Changing rudder post angle

Discussion in 'Boat Design' started by Vineet, Jun 13, 2022.

  1. Vineet
    Joined: Apr 2021
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    Vineet Junior Member

    I have heard of people changing the rudder post angle to be more vertical as a way to reduce pressure on the helm, particularly on heavier displacement hulls sailing downwind, so that the rudder is not fighting the buoyancy of the stern.
    Do any of you have experience or knowledge of modifying a transom hung rudder that is sloped to make it vertical or closer to vertical?
     
  2. wet feet
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    wet feet Senior Member

    I have changed a swept back dinghy rudder to have a vertical leading edge.That wasn't what I understand your question to be but what exactly are we discussing?The buoyancy distribution of the hull won't be changing if you alter the angle of the rudder's operation and neither would I expect any alteration in the amount of weather helm.You might bring about a very small alteration in the turning moment because of any difference in the distance between the CLR and the rudder's centre of area.

    The pressure on the helm might be a result of the area of the rudder not being optimally balanced around the pivot axis of the rudder.A couple of pictures of the boat under consideration would be helpful.
     
  3. Vineet
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    Vineet Junior Member

    I'm having issues with uploading photos so I'll add those in a bit. It's not the weather helm I'm looking to address. I'm looking ease the pressure on the helm when sailing downwind on a heavier displacement hull when the stern gets pushed around from following waves instead of lifting and getting closer to planing as on lighter displacement hulls. So it's not weather helm I'm interested in addressing. The rake of the rudder post, with the bottom of the rudders' leading edge being further forward than the upper leading edge, wants to pull the stern downwards when the rudder is turned, thereby fighting the buoyancy of the stern and adding pressure to the helm.
     
  4. wet feet
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    wet feet Senior Member

    Is the boat a long keel traditional design?If so,adding a skeg aft for a modern style of rudder might work.An unsupported spade rudder might also work,but would be more vulnerable to damage.
     
  5. Vineet
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    Vineet Junior Member

    here are some pics describing what I am referring to. The image with with full profile image (Deben 4 3/4) is the boat I'm considering altering. The ballast on that photo is cast iron. I would change it to lead, leaving more deadwood. The photo of just the aft profile (Vancouver 27) is what I'm considering to alter it. I wouldn't want to alter the hull lines so I am wondering if a rudder post could be fitted to go up vertically through a portion of the transom. I would also consider cutting away some of the deadwood as in the Vancouver 27. Thoughts?
     

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  6. messabout
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    messabout Senior Member

    The reaction of the boat will be a tendency to submerge the stern to some degree. The severity of the reaction will be a function of the swept angle of the rudder and the velocity of the boat. Think of the rudder as a dive vane. Consider the angle of attack of the turned rudder, it is generating negative lift. Small tiller inputs may not change things very much, but a larger input could cause a noticeable reaction.

    Observe that such a shaft pivot angle will cause the rudder to create +/- lift. Anything that creates lift also creates drag.

    With all that armchair reasoning, many older boats did have negative rake of the rudder post. Those old-time designers and builders may have known some things that I do not know.
     
  7. Rumars
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    Rumars Senior Member

    Assuming this is an existing boat, about the only thing you can do is alter the rudder pintles position. This means a new angle for the top pintle, a steel frame to move the middle one out until the rudder is vertical, and either a strap exending the keel for the bottom one, or going to a spade rudder without bottom pintle. Everything else requires altering the hull lines.
    After modifications you will probably have better maneuverability in reverse and possibly better rudder authority. A balanced spade rudder will also give you lover forces on the tiller.

    Changing the ballast to lead is not going to give you anything, the change in CG will be marginal. Lightening the rig by using more advanced materials will have a more significant effect, and that's where you should spend the money.

    I say leave the boat as it is and either learn how to live with it or buy a new one with a more modern underwater shape.
     
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  8. wet feet
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    wet feet Senior Member

    For a boat of it's age and value,a major rudder alteration exercise would consume time,money and effort that might be considered disproportionate.If hard steering is a feature,it might be simpler and less complicated to add a bowsprit and move the CE forward a touch.Alternatively and taking into account what was mentioned in post #6 ,it might be feasible to adjust the shape of the rudder to create more lift and less drag.I suspect the boat was built with a rudder that has very little resemblance to an efficient foil section and if it moves from being a slab of wood with the corners knocked off to something a touch more efficient,the steering loads may reduce.

    Alternatively,the final sentence of the previous post should be considered.
     
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  9. Scuff
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    Scuff Senior Member

    Doesn't the negative swept angle try to push the bow down as it's moved off centerline?
     
  10. messabout
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    messabout Senior Member

    ^ Scuff; Point your finger down toward the floor at about a 45 degree angle. Now rotate your hand in either direction. Observe that the flat of the palm would be inclined to push the stern of the boat down.
     
  11. Vineet
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    Vineet Junior Member

    Hello messabout, that is what I understand. It thereby is fighting the buoyancy of the stern.
     
  12. Scuff
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    Scuff Senior Member

    If the foil is still producing lift doesn't that force attempt to push the bow down? I did consult on a rudder for my trimaran and the design provided had 25* of negative rake (transom angle). My concern was with ventilation but the NA said another concern would be the tendency to push the bow down when moved off the centerline a significant amount. I do see how the drag itself could do the opposite. I ended up ditching the stock design. Good discussion.
     
    Last edited: Jun 16, 2022
  13. rangebowdrie
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    rangebowdrie Senior Member

    That pic of the original rudder plan shows it to be of the old "heart shaped" design.
    In that type of shape, the center of effort is quite far back from the leading edge, and the greatest concentration of area is close to the surface where it has less effect.
    Before I'd change the angle, I would look at what is popularly known as a "Ted Brewer rudder", in which the aspect ratio is increased and more of the area is towards the bottom.
    An increase in aspect ratio lessens the amount of input force needed, it becomes more effective, having more of its area in deeper water, and has less drag.
     
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  14. Kayakmarathon
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    Kayakmarathon Senior Member

    Vineet, there are two problems to be solved. force on the helm, and transom sinkage during a turn. Reducing force on the helm is accomplished by locating the rudder post as close to the center of lift of the rudder. Your examples show the rudder post on the leading edge of the rudder. Instead, locate the rudder post 25% aft of the leading edge. The tradeoff of reduced helm force is increased chance of weeds collecting between the hull and the rudder. Reducing the transom sinkage during a turn can be achieved by a swallowtail shaped rudder (opposite of the "heart" shaped rudder mentioned earlier). Redesigning the hull to accept a swallowtail rudder would requite a lot of $tructural analy$i$ (money).
     
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  15. rangebowdrie
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    rangebowdrie Senior Member

    When the rudder post is not angled/slanted all that much there really isn't that much force being directed into "submerging the stern", it takes a pretty radical angle, (look at a 1930s race boat,) in which rudder post angles were approaching 45 degrees.
    And in designs like that the long stern overhangs pick-up lots of buoyancy with just a minimal amount of contact with the water.
    In contrast, with a moderate angle and the greater area towards the bottom of the rudder a reasonable amount of force is generated that tries to rotate the hull to a more upright position, (decrease heeling).
    Of course, this is with rapid and short-lived helm travel.
    Any helm movement builds drag.
    An exception: when going to weather with well-tuned/trimmed sails, it is advantageous to carry ~2 degrees of weather helm, this helps the boat lift to weather while not inducing dis-proportionate drag.
     
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