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Author Topic: Rocking and Rolling - Bilge Keels  (Read 262 times)

GG

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Rocking and Rolling - Bilge Keels
« on: October 15, 2019, 01:28:09 PM »

Having just spent some time carefully ballasting the latest model, it was "tender" to start with but now definitely knows which way up to float, I was reminded of something that happened some years ago.  It was something of a surprise to hear that a modeller who had built a destroyer from one of my plans reported that it was unstable.  This worried me and the prototype was quickly refitted and retested.  No instability could be detected but it was clear that the model would noticeably heel when the rudder was applied.  Suddenly removing the rudders deflection and the model immediately rolled upright but oscillated a few times before settling down. Was this the instability the modeller had reported?


This led to me carrying a few heeling trials in the bath, when Mrs Guest was otherwise engaged of course!  The models were pushed down on one side until the edge of the deck was at the water level and then released.  Whilst all the models (thankfully) sprang back upright, some came to rest with the briefest of wobbles whilst others overshot and oscillated through a series of diminishing cycles.  The first was an example of a heavily damped oscillation in which the energy in the bodies motion is quickly dissipated.  The second is a lightly damped one where the energy cycles from potential to kinetic back to potential and so on, with little loss of energy in each cycle, Fig 1.

[/size]It was obvious that boxy hull sections favored a damped oscillation whilst more rounded sections displayed less damping.  The reason for this is easily understood if you consider trying to rotate rods in water, one with a round and the other a square section, Fig 2.  The round section will turn easily can create little disturbance in the water.  The corners of the square section would churn up the water and oppose the rotation.Full size vessels often incorporate Bilge Keels to dampen their rolling motion.  I'll confess that I rarely fit them into my models because they can be vulnerable to damage and a large positive Metacentric Height is favored which makes them more reluctant to roll anyway.  But, in this case it seemed worthwhile to experiment and see what effect adding bilge keels to the destroyer that started this work might achieve.The keels were just "L" shaped plastic strips held onto the hull with adhesive tape, not elegant but good enough to show any effects, Fig 3.  Sailing with the keels in place showed that the model would still roll by the same amount as before when the rudder was applied.  On returning to a straight course, the model would return to an upright attitude but now with little oscillation being observable. Removing the bilge keels and sailing the model again and when straightening out of a turn the model would oscillate a few times. But, this "overshoot" when rolling back upright was only obvious when the rudder movement was sudden.[size=78%]
[/size]You sometimes hear the comment that "bilge keels make a model more stable" but this is not true.  If you have a model that will roll over when the angle of heel exceeds say 45 degrees, then, bilge keels or no, exceed 45 degree and it will still roll over.  Yes, bilge keels slow the response to heeling disturbances but this ought not to be confused with a models stability.  This work reinforced my preference for giving models large Metacentric Heights.  Some people avoid this on the grounds that the model becomes "Stiff", that is it will spring back rapidly when heeled and become uncomfortable for passengers.  This is true but in my experience a "Stiff" model is harder to start heeling in the first place plus safer in the event of sailing accidents and I have yet to consider passenger comfort when sailing my models.     [size=78%]
[/size]The other thing learnt was that rolling can be reduced by applying rudder movements in a restrained fashion. After all we now have proportional control with RC gear and not the "all or nothing" operation of older gear.[size=78%]
[/size]But, for scale fidelity, you still need to add bilge keels![size=78%]

[/size]Glynn Guest[size=78%]
[/size]Glynn Guest[size=78%]
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