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Old 03-11-2013, 08:30 PM   #227
Marin
Scraping Paint
 
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Join Date: Oct 2007
Posts: 13,745
Passive stabilizers are mechanical devices, usually "birds" or "fish" that are hung down into the water from heavily constructed outriggers that hinge out from the side of the boat. Much like the trolling poles on a salmon troller.

The "birds" are finned weights that are suspended a few feet below the surface and "swim" alongside the boat. Because of their shape, they resist the rolling movement of the boat. They are passive because they don't do anything other than hang down from the outriggers.

Active stabilizers are just that. They are usually in the form of powered fins attached to the bottom of the boat. Electrically or hydraulically driven, they (usually) have a gyro mechanism that senses the boat's rolling and pitching movement and this information in turn directs the fins to move to counteract it. So if the boat rolls to the left the fins will move to counter the roll.

Active stabilizer systems are used on boats as small as a Grand Banks 42 up to vessels as large as a 1000' cruise ship.

The advantage of passive stabilizers is they are very simple. No motors or computers or gyro sensors, etc. The advantage of active stabilizers is they actively and rapidly counter the movement of the boat and so tend to provide a smoother ride than passive stabilizers.

The disadvantage of passive stabilizers is they require a properly designed, heavy-duty structure to support them and take the high strain of resistance from the birds in the water. The disadvantage of active stabilizers is the cost, the complexity of the system, their vulnerability to being hit by debris in the water or obstructions in shallow water, and the added service and maintenance requirements imposed on the boat owner.

The first photo below shows a pair of stabilizer "birds" on the Nordhavn across from our slip in Bellingham. The second photo is of the same boat showing the heavy outriggers or stabilizer poles that are hinged outboard and suspend the heavy birds in the water.

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