464 w/ and w/o foils — By Capt. Bill Pike —
Cuttin’ Edge Cat
|Can a foil boat go faster? We run the numbers on a couple of multihulls to find out.|
I’m a sucker for this kind of thing, really: Two midsize powercats, on loan to PMY from NauticBlue Power Yacht Vacations, an offshoot of The Moorings, the worldwide charter outfit. A testing venue shimmering with scenic beauty. And finally, an immense and groovy pile of test gear, complete with freshly calibrated fuel-flow-measuring equipment and enough compression-type fittings to stock a diesel-repair shop.
The objective was simple—to scientifically quantify performance differences between two NauticBlue 464s, one with two big, stainless steel hydrofoils (see illustration below), and one without. Uniformity was key, of course. So both boats had to have the same engines—twin 370-hp Yanmar 6LYA-STP diesels—and the same water and fuel levels (full) and be judged under the same sea conditions—one-footers, more or less—on the same stretch of water: Road Harbor, Tortola, British Virgin Islands. Prop diameter and pitch were the only parameters that were out of synch. While my conventional 464 had two 22x31 four-blade wheels, the hydrofoil-equipped model sported oomphier 23x33 1/2 props, presumably to both generate and harmonize with improved speed and performance.
One other detail that’s worth mentioning up front: The 464 with foils had a semiadjustable feature, meaning that while her forward foil was fixed, her aft one was movable. More specifically, her stern foil could be hydraulically trimmed down from the horizontal by approximately 17 degrees for an optimized angle of attack via a toggle switch at the upper helm station. Before departing for sea trials from NauticBlue’s facility on the shores of Road Harbor, I dove on the hydrofoil 464 with snorkeling gear to examine the stern foil, noting that its ends were engineered to pivot between the boat’s demihulls via a simple trim-tab actuator, a pulley system, and a length of cord. Not fancy perhaps, but simple and effective.
It was a gorgeous day on Road Harbor. While I noted no obvious handling differences between the two test boats in open water, the data I recorded was dramatic and thoroughly validated claims made by the originator of the foil system on the foil-equipped boat, South African mechanical engineer Karl Gunter Hoppe. The boat exceeded the speed of her more conventional sistership by Hoppe’s predicted 40 percent and then went on to trounce the poor thing by a whopping 47 percent. Check it out (see data boxes and acceleration curves, this story)! The hydrofoil 464 achieved 31.5 mph at WOT, then upped the ante to 33.3 mph with the foils fine-tuned. The conventional cat did 22.6 mph, tops.
This article originally appeared in the May 2004 issue of Power & Motoryacht magazine.