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Rudder envy





I made a new rudder designed to keep the boat tracking straight. It is ridiculously tiny. The photo below does not do it justice. It's 1" wide, and it works like a charm. I found it very easy to make slow, gradual turns and keep the boat tracking in a straight line. Without any rudder at all, it wants to turn left, so this small little wonder works very well.



I also had the hull finished and painted. It is WAY smoother, and contrary to most of the advice I have been getting, is NOT responsible for any appreciable speed gains! I really don't think that at these speeds, surface finish is all that important.

The good news is that we are now very close to design specs for speed!


I found that the only way to get accurate speed/power data was by doing large GPS speed averaged loops around the perimeter of the lake. They are VERY accurate. I was able to repeat the same configuration more than once and get the exact same average down to .1 kph. I use an SRM power meter connected to the cranks, so I am able to output a constant power level and record my average speed over a set course with a GPS. Repeating this with small changes to the boat allow me to quantify the effect those changes have.

Here are the results of today's testing:
  • thin prop, large rudder, left hand loop, 150 watts, no paint on hull, packing tape on hull (test from last week) = 11.1 kph
  • thin prop, large rudder, left hand loop, 150 watts, painted hull = 11.2 kph
  • thin prop, tiny rudder, left hand loop, 150 watts, painted hull = 11.7 kph
  • thicker prop, tiny rudder, left hand loop, 150 watts, painted hull = 11.7 kph
  • thin prop, tiny rudder, left hand loop, 150 watts, painted hull, prop strut pulled close to the hull by a cord ( I wanted to see what would happen if we moved the prop closer to the hull) = 11.6 kph
  • thin prop, NO rudder at all, left hand loop, 150 watts, painted hull, = 11.8 kph
  • (I had to stop and start the GPS 2 times to paddle correct course, but I don't think it mattered)
  • thin prop, tiny rudder, RIGHT hand loop (counter to the way the boat naturally wants to steer), 150 watts, painted hull, = 11.1 kph (I had to use the dip steer rudder a couple of times to stay on course, but I still think this was slower because it went against the natural turn of the hull/prop)
  • thin prop, tiny rudder, left hand loop, 150 watts, painted hull, windy (small ripples on water) = 11.5 kph
The tiny little rudder worked very well for keeping a straight line, doing a gradual turn around the lake or for correcting my heading, but was useless for doing any kind of turns. I rigged up my old large rudder up with a spring and pull cord and hung it off the stern for when I needed to do big turns and it worked, but nowhere near as good as when that large rudder is under the hull turning in the rudder tube. I don't have a photo of it - I'll take one next time.

For general touring around and training, the large rudder in the rudder tube works fine. For the record attempt, and testing where I will steer around a large, gradual, round course, the small rudder works fine and I don't even need the dipping rudder.

Pulling the prop strut in tight to the hull using a cord did not do anything to correct the direction of thrust. In fact, I took a close look at the thrust direction when spinning the prop while held back to the dock and the thrust is very straight - I don't think that is the problem. I believe it is due to the thrust being at the side rather than right down the middle, and the cost of putting the thrust in the center would probably cost more than the couple of watts it probably costs now.

To summarize, the paint and body work added some weight because I had to add one washer to lift the outriggers, but the paint made it only .1 kph faster than packing tape, and the packing tape improvement over the unpainted hull wasn't quantified in the loop test, but the straight-line tests showed no difference. The small rudder compared to my large rudder is worth .6 kph (that's huge), and a bit of wind (maybe 10 kph) was worth a reduction of .2 kph. The wind effect could also be fairly significant because average wind speeds typically reach highs of 15 to 20 kph during mid afternoon around Calgary.



This is a cord we strapped to the rudder strut to pull it toward the hull. It didn't change the left steering tendency, and slowed it down (probably due to the drag of the cord) by .1 km/hr


Greg Bradley going for a spin

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4 Responses to “Rudder envy”

  1. # Anonymous richeaston

    greg, you need a thin petg fairing and tailbox, since your body is draggy. this will help in windy conditions, also. the weight should be minimal and worth it.
    rich easton  

  2. # Blogger Adventures of Greg

    Rich: at 11 to 12 km / hr, do you really think that a fairing would be worth it? The objective is to plan the record attempt for a day with low winds, so I am hopefuly that we can mitigate the conditions somewhat.  

  3. # Blogger biff

    The Hull looks great, I am a little surprised that it didn't make a bigger difference. I was expecting 0.5kph, but it looks really nice. How much weight did it add?

    Also about the u-joint. I was going to make a comment about that when you posted how quickly the new one broke. When something breaks it is usually because it is under more force than anticipated and if it is moving that means more power loss.  

  4. # Anonymous Anonymous

    The left turn tendency is caused by two things, I suspect:
    1 The offset propeller position
    2 "Propwalk" - the left travelling (seen from behind) propeller blade (ie the one at the bottom of the circle) is further from the surface than the right-travelling, top one - the deeper blade is in "grippier" water than the other, so as it travels to the left it pushes the boat to the right (as well as forward of course). Since it it is attached to the boat aft of the centre of lateral resistance, the bows of the boat will go the opposite way - to the left.

    Why not shift the prop to the other side so that these two effects cancel out to some degree, rather than add to each other?

    While were at it...I rekon that you should drop the UJ altogether and accept the marginal loss in power of having the prop at the same angle as the shaft itself - if it isn't there it cannot break?  

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