Rudder Size vs Lee Helm

Discussion in 'Sailboats' started by ancient kayaker, Nov 7, 2013.

  1. ancient kayaker
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    ancient kayaker aka Terry Haines

    I've got into an argument about rudder size and lee helm that I cannot win because I am not by any stretch of the imagination an authority on sailboats.

    In my opinion rudder size has very little to do with lee helm and a larger rudder is not the solution for the conditon. I am also of the opinion that lee helm is 100% a bad thing.

    I also believe it can be dangerous to attempt to overcome lee helm with a huge rudder. If the rudder area is comparable with the board's area and the boat has lee helm there is a risk of forcing the board into a stall condition, losing all control over direction.

    What is a reasonable relationship between rudder area and daggerboard area? I can see how the ratio might vary between different boat types and sizes, but it seems to me the rudder should always be significantly the smaller of the two.

    Who's right? BTW we're not discussing some weird experimental craft, just your typical small sailboat.
     
  2. sharpii2
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    sharpii2 Senior Member

    I'd say you're about 80% right.

    The bigger rudder would not fix the problem but would make the boat more controllable. You'd at least be able to point the bow the direction you wanted. The effectiveness of a rudder is it's size times the distance between its Center of Area (CA) and the effective CA of the board or keel.

    So, the bigger rudder could overcome the control issues of a lee helm.

    What it can not do is totally eliminate the disadvantage of such.

    The rudder would be creating lift to leeward to force the bow to windward. This, in the point of view of sailing upwind, is subtracting lift to windward.

    If the problem is big enough, the rudder would have to be so huge that it would be creating more lift to leeward than the board is to windward.

    The reverse of this situation is a hull that has a weather helm, but little or no keel and a huge, efficient rudder. The rudder would then be creating lift to windward as it tries to force the bow down wind. Yorkshire cobbles work that way. The big rudder is easy to un-ship, when it is time to come ashore. The boat is then rowed there backwards with its bow facing the surf.

    Lee helm is not always a bad thing.

    Suppose you want to sail down wind for thousands of miles, and you want your boat to be directionally stable enough not to need a vane or auto pilot. Setting sails in such a way as to deliberately cause a lee helm is a way to accomplish this. Such was done with SPRAY by Slocum, so he could go weeks without ever touching the helm. That was partly what the long bowsprit was about.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Well it depends. A lot of small boats carry a fair percentage of their lateral area in the rudder. If this is the case, increasing area aft is a quick fix. Then again, just scooting you butt aft a foot might be a small boat needs too. I've never seen a sailboat do anything close to reasonably well upwind, without a fair bit of weather helm.

    It helps to be more specific about the boat, as the design approach can be accessed. In larger craft where "cheek trim" isn't going to have much affect on the boat, you'll want better balance with the appendages/sail plan. In these cases, it's usually better to just place some additional area aft, such as a skeg. This isn't always practical, so the rig gets raked first, usually followed by a bigger rudder, as more practical approaches to a lee helm situation.

    Bolger was famous (or infamous, depending on how you look at it) for employing a massive rudder, to carry a significant percentage of lateral area. Of course, these are just the boats, you're not talking about, but some designers, myself included, do expect a portion of the lateral area in small craft, to be absorbed by the rudder. Now we get into the boat type. If she's clean underneith, meaning no skeg, no fin, no deadwood, just a set of appendages, then sizing them can be critical, but if there are skegs, stub fins, maybe a little hunk of deadwood or keel batten, etc. this is less true.

    Again, it all depends - are you confused yet?

    http://sponbergyachtdesign.com/Keel and Rudder Design.pdf

    This might be helpful.
     
  4. MikeJohns
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    MikeJohns Senior Member

    The old traditional Spanish Gamella caries just about all the lateral area in a very large fwd raked rudder. They apparently sail quite well to windward, funny old world.

    Any reasonable sized rudder moves the 'effective' CLR considerably with small deflections. 3 degrees of rudder angle on nearly all underbody shapes with a reasonable rudder area moves the CLR by close to 10% LWL.
     

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  5. ancient kayaker
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    ancient kayaker aka Terry Haines

    Thanks for the info. I didn't want this guy to do anything rash . . .
     
  6. BobBill
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    BobBill Senior Member

    And thanks from this boyo too.

    I am crafting a rudder foil for an outrigger using orig Hobie 18 rudder as template and planned to make it a bit longer than orig and see what would happen weather helm-wyze...keeping in mind the sail would no longer be centered between hulls but on the port main hull and way forward ala Malibu Outrigger, and the s'board hull being an asymmetric Hobie 16 with standard Hobie rudder (same for 18 And 16).

    As always, you guys fill in my blanks so I do not get too far adrift and spend time on the hard fixing my goofs.
     
  7. Petros
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    Petros Senior Member

    there are several things that cause weather or lee helm, a large rudder with all of the area aft of the hinge line could make it worse. So you are correct in cautioning them about changing the rudder size as a means of correcting lee helm. It will give more rudder power and control, but that is unrelated to lee helm or weather helm.

    Usually the best way to correct it is by raking the mast. A larger center board or dagger board could help too, or changing the location of the center board, depending no the relationship with the sail location. Also changing the location of the rudder hinge line relative to the rudder area will change it as well.

    There are lots of factors, and just throwing on a larger rudder will not likely stop lee helm.

    As far as size goes, usually the smaller the better for less drag, but than you loose rudder control at lower speeds, and you have a higher risk of the rudder stalling. So larger is generally not harmful in terms of control (at the cost of slightly high drag), but by itself is unlikely to correct lee helm.
     
  8. MikeJohns
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    MikeJohns Senior Member

    Petros
    Balance of the rudder will reduce the torque but it wont make any difference to the CLR. the rudder lift characteristics aren't altered at all by the hinge line position if you think about it.

    As far as size goes the issue is a compromise of wetted surface drag vs the Lift Drag relationship. But a larger rudder can be beneficial to actually reduce drag. Molland illustrates this here: (see attached pdf)
     

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  9. gggGuest
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    gggGuest ...

    People get very confused about weather helm/lee helm: there are at least three separate phenomena which cause the rudder to pull/push on the tiller.

    As Par quite tightly points out, many boats carry significant sideload on the rudder. After all if you are dragging that foil through the water it may as well do something useful all the time. A rudder that is carrying sideload will tug on the tiller in line with the load on it, and the greater the area behind the pivot or the further the area is aft of the pin the harder but in spite of much confusion this is not weather helm.

    Second of course is heel. Most sailboat hulls turn to a greater or lesser extent when heeled, and this can be very marked indeed and need a lot of correction to hold the boat straight.

    The third of course is the true weather helm - an offset required on the tiller to keep the boat straight on course when sailing bolt upright. Its not nearly as common as might be supposed, because given a boat with separate centreboard and rudder moving the rig back and forward just changes the relative distribution of side load on the two foils, and will not require a steering offset until the rudder is overloaded - as highlighted in the Moller paper quoted above.

    I don't know that there can be a rule of thumb for rudder area/board area proportions, because it depends so much on other factors. At a rough guess more than about 75% of daggerboard area would be rather unusual.

    I think before one starts major alterations one has to have a reasonable understanding of exactly what the problem is, and as can be seen from the above so called weather/lee helm is a much more complex phenomenum than many people believe.
     
  10. MikeJohns
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    MikeJohns Senior Member

    Heel has less of an effect than you'd expect. For a common displacement hullform, from memory 45 degrees of heel introduces a movement of the theoretical "CLR" of around 6% LWL. If you compare that with just 3 degrees of rudder shifting the CLR by 10% you see the importance of relatively small rudder angles in balancing your sail area.

    Often the rounding up of an overpowered craft is not so much the CLR shift but because the rudder has lost all effective lift due to several factors. And that comes back to the huge contribution the rudder has on CLR.

    Downwind in a modern wedge with a bit of a sea running is the course that really tests your rudder lift and stall characteristics. Bigger rudders with lower loading and delayed stall are very beneficial at times and can have significantly lower drag because they produce more lift at lower angles of articulation.
     
  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    It was meant as a quick question but I appreciate the way you are taking the discussion further. This is my understanding so far -



    Fundamentally Lee Helm results when the sail's effective center of effort is ahead of the effective center of lateral area.

    A bigger rudder may well increase the amount of steering power available but by moving CLA towards the stern it will, I think, make things worse.

    Lee helm is unsafe because if the tiller is released the rudder will feather and the boat will head downwind.

    Increased leeward force available from a bigger rudder will require more lift from the board which may put it into stall, and again the boat will head downwind.

    The proper remedy would be anything that moves the sail area forward or moves the CLA towards the stern.

    Heeling assists a turn to windward by shifting the sails thrust to leeward. Also most hulls turn away from heel - we toothpick jockeys often steer like that.



    There will always be exceptions such as sailing canoes which often lack a board or visible keel and of course catboats seem always to have gigantic rudders. However the great majority of my 100 or so sailboat study plans show rudders which are significantly smaller than the daggerboard or keel, although less so on smaller boats.

    With a boat with the much more desirable weather helm, the rudder's force is in the same direction as the board's lift. It might be feasible in such a situation for the rudder to have as much or more area than the board for some boat types, in fact it would probably reduce leeway.
     
  12. BobBill
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    BobBill Senior Member

    Thanks ancient K, from an ancient outrigger driver.

    In my case, the clr and ce are near correct, if not a bit asymmetric in favor of weather...rig is semi-crab claw ala Malibu Outrigger, but I may add option for very high aspect unstayed narrow soft wing type main...if I can figure out how to make a tall carbon spar...28 feet or so...

    I had read that all things being equal, if one was to length a rudder foil, drag etc aside, weather helm might increase. I inquired here because I am crafting a rudder for the main hull (vaka), using orig Hobie 18 rudder as template and have a few inches of wood beyond the template's length.

    With the template rudder on the outrigger, I thought to lengthen the vaka rudder a few inches just for the heck of it to see if the result would increase weather helm.

    Twas just one of those thoughts that come up while sanding my day away etc.

    I had thought to foil it, but that seems too much hassle sailing and would bring up a raft of flotsam here, you get my drift.
     
  13. PAR
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    PAR Yacht Designer/Builder

    Increasing the rudder's length wouldn't increase weather helm, it would tend to decrease the "feel" of weather helm with the additional area aft. Adding (effective) lateral aft or adding sail area forward, will ease weather helm (assuming it is weather helm).

    If you increase aspect area in either your rig and/or your appendages, you also need to adjust your lead, typically adding to it, as aspect goes up. I have a semi hard wing sloop with 21% lead and a gaff sloop with 13%, so the difference can be significant.
     
  14. BobBill
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    BobBill Senior Member

    Got it...I don't have much room left to move rig forward...so likely will sit with the semi crab claw and maybe, just maybe add small job on a stick...but that is later. Got to get it finished first.

    Guess I will go with current rudder length...factory same for both 18 and 16 cat hulls.

    Thank...here are pics for mind's-eye.
     

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  15. ancient kayaker
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    ancient kayaker aka Terry Haines

    - are you thinking of a lifting foil, Bruce foil, or just the rudder profile?
     
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