Stupid little rudder !!!

Discussion in 'Boat Design' started by Little Iris, Jul 7, 2014.

  1. philSweet
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    philSweet Senior Member

    Little Iris, By any chance has she been re-propped? My first instinct is that the current prop is a couple of inches bigger and lower pitched than the original. This may have improved fuel efficiency, but it could also have created a problem for maneuvering. If you originally had a smaller prop, when you blipped the throttle to effect a turn, you got a lot of rudder and a little thrust. With the bigger prop, you aren't able to blip the throttle because the thrust is now twice what it was for the same amount of steering. I'm just thinking if you still have the old prop lying around, that would be a pretty cheap fix - at least in the short term. (And if my guess is correct, it would help if we had the specs for the old and new props, and a displacement figure.)
     
  2. UNCIVILIZED
    Joined: Jun 2014
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    UNCIVILIZED DIY Junkyard MadScientist

    Sir, you do appear to be in a bit of a fix with so much ship in front of said small steering device. And I'm throwing these ideas out there for some evaluation for the professional NA's in the crowd. But might one of the following help:
    - Add a Kort Nozzle for the propeller, thus adding to how much flow you get over the rudder when you're trying to use directed prop blast to assist with manuvering at low speeds.
    - Add end plates at the top, & or bottom of the current & or a new rudder. Thus preventing/lowering end plate slippage. Kind of like one sees on the tips of some aircraft wings. Yes, I forget the tech term, & I know, completely different foil sections, operating speeds, etc., etc.
    - Is there any reason in addition to the other suggestions for upsizing the rudder within it's aperture, to not extend it's aft end all the way up, so that it's profile matches that of the hull in that area. Therefore, no room for lost thrust over the top end of the rudder.
     
  3. daiquiri
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    daiquiri Engineering and Design

    Yes, I have seen the effect of the prop pitch on the rudder effectiveness in action and it can indeed be important. That's a thing to check out for sure.
     
  4. daiquiri
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    daiquiri Engineering and Design

    This is a good hint too. For a rudder of this shape, the maximum angle of deflection should be around 35°, better yet 40°. Increasing the angles to this value should give a significant improvement in steering effectiveness.
     
  5. Little Iris
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    Little Iris Junior Member

    I am not aware of that the prop would have been changed. I can not find the diameter and pitch for it. I am in the process of installing hydralic steering. Maybe I can make the rudder turn 40 degrees?
     
  6. daiquiri
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    daiquiri Engineering and Design

    If doable, that could imo be the simplest way to improve the rudder efectiveness.
     
  7. WecBoat
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    WecBoat Navy Blue

    If the support can take it, any kind of more area will be good.

    I have a boat like that for 35 years now. This kind of boat need a lot of throttle, I mean a lot. Use all your power for a short moment. Very short. Full wheel, full throttle, very short time. Your propeller have a step, right or left, use it, I mean a lot. Go see the fisherman "parking" their boat, you can learn a lot from them.
     
  8. PAR
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    PAR Yacht Designer/Builder

    WecBoat, we call this "back and fill" and it's a common single screw, straight shaft tactic, that with a little practice can make a world of maneuvering difference.
     
  9. WecBoat
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    WecBoat Navy Blue

    Yes, "back and fill" is the name of this maneuver. When you get it, the boat obey.
     
  10. Little Iris
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    Little Iris Junior Member

    Like this!?

    I found this picture and this rudder have the top filled in and fish-tails. Maybe this is close to what you all have suggested?
     
  11. daiquiri
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    daiquiri Engineering and Design

    Mattias,
    May I suggest you to try simple solutions first? Unless you have some money to throw away, of course. ;) Try increasing the rudder angle range and see what happens. Perhaps it will be sufficient and no other modifications will be necessary. If not, you can then start adding other features, like more area, end plates, fish tail, NACA profiles or whatever you wish.
    Cheers
     
  12. MikeJohns
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    MikeJohns Senior Member

    just alter it to a similar shape to this.
    A rudder like this will give lift up to 40 degrees before it stalls , has a slightly higher lift than a NACA foil and similar drag coefficients. If the continuation of the stock through the plate is omitted it's characteristics are just flat plate. Still with a higher lift at low angles and still useable up to 40 degrees articulation before stall.

    As far as drag is concerned underway you are always better off with a slightly larger rudder than a larger angle with any non zero rudder angle. Single prop boats always have some rudder underway.

    I've seen marked improvement in the top fill in as PAR first suggested.

    [edit...add ]
    For the fluid dynamicists the pipe welded as the structural continuation of the stock improves a plate rudder, reducing it's drag to something very close to a NACA 00xx foil at all angles up to stall.
     

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  13. AndySGray
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    AndySGray Senior Member

    Sun is shining and boat is on trailer - Hmmm

    Maybe, but not yet. It's the middle of July - launch the boat (and launch the lawnmower too if, as it apears, it has become a distraction to boating).

    Throw a couple of extra fenders on there and practice the back and fill - remember the gear you'll use most is neutral - Steer then Gear.

    Most of the guys and girls on this forum would jump at the chance to be on a boat like that even if the steering consisted of a 2x3 with a piece of ply roughly nailed to the end featuring a message about saving the whales.

    Think about aeroplane pilots - hardest part of flying is takeoff and landing - but those probably total less than 5 minutes of any flight - novice pilots do a lot of 'touch and go' drills - come in for a landing and as soon as the wheels are touched down, power up and turn it into a takeoff.

    Find a quiet dock and do the same - bring the boat in to the point where you could step ashore, then push off, do a figure 8 or donut and repeat. Dont underestimate how long it takes to learn the ideosyncrasies of the vessel - the old fishermen didn't pick up that skill by osmosis, but by practice.

    I used to take out dive trips on a 44' single screw and became so familiar that docking was a given. Then the prop got dinged and our spare was LH version of the RH which was fitted - gearbox linkage was flipped over and all was the same in theory - in practice took a very stressful 2 weeks to get back to been OK with docking it - did not realise how much prop walk I used to take for granted :eek:
     
  14. daiquiri
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    daiquiri Engineering and Design

    Mike, I would really like to see some hard data which could corroborate this claim. I couldn't find anything about it in my textbooks.

    Actually, I have some conflicting data about low-speed flat-plate vs. streamlined-section rudders, in which some authors report higher lift of the flat-plate rudder, the others report lower lift. However, they all report higher drag. I suspect that there is a strong inconsistency in the Reynolds number relative to various tests. It is a well-known fact in aeronautics that for very low Re (laminar flow) flat plates are unbeatable, but this advantage rapidly drops and becomes negative as the Re increases.

    I can imagine that a cylindrical stock along the span length of the flat-plate rudder blade can influence the flow pattern and hence the resulting forces. But whether it would enhance or degrade the rudder's characteristics - that, imo, will strongly depend on where the stock is placed along the chord length.

    Cheers
     

  15. Petros
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    Petros Senior Member

    flat plates can make effect low drag rudders, until you defect them too far. then they become ineffective.

    I suspect the full length pipe will increase drag of the rudder, but it would create a standing vortex that will make the rudder more effective, and VERY stall resistant. It would be a simple way to make it more stall proof and more effective I suspect, at the cost of a little drag (on a boat that large the increase in drag would be hardly measurable anyway).
     
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