather powerful, but that's another story), use more power and the tiller is pulled across to full deflection to one side or the other. You can hold it in the middle but move it a bit to the side and it yanks across. That being said, tickover power is enough for most situations and moving around our marina, spinning on the spot, reversing out of moorings is fantastic. In terms of open or closedness the rudder is bistable. It will stay open if you leave it open or will stay shut. Put it part closed and it tends to migrate to closed. This is perfect for what I want as I never really want it half open, I've a throttle for that. A problem I've just encountered is that one of the actuator arms is slipping on the shaft, allowing the shell to move. (probably from bashing the rudder against a lock gate, the rudder is better protected by the stern of the boat than a conventional rudder, but still protrudes slightly). This has been resulting in it not fully opening so I've been going along kicking up white water without going anywhere fast. Also I should have made provision for lubrication and I shouldn't have made the bushes so tight, as the rudder is stiff to operate even when you're not moving. So basically I want to redesign it so be better engineered and stronger, to require less force to operate in reverse, to self-centre in forwards, but retain all of the positives. This has got me thinking about the exact nature of the kitchen rudder.
    First off I understand about the effect of the pivot position on a conventional rudder. If all the area is behind the pivot the rudder is heavy to operate and wants to self centre, if it's fully balanced then almost not effort is required to move it but it doesn't self centre, and with more area in front than behind then it's unstable. But how does this apply to a kitchen rudder? I assume it's similar. My design has fairly equal area behind and in front the pivot and it's quite light to use in forwards but has no self centring tendency. The problem is, in the reverse position, I need to add area somewhere to make it more stable and better balanced, but where? The other thing that occurred to me is the force to open and close it. Bernoulli's principle (faster moving fluid has lower pressure) should tend to suck the clamshells closed. I think that's why a screw jack is commonly used since that will hold them open against any pressure. I was expecting this might be a problem and included a pin to lock it in the open position. However my rudder holds open easily. I think this may be due to the lips I added to the back of the clamshells. Actually these were a bit of an afterthought as a bodge to get the shells to close fully, but I think I may have lucked onto something there, they must push the clamshells apart, so I think that's why the rudder holds in the open position.
    Another thing is the rudder forms a duct around the left and right side of the prop, constraining the thrust in those directions, but there is nothing above and below it. I'm thinking about adding curved plates above and below the rudder, fixed to the boat and sticking out behind it to do that. More importantly for me, they'd protect the rudder from more bumps against lock gates!
    Does anyone else have any thoughts/comments on the shape of shells? It's clearly complicated and I was never going to get it right first time, but I've got some ideas that'll hopefully improve it. In an ideal world I'd produce dozens of subtly different shapes and try them all out, but I have to pay to have the boat out the water to change it, and it's gets expensive if I need to do it too many times!
     
  2. ben2go
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    ben2go Boat Builder Wanna Be

    If you remove the lips on the rudder's edges will help with forward thrust.The clam shell that is mounted inside at top should be mounted inside at the bottom instead of one stacked on top of the other.This will help the two halves close.The one mounted inside will have to be slightly smaller than the other.
     
  3. timswait
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    timswait Junior Member

    Removing the lips should indeed increase forward thrust, but would it also make the rudder try to pull closed? I'm thinking of changing the lips to be longer but less angled. Hopefully that way they'll still produce the torque to hold the clamshells open but won't restrict the flow in full ahead so much. Good point on having one shell inside the other, I can see how I could make that close better. I was going off this diagram which shows one shell offset above the other.
     
  4. sean-nós
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    sean-nós Senior Member

  5. TerryKing
    Joined: Feb 2007
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    TerryKing On The Water SOON

    Everything I've read so far says it's most effective for fairly slow speed boats, and the drag is excessive for fast boats such as you are building.

    But it sure would be nice to be able to keep the simplicity of direct drive and then be able to 'neutral' and 'reverse' with "something".. :)

    I think "it" would have to retract up out of the main prop stream, and have just small rudderlike sections still in the water. Maybe...

    Anyone else seen details on high-speed use???
     
  6. sean-nós
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    sean-nós Senior Member

    Thanks Terryking if the only problem is drag I could live with that I hope to run a v8 in her and can't see been as bad as a waterskier, Top speed is not my main goal with this boat I just want to look good,sound good and be able to go backwards :) They say that going backwards is for people who make a mistake going forward well thats me.
     
  7. timswait
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    timswait Junior Member

    I'm sure it would 'work', although I doubt it would be optimal. As said above the general wisdom seems to be that the shells have a lot of parasitic drag which impedes performance at higher speeds. A lower speeds the ducted prop actually improves propulsive efficiency, but at higher speeds the additional drag overides that. I've heard a figure of 12-15 knots being talked about as the cross-over point where the rudder stops helping and starts hindering performance, although I don't know if there's any science behind that or whether it's just rule of thumb.
    Someone was describing and alternative arrangement where a conventional rudder is used and the thrust reversing bucket is lowered down the rudder shaft when it's needed. It's positioned above the waterline when it's not needed. I can't remember the name for that arrangement. Doesn't sound exactly an elegant solution, but it does prevent that parasitic drag element.
     
  8. timswait
    Joined: Jul 2008
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    timswait Junior Member

    I'm also planning on changing the prop while it's out the water. I currently have an 18x12", but the engine struggles to get much over 1500 rpm flat out. Obviously it's not generating much power at this speed (it's a car engine so redlines at over 4000). To let the engine rev higher I'm going to drop the pitch to 10". I'm not sure whether to also increase the diameter. I can physically fit a 1 inch larger prop, this will reduce the tip to inside of the rudder clearance from 1" to 0.5". Is this a good thing? Also should I go for a turbine or an equipoise prop? The maximum boat speed is less than 10 knots.
     
  9. timswait
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    timswait Junior Member

    Anyone know anything about props, is this a good idea or should I stick with 18" diameter?
     
  10. jholman
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    jholman Old Bayliner Montego

    I wonder if you could adapt a hydraulic trim tab mechanism, tipped on its side, for this device since it would make a new forward/reverse control? You'd still need manual redundancy of course.
     
  11. jholman
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    jholman Old Bayliner Montego

    Old Movie

    Just watched "The Man Who Never Was". About 50 mins into the movie there is a three second sequence of an naval pinnace coming alongside a warship and the helmsman is cranking away at his Kitchen Rudder.

    Just a little bit of trivia!
     
  12. timswait
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    timswait Junior Member

  13. ben2go
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    ben2go Boat Builder Wanna Be

    Sweet deal.Looks like you got it working purdy damn good.
     
  14. duluthboats
    Joined: Mar 2002
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    duluthboats Senior Dreamer

    Tim it looks great, was it worth the effort? What do the other boat owners think of it?
     

  15. timswait
    Joined: Jul 2008
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    timswait Junior Member

    Very good question! It's hard really to say if it's worth the effort, I'd say yes but partly because I enjoy the challenge of designing and building things like this. The manoeuvrability of the boat is certainly much better than a standard narrowboat. Although it's not really any better than could be achieved by fitting a bow thruster. In materials terms it's much cheaper than fitting a BT, but not if I account for all the time that went into it (although as I already said, I quite enjoy that so it doesn't really count). Also it means no compromise on the size of the water tank, a BT would mean I'd have to have fitted a smaller tank, and I live on the boat so that's important.
    The biggest problem with it is the force it requires to use it in reverse. I'm a fairly fit 28 year old so it's not too much of a problem for me, but it would be too heavy for many people. Even I can't hold it closed and steer it against full throttle on the engine. This also means that while boat stops very quickly, it's not spectacularly quick as I can't use full throttle to brake with. It is very useful to still be able to steer when braking though, especially if you come round a tight corner or through a narrow bridge and find another boat coming the other way! Not having much sticking out behind the boat to catch in locks is also very handy, as the boat is 60 feet long and many of the locks we use are also 60 feet!
    Other boaters have all been positive if they realise what it is. It gets lots of questions when it's out the water, but when it's in the water most people just think it's a fancy hinge to fold the tiller out the way!
    Overall if you like messing around with mechanical things it's a great project and I think the design has a lot of mileage in it for improvements. On the other hand if you want an easy life and do a cost benefit analysis of it then you'd probably go a more conventional route!
     
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