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Workboat Annual According to Gruzling, the Hydralift skegs work in a number of ways. They have a much higher aspect ratio than regular skegs, which makes them more efficient foils; that is they have a higher lift to drag ratio than regular foils. Although the horizontal foil was origi- nally put there to increase the strength of the skegs, it also generates lift from the water flowing up the rake. The direction of this lift is not only up, but forwards as well, so it actually provides some thrust. Gruzling says that the vertical foils interact with vortices formed as the water fills in from the sides, also reduc- ing the drag . There seems to be both scientific and anecdotal evidence that the Hydralift skegs work. Gerry Stensgaard, man- ager of the Ocean Engineering Center at B.C. Research, Inc., a towing tank in Vancouver, Canada, says that when they test barges, they find that normal skegs typically add 30 to 50 percent more resistance over the bare hull drag. From a number of tests, they have found that the Hydralift skegs add 10 percent or less resistance, and theoretically could reduce the overall resistance to below the bare hull resistance. He gives some specific examples: one barge design, in which regular skegs increase the resis- tance by 23 percent, Hydralifts increase it by only four percent. He says that these model test results would suggest a speed. Another example Stensgaard gives is a barge where normal skegs would increase the resistance by 50 per- cent, the Hydralift resistance reduction Invented by Joe Gruzling, President of Nautican Research more than 20 years ago, Hydralift skegs add 10 percent or less resistance. three quarter to one knot speed increase for the same horsepower. Gruzling states says the other way to look at this is the tests predict a 20 percent saving in horsepower for towing at the same translates into about a two knot increase for the same horsepower. In the real world, Bob Shrewsbury, President of Western Towboat, says that they can get at least a half knot increase in speed, though they "typically reduce power and tow at the same speed". Interestingly, it turns out that one class of barge that Western Towboat tows for ARM is the first example that Stens- gaard gave, where he said the results predicted a three-quarter to one knot increase. John Armstrong, VP of Marine Operations at Seaspan Interna- tional, says that they enhance the sea speed, increasing it by one quarter to half a knot. Gordon Taylor, Operations Manager for Dunlap Towing, also says that their "tow speed is a little better, typically half a knot." Others report that one can back off the throttle by 10 per- cent to make the same speed, which, using a standard propeller curve, com- pares closely with the tow tank results of a 20 percent horsepower savings. These independent reports seem to give a con- sistent comparison between real life and theoretical results, that is within engi- neering tolerance. The story that Shrewsbury feels really illustrates the performance of the Hydralift skegs involves two Western Towboat tugs: the Western Titan and the Gulf Titan which happened to be side by side in Principe Channel, a straight off the B.C. coast, each with a barge under * Take a closer look inside our new tank gauging system, Saab TankRadar® STaR. It will bring intelligence and integration to your tank monitoring—offering a superior range of functions. The new STaR system features a 3-in-1 solution that integrates Level Gauging, High Level and Overfill Alarm systems in a single unit—designed as independent, intrinsically safe, measuring channels. As a result, it's easy to install, maintain and operate. STaR takes advantage of a new sensor technology for safer and more reliable loading and discharge. And it's future-proof—the integration capabilities of STaR ensure compatibility with new and emerging software. STaR is the latest proof of our technology leadership. No wonder that nearly 60% of all the new tank gauging installations in the marine market are designed and delivered by us. EUQROSEMOUNT Phone: +46 31 3370 000 • E-mail: sales.marine@marine.saab.se Web: www.saabrosemount.com SHAPING THE FUTURE IN TANK GAUCINC Circle 305 on Reader Service Card or visit www.maritimereporterinfo.com 46 Maritime Reporter & Engineering News
