Page 46: of Maritime Reporter Magazine (November 2002)

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 *

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