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m
→Bilge Keels
* Flopper Stoppers
=== Bilge Keels ===
Bilge keels are a type of fin attached to the chines of the hull. They serve as passive roll stabilizers, by offering resistance to the water when the trawler rolls. They should be located as far aft as possible, to reduce roll and improve stability.<ref>Cape Horn Yachts, http://capehornyachts.com/</ref> Long low-aspect keels can reduce rolling by 35-55% [43]. <ref>Marine Technology, see http://www.kastenmarine.com/roll_attenuation.htm/</ref> (Aspect ratio is the ratio of width to height, e.g., 4:3 is 4 units wide by 3 units high.)
Bilge keels can be made strong enough to support the hull and keep the boat upright when it is accidentally or deliberately grounded. To support grounding, both the keel and bilge keels are engineered three to four times stronger than ABC requires. Failing to engineer the keels adequately can cause bilge plates to crack.<ref>Ship Construction, D. J. Eyres, Butterworth-Heinemann, ISBN 07506807092007</ref> Such keels will offer some protection to accessories attached to the main keel. Bilge keels can also be designed as short angled fins.
The downside of bilge keels is that they increase drag slightly. Hopefully the extra drag will be offset by the performance of a bulbous bow.
=== Active Stabilizers ===
Active stabilizers are another type of roll-damping fin. They have electric or hydraulic motors so that their angle of attack in the water can be adjusted dynamically, a little bit like wing flaps on an airplane. Electro-mechanical sensors and a control system make automatic adjustments to the fins. Actuators can be electric or hydraulic. The plates on the hull must be strengthened where the stabilizers are attached. As mentioned before, active stabilizers are more effective on a round bilge hull than on a hard chine hull. They should be located close to the pivot point of the hull, typically just aft of the maximum beam.