Rudder stops question

Discussion in 'Boat Design' started by ted655, Aug 13, 2006.

  1. ted655
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    ted655 Senior Member

    While I realize a boat will turn if the rudder is at a right angle to the boats heading. This is "drag" on that side of the boat, rather than steerage. What angle from the centerline of the boat is considered steering, not drag? Stops should be installed to prevent the rudder from exceeding that angle? OR, is there a reason that a 90 degree rudder is ever necessary? Thanks.
     
  2. Greybarn
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    Greybarn Junior Member

    We typically install stops at plus/minus 30 degrees. This is usually plenty of rudder angle to make any kind of turn. The one exception is a slow speed, very tight turn required before the start of a race.
     
  3. jehardiman
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    jehardiman Senior Member

    Ted;

    You haven't said what type of rudder, and that makes a difference. Maximum acceptable rudder angle of a spade rudder on a twin screw crewboat is very different from the pintle hung rudder on a full keel yawl. Additionaly there may be very good reasons for going to 90 degrees...such as being able to pull the prop shaft without removing the rudder...
     
  4. KFB
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    KFB Junior Member

    Stops are typically installed at 35 deg P&S in typical twin screw rudder installations. I can't think of any good reson to allow more than that, and plenty of reasons not to. The rudder shaft centers should be offset from the tailshaft line to allow for the removal of the tailshafts without dropping the rudders out, turning any rudder that is in line with the propshaft gets you nowhere regardless of how great the angle is...
     
  5. ted655
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    ted655 Senior Member

    Oops, sorry. Flat btm., Displacement scow hull, 12' X 52', slow speed, single screw.
    Past 30 to 35 degrees and efficintcy falls off fast?
     
  6. jehardiman
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    jehardiman Senior Member

    Many single screw work and fishing vessels with shoe type plate rudders have holes in them to withdraw the shaft. Quite common in the single screw commerical world where offset rudders are impractical to say the least.

    Oh yeah. As has been pointed out, most rudders/airfoils are well into stall about 30-35 degrees and beyond that any lift (i.e. steering force) falls precipitously. If the rudder is well sized and you are still not getting the response you desire, then there are some towboat tricks that could be used, but most of those are designed in or require major reconstruction of the stern.

    And then also there are your expectations. What is your background comming into this vessel? Is is not handling like your previous craft? Is this your first vessel of this type? From what you describe, you basicaly have a powered barge, so I would expect it to handle like....a barge...:D
     
  7. ted655
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    ted655 Senior Member

    Nothing wet yet. Just design info.
    I have a "stump" skeg on one of my boats. It slides into groves in the bottom of the hull and a pin holds it in place. It mounts just ahead of the outboard. When running in water that is full of hidden obstructions it is great prop protection. BUT.... it takes twice the the space to turn the boat. From this I suspect that adding a full length skeg to my barge would have the same detrimental effect. But what about a keel board that was prominet at the bow and tapered to nothing by the mid hull length? Say 6" at the bow, tapered to 0 amidships? Would such a thing act as a pivot point, helping prevent bow slippage and turning more efficiently?
     
  8. jehardiman
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    jehardiman Senior Member

    Any appendage at the bow will tend to decrease directional stability. Any appendage near the stern will tend to increase directional stability. Now how decrease/increase is a big question that can take a fair amount of investigation (i.e. money) to predict in advance. The problem is that you need to make sure that the vessel retains "adaquate" directional stability for the rudder force at very small angles of attack, otherwise she will "gripe" and you will have to be constantly steering her, which is a huge pain.

    Do you want maneuvering at speed or at near zero speed through the water. For slow speed maneuveing a bow thruster or twin screws may be the answer
     
  9. Frosty

    Frosty Previous Member

    I am very interested in this as I have had many steering problems in the past on boats, usually above speed of 10knts though.

    I dont know what 'appendage' is or 'gripe'?

    Costantly having to steer is something I would like to understand more of.
     
  10. ted655
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    ted655 Senior Member

    :) An appendage means anything that you add to the hull that sticks down (or out). Like a rudder, board, keel, skeg or leeboard.
    Grip is when something sticks, grabs or holds onto something.
    Something with a fairly wide area, sticking down in the water, fastened to your hull & just ahead of your prop/rudder, will tend to keep your boat from changing coarse untill you turn the rudder enough to overcome the effort of this "appendage".
    The trade off to this is you increase the time/ distance it takes to make a turn.
    All sorts of appendages have been invented & created and located on the hull.
    In my case I was looking for a way to turn sharper at slow speed. Like you, I still want to not correct my coarse every few seconds. I may not get both. Every thong in boating is a trade-off I guess.:D
     
  11. jehardiman
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    jehardiman Senior Member

    The "gripe" is the forefoot. It "grips" the water and makes the vessel go straight (or so it was believed)

    "To gripe" is the tendency of the vessel to want to rotate around the forefoot, usually in a sailing vessel to the windward. It is caused by there not being enough directional stability, such as too deep a forefoot. In motor vessels it means that the vessels heading wants to wander and requires constant rudder inputs to keep her on course.
     
  12. Frosty

    Frosty Previous Member

    Oh boy this is really good stuff. I wonder If I could be so forward as to ask if you would please visit my thread 'advise needed for foam job'. There is a picture of my boats bottom. At 20 kts she becomes a handfull, she is capable of much more but I cant hang on to it. At 17kts she is as steady as a rock, I can go below to make tea and she will still be on course with a wake as straight as an arrow.

    I have experimented with toe in and toe out of the rudders,( similar but not same as picture) with apparanty no reall affect at all to steering sensitivity. The bulbous bows were found to be the maim reason for this however it still remains. In following seas its a battle.

    Ted655 Apologies and excuse me if I am taking your thread I wont dwell on the matter.
     

  13. MikeJohns
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    MikeJohns Senior Member

    Yes it's a term from the days of sail not used much these days, describes the action of a sailing ship with too much canvas set fwd. often alternating between lee and weather helm. The solution to Griping was to drop some of the fwd sail.
     
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