Improving IOR Stability?

Discussion in 'Sailboats' started by jakmang, Mar 6, 2013.

  1. Paul B

    Paul B Previous Member

    Well, you have not provided one single photo to back up your assertions. The photos that have been posted seem to indicate you are wrong. So please, post a few images of J24s and IOR boats with their bows and decks underwater, beginning to broach.

    Photos don't lie, but some people do...
  2. Paul B

    Paul B Previous Member

    There's the rub. He used the J24 because he thought that proved his point. He thought a J24 was an IOR design.

    I have no idea why some people have the need to post about things they have no experience with. I suppose they think it makes them seem like experts. It is just the opposite.
  3. Paul B

    Paul B Previous Member

    IOR raceboats moved from the cruiser/racer ideal toward full racing because they were racing, and winning is the goal of most people who race.

    The biggest change was in sail material. Most available sail materials up until we had film sails had too much stretch. So the more you could bend your mast, the more you could change the shape of the main to keep up with the stretch. The bigger the main, the more stretch, and this was a problem. So until the sail materials caught up, having a big fractional main was difficult to have "right" across a wide range.

    In something like the 12s they were daysailing boats and had a quiver of mains. They would get the daily weather report and choose the correct main for the condition. Hard to do in an ocean race.

    In the early days of the IOR (early 70s) sparmaking was pretty crude. As the technology progressed the spars became more adjustable and easier to adjust. Masts went from being "telephone poles" to very flexible and controllable.

    As the rigs moved to fractional from masthead jumpers were used. This allowed the top of the rig above the hounds to become quite small. To keep the top of the mast from falling off we used jumpers with an included angle of about 120 degrees or so.

    These jumpers were not there so much to keep the headstay tight. The headstay was tensioned by the running backs. They controlled the athwartships bend of the spar, and also kept the top of the mast from overbending when backstay was applied.

    As sail materials have continued to improve we now don't need our masts to bend as much as they had to in the past. This has simplified things a lot.
  4. michael pierzga
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    michael pierzga Senior Member

    modern boats, like the TP52, have very simple masts.... running back stays are prohibited for simplicity , head stay tension via aft swept spreaders and mast bend permanent backstays.
  5. gonzo
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    gonzo Senior Member

  6. Petros
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    Petros Senior Member

    looks to me you would solve the broaching by installing two rudders, at the turn of the bilge, at about 40 deg angle outward. even if you bury the bow this would prevent the hull from broaching.

    It appears to me that the rudder comes out of the water when it is heeled over, the big beam lifts the center line rudder out of the water. Burying the bow makes it worse, but you do not need the bow buried to get it to broach. You lose directional stablity as soon as the hull heels and the rudder comes out of the water.

    A much deeper single rudder would help, but two rudders angled outward should stop the broach. I would also still install the side strakes, which will also improve directional stability with minimal changes to the hull.
  7. mudsailor
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    mudsailor Junior Member

    Unless you wan to race the boat hard I would not change a thing....Under 99% of the conditions you will normally sail in there will be no issue, for the remaining 1%, head up 10 deg or drop the kite, either will work.
  8. MikeJohns
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    MikeJohns Senior Member

    No ones mentioned the flow field.

    Of course there’s a sequence of events all interrelated. The boat is traveling fast ‘pulling’ up a big stern quarter wave and altering the flow over the rudder in the process. Rudders are normally very efficient foils with high turning moments but the hull itself can make the rudder ineffective.

    The effectiveness for any reasonable rudder design depends foremost on the flow across it. The hullshape of a sailboat and particularly it’s quarter wave characteristics (and related 3d flow field) are of particular interest when it comes to rudders.
    Detrimental heeled volume distribution and the resulting flow field are significant in loss of rudder turning moment. You have to consider what 3D flow the rudder actually operates in. As velocity increases the quarter wave grows and the inflow vector for the rudder shifts considerably. This is the water moving back into the space as the hull moves through the water.

    As Paul said early on the best fix from a hydrodynamic perspective is to change the hullshape.
    Moving the rudder aft as far as possible (even off the transom) or adding a skeg might help but you’d want a good study of the flow field, or a model test, or find someone who successfully modified a similar boat and copy their design.
  9. sean9c
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    sean9c Senior Member

    Very enlightening crayon drawings of different configurations. I'm assuming you are using the authors description of drawing B. Unfortunately it's not an accurate definition. The correct definition would be a full keel with a cut away forefoot.
    Anyway, knock a boat with that keel/rudder down flat and it's rudder would be out of the water.

  10. gonzo
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    gonzo Senior Member

    They are the original designs called "finkeels". Any boat will reach a angle of heel when the keel is out of the water. However, these finkeels maintain steering at a higher angle of heel and bow down attitude.
  11. sean9c
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    sean9c Senior Member

    Ya notice how far off topic you've drug this thread?

  12. Paul B

    Paul B Previous Member

    How do you think this would sail at the normal 15 degrees of heel?
  13. gonzo
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    gonzo Senior Member

    Twin rudders work well in scows and hulls with really wide sterns. They would probably need some kind of external structure to be fitted on a pinched stern hull.
  14. Paul B

    Paul B Previous Member

    Sadly we have participated in this, trying to prevent disinformation. You can lead a horse to water...

    Hopefully the OP got something useful out of this.

    Attached is one of my favorite photos from the IOR era showing the classic round down. Note the bow NOT under water, and the rudder out far enough that it is useless. Boom held by the runners, pinning the boat down.

    This is the mode that most often caused gravity storms in those days. I heard this boat somehow did not blow the rig out, but did need to replace the kinked rig afterward.

    Attached Files:

  15. jakmang
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    jakmang Junior Member

    I have learned a lot in a few days. You all have reinforced my thoughts that I should probably not ruin what is still a nice little boat. I will look for something more suitable for my purposes.

    Thanks to all for the information.

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