Jeanneau Sunfast 36 - Directional instability

Discussion in 'Sailboats' started by Petr Podobsky, Oct 19, 2024.

  1. Petr Podobsky
    Joined: May 2024
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    Petr Podobsky Junior Member

    With my friend we recently bought a Jeanneau Sun Fast 36 and sailed her from Norway to Poland. With broken autopilot steering was a bit of a nightmare especially sailing downwind on waves.

    As we plan some longer trips we want to fix that handling.

    Based of my understanding of the problem and also on this thread Directional Stability https://www.boatdesign.net/threads/directional-stability.7613/ we want to add some little fin in front of the rudder. It should basically substitute the skeg.

    The questions are:

    1. How big the fin should be? (It will change the position of CLR, but we could also add some fin in front of the keel)

    2. How far it should be from the proppeler?

    3. What are the problems caused when the fin will be too close to the proppeler?
    Thanks

    Petr
     
  2. jehardiman
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    jehardiman Senior Member

    Have you checked the mast rake and tune? Do the sails still have good shape? Does she sit her lines or is she bow heavy? I'd work with the rig and trim a season or two before considering messing with the underwater profile.
     
  3. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    +1 re J E Hardiman's comment above.

    What sail(s) were you using while sailing down wind, and what was the wind speed (approx), and the sea state?
    Were the waves from one direction, with long wavelength, or were they 'confused' from different directions?

    You already have a semi-balanced spade rudder, hence I don't see what effect a skeg in front of it will have.

    Here is the side elevation drawing for your Sunfast from Sailboat Data -
    https://sailboatdata.com/sailboat/sun-fast-36-jeanneau/

    She has a relatively large mainsail; if I was sailing her down wind, or on a broad reach, I think I would be more inclined to use just a headsail (especially) in stronger winds, and maybe with a well reefed mainsail in lighter winds, especially if the seas are confused.

    sun_fast_36_drawing.jpg
     
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  4. CarlosK2
    Joined: Jun 2023
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    CarlosK2 Senior Member

    "With broken autopilot steering was a bit of a nightmare especially sailing downwind on waves."

    ---

    typical of the standard modern (1970-) yacht

    with Yaw, Roll and Pitch the hull produces a lot of lateral force on the bow, plus a huge centerboard/Keel produces lateral force forward of the center of gravity

    IMG_20241020_131637.jpg

    note that in this estimate by JW Slooff (a great expert in fluid mechanics who designed the famous keel of the Australia II) Pitch is not included, only Yaw and Roll/Heel

    If we also add the orbital current of the Wave (which surely in the North is short in relation to the length of the yacht) the matter unfortunately has no solution

    ---

    (The modern yacht is a good metaphor for this beautiful asteroid we live on and horrible world of apes and troglodytes

    Everyone insists on either closing their eyes to avoid seeing reality or justifying the unjustifiable or, on top of that, blaming the suffering user instead of the creator and the sociopaths who rule this world)
     
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  5. CarlosK2
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    CarlosK2 Senior Member

    Let's face it: how many yacht designers know how to calculate the Munk Moment of a yacht with paper and pencil?
     
  6. CarlosK2
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    CarlosK2 Senior Member

    IMG20241020135203.jpg

    The Munk Moment of a heeled yacht's Hull can be (in a crude aproximation) a figure as frightening as 2 Leeway/Yaw in radians multiplied by the dynamic pressure (q = 0.5 x Velocity^2 x Fluid density) times the Displaced Volume times 0.8

    And then we would have to add the keel, a keel that works forward (!) of the centre of gravity of the yacht and is therefore destabilising

    But if we add Pitch ... we have a case of "cross flow" (see the two books by Hörner, Fluid Dynamic Lift, Fluid Dynamic Drag) and the coefficient of "cross flow" according to Hörner can go from 1.1 ... to 1.8 (!)
     
  7. CarlosK2
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    CarlosK2 Senior Member

    IMG_20241020_151528.jpg

    (Pierre Gutelle, The Design of Sailing Yachts)

    The helm of a modern yacht is a loner facing a gigantic task, let's look at the list

    (1) The orbital current of the wave, current that flows in opposite directions in the trough and on the crest

    (2, 3, 4) A tremendous family of three variants of the Munk Moment: Hydrodynamic Yaw Moment of the Hull due to Leeway/Yaw, + Roll/Heel + Pitch

    Max Munk - Wikipedia https://en.m.wikipedia.org/wiki/Max_Munk

    (5) The destabilizing moment of a huge and efficient Keel that produces lateral force forward (!) of the center of gravity of the yacht

    (6) The aerodynamic moment of the sails, and

    (7) The effect of the keel on the rudder: the "downwash" of the keel on the rudder that should be called 'sidewash'
     
  8. Petr Podobsky
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    Petr Podobsky Junior Member

    Thanks for your interest. I will answer in order:

    1. Mast and rig seems to be in wery solid shape.

    2. Sails are almost new. Mainsail is carbon. Headsail is Dakron.

    3. On the water she is sittig correct.
    1. We used both only headsail and headsail and mainsail sailing downwind. I didn´t notice any significant difference.

    2. Wawes was both from one direction or from two directions. Of course wawes from two directions was more complicated to steer the boat.

    3. We experienced rough sea, about 2m+ short and steep wawes and strong winds. And also less hostile enviroment. Of course rough sea was worse.

    Our other “feelings“

    1. When the boat heels, the radius of the turn even diminishes.

    2. She is directionally unstable even when motoring on flat water. I mean she is not holding the course for few seconds, if the rudder wheel is free.
    She is simply too sensitive. I cannot imagine someone unexperienced to steer her. Maybe the surface of the rudder is too large. I think I have steered some balanced rudder boat but never experienced something like this.

    On the other hand. The steering wheel has too long run. (It took too long time to steer her from one direction to other) Steering wheel is not original. It was added by some previous owner. This we will fix easily.



    As we both can work with laminate, and boat will be soon of the water, it is relatively easy for us to add some stabilization fins.

    My friend tend to add some fin closer to the bow of the boat. We could also add some stabilization finn on the stern of the boat and there we can make it adjustable or removable when needed.

    I´m triyng to understand the above given formulas, but what is the conclusion? (it is hard to solve :))

    Some fin closer to the bow of the boat make sense for me in case of the wawe comming from the stern ...

    Other directions of sailing was ok.

    What would be your sollution?
     
  9. gonzo
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    gonzo Senior Member

    You are refering to two different issues.
    1. Directionalyl unstable while motoring. That has to do with wash of the propeller. The water is flowing on a spiral and is typical of balanced rudders behind a propeller.
    2. Directionally unstable downwind sailing downwind on large waves. That is typical of hulls that are fat in the middle and have fine entry. Slowing down the boat helps (reduce sails). Otherwise, change course to prevent going straight downwind and make to tacks to your destination.
    You describe an undesirable aspect that are inherent in the design. Also, a helm with too much reduction is making the correction slower. You have not indicated if it is hydraulic or mechanical. Either one can be modified to have less turns. Expecting the boat to stay on course with nobody at the helm is not realistic. However, there are methods to make a boat self-steer with the sails only. Downwind, use two headsails with spinnaker poles. They need to have the clew further forward than the tack. Upwind or in a reach, a small jib set from the topping lift to somewhere in the foredeck will also self -steer. You need to back it up. When the boat points up too high, the main and jib get backwinded and the small jib is square to the wind which causes the boat to steer down. When the boat points lower, the opposite happens. I sailed one of my boats alone from Nicaragua to eastern Colombia like that. I lost some speed though.
     
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  10. Petr Podobsky
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    Petr Podobsky Junior Member

    The helm is mechanical.

    1. unstable while motoring - some fixed fin in front of the rudder should help, right?
     
  11. bajansailor
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    bajansailor Marine Surveyor

    Maybe - however I used to sail on a friend's boat which had tiller steering, and the rudder was mounted on a substantial skeg.
    When she was under power, if you let go of the tiller she would immediately spin around violently, and while under way one could feel the side forces acting on the rudder from the propeller wash via the tiller.
     
  12. CT249
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    CT249 Senior Member

    What year/model?

    I haven't done much time on Jeanneau 36s but most of the earlier ones were OK downwind as far as I know. What was the nightmare? Did it want to round up (to windward), round down (gybe), or both? What similar designs have you sailed and what is your sailing background? What sails?

    My own boat (same hull as the J/35) is not too dissimilar to the 1994 Sun Fast 36 designed by Briand and while it wanders back and forth when fast running, it's not a nightmare. However, if the sails were wrongly trimmed or I was trying to hold a very precise course, it would be a handful.

    In general Briand boats were known as being at their best downwind in strong winds so he knew how to design a boat that handled well. It would be best to look at the issue very deeply from all angles before spending money.
     
  13. CT249
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    CT249 Senior Member

    Interesting. I switched my old boat from skeg-hung rudder to spade and suffered the same issue under power. I assumed it was caused by the shape of the spade rudder but if the same thing happens to a skeg-hung rudder I must be wrong.
     
  14. gonzo
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    gonzo Senior Member

    It is the way the flow of the water affects the rudder. The effect is much less in full keel boats with an attached rudder.
     

  15. tane
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    tane Senior Member

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