Moving the rudder aft

Discussion in 'Sailboats' started by Brent Swain, Aug 31, 2010.

  1. Brent Swain
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    Brent Swain Member

    When I singlehanded my first boat, a Kinney designed Pipe Dream, from BC to New Zealand , a boat with a short keel with attached rudder, I found her almost uncontrolable downwind.
    In New Zealand, I pulled the rudder off the short keel, and gave her a skeg mounted rudder six feet further aft. While that had minimal effect on balance, the difference in downwind control was huge.
    I have since heard of people getting similar results on many such designs, with drastically improved downwind control. Friends who circumnavigated in an Alberg 37, said it was scary trying to control her downwind. I suggested they do the same . Instead, they put a dagger board just ahead of the rudder post, and said the improvement was huge. They did say however, they wished they had simply removed the rudder and shipped one on a skeg, much further aft.
    I wished I had put my rudder on the transom, greatly simplifying self steering and inside steering, as I have done on all my boat since.
    Anyone planning to head offshore with a short keel with attached rudder, should consider making the change in advance.
     
  2. larry larisky

    larry larisky Previous Member

    as for the rudder, brent, i repeat for the one who send me this message
    you have to be a dumb *** for not controlling your own boat.
    is that too rude or too rough
    sender did you read brent post lately.
    obviously not or you wrote them yourself, and you wrote the message above.
    i don't know i am perplex.
     
  3. ancient kayaker
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    ancient kayaker aka Terry Haines

    - maybe not. Moving the rudder aft could easily have made the problem worse by moving the center of lateral resistance even further aft. What makes more sense to me is that the increased rudder leverage has made the balance less critical than before. It sounds like you may have a boat with lee helm. IMHO the rig should be the first place to look. If you already did that then my comments do not apply ...
     
  4. messabout
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    messabout Senior Member

    Back off a ways Larry! This is a forum for attempting to be helpful. This is not a suitable place to belittle anyone.
     
  5. gggGuest
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    gggGuest ...

    I suspect its rather more complicated than that...

    A separate skeg and rudder works a bit rather differently to a boat with the rudder on the keel, because both are foils capable of contributing lateral resistance, and it seems - in dinghies at least - that the centre of lateral resistance can be moved between the two a considerable amount without changing tracking and genuine weather helm.

    A lot of people seem to confuse the side load on an unbalanced rudder with weather helm. Ain't necessarilly so. An unbalanced rudder taking up a lot of sideload - especially without skeg - may be tugging significantly on your hand without being offset from the centreline.

    I trust I can make this contribution without being flamed by the less adult members of this forum...
     
  6. ancient kayaker
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    ancient kayaker aka Terry Haines

    Perhaps I am one of them. It always seemed to me that if a leewards force on the tiller is needed to keep the boat from heading downwind, then releasing the tiller will allow the boat to do just that. Can you expand your explanation, please? I want to see where I am misunderstanding.
     
  7. Milan
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    Milan Senior Member

    Pipe Dream is typical design from her era. (Kinney designed her in the early sixties I think, when he worked for Sparkman & Stephens).

    That was period when boats had graceful, elegant lines, long overhangs, short waterline, balanced hull forms, rudders attached on the trailing edges of the keels.

    Keel was short in her time but these days, in the fin keel - spade rudder era, calling her short keeled boat gives wrong impression to the most of the readers.

    At that time, designers already began searching ways to minimise wetted area, within limitations of solid wood construction techniques. So, keel was cut at the front and back. That places rudder forward, giving it rather small leverage, but preserving structural strength. Rudder’s turning force is further weakened by the sharp angle – rudder tries to push stern down as much as to turn the boat. That downward force tend to exaggerate rolling on the downwind courses. Boat reacts rather slow to the rudder movements.

    Placing rudder further aft in more vertical position certainly gives more control.
     

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  8. Eric Sponberg
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    Eric Sponberg Senior Member

    I had a similar project recently with a Spencer 42, a design of the same era as Pipe Dream and the Albergs. Copernicus had a keel-hung rudder, and the owner hired me to design and engineer a spade rudder. The project worked well, and I have just posted a story about her on my website. Included is some description from the owner of how well it worked:

    Link: Copernicus Rudder

    I hope this helps.

    Eric
     
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  9. Petros
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    Petros Senior Member

    I think the rudder attached to the full keel is now an obsolete design. Virtually no modern design does this any longer unless the designer has nostalgia for old ships.

    I do not believe there is a single racer or even a performance cruiser that would use such a configuration today. There is a good reason the rudder is moved aft and is a separate structure for the keel, it is more efficient, it has better control and has a number of other virtues. It is just the improvement of the science (or perhaps from applying modern fluid mechanics to hull design) of boat design, making anything else obsolete.
     
  10. gonzo
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    gonzo Senior Member

    Not all boats with fin keels and attached rudders behave badly. Stern hung rudders, which have been around for centuries, also have drawbacks. One of them, is that in heavy seas, specially running, it can come out of the water. Deep forward rudders don't have that problem.
     
  11. Petros
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    Petros Senior Member

    maybe so Gonzo, but it is still obsolete.
     
  12. gggGuest
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    gggGuest ...

    Well, consider a hypothetical completely balanced rudder . If you have to pull the tiller 15 degrees to windward off the centreline to keep the boat in a straight line you have weather helm: yes? If you have to push it 15 degrees down to leeward to keep the boat in a straight line you have lee helm, even if in both cases it doesn't take any effort to hold the tiller in that position.

    Now consider a boat with rudder and centreboard of equal sizes and sections, no skeg and 100kg of leeway force to resist from the rig. For the best use of these two equal foils there should be 50kg sideload on each foil. If the tiller is exactly on the centreline and the boat is tracking straight then we have a perfectly balanced boat: agreed? However if the blade is pivoted at the leading edge then there is 50kg of force attempting to push the blade to windward, so to keep the boat on track the helm must exert say 15kg of force (depending on tiller length) to keep the boat in a straight line. In order for the boat to have no load on the tiller when balanced you'd have to move the rig so far forward that all 100kg of sideload was on the centreboard and none on the rudder, which would be inefficient.

    What makes this even more confusing for the average sailor is that most of the time he has his tiller pulled to windward not because the boat is fundamentally unbalanced when bolt upright, but because the boat is heeling over 20 degrees, the centre of water flow is not remotely in line with the centreboard and the deflection he's putting on the rudder to keep the boat straight is not only keeping it straight,but also lifting the stern and pushing the bow down and making the imbalance even worse...

    Nothing about sailing technology is simple if you examine it in sufficient detail! Fortunately we need to understand almost none of it to sail a boat.
     
  13. Brent Swain
    Joined: Mar 2002
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    Brent Swain Member

    Once I moved the rudder aft the change in balance going to windward was minimal and she still had a slight weather helm, but far less.
    There are many older designs which were built far more strongly than many could afford to build them to day , which have this type of underwater profile. Older Spencers are a good example. Moving the rudder aft on a skeg gives one an affordable , well built boat, at a fraction the cost of a new one, without the drawbacks of the rudder being too far forward. With the rudder on the keel, the smooth aft sections act like a surfboard in a following sea. Stopping the crossover flow makes all the difference.
    What having the rudder further forward gives you in keeping it in deeper water, is more than lost buy having far less leverage there, and the stern acting like a surfboard without a skeg.
     
  14. dskira

    dskira Previous Member

    Brent you can't really answer nothing about rudder since you don't know the stability, therefore the radius of your boat.
    I asked a simple question, the 101 of naval architecture and your never answered.
    That is your problem, you do not know what you design, and it is my duty to report you to your customer as a fraud.
    (read origami post)
    Daniel
     

  15. Brent Swain
    Joined: Mar 2002
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    Brent Swain Member

    Dan and Larry will automatically disagree with anything I say, just for the sake of disagreeing.
    Should one only build boats designed by or get ones boat building and design advice from, adults?
     
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