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Old 06-28-2018, 05:57 PM   #21
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For whatever reason, Nordhavns seem to be very rolly boats, at least judging from how much more they rock around in a slip than nearby boats do. I hvae been told that Norhavn explains this by saying that they have designed the boats to have a "resonant frequency" that exposes the boats to excessive rolling in calm conditions, while correspondingly diminishing the rolling in heavier seas. I am not sure that is even possible, but if true, it may be that use even in the ICW would benefit from stabilizers.

OK! here goes my controversial posting that might shock some:


Simple hydrodynamics lesson: to be stable, floating object waterline need to stay parallel to the water.
With other words - rolly boat is trying to stay parallel to the water when even slightest wave passes underneath the boat. The rolly boat has more dynamic stability than the boat that do not roll. Described above is only one of the many factors that influences boat stability, so please let's not argue about it. Some might not agree, but this is the principle and if you don't believe me, please read your physics textbooks from high-school or many books by nautical engineers on boat stability.
While having rolly and snappy boat helps stability, it does not help the human comfort. The opposite, one feels quite bad being thrown around an quick rolling snappy boat.

The active fin stabilization systems usually use predictive algorithms to try to prevent the boat from starting to roll. Once the boat is rolling there is very little they can do for stopping the momentum. the best effect will be just slowing down the roll period to acceptable (for the humans on the boat) level.

Any respectable Active fin manufacturer will NOT claim that their system improves boat stability.... because it does NOT. It helps "poor" people inside feel better - and this is the real benefit from having active fin stabilization system.

And remember, almost any boat will be able to handle much more than it's occupants.

In conclusion: the easier the boat rolls the easier will be for stabilizer fins to return the boat in the desired horizontal position but not parallel to the water underneath.
cheers!
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Old 06-28-2018, 06:48 PM   #22
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....If you stay in calm water, why buy a Nordhavn?....
Because they are really really cool!!
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Old 06-28-2018, 08:01 PM   #23
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In conclusion: the easier the boat rolls the easier will be for stabilizer fins to return the boat in the desired horizontal position but not parallel to the water underneath.
cheers!
So, rolly boats roll more in relatively calm conditions (like ICW) and would therefore benefit more from stabilization in those conditions but the stabilizers wouldn't have to work too hard?
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Old 06-29-2018, 02:40 PM   #24
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So, rolly boats roll more in relatively calm conditions (like ICW) and would therefore benefit more from stabilization in those conditions but the stabilizers wouldn't have to work too hard?

Not really! That's the beauty of the of the active fin systems. They work only when they have to. If it takes very little to roll the boat, it takes very little to prevent the roll. Usually the response of an active system is controlled by GAIN setting and the respond will be proportional to the trigger. All Stabilization systems that I know of have something called proportional–integral–derivative controller PID. More info here: https://en.wikipedia.org/wiki/PID_controller. The autopilots employ the same principal.

So the short answer is No, the stabilization does not work any harder as as I mentioned before. It tries to predict the roll and stop it before it occurs. The deflection of the fins will be dependent on few internal sensors and the settings of the parameters for the PID control
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Old 06-29-2018, 02:49 PM   #25
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Not really! That's the beauty of the of the active fin systems. They work only when they have to. If it takes very little to roll the boat, it takes very little to prevent the roll. Usually the response of an active system is controlled by GAIN setting and the respond will be proportional to the trigger. All Stabilization systems that I know of have something called proportional–integral–derivative controller PID. More info here: https://en.wikipedia.org/wiki/PID_controller. The autopilots employ the same principal.

So the short answer is No, the stabilization does not work any harder as as I mentioned before. It tries to predict the roll and stop it before it occurs. The deflection of the fins will be dependent on few internal sensors and the settings of the parameters for the PID control
I think you misread my question. I suspected that the stabilizer would NOT have to work as hard on a rolly boat (which you are now confirming), and I asked whether a rolly boat would not be more in need of stabilization in light condtions such as ICW, which based on your prior comments it seems it would.
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Old 07-02-2018, 01:42 PM   #26
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I also have a rolly, round bilge hull that has great stability but in certain conditions gets really rocking and rolling. Messy inland waves aren’t a problem as they usually do not have a regular frequency.

The rolling is at its worst when there is a steady swell with a frequency that matches the boat’s natural roll frequency.
It’s a bit like pushing someone one a swing. If you push at random intervals you won’t gain a lot of momentum. If your pushing is in time with the swings frequency, a lot of movement is generated.
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