possible to decrease rolling motion?

Discussion in 'Stability' started by kapnD, Sep 25, 2017.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    I think most people prefer a more snappy motion, as being less sickness-inducing.
     
  2. PAR
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    PAR Yacht Designer/Builder

    A hull like that will always back like crap (if at all) on a single, straight shaft. She looks to shallow for a conventional bow thruster, though a "drop down" style might be fitted. This would help backing and generally maneuverability too, particularly in tight quarters. The rolling issue will likely be a bit of experimentation, before you find one or more approaches that work well.

    I didn't attend EHS, getting here relatively recently (couple of decades back). It must have been quite a shock to you, coming back after all those years. Some things don't change though, as most of the local women still have most of their teeth and the good 'ol boys network is in full swing. Don, next time in town, stop by for a beer.
     
  3. kapnD
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    kapnD Senior Member

    The motion of the boat doesn't bother me in the least, but some passengers have complained bitterly. (I've never been seasick, but it sure looks miserable!)

    Yes, I have contemplated thrusters, but haven't found one that fits my budget, but -- thread drift...
    There are dozens of discarded jet skis in my neighborhood, I have entertained dreams of mounting a salvaged motor/pump in the engine room with the discharge piped through two big thru-hulls near the bow.
    One electrically controlled Y valve to direct the thrust?
    Too bad there aren't any diesel powered jet skis!
    PAR,
    The changes in Eustis are purely physical, the undercurrent remains the same as it ever was. Must be a lot of good dentists there.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Yeah, Eustis was an eye opener for me. I moved up from Marco Island, after some years in St. Pete and settled initially in Mt Dora. Mt Dora was nice back then. I got an apartment between 4th and 5th on Donnelly and had three bars within 100 yards, to stagger to. Back then in the summer, you could lay down on Donnelly St. and not get run over by a car for at least a 1/2 hour. Then it was listed as the best small town in America to live, two years in a row. The population jumped from 3,000 to 10,000 in a few years and it's still bursting at her seams. Taxes sucked, so I found a nice hunk of woods on unincorporated county land. Literally a 5:1 difference in taxes. My neighbor has multiple stomachs.
     
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  5. rxcomposite
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    rxcomposite Senior Member

    I have been a merchant mariner in my early days and most of us would swear we like the gentler rolling motion that than a snappy one. Our ship carries ore or logs. A snappy one would feel like having your head in a vice grip and unexpected slamming into the walls. A rolling causes a light headedness, something like a prolong roller coaster ride. In both cases however, it feels like somebody is gripping your throat.:eek:
     
  6. rwatson
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    rwatson Senior Member

    You could always go for small stabilizers.

    They could take the form of sealed daggerboards, perhaps at 45. that can be deployed partially or fully on demand. You could even have the case partially extending past the hull line, as a streamlined extension to provide a bit of permanent directional stability.

    As an engineering exercise, it would sure beat trying to re-position the motor.
     
  7. gonzo
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    gonzo Senior Member

    A traditional solution for backing up is the addition of two little rudders on each side and a bit forward of the propeller.
     
  8. kapnD
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    kapnD Senior Member

    Gonzo, if you blow up the photo in post#4 you can see a small cylinder ahead of the propeller and above the shaft.
    This is original equipment on the boat, it's some kind of a "reversing rudder".
    It is linked to the rudder, and swings in the same direction as the rudder, but I doubt that it has much effect.
    I am going to remove it on next haulout. The boat already misbehaves badly in reverse, and I don't think it can get much worse without that little complication dragging along, collecting barnacles and disturbing the flow over the propeller when moving forward.
    Rwatson, that is a very cool idea!
    The boat already looks kind of like a whale, so it is fitting that it should have pectoral fins.
     
  9. Dolfiman
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    Dolfiman Senior Member

  10. PAR
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    PAR Yacht Designer/Builder

    There's basically three stabilizer routines; fins, gyro and Magnus effect. Fins and retractable setups tend to work better on displacement speed craft, mostly because of drag, gyros work better on faster vessels.
     
  11. rwatson
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    rwatson Senior Member

    That rotor solution is a new one to me

    "Koop explains that this is one of the huge benefits of this Magnus RotorSwing system. Fin yacht stabilizers will not provide any significant roll damping at slower speeds. The Magnus system however works highly efficient from 3 knots all the way till 12 knots. From that speed and up, the Rake function will take care of higher speeds."

    Stabil.png
     
  12. Barry
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    Barry Senior Member

    Or a set of paravanes, makes the boat look "fishy" but effective at slow speeds, some drag cost
     
  13. Dolfiman
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    Dolfiman Senior Member

  14. kapnD
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    kapnD Senior Member

    Rotor Swing is certainly impressive, but I’m not even going to look at the cost of the system!
    Also looks to be a major PITA to keep clean from marine growth.
    Seeing the video where the vanes become exposed suggests that they are poorly placed, at least on that particular installation, I have seen birds pop out from too shallow rigging, and the resulting slew was pretty scary.
    Without any refined engineering skills, my gut says that the rotating vanes would do well if mounted directly under the keel, pointing straight down at maximum distance from the axis of roll rotation.
    I will continue to investigate more passive solutions, and have acquired a ton of lead ingots to be placed strategically.
    I just re-read Michael Kastens publications re roll damping, and think I’ll start by placing the lead along the sides at the widest part of the hull, then move it to the center for comparison.
    I have tried placing the lead across the transom (on deck, where it landed when first loaded), it only decreased the waterline by a couple of inches, and has had very little effect on roll, or anything else for that matter.
    I may be concluding soon that the vessel is vastly under loaded, and while it is not creating a particularly dangerous situation, the light condition may be a big contributing factor to the rolly motion in quartering seas.
     

  15. PAR
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    PAR Yacht Designer/Builder

    Magnus effect devices have to rotate horizontally to effect roll. They also have to project away from the centerline to be effective and the farther from the centerline they can be mounted the more effective they become.

    Weight, to be effective enhancing the righting arm (stability), needs to be as low as practical and on the centerline, assuming it's fixed. Now, if you had a sled, that could be dragged from one side of the bilge to the other, this weight could be made more effective, but you'd have to shift this weight on each tack or jibe.

    If this was my boat, I'd fill all the tanks and load her up with the usual thing you'd have aboard, including a well fed crew. How does she sit on her LWL, does the roll seem better or worse? I think you may be correct in that she's way under loaded. That hull shape makes a fair bit of sense to me and she looks like she would have a fairly slow roll moment, which is desirable and she also looks like she can load down uniformly (also a good thing) while maintaining this reasonable roll attribute. This is common among well thought out working sea boats. They need to drive light and loaded and get home, so a level of moderation in most regards, is how the hull ends up. Yeah, not as fast as she could be, not as stiff, maybe not even with as big a load as similar other designs, but she'll bring your soggy butt home in a nasty slosh, maybe a little more motion sickness than other boats, but you're home, safe with a full load, while the other more stiff and faster boats are still on their way in, hoping they don't capsize before passing the breakwater.
     
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