Rudder change - please advise, who can

Discussion in 'Boat Design' started by BarryG, Jun 19, 2008.

  1. BarryG
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    BarryG BarryG

    I posted this thread, then edited the attachment, and it seems I lost it. I re-post.
    The boat is a Bruce Roberts “Mauritius”, a centre cockpit, cutter rigged sloop, 43ft LOA, 33 ft waterline, 17 tonnes laden for cruising.
    The keel is long, starting just fore of the mast, and the aft end is angled as per the rough sketch. (not to scale). The rudder stock is also angled and is supported by a nylon-bushed pintle at the lower end.
    The boat is heavy on the helm, partly because the wheel is relatively small, being bulkhead mounted on the port side of the companionway, an arrangement which I shan’t change.
    She doesn’t hold her course well even when hand steered, and though the Aries wind vane can keep her on an average course, it works hard to do so, and she progresses in very long sweeps. She is also sluggish and slow to respond to her rudder, making her awkward to manoeuvre in tight spots. I suspect the rudder area is too small, but the angle at which it’s set doesn’t help either.
    I have in mind to change the angle of the rudder stock to make it hang vertically and to balance the rudder by building about 15% of the total area forward of the pivot line. I hope that would lighten the helm. It would also push the rudder aft and I hope that would improve her directional stability. Since the rudder will have to be built anew, I would like to ensure it is as big as it should be.

    Please could somebody advise:
    1. Will I be creating an unforeseen problem by making this change. The mechanics I can cope with, but I’m referring to handling, performance or safety.
    2. What guidelines might I use to arrive at a suitable rudder surface area.
    3. Will the foil shape of it be very critical, bearing in mind she is a heavy cruiser, and not racing?
     

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  2. PAR
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    PAR Yacht Designer/Builder

    Yes, your asking for trouble if you attempt to add "balance" to a keel hung rudder. It will act more as a brake then a rudder with your projected 15% balance.

    It would be easier to offer advise if you could answer a few questions.

    What is the rudder deflection when on a closed hauled course in moderate wind strengths (say 10 knots)? This can be measured at the quadrant or better at the emergency tiller location. I need the angle the rudder must be held to maintain a straight course in these wind strengths.

    How worn out are your sails? Their age?

    Have you attempted to rake the main forward to ease the helm pressure?

    What is the steering gear you're using? This would include the quadrant size, and helm type and model, etc.

    This is a long keeled craft and steering isn't going to be particularly stellar in any event, but the helm should have nice balance when in moderate winds with well cut and set sails.

    Standing up the rudder post and getting much of the lower portion of the rudder in slightly better flow will help, possibly with more area and a better foil shape, but it's important to discover what the problem actually is.

    Worn out sails can increase helm pressure as can a rig that is raked too far aft.

    Has this been an issue since the sails were new or is it a recent appearing problem?
     
  3. diwebb
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    diwebb Senior Member

    Another option to look at if there is sufficient space behind the existing rudder is to add a second spade rudder and relegate the existing rudder to trim tab status. I have seen this done on a Lapworth 50 and it worked well. I also proposed it for a 50 ft Luders that was having similar problems. The increased lever arm for the rudder, being further aft, allows for more effect from less area, so the rudder does not have to be so large. There is also the additional factor that if it does not work out you still have the existing rudder to work with. Also for a long distance cruising boat you have redundancy if one rudder fails. You could also couple the rudders together with a removable coupling as the DeRidders did on Magic Dragon. This would allow for multiple arrangements for self steering and trimming options. If you do not like spade rudders then a skeg could be added as part of the new installation.
     
  4. BarryG
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    BarryG BarryG

    Thank you. I am not where the boat is at present so can't answer your questions on rudder angle. The sails are two years old but the problems were there with the previous set, and when these were new. The mast is very nearly vertical, a rigger in New Zealand having adjusted for the reasons you suggest.
    If I feel I should go ahead and do some changes I shall only do so after exploring these sorts of details, and would be grateful if I could come back to you in that event
    Regards
    Barry Garnett
     
  5. BarryG
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    BarryG BarryG

    Thanks for the interesting idea, but a second rudder sounds, at first take, like a second set of problems. Having circumnavigated and sailed over thirty thousand miles in the boat in the last five years I'm inclined to leave the rudder problems as they are, if my proposed fix is just going to complicate matters.
    Regards
    Barry Garnett
     
  6. diwebb
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    diwebb Senior Member

    Hi Barry,
    you may want to contact Jane and Michel DeRidder of Magic Dragon, which has twin rudders. They can be contacted via www.setsail.com/s_logs/deridder/dragon.html or it you look on www.trademe.co.nz you can see photos of Magic Dragon in the moored sailboats for sale section. The DeRidders have been cruising Magic Dragon for over 40 years and have had no problems with the rudders as I understand from them. They can also be contacted by phone on 027 260 9009 please add the New Zealand prefix from wherever you are phoning from. They are a very friendly and knowledeable couple and very willing to talk about the design of Magic Dragon.
    You may find that building a second rudder is actually cheaper and less of a problem than trying to rebuild the existing rudder. The one thing to bear in mind is that the further aft the rudder is the better control you have. I owned a Kettenburg PCC when I was in California and she was a bit of a beast to handle off the wind although fine going to windward, I heard of one owner who replaced the rudder with a skeg mounted rudder of about half the area of the original mounted at the aft end of the waterline (about six feet further ) and vertical instead of angled forward at about 40 degrees. This completely cured the steering problems and in fact made the boat faster in most conditions because of the reduced wetted area and removing the braking effect of the rudder when angled to counter weather helm.
    Anyway please look at the idea as I know that it has worked for other boats and could be a viable solution for your boat. All the best with the project.
    David
     
  7. PAR
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    PAR Yacht Designer/Builder

    Have you contacted Bruce Roberts? There is a pretty substantial network of Roberts owners and builders that also may be of some help.

    It sounds like you've looked at the other things that could be causing the problem and some changes may be in order.

    I understand there are two versions of this boat, the later version has a fin with skeg and rudder. The earlier version, which is what I think you have (also called the Norfolk 43) has the long keel (with three different drafts available) and the inclined keel hung rudder like yours.

    I've heard reports of similar issues with this particular design.

    Standing the shaft upright to vertical and possibly increasing it's area will surely help. I would do it similar to the your right-hand drawing, without balance and with increased area on the lower portion of the blade.

    Again you should contact Bruce and see if he's drawn up some "adjustments" to this design, as I'm sure this isn't the first time he's heard it. If not, it would be good advise to have a designer go over your suggestions and work up scantlings, so it doesn't cause more problems then it solves.
     
  8. Guest625101138

    Guest625101138 Previous Member

    A rudder at the end of a long keel is not the most effective means of turning a yacht but all things are relative.

    With a short keel and stern hung rudder they operate in relative isolation as foils so the turning moment is easy to calculate. I have not done numbers for your situation but it could be done. However that does not stop me from making a comment or three on proposed changes. (Having tried to steer a 36ft Roberts downwind in a steep sea with a similar set up but further complication with overgeared hydraulic steering I have some idea of your plight - particularly if you are comparing with the helm of a yacht with fin keel, spade rudder and flat section aft.).

    Extending the rudder aft will increase the moment working against the shaft. Partial balance as you have drawn will reduce the torque required - so sort of offset each other and may not gain the required reduction in steering effort.

    Lengthening the rudder aft gains area but reduces the foil aspect ratio thereby increasing the loss to induced drag so again there are offsetting factors and may not improve responsiveness.

    Separating the rudder from the keel and using a high aspect thick section rudder seems to offer real potential. You can set it up so shaft is close to balance. The centre of lateral area is moved aft by virtue of taking the rudder aft - thereby reducing tendency to weather helm. You can use a smaller rudder area so less drag. You can use a very powerful section like a NACA0020 with maybe aspect ratio of 4 to 5.

    If you see great complication in moving the rudder then you may find a shorter thicker rudder in the existing position actually generates better steering force with less torque. It may not actually be thicker in absolute terms but in percent of chord length.

    You could do an interesting exercise to determine the best rudder geometry for your existing pivot points. Normally somewhere between a 20 to 25% section will generate the highest steering force for the lowest in-line drag.

    Point of this is that unless you do the numbers, at least to first order issues, your intuition could lead you in the wrong direction. Better still, like PAR suggests, if you can just copy a change that is known to work.

    Rick W.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Balance at the top of that rudder will surely act as a brake, though you may get some helm relief if the lower portion has some balance (I doubt it). I've had some experience trying to solve these types of issues and I've never been able to develop a partly balanced blade, that close to a deadwood assembly (deceased platting?) that worked.

    Additional area lower will pick up induced drag, but at this point in the flow, likely will reattach, with enough area and over come the slight drag (which isn't necessarily a bad thing in a cruising rudder). This is much like the elliptical foil ends, some famously talked about on Spitfire wings. Yep, the shape is better at induced drag, but the additional area of a square tip (or in your case the bigger lower section) makes up for the drag with more response. Interestingly enough, very few of the aircraft after the Spitfire used elliptical tips, preferring to suck up the lose in favor of the additional lift square tips can provide. The P-51 easily out preformed the Spitfire once the engine matched it potential. The wing tips where quite square and the plane is still a preferred pylon racer to this day. So much for elliptical tips and the excessive induced drag they mitigate. I know I just pissed off a bunch of wing tip hugging lubbers, but performance is easily quantifiable, in spite of theoretical debate.

    Yes, it would be nice to have good separation and a spade, but the rudder is currently just about at the end or LWL, so separation is a questionable thing on this old style Norfolk. A better section as Rick has suggested would be helpful.

    I know Bruce has had issues with this and other rudder assemblies. Not that he can't design a rudder, but has had issues arise with the volume of designs he has produced. His designs generally work well, within the performance envelope he envisioned. Call him or contact the owners groups. They'll have solutions that have already gone through the "mill" and you'll be on your way.
     
  10. BarryG
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    BarryG BarryG

    To all who have responded to my question, thank you. You have been a great help in ponting me towards help in the specifics should I pursue this cnange.
    Barry Garnett
     
  11. FAST FRED
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    FAST FRED Senior Member

    A simple method of "power steering" that is frequently used on aircraft is called a tab, or balanced tab.

    The concept is a trim tab is hung in the back of the rudder and when not being used by either an autopilot or self steering gear the tab works to increase the deflection of the rudder.

    As you move the main rudder the tab moves in relation to the rudder , helping it turn , so reducing the helm effort.

    The trick (trial and error) is getting the relationship between the rudder and tab just right.

    FF
     
  12. dougfrolich
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    dougfrolich Senior Member

    Nice one Fred. Here is an example I just finished.
     

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  13. Tim B
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    Tim B Senior Member

    I liked the "add new rudder aft" idea, but you need to get the new one reasonably far aft to make a big difference. If you're worried about having two rudders, just remove the current one and fair the hull. You will need some sort of structure aft to take the rudder loads, though. A simple design to ABS rules should be ok.

    Can I just make a small point on the fluid dynamics side of things? It's a long keel cruising yacht. The drag is not going to be significantly affected by what you do to the rudder. Certainly suggesting that the induced drag will change dependant on the rudder design is irrelevant in this case. For well-separated foils on high-performance yachts, it will make a noticeable difference. Just not in this case.

    The foil section will not be really significant for the reasons above. Aim for an NACA section that is thick enough to handle the stresses. You can work out the required area on a simple moment balance (of what you have now, and what you propose)

    Cheers,

    Tim B.
     
  14. Petros
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    Petros Senior Member

    Your proposed change I suspect may actually reduce rudder effectiveness and increase drag, despite the larger size.

    The part of the rudder ahead of the pivot will only cause the flow to separate off the aft end of the long keel. The larger gap between the back of the keel will allow the pressure difference to bleed through.

    Also by making the rudder larger you might also risk having the wheel snatch out of your hands in a following sea. The forces transmitted to the steering gear from wave action will be larger.

    Three simple things to try with your existing rudder is to put in a gap seal between the rudder and the aft end of the keel (this keeps the differencial pressure from bleeding though the gap). You can also try adding an "end plate" to the bottom of the rudder (that will increase the effective area and aspect ratio). Also fairing the keel to rudder transition area should help keep the flow attached around the rudder area to improve the turning power (especially at the gap area). Having a smooth curving surface all the way to the trailing edge from the aft end of the keel is important to get maximum effectiveness.

    None of these changes are difficult, have little risk of a bad result, and should help the rudder authority. You can even do quick and dirty tests with duct tape and putty to see if it works. If you are going to add area, add it to the trailing edge, though I would try this last.

    Good luck.
     

  15. diwebb
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    diwebb Senior Member

    One comment on balanced area on a keel attached rudder. On my boat Arcady, a modified Bruce Roberts Offshore 38, I had a keel attached rudder with the propeller set quite well forward of the leading edge of the rudder and balanced area projecting forward in to the prop aperture. This did help in reducing helm forces but the biggest advantage was the improvement in handling under power. Even though she had a full keel she could turn in little over her own length as all of the prop wash was deflected by the rudder. I also had a trim tab on this rudder and as previous commentaries have said this helped with helm effort a lot, if it was pinned in the fore and aft direction.
     
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