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HullFeatures

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Roll Acceleration
== Roll Acceleration ==
Roll acceleration is the force of gravity (G force) you experience during a roll. High rates of acceleration are very uncomfortable, stress the body, and make it impossible to sleep. Marchaj <ref> Marchaj, Seaworthiness, The Forgotten Factor, chapter 4, "Boat Motions in a Seaway"</ref> has proposed four physiological states: Imperceptible, Tolerable, Threshold of Malaise, and Intolerable. Malaise starts at 0.1 G, Intolerable starts at 0.18 G.
 
== Roll Damping Systems ==
Roll-damping systems, as the name implies, are designed to reduce the roll of a vessel. Reducing roll increases comfort. Roll-damping systems are passive or active, and can be internal or external. The main types are:
* Bilge Keels
* Active Stabilizers
* Ballast Stabilizers
* Paravanes
* 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%.<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.
=== Ballast Stabilizers ===
Ballast stabilizers were once common only on large cruise ships but have begun appearing in European yachts and a few large trawlers like Cape Horn. A ballast stabilizer consists of two interconnected water tanks, one on either side of the centreline. As the boat heels a pump transfers water rapidly between tanks to counterbalance the rolling motion. A variation on this theme is to use sliding weights.
 
Obviously the pumping systems should have excellent redundancy. You wouldn’t want water ballast on the wrong side of the boat at the wrong time.
=== Paravanes & Flopper Stoppers ===
A discussion of roll damping would be incomplete without mentioning [https://en.wikipedia.org/wiki/Paravane_%28weapon%29 paravanes] and flopper stoppers. Paravanes are long poles extended horizontally from the sides of a trawler, with winged paravanes that reduce the boat’s rolling inertia when underway.
 
Flopper stoppers are similar, but with flotation devices on the ends for use at rest. For either, the supporting mast structure raises the centre of gravity, which decreases ultimate stability.<ref>The Use of Roll Damping Paravane Systems (Paravane Stabilizers), Transport Canada, http://www.tc.gc.ca/marinesafety/bulletins/2000/15_e.htm/</ref>
 
Paravanes work the same way a high-wire performer uses a balance pole, or you use your arms when play walking down a beam or curb. They originated on fishing trawlers, which anyway have booms to set and raise nets. They can be very effective.
 
At other times, the booms can bounce, or even smack the boat, so people weigh them down with chain. They’re also cumbersome to set and raise. If a boom is lost in bad weather, the boat can capsize from the imbalance. You might want to consider them only if you’re converting a fishing boat that was designed for them.
== Maximum Hull Speed ==