Rudder blade design - problem

Discussion in 'Hydrodynamics and Aerodynamics' started by WildCard, Apr 16, 2020.

  1. WildCard
    Joined: Apr 2020
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    Location: Germany

    WildCard New Member

    This is my first post, I hope it's ok to ask for help/thoughts here.

    My problem is with the rudder on a Seaquest 36 with tiller steering. I own this boat for a year now, and love it. But I suspect there's a problem with the rudder design.

    The original rudder of the boat was very narrow. I have attached a pic of this rudder (it's from a another Seaquest, but mine had the same rudder originally). The preowner said that this blade was to narrow, not giving enough control. So he had a different rudder mounted, with a bigger blade. I have a pic of this current rudder attached, too. The pre-balance is 18% according to Jefa. This is a standard racing rudder from Jefa, it is their 35R-profile, this one: The pre-balance is 18% according to Jefa.
    https://jefa.com/ftp/rudder/rudder_blade/RUD35R.PDF

    With this rudder, she is relatively difficult to steer, I mean, it takes a lot of strength to move the tiller, there seems to be quite some pressure on the blade. It's not the sail trim or rudder pressure from the sails, because it is the same when under engine. When at rest in the berth, the rudder can be moved easily. But as soon as she moves and gets up to speed, the rudder is hard to turn. Steering "with two fingers on the tiller extension" is not possible when underway. The previous owner told me it has always been like that with the 35R-rudder.

    The rudder bearings are good, I have checked them this winter. They are JP3-bearings, a normal bearing at the top and the lower one is a roller bearing.

    I would like to hear your thoughts on this problem - is there any possibility of making the steering easier? I wanted to install a Raymarine Evo-tiller pilot, but I think it would not be able to handle the force neeeded to steer. And I would like to steer her with "two fingers on the tiller", as I was used to with tiller steered boats...

    Any help appreciated.
     

    Attached Files:

  2. jehardiman
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    jehardiman Senior Member

    So the rudder stock is angled, correct? Wheel or tiller (I assume the latter given you comments)? Any sign of wear against the hull or inside the rudder tube?
     
  3. WildCard
    Joined: Apr 2020
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    WildCard New Member

    Yes, tiller steering. And correct, the rudder stock is angled. I have a drawing of the original rudder blade and the rudder installation, there you can see the angle:
    https://teamswashbuckler.files.wordpress.com/2020/04/rudder-original.pdf

    The rudder stock was good, no signs of wear there or against the hull.
     
  4. jehardiman
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    jehardiman Senior Member

    There is a reason you rake the shaft and a reason to sweep the blade, but I think this has gotten too far off the mark having both. Realistically, as long as the rudder stock is not bending or binding, to reduce rudder torque you are going to have to get a new blade with the shaft axis closer to the center of pressure (not area, not sure what the center mark is on the drawing).
     
  5. WildCard
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    WildCard New Member

    Thanks for your thoughts. Can you explain a bit more what you mean by "shaft axis closer to the center of pressure"? Not sure I understand what you mean...
     
  6. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    I think that jehardiman is effectively suggesting that you need more side profile area forward of the rudder stock, to improve the balance.
    I am wondering if it would be possible to experiment with adding a 'nose cone' (for want of a better term) to the rudder - shape it out of foam, glue it on (with glue that can be 'undone' again if you want to make further changes) and fair it reasonably smooth, and then test it under way with the engine?
    You shouldn't have to test it drastically, eg by turning it hard over at maximum hull speed - just turning it maybe 10 degrees while motoring at two or three knots should tell you if it is an improvement or not?
    If it is not an improvement, then maybe add a bit more?
     
  7. jehardiman
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    jehardiman Senior Member

    The center of pressure (CP) is the point on the foil centerline where the force on the foil is resolved into a single vector, i.e. if you were to restrain the foil at the center of pressure there would be no moment. The problem is that the center of pressure moves with the angle of attack, generally moving "forward" as angle of attack increases. In testing foils are generally constrained at the 1/4 cord point, that is 25% of the cord back from the leading edge, and the moment about the point is measured to determine the center of pressure for the angle of attack. In the case of the RUD35R shape, the sweep of the foil causes the summation of the CP to be significantly aft of the shaft axis. Moving the shaft axis aft closer to the CP would reduce the torque needed to rotate the blade. A well designed relationship between CP and shaft axis should have the maximum moment to turn the rudder at 0 degrees, decreasing until at some high (30+) angle of attack, the CP goes forward of the shaft axis. However, with your install not only is the CP well aft, the shaft itself is raked. Generally raking the shaft increases tiller torque by reducing the effective length of the tiller.
     
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  8. WildCard
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    WildCard New Member

    Thanks for the explanation. Do I understand you correct - more rudder blade area forward could help? The same as bajansailor proposed? That would effectively increase the pre-balance of the rudder (18% as of now), right?
     
  9. jehardiman
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    jehardiman Senior Member

    More area ahead of the shaft is the obvious physical manifestation of moving the shaft axis aft towards the CP, but that may not be the answer in this case. As well as determining shaft torque, the CP (this time the distance from the CP to the shaft bearing) also determines the location of the force that produces rudder shaft bending moment in spade rudders like yours. In this case, adding area to the existing rudder will increase the shaft bending moment, how much and if that is acceptable on the shaft and bearings would need a detailed analysis.
    <edit to add the CP dimension that matters to the shaft bending problem>
     
  10. philSweet
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    philSweet Senior Member

    First of all, you aren't going to steer any boat that size with two fingers on the tiller. I sailed a similar sized tiller steered boat a couple thousand miles from St Croix to Montauk and the usual system was to have two people sitting across from each other with their feet on the 7 foot tiller. Under fine conditions, ropes could replace one of them. Two hours at the helm will be quite enough in any kind of weather at all. When setting up for a tiller-minder autohelm (or any other kind), you want to the opposite of what you are thinking about. It is better to run a bigger rudder with higher loads, but much better authority, than to run a smaller one that needs more throw. You can also just run a conventional quadrant-type AP, which would be better IMO on that size vessel. Note that autopilots don't get tired holding the tiller in one place, so sailing in a seaway isn't a problem if the boat tracks well. Beating in a seaway could easily have tiller end loads going from about 0 to 70 pounds every six seconds (call it a 500 footpound variation), and this would be considered normal. By making steering corrections at the load lull point and letting ropes hold the tiller at the load peaks, you can save massive amounts of energy, but most APs can't be trained to do that.

    So basically, you can't get rid of the dynamics in the rudder caused by the action of a boat in a seaway. You can only improve the tracking of the vessel through sail trim, ballast trim, and letting the rudder (and boat) move through an orbit with the seas. Use a watt meter to track the power draw of the AP and find out the trim that minimizes power consumption.

    Static or average loads while reaching in fine conditions are another matter. You change the balance of the boat with sail trim to establish a slight windward tiller angle, typically about 2 degrees, but for AP use, set it based on best tracking stability. High performance boats might be fast, but not track well when set like that. An AP may perform best with a different balance than what a skilled helmsman wants.

    If you look at the specs of direct drive APs, they usually have a torque rating based on a throw of about +/- 30 degrees. A 100 kgm rating covers boats from 20' to 45' according to Jefa, but I would guess the 45'ers are not being sailed offshore in 20' seas with 45 knots across the deck. Still, my 70 pounds at 7 feet only works out to about 68 kgm, so there is something in reserve there. Make sure your steering system can resist the tiller forces of these systems. If the AP is pushing 500 pounds on you tiller at 1.5 feet, that's 500 pounds on the rudder bearing, compared to only 100 pounds if the same torque is made at the end of a 7.5 foot tiller.
     
  11. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    First of all, you aren't going to steer any boat that size with two fingers on the tiller. I sailed a similar sized tiller steered boat a couple thousand miles from St Croix to Montauk and the usual system was to have two people sitting across from each other with their feet on the 7 foot tiller. Under fine conditions, ropes could replace one of them. Two hours at the helm will be quite enough in any kind of weather at all. When setting up for a tiller-minder autohelm (or any other kind), you want to the opposite of what you are thinking about. It is better to run a bigger rudder with higher loads, but much better authority, than to run a smaller one that needs more throw. You can also just run a conventional quadrant-type AP, which would be better IMO on that size vessel. Note that autopilots don't get tired holding the tiller in one place, so sailing in a seaway isn't a problem if the boat tracks well. Beating in a seaway could easily have tiller end loads going from about 0 to 70 pounds every six seconds (call it a 500 footpound variation), and this would be considered normal. By making steering corrections at the load lull point and letting ropes hold the tiller at the load peaks, you can save massive amounts of energy, but most APs can't be trained to do that.

    So basically, you can't get rid of the dynamics in the rudder caused by the action of a boat in a seaway. You can only improve the tracking of the vessel through sail trim, ballast trim, and letting the rudder (and boat) move through an orbit with the seas. Use a watt meter to track the power draw of the AP and find out the trim that minimizes power consumption.

    Static or average loads while reaching in fine conditions are another matter. You change the balance of the boat with sail trim to establish a slight windward tiller angle, typically about 2 degrees, but for AP use, set it based on best tracking stability. High performance boats might be fast, but not track well when set like that. An AP may perform best with a different balance than what a skilled helmsman wants.

    If you look at the specs of direct drive APs, they usually have a torque rating based on a throw of about +/- 30 degrees. A 100 kgm rating covers boats from 20' to 45' according to Jefa, but I would guess the 45'ers are not being sailed offshore in 20' seas with 45 knots across the deck. Still, my 70 pounds at 7 feet only works out to about 68 kgm, so there is something in reserve there. Make sure your steering system can resist the tiller forces of these systems. If the AP is pushing 500 pounds on you tiller at 1.5 feet, that's 500 pounds on the rudder bearing, compared to only 100 pounds if the same torque is made at the end of a 7.5 foot tiller.
     
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  12. Kim Klaka
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    Kim Klaka Junior Member

    Err... isn't it the other way round? The centre of pressure moves AFT as the angle of attack increases in all the experimental data I have seen (Whicker & Fehlner, Thieme, Molland etc.).
     
  13. jehardiman
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    jehardiman Senior Member

    No, you need to read complete context for this thread. His rudder shaft rake and sweep is not similar to a 1/4 cord line.
     
  14. Kim Klaka
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    Kim Klaka Junior Member

    The 1/4 chord line is merely a reference line for measuring moments. Are you saying that if the rake angle is large, CP moves forward with increasing AoA, i.e. opposite of when rake is zero (as in most wind tunnel tests). If so, is there experimental data to support the statement? This could be very enlightening.
     

  15. jehardiman
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    jehardiman Senior Member

    Are you a naval architect? Have you studied engineering?
     
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