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Thread: Rudder Dynamics

  1. #1
    VDRIVERACING
    My 21 cruiser wants to turn right all the time. I have been fighting this forever, and had improvements off and on, but never really found the answer. I've bought more props, and adjusted plates more than I care to think about.
    Tonight at the shop--think tank--some of us got to talking, beer in hand, about the attributes of rudder shapes and post centers and how trailing or bottom edge angle affect handling. I have a ski race rudder from Barber Welding, and since I am due to pull it out for an inspection, this seems like a good time to think about mods or replacement.
    I run off the flywheel; left hand prop. I'd like to hear from those who have experimented in this area, and have some thoughts to share.

  2. #2
    Itsahobby
    My first boat was a Spectra it had a Glenwood(huge) brass rudder on it. I noticed my Brass prop had a big crack in it 1 week before a trip. I took it into a shop to get the prop replaced and was talking to the owner telling him how the boat pulled to the right. He grabbed a grinder and beveled the left hand trailing edge of the rudder about 4 inches from the top down.
    This seem to ease the pulling, but never completely got rid of it.
    I'm currently running the same rudder on my RaysonCraft just because I got low on funds during the build. I really need to retire that rudder
    I've heard the Schiada Rudders help with the pulling, also.
    Jerry

  3. #3
    Marty Gras
    If you have the shaft at "zero degrees" to the hull bottom, I have found that going to a 3" or 4" longer blade helps with the torque pull. It also makes the steering more responsive with less input. The longer blade is attacking the water at or near 90 degrees when the boat is at speed, so it should not effect the "lift/drag" very much. You can always sharpen the right leading edge a little more than the left, and it may give a little less pull.

  4. #4
    suckerpunch
    I have been playing with this for years, on all of our hydros, some customers runner bottoms, and my own lake boat. I think what your boat is doing is certainly normal, but for a lake boat it is annoying. My test results have shown up to 6 mph in just rudder design itself. The best steering rudder, is the slowest .(Full wedge with razor leading edge) The fastest rudder that barely steers the boat has a bull nose leading edge and a sharp trailing edge. The same goes for the struts. We build our struts with a bull nose leading edge, and a smaller tapered trailing edge, so that the water flows to the prop, and not away. Its the same concept as a nose cone on an outdrive. If I run a prop that does not pull hard to the right in my hydro, I have a suck ass prop on it, and it always slows down. It drives nice but slows down . I run totally opposite from all other racers. I run very high pitch 4 blades, with just 1.09 percent in the box. She runs through the traps at 7800-8000, no trans, at 135-140,in 8 seconds. I am telling you this because I took this info to my lake boat. I have a 23.6 eliminator eagle v-drive. I haven't run it much, but when I did , I always played with it. I built a very large,12.5 high pitch 3 blade for this boat. It had a blown 488 low boost , two 1150s typical lake deal. As soon as I put this real prop on it, she pulled hard to the right. To help correct this, and not put a suck ass inefficent prop on it, I changed the shape of the rudder, cut the overall lenth, and could live with it. I corrected the leading edge to turn a little left, and put a 1/8 radius bull nose on it. This is tried and true from the track to my own stuff. I am almost done with my new engine for the cruiser. I will be building all kinds of new props, s/s rudders, and testing them for everyone to have a better, more efficent system. I laugh my ass off when people rattle off prop numbers, they dont even have a clue that some prop builders stamp what ever they are asking for at the time of purchase. Those numbers do not desribe, blade size, cup, rake, foward blade, ect. I do sometimes rattle on, but I hope this info can help you with something on your boat. Joe

  5. #5
    Sangster
    I have been playing with this for years, on all of our hydros, some customers runner bottoms, and my own lake boat. I think what your boat is doing is certainly normal, but for a lake boat it is annoying. My test results have shown up to 6 mph in just rudder design itself. The best steering rudder, is the slowest .(Full wedge with razor leading edge) The fastest rudder that barely steers the boat has a bull nose leading edge and a sharp trailing edge. The same goes for the struts. We build our struts with a bull nose leading edge, and a smaller tapered trailing edge, so that the water flows to the prop, and not away. Its the same concept as a nose cone on an outdrive. If I run a prop that does not pull hard to the right in my hydro, I have a suck ass prop on it, and it always slows down. It drives nice but slows down . I run totally opposite from all other racers. I run very high pitch 4 blades, with just 1.09 percent in the box. She runs through the traps at 7800-8000, no trans, at 135-140,in 8 seconds. I am telling you this because I took this info to my lake boat. I have a 23.6 eliminator eagle v-drive. I haven't run it much, but when I did , I always played with it. I built a very large,12.5 high pitch 3 blade for this boat. It had a blown 488 low boost , two 1150s typical lake deal. As soon as I put this real prop on it, she pulled hard to the right. To help correct this, and not put a suck ass inefficent prop on it, I changed the shape of the rudder, cut the overall lenth, and could live with it. I corrected the leading edge to turn a little left, and put a 1/8 radius bull nose on it. This is tried and true from the track to my own stuff. I am almost done with my new engine for the cruiser. I will be building all kinds of new props, s/s rudders, and testing them for everyone to have a better, more efficent system. I laugh my ass off when people rattle off prop numbers, they dont even have a clue that some prop builders stamp what ever they are asking for at the time of purchase. Those numbers do not desribe, blade size, cup, rake, foward blade, ect. I do sometimes rattle on, but I hope this info can help you with something on your boat. Joe

  6. #6
    VDRIVERACING
    Thanks for the feedback. I pulled the rudder out today, and will drop it off at Barber Welding this week for mag and massage. Kind of like a rudder day spa.
    Some good info from Suckerpunch about drag. His comments about a bullnose design to reduce drag, reminds me of a study the U.S. Navy did on hull drag. They used a parabolic hull shape, where the the front of the hull was widest, then narrowed in the center, and then wide again toward the stern. They found this reduced drag by pushing water away from sections of the hull.
    Let's see if we can find a happy medium between the battle of the steering wheel, and suffcient rudder response.

  7. #7
    lilrick
    You should try working out too!! Those 10 lappers are grueling I would imagine.

  8. #8
    Senior Member
    Join Date
    Feb 2009
    Posts
    1,105
    Have you thought of installing a steering box in your cruiser? I put one in a Schiada that I owned and it worked perfect. Abolutely no torque steer at all. I put the steering gear at the rear of the boat and used a double cable steering setup to drive the box. Just a thought...
    Steve

  9. #9
    VDRIVERACING
    Have you thought of pinstalling a steering box in your cruiser? I put one in a Schiada that I owned and it worked perfect. Abolutely no torque steer at all. I put the steering gear at the rear of the boat and used a double cable steering setup to drive the box. Just a thought...
    Steve
    Intersting thought. GN rules appear to allow gear boxes, but I'm not sure if that means a transmission gearbox, or a steering gearbox. I don't think I have seen anything but cables and pulleys. Not sure why that is...

  10. #10
    VDRIVERACING
    You should try working out too!! Those 10 lappers are grueling I would imagine.
    Ya mean like a size 10 lap? That would be a workout. I have been remiss and driving past the gym. Maybe if I go inside?:jawdrop:

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