BOATKEEPER

Rolling Chocks May Give Comfort—But Not Stability From Pacific Fishing, March 1998 By Terry Johnson, University of Alaska Sea Grant, Marine Advisory Program 4014 Lake Street, Suite 201B, Homer, AK 99603, (907) 235-5643, email: [email protected]

o paraphrase Introduction to Naval Ar- effect. As a boat rolls, the chine area swings positioned they work quite well at , Tchitecture, it has been the design- in a large arc, passing quickly through a and even better underway. er’s unhappy lot through the centuries to volume of water. The bilge interrupts There is a drag penalty for chocks, and try to devise ways of damping the objec- that flow of water around the chine, and a it’s there whether you need the roll control tionable roll of vessels in a seaway. low-pressure eddy behind the fin absorbs or not. But the increase in fuel consump- Modern technology has provided a energy of motion. The damping effect is tion is estimated at less than 10%. range of roll-damping options, including velocity-sensitive, like a shock absorber; How to position them in the streamline? active and passive ballast tanks and gyro- the faster the boat tries to roll, the stronger Well, you can pay a naval architect a couple activated fins, but most fishing vessels still the damping effect. Hard-chine boats and grand to study the and draw up a set of rely on one of three traditional, passive, others with high initial stability benefit less plans, or you can eyeball it. Most people, low-tech roll-stabilizing systems. from bilge than do round or soft chine including some successful boatyards, just Steadying sails have long been used on hulls, because the harshness of the hard eyeball it. motorized fishing vessels all over the world. chine edge creates its own turbulence and The fins should taper at both ends (rather “Flopperstoppers” or paravane stabilizers, damping effect. than start and end with sharp corners) and familiar to most West Coast fishermen, should have a rounded or flat outer edge work well underway and fairly well at an- bviously, the greater the total area of rather than a knife edge, to avoid damage chor, but are noisy, difficult, and sometimes Othe keels the more roll resistance they to and by fishing gear and other obstacles dangerous to handle, and may be impracti- provide. They can be made fairly long— in the water. On metal boats the keels are cal around nets or lines in the water. half the length of the boat or more, depend- essentially just metal plate edge-welded to Bilge keels, or “rolling chocks,” the ing on the curve of the hull. However, they a sacrificial doubler plate welded to the hull, long, rigid fins that protrude from the chines should extend forward only to the point with pipe or half-round welded to the outer of most and larger fishing vessels, are where they start converging due the curve edge for safety and rigidity, and possibly increasingly being designed or retrofitted of the ; any more and they would add with a few braces or stiffeners. On fiber- onto smaller vessels. Effectiveness varies, drag when underway. Besides, they would glass hulls, the yard can use a molded piece, depending on design and hull type; but a be ineffective if they were to come out of shaped in cross section like a V, which is growing number of owners are specifying the water when the boat pitches. laminated to the outside of the hull. them on the understanding that they can Commonly they are only eight inches make a significant improvement in a boat’s or so in width to avoid extending beyond he layup is about five inches wide at the comfort level. the “envelope” of the hull, where they could Tbase, tapering to two inches at the outer How much improvement? It’s hard to be damaged by a trailer, carriage, or at the edge. Or, a foam board mandrel can be quantify. Little controlled testing of bilge dock. If the keel is positioned down at the glued to the hull, which is then encased and keel effectiveness has been done, and re- turn of the bilge, and is placed perpendicu- laminated to the hull with multiple layers sults have not been published outside spe- lar to the surface of the hull at that point, it of matt, cloth, and resin. Some yards that cialized engineering journals. I surveyed can be made longer without extending far use hollow molded pieces fill the interior designers and builders on both coasts and enough to attract damage. Some yards put with liquid resin, which hardens and pro- none could refer me to published perfor- bilge keels as wide as 16 to 18 inches on a vides a stiffer, stronger fin. mance data. Some naval architects don’t 60-foot hull. The keels should be attached only to the believe they are very effective at all, or cite The trick is to place the keels such that exterior surface of the hull, not through- roll magnitude reduction of only 10%, but they are within the water flow streamlines bolted, so that they will collapse or shear other designers and builders report reduc- of the hull so they produce minimal drag. off without opening the hull if they hit tion of 30% to 50%. Some boatyards are This can be a problem if the boat operates something. doing a brisk business retrofitting bilge over a wide range of speeds, as do most semi- Because bilge keels or rolling chocks keels for owners who have observed im- displacement hulls, because the streamlines reduce the roll amplitude, it’s tempting to proved characteristics on other vessels. change with speed. Some engineers place think that they increase vessel stability. Not Success seems to depend on at least rolling chocks high on the chines to produce true. They make a boat more comfortable, three factors: the characteristics of the hull, a little lift, and they claim they work best but not safer. If your boat seems excessively the size of the keels, and their exact posi- when underway. But the more widely held tender, chocks could make it more sea- tioning. view seems to be that they should be posi- kindly, but you should spring for a stability Bilge keels work primarily by absorb- tioned lower and at a 45° downward angle, test to ensure that it won’t some day roll all ing roll energy through the viscous-eddy which precludes any lift effect. If correctly the way over. ✦