Reprinted from SailTech-89, Proceedings of the Eighteenth Annual Conference on Sailing Technology, Volume 35, pages 29-39. The conference was sponsored by the AIAA and SNAME, and held at Stanford University, October 14-15, 1989. In prior years, these conferences were entitled The Ancient Interface. (Please note a new address for David A. Keiper: 123 South Pacific Street, Cape Girardeau, Missouri 63701.) HYDROFOIL CRUISING AND DAYSAILING David A. Keiper -4'707 Box- 1-8081- -San-Frai+c&eo.i-CA- 9414-8 - Abstract Results of further sea trials of the 31'4'' hydrofoil trimaran "Williwaw" are presented. Modifications for the next hydrofoil cruising yacht are discussed. The design of, and performance results with a 14-foot cartoppable hydrofoil sailboat are presented. 1. INTRODUCTION Francisco Bay, striking while the boat was not moving, led to capsizes. With float-hull buoyancy The design of, and early sea-trial results with the of 40% of loaded weight, the boat had a 31'4" L.O.A. hydrofoil cruising yacht "Williwaw" successful 500-mile shakedown cruise off the were described at Ancient Interface I11 [I]. California coast. However, I still felt there was Some further improvement modifications were too much risk of capsize. On all ocean passages, described at Ancient Interface VI, and movies "Williwaw" had a float-hull buoyancy of about were shown of "Williwaw" in action [2]. Since 60-70% of loaded boat weight. With 20,000 miles that time (1975), there were an additional 12,000 of hydrofoil cruising behind me, I now feel that miles of sea trials, which included both heavy it is safest if float hulls can carry 100% of weather sailing and tradewind sailing, from loaded boat weight. California to New Zealand, and return to Hawaii. Figures 1 and 2 show the boat flying on its However, compared to conventional trimarans, the hydrofoils. float hulls of a hydrofoil trimaran can be placed higher, and be made shorter and fatter, with more I subsequently developed a hydrofoil daysailer, the "Stormy Petrel," a 14-foot hydrofoil trimaran that is both cartoppable and can be easily set up on and launched from a beach by one person. It was thoroughly tested for two seasons in the Golden Gate region of San Francisco Bay. 2. ALTERNATIVE DESIGN APPROACHES Although my approach to the design of the hydrofoil cruiser was covered at Ancient Interface I11 [ 11, some of the discussion needs to be extended and compared to the approaches of others. Many other experimenters advocate the hydrofoil monohull, that is, dispensing with the float hulls, FIGURE 1 in an effort to save weight and wetted surface. "Willi~aw,~~in its first trials, had minimal float- SMOI<ING ACROSS HAWAIIAN WATERS AT 18-2'0 KNOTS. MOST OF THE hull buoyancy of the order of 20% of loaded SPRAY COMES FROM THE HARD-WORKING weight. However, sharp wind gusts on San LEEWARD HYDROFOIL. only significant change was installing torpedo- water and went under about 8 feet, solid water most of the way to the mast step. shaped bodies at the lifter-strut junctions on the Oddly, no spray was thrown up from the deck. bow foil 121. I looked apprehensively at the masthead, and saw that the mainsail peak was still drawing In a 17-day tradewind voyage between Hawaii and wind. Williwaw sat with her nose under for Samoa, "Williwaw's" foils were set continuously about 2 seconds. I saw no tendency for the for 12 days. ' During a 10 day period, which stern to lift up. Then the bow popped out, and she accelerated forwards. Rather than included a doldrum's crossing, she covered 1650 being down in a hole at this moment, we nautical miles. Much later, on the final leg of seemed to be in a small wave trough. the return voyage to Hawaii, "Williwaw" averaged In my mind, I tried to put together all that i had just witnessed. I realized that we had 172 miles per day over a three day period, with just gone down the sort of wave that hydrofoils set continuously, while being solo- pitchpoles yachts, whether monohull or multi- sailed. hull. Such a wave cannot exist for long, perhaps 10 to 15 seconds at most, but we were When "Williwaw" left New Zealand for Rarotonga, there at the moment of its existence. The a disturbed SW air stream dominated the weather average height of the seas was not so great that any yacht would have had a drogue out. patterns. It was producing frequent gales with Going down the slope at 20 knots, the bow shifty wind directions, though generally on our was high off the water, with the boat at a stern quarter or aft. Typically, at the beginning trim roughly parallel to the wave slope. Either a monohull or a multihull would have of a gale, if we had full sail up, "Williwaw" had its bow down near the water, and a would average about 18 knots, with speed bursts negative trim angle relative to the slope. A to 25 knots. However, after an hour of gale yacht tends to pitchpole (i. e., go end over end) if the bow deck is below the center of winds, some waves developed extremely steep gravity of the boat, along the line of travel. backs. "Williwaw" would slam into these walls of Even though Williwaw had a very low bow water. After a few slams, we always reefed deck, the bow foil kept the bow well up, and helped prevent pitchpoling. When Williwaw sails, and then found reasonable boat behavior at began to dig in, the large reserve lifting area a lower speed average. on the bow foil would have given a tremendous righting impulse. Even if we had had A freak wave was encountered about 500 miles sufficient angular momentum for pitchpoling, out of New Zealand. Here is an account of the the rudder foil would have resisted it by going events on the morning of June 13, 1976: into negative lift. Thus, a number of factors related to the hydrofoils helped prevent At 9:30 AM, our first squall-du-jour was pitchpoling. was coming on. After several squalls passed, Just before the freak wave, I had gotten out we shook the reef out of the mainsail. Then the sextant and chronometer, getting ready to at 11 AM, we got a big squall with Force 7 take a Noon sun sight. These, and many other gusts. The waves were pretty nasty, but we items inside the boat had been thrown had excellent rudder control. The boat forward. The sextant, a heavy World War I1 occasionally dropped from the foilborne state, Air Force sextant, had put a deep dent in the slowing down somewhat rapidly. No doubt the mastframe. Back in the cockpit, I had, of shifty winds of the past day were generating course, braced myself, but didn't think that unusual wave interference patterns. the deceleration had been drastic. One freak wave is burned into my memory The observed behavior of "Williwaw" on that forever. I was at the helm as we came over a wave crest doing about 8 knots, which is a freak wave suggests that hydrofoils can offer bit less than one-half foilborne. Then "Willi- some degree of protection against pitchpoling. waw" nosed down about 45O, and I was looking "Williwaw's" particular foil configuration, using a down into a hole in the water, about 40-feet deep. There was no rounding between the bow hydrofoil unit, probably helped. slope and the trough. I could feel my hair Some discussion of "Williwaw's" structures is standing on end as Williwaw began accelerating like wild down the slope. We hit the bottom appropriate, as it succesfully cruised 20,000 miles, doing about 20 knots, and her bow cleaved the and was the only offshore sailing hydrofoil ever The starboard lateral foil twice suffered bending Among the improvements over "Williwawt' will be of the main struts in such a way that lifter higher LIDS for the hydrofoils. Higher LIDS can angles of attack were reduced. It was straight- be achieved by using aluminum alloy foils of a ened out ashore by leveraging with stout timbers. higher yield strength (allowing proportionately The main struts had been somewhat weakened by greater lifter spans, i.e., fewer struts), using a the heating necessary during welding. Strut proper strut extrusion, and "cleaner" foil design alloys were all 6061-T6, whereas the lifters were overall. Proportionately deeper foils will allow 6063-T4. The lifter extrusions had been aged the boat to fly higher off the water, keeping it many years, and were probably in about the T5 from being slowed down as much by hulls hitting condition, which means that the yield strength wave tops. Other improvements relate to a more was about 15,000 psi. Because of the modest efficient sailing rig, better hull structures, yield strength of the lifters, the foil units improved float-hull shapes, and a relatively wider required an excessive number of struts, which boat that can stand up to the strong apparent certainly added to water drag forces. winds generated in high speed sailing. The boat "Williwaw" was destroyed at anchorage by storm should be able to fly much closer to the wind, waves entering Hanalei Bay, Kauai, Hawaii, on and exceed true wind speed more substantially on October 15, 1977. I was 5,000 miles away at the reaching courses. The wind threshhold for flying time, and had selected the wrong person to be should come down significantly, making the caretaker. hydrofoils useful a greater proportion of the time, and allowing the boat to make much better speed 4.