rudder foil selection

Discussion in 'Boat Design' started by Hravn, Apr 18, 2005.

  1. Hravn
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    Hravn Junior Member

    I'm looking at comparisons between a Naca 0015 (existing rudder) vs a 63010 (proposed replacement) foil for a transom hung rudder on a smallish sailboat. Both vacanti software (loft97) and xfoil suggest a ~20% drag reduction and ~10% lift increase in the 2-8 knot range @ 3 deg leeway with the 63 series section over the 0015. That or close in the real world would be pretty sweet. Any idea how these numbers would translate into real rudder performance ? I've been looking for published experimental results in these ranges, but have not found any. any help would be much appreciated.

    a secondary question, is a 63010 foil significantly harder to acheive ? It does not look to be a problem, but haven't built one yet.

    thanks.
     
  2. Doug Lord

    Doug Lord Guest

    ruder angles

    Seems to me to better analyze rudder performance you'd want to check it out at much higher angles and compare the stalling angle of both.
    Is it a light weight boat or a keelboat?
     
  3. RWL
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    RWL Junior Member

    Foil efficiencies that might help

    I did a comparison of kayak rudder foils at speeds similar to what you are looking at, a couple of years back. I will attach the Excel spreadsheet that shows the comparison of efficiencies for four foils. I used the standard flat aluminum plate as typical for kayaks, and compared it to 63012 and 0010 foils with aspect ratios of 2:1, as well as a high aspect ratio 0010 foil (4:1). As you can see, the efficiency of the 0010 foil and 63012 foils are pretty similar, while the high aspect foil does some very funny things after stall angles are met. If the file uploads properly, you can look at the supporting spreadsheets if you can decipher what I was doing.

    RWL, a displaced North Wester
     
  4. Hravn
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    Hravn Junior Member

    Possibly so, I'm new to this and trying to get a handle on things for comparison. Its a 3800lb, 27 ft keel boat. What AOA regime would be most critical ? How does a stall angle difference of a few degrees translate into actual performance ? Is the rudder not typically stalled in anything above about 10 deg off centerline (+- a bit for the changing motion of the stern) ?

    I'm leaning towards strip planking myself a 63010 foil rudder and putting it on the boat to see what happens.

    thanks.
     
  5. RWL
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    RWL Junior Member

    oops, forgot to add the data file

    here is the Excel file that I mentioned.
     

    Attached Files:

  6. Skippy
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    Skippy Senior Member

    Yes, definately higher angles. Leeway is relevant to the keel, but the rudder typically acts at much higher angles of attack. It would be good to know more accurately how fast and how small the boat is. Even with the 63 series, 63015 or 63012 will have a higher stall angle, which is important for the rudder.
     
  7. RWL
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    RWL Junior Member

    forgot to give you air-to-water relationship

    The 32 metres/sec speed is in air, which the equivalent of 3.5 knots in water.
     
  8. Hravn
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    Hravn Junior Member

    Hi;
    Thanks for the spreadsheet, came through fine. I'll have a look at it.

    I can usually top out at about 6.2 knots upwind, and haven't gone faster than 11.5 downwind, but 2-8 knots is a pretty typical range. The rudder has a 14.5 inch chord with a span of 50 inches, so we're looking at a 3.5 aspect ratio, stern hung vertical section.

    thanks.

    todd.
     
  9. Hravn
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    Hravn Junior Member

    very nice !
    If I'm thinking about the chart properly the 2:1 aspect rudders are stalling earlier but providing more lift (or a higher l:d anyway) than the 4:1 0010 until it stalls and then they are essentially the same ? That seems conterintuitive, I would have thought that the high aspect foil would do better. How does this translate into actual lift #s as opposed to the ratios ?
    I'm not clear enough on the raw math to fiddle with the calcs and change sections, can you upload the 0010 section calculations ?

    I was looking at absolute numbers to compare sections with the same span and chord as I wanted to keep the lift #s at or above the existing design while lowering drag. The rudder definitely provides a fair amount of lift upwind and the last thing I want to do is decrease it.

    This data also seems to back up my earlier point that the rudder is essentially stalled above 10 deg or so and that l:d ratios anyway are similar after that point.

    sorry for all the questions.
     
  10. RWL
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    RWL Junior Member

    now if I can get my brain cells organized and remember.

    The chart shown is purposely "dimensionless", so it shows ratios of efficiency for the foils over a range of attack angles (-20 to +20 degrees). The highest efficiency is around 4 to 6 degrees with the lift force of the foil being about 12 times the drag force. The drag and lift charts are in the other pages of the spreadsheet, but they will not give you absolute numbers either. The three "true" foils all stall at about 12 degrees, and the flat plate stalls about 6 degrees. If I remember correctly, the high aspect foil had slightly less drag, but much less lift, so its efficiency was lower. The disconcerting thing was that after stall, the flow over the high aspect foil was so detached that effectively there was no lift, which means no steering!. The only way to re-attach the flow is to put the foil back to center. Interestingly, this is exactly what used to happen to the Viking 33s of C&C design years ago when you got them on a power reach. The tiller would get closer and closer to your nose and then they would round up. Absolutely no use in trying to steer out of it, just put the tiller back on center and wait for the boat to recover (an crew also). Once again, trying to remember what was said at the time, I think the concensus was that the gain by going to a 63### foil over the old style 0100 was minimal in a rudder, but probably more relevant to keel design. I didn't try any other high-aspect rudder foils in my experiment, since no one was trying that for kayak rudders at that time. Not sure what would happen at the higher speeds you mention, as our wind tunnel couldn't go fast enough for the Reynold's numbers we were working with and give a correlation to water.

    RWL
     
  11. RWL
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    RWL Junior Member

    How did you comute your foil aspect ratio?

    Just a quick note on aspect ratio. All of my foil ratios were computed from the water surface to the bottom of the foil. I only note this, because I got into a discussion with a sailing friend a while ago, and he was measuring his rudder (transom hung) from the top to bottom.

    RWL
     
  12. Hravn
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    Hravn Junior Member

    I'm measuring from the waterline down: 50 inches, on a 14.5 inch chord. Heres the (interpolated) data from loft97 @3 degrees leeway, 5 knots

    drag lift L:D ratio
    0015 3.7 80 25
    0010 3.2 80 26
    63010 3.0 90 27

    I'll take a harder look at the stall angles and lift.

    thanks.
     
  13. alex_sailor
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    alex_sailor Junior Member

    The 32 metres/sec speed is in air, which the equivalent of 3.5 knots in water. Sorry, this calculation is out by about an order of magnitude. 32 metres/sec = 115200 metres/hour divide that by 1849 metres (METRIC DISTANCE IN A NAUTICAL MILE) =62.3 kts.
     
  14. Perm Stress
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    Perm Stress Senior Member

    63- series is a laminar flow foil, so it is characterized by "drag bucket" at angles of attack smaller as ~3degrees for infinite span, and may be ~5 degrees at real aspect ratios.
    Outside drag bucket it is no better as turbulent 0015 you want to replace.
    On the other side, max lift is (could be) lower, stall angle also lower, just the opposite qualities one want from the rudder.
    In short, only if you are SURE your rudder will never see angle of attack above ~5 degrees, replacement is worthwhile. Are you sure?
     

  15. DCockey
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    DCockey Senior Member

    I've seen a recomendation to use a 00xx series airfoil for the rudder for the reasons Perm Stress gives, and if performance is a major concern to use a laminar flow section with a broad drag bucket for the keel where the angle of attack in a performance boat is generally low.
     
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