Stupid little rudder !!!

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

  1. MikeJohns
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    MikeJohns Senior Member

    I've recently moved and all my archives are still boxed as are my books.

    Even if you just look at free stream data from actual tests such as tabulated by Molland (in Marine control surfaces) you'll immediately see that a flat plate has a higher practical lift coefficient than a NACA foil. In fact in water, the NACA foil only has a better lift coefficient across most of its range in water (even in a perfect free stream) when used backwards !
    Small to moderate changes in Drag are not important in this particular application. I think any advice to change a flat plate rudder for a NACA section for this sort of motorcraft is misleading and you'd have to look at what you are really trying to achieve.

    Also consider that a rudder operating behind the prop is not in a free stream and spanwise has completely different inflow angles and it's operating in highly turbulent flow. Fatter NACA sections where you can force some reattachment aren't suitable for this type of craft. They work well for ships.

    I added comments about the "cylinder and splitter plate" for interest. Yes the effective shape is obviously created by the bound vortices. But the real message is that the cylinders presence isn't detrimental as many people imagine it must be. Applied real world fluid dynamics is necessarily based on observation and real world testing. Fluid dynamics is not precise and it's certainly not aerodynamics

    Maybe read Blevins "Applied fluid dynamics" ? Also Mollands " Marine control surfaces"
     
  2. gonzo
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    gonzo Senior Member

    I think that increasing the blade area forward of the rudder post could help too.
     
  3. Little Iris
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    Little Iris Junior Member

    What would the area forward be in %? It seems to be enough space between the prop and the rudder. What would the minimum clearance be?

    Thanks,

    Mattias
     
  4. MikeJohns
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    MikeJohns Senior Member

    Mattias

    Clearances, three-bladed propeller (as the Minimum % of propeller diameter)
    Prop tip to hull 17%
    Prop tip to lower support 4%
    Deadwood to propeller 27%
    Propeller to rudder 10%


    AdHoc posted earlier that you could put 20% of the area ahead of the pivot in this application.
     
  5. PAR
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    PAR Yacht Designer/Builder

    A 20% balance needs to be justified, as you can easily over balance a blade this way. Judging by the photos, this would double the current balance and I'd be inclined to use considerably less then 20%. I also think some of the minimums are "too" minimum for comfort, though you certainly can get away with it, especially in slow speed craft.
     
  6. Little Iris
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    Little Iris Junior Member

    What would happen if the rudder gets "over balanced"? We never go faster than 6.5 knots (S/L 1.2).
     
  7. PAR
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    PAR Yacht Designer/Builder

    The helm will become progressively over sensitive, where little inputs can cause wild swings of the blade, because of the leverage the prop wash has on the balanced portion. If you have to error, it should be on the too little side of the balance thing, as the penalty for too little balance is just helm effort, while too much can be difficult steering.
     
  8. MikeJohns
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    MikeJohns Senior Member

    The center of pressure for a rectangular flat plate in water moves aft approximately (0.2+0.3 sin(a))C where C is chord length and a is rudder angle.

    The main concern is chatter in the steering gear if there is any linkage slop as the C.o.P. moves about the neutral point. Also the feel of the rudder for systems with feedback like a tiller or cable steering at small angles of rudder.
    Whether C.o.P. forward of the stock is an issue depends on the type of steering gear. It's not unknown to put significantly more than 25% ahead of the pivot for specialist craft.

    The more rudder you can deflect into the prop stream in a craft like this the better the low speed maneuverability.
     
  9. philSweet
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    philSweet Senior Member

    As a previous poster pointed out, the idea is to steer, then gear. Lead has very little to do with the basic problem of low speed maneuvering. It mostly has to do with the power required to swing a rudder quickly for collision avoidance. It is a cruise speed issue more than a slow speed issue. Or perhaps a better way to say it is that it is a control power issue. If the current arrangement can be swung lock to lock quickly and easily during low speed maneuvers, I wouldn't worry too much about the (lack of) lead.

    Little Iris, can you answer a few more basic questions.

    1. If you are ticking along at 3 knots and put the helm over, what is your turning diameter?

    2. If you are ticking along at 3 knots and put the helm over and goose the throttle, how does the boat respond compared to leaving the throttle alone? Does the boat accelerate easily through, say, a 180 degree turn? Or does the rudder keep the boat bogged down to 3 knots?
     
  10. AndySGray
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    AndySGray Senior Member

    Dont be too afraid to overbalance, then reduce

    Adding metal is way harder than removing it, so unless you have a welder at home it may be more practical to go with the 20% (or more), suck it and see, drop her back on the trailer, then 5-10 minutes with a sawsall or an angle grinder/zipcut disk will take you to 18%. Heck, you can even do that on the ramp if it's quiet.

    Re-Launch and repeat as needed. For most 20% to 18% to 16% is a very do-able progression but adding 2% each time will be a couple of hours hard work even IF you have a welder (You shouldn't - they don't work well on wooden boats :p).

    How about using a few bits of old plywood to test the proposals here - drill a few holes and bolt to the existing rudder - Fabricating a new rudder will be pricey - you don't want to go there till you're happy it will do what you want.

    That said, I really like the PhilSweet diagnostic test above. Kudos to you Sir.
     
  11. MikeJohns
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    MikeJohns Senior Member

    A flat plate At only 10 degrees has it's center of pressure at 25% aft of the Leading Edge, compared with only around 18% for a NACA foil. A flat plate can have more area fwd of the pivot than a NACA foil.

    The more rudder area you can deflect into the prop stream in a craft like this the better the low speed maneuverability.
     
    Last edited: Jul 13, 2014

  12. Little Iris
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    Little Iris Junior Member

    I will do the test in the next couple of days and post the results!
     
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