Stormy Petrel
Cartop Hydrofoil Trimaran
By David Keiper
[Reprinted from MAIB.]
While 5,000 miles away in upstate New York, I heard by telephone about Williwaw being wrecked in Hawaii. At the time. the loss didn’t have much impact on me I felt like I had had sufficient tests of the boat in 20,000 miles of cruising. I had the design of a 35footer fairly well along in 1976. However, I had hoped to build the next hydrofoil sailing yacht with the proceeds of the sale of Williwaw. Unfortunately, I did not have hull insurance on Williwaw, and so ended up with nothing.
In 1982, my thoughts resumed to hydrofoil sailing temporarily. For many years I had been carrying around in my head some ideas for the design of a small hydrofoil trimaran that could be carried on the roof of a car. Also. in 1982 I got married. During the summer of 1982, my wile .and I had a race going on to see who could launch a new creation first, me a 14foot hydrofoil trimaran, or my wife a baby. My wife won, launching our son Steven on August 21. I didn’t launch my boat, named Stormy Petrel, until September 18. Even though my wife won by launching Steven first, I noted that my boat seemed much better developed at launch time. I flew my boat successfully within weeks, but Steven couldn’t even walk for a year
Stormy Petrel could be carried on top of my car and set up and launched from a beach by one person. While sailing the boat in the Golden Bate region of San Francisco Bay, I got speeds of at least 30 knots in several longish runs. In more ideal conditions, greater speeds are possible.
The boat uses a threefoil system made of 2″ chordlength hydrofoil extrusions which were already on hand, remnants of the hydrofoil extrusions I had manufactured ten years earlier for small sailing catamarans. While developing catamaran hydrofoils, I had become frustrated when the boats themselves tended to come apart after extensive tests with hydrofoils. On Stormy Petrel I wanted structures that would hold up under rigourous testing in the Golden Gate Straits. I choose the trimaran form. with a single crossarm that could be rotated 180 degrees for setting or retraction of both lateral foils at once. With floats only five feet long it was possible to rotate the floats as well as the foils. A lever next to the mainhull controls lateral foil setting and retraction There are a number of adjustment holes for setting the lever in place so that one can set the lateral foils at different angles of attack.
At the stern of the boat is a single wooden rudder which has two positions. One way, it gives a 10″ deep rudder blade for launching and sailing in light airs. Swung upside down, the blade runs 24″ deep and has a pair of hydrofoil stabilizing fins. There are three different angle settings for the rudder foil.
The lateral (main) hydrofoil units, port and starboard, are of ladder form with six rungs, each with a 2″ chord and a span of about 13″. Dihedral alternates 30 and 45 degrees with adjacent rungs, thereby stiffening up the foil units. There is also a larger triangular foil mounted to the float which assists during liftoff and also braces the main foil strut foreandaft.
Angles of attack on the lifters increase progressively as one goes up the rungs of the ladder. That way the lowest two rungs can have lift coefficients appropriate for high speed, and the upper rungs can assist lift off better and also provide more reserve lift for better longitudinal stability of the boat. At high speed, with most of the lateral foil rungs out of’ the water, the boat has roughly level trim.
Even though the boat has a 14foot beam, it can be carried on a car roof rack by placing the crossarm unit parallel to the main hull. The crossarm unit has an aluminum pipe with float hulls and lateral hydrofoil units permanently attached to the ends. Main hull and crossarm unit, each weigh about 70 pounds. With only one person, the boat ear be taken off the car, be assembled on the beach and be ready for launching in perhaps 2() minutes, without rushing.
The boat has a 19.5foot tall rotating mast with a stay to the mainhull bow and a shroud to each end of the crossarm. The fullbattened sail has an area of 95 square feet. The boom can he hauled in with two sheets, the primary one with a 3to1 mechanical advantage. A secondary sheet multiplies the advantage to 9to-1 for fine trimming of the sail while foilborne. While the boat is going fast on its hydrofoils, the sail force becomes extremely large and the higher mechanical advantage is necessary to be able to haul in the sheet.
In case of a knockdown or capsize, neither of which have happened, the sealed mast will help keep the boat from going upside down. A float hull would fully submerge as long as the weight of one crew member is on the boat and crew weight could leverage the boat back up.
In light airs the boat is sailed with foils retracted. With no wind and/or adverse tides the boat can be paddled as easily as a canoe. To keep the boom from flopping hack and forth, the boom is lifted out of the way with a topping lift.
Foil performance becomes interesting while close reaching with six or seven knots of wind in flattish water. It requires about ten knots of wind to get fully foilborne with one person aboard, but more wind is needed if the water is rough. With two people aboard the bottom of the mainhull can’t get completely free of the wave tops, although the boat can still get to 20 knot speeds with winds of 25 knots.
The highest speeds seen so far were at least 30 knots, on two occasions sailing in the Golden Gate Straits with long. gentle sea swells coming in plus about one foot of random chop. probably from assorted reflected waves. The foggy wind was gusting up to 20 knots but wasn’t producing chop. On both runs Stormy Petrel took off with wind just ahead of the beam heading north parallel to the Golden Gate Bridge. After becoming foilborne doing 15 to 20 knots I turned the helm so that the boat headed about 20° further off the wind. The boat immediately accelerated but the apparent wind still came a few points off the bow. I then turned the boat another 20° off the wind and the boat accelerated further. When I turned the helm even further off the wind we accelerated a third time, with the true wind on the quarter, as judged by the fact that we were now headed toward Angel Island. However, the apparent wind remanded a few points off the how. The boat was lifted high, the leeward lateral foil perhaps averaging eight inches in the water, the windward foil just out of the water, and the rudder stabilizing fin perhaps running ten inches deep. The water went by in a blur under the boat. The boat seemed to be in synchrony with the long swells much of the time, but the slopes were so low that the swells could not have contributed much to the speed. Both of’ these highspeed runs were interrupted after about a halfmile by the sheet automatically popping out of its cam-action cleat, an event that has never happened otherwise.
The boat’s behavior in the fierce gusts of Hurricane Gulch, Sausalito, was aweinspiring. When the gusts struck while stopped with the sheet somewhat free, I leaned way aft on the boat yet still the mainhull bow deck was driven down to the water’s surface as the boat accelerated. Tremendous clouds of spray went up since the wind kicks up a onetoot chop almost instantly. The boat suddenly squirts out of the water fully foilborne and accelerates to about 25 knots. There is usually no chance to haul in the sheet and make the most of the gust before it dies.
Occasionally. the boat became completely airborne while passing across ferryboat wakes. When that happens, one must release the sheet before the sail forces can roll the boat over. The boat comes down to the water rightside up. There is another situation in which I ve also had to release the sheet. In moderate wind conditions, Force 3 to 4, with flattish water, there is less foil drag and less sail forward pitching moment. Hydrofoiling with sail sheeted in, the boat can start to do a backward capsize, pivoting along the line joining leeward foil and rudder stabilizer. If I can get my weight forward (or to windward) quickly enough, the sheet doesn’t need to be released. I believe the boat could be improved with a hiking board or a trapeze.
In these same moderate wind and wave conditions Stormy Petrel is able to match the speed of windsurfers. yet the helmsman can sail in easy comfort on the hydrofoils. In other conditions, the windsurfers go faster. A lack of adequate hull clearance in rough waters seems to limit performance of Stormy Petrel. Foil draft at rest is 24 and rnainhull draft is 8″. Deeper foil draft would also allow the boat to fly better with two persons aboard.
Stormy Petrel was thoroughly tested in two seasons in the Golden Gate region of San Francisco Bay. The boat has never produced any gutwrenching decelerations. If anything, the forward accelerations are more frightening. In strong wind gusts I’ve just hung on to the boat, terrified, but the boat behaved properly: that is, accelerated forwards and flew stably on its hydrofoils.
After the tests of Stormy Petrel, I felt that hydrofoils are extremely practical, as well as great fun, for daysailing. As of 1995, Stormy Petrel is still going strong. It is in the hands of Jim Montgomery who sails the boat on Lake Erie. He is planning to increase the sail area somewhat because of the generally lighter wind conditions around there. He will also be trying out a hiking board, which should increase the boat’s performace.
As for me, I still think I prefer 30 or 40 feet of boat length under me for I’m basically a lazy cruising sailor. In these larger boats I always use a fourfoil system to achieve stability in all sorts of seas encountered offshore.
Flying My First Sailboat
By Jim Montgomery
Most sailors start out at an early age learning to sail on dinghies and other small boats. This is certainly traditional and a most proper way for anyone to get into the sport. On the other hand, some of us older folks jump right in and learn to sail on a multihull.
Last year I purchased a 14′ x 14′ (yes, that’s the beam) trimaran with hydrofoils. Dave Keiper designed and built Stormy Petrel for San Francisco Bay conditions, windy and rough more often than not. The rotating mast sail is only 95 square feet because of those conditions and can’t be reefed while sailing. This caused some worry but later proved to be a needless concern.
My first sail in Presque Isle Bay off of Erie, PA, began with a dry mouth and nervous anticipation as I assembled the knock down style 180 lb. boat on the beach. After checking everything about five times (this was my very first sail ever) the boat was launched…and, my Lord, I was actually sailing and rather quickly at that! After clearing the shallows I rounded up into the wind to rotate the aluminum crossbeam 180 degrees so that the hydrofoils were in the water, Stormy tore off on a reach gathering speed and at about 12 knots it came out of the water with only the lateral hydrofoils and the rudder foil in the water. Dave told me what to expect when I first took Stormy out so all this seemed perfectly normal to this novice but yet a little scary! The flight was incredibly smooth with the foils cutting through the bay chop like a knife and a most delightful humming sound was coming from the foils. Lack of proper sail trim cut my ride short but, as the afternoon wore on, I started to learn how to sail this strange craft. Sometimes when a wind gust would hit, Stormy would get pushed down slightly and then just rise out of the water on the foils and accelerate very quickly with little heeling. Other small boats on the bay that day were heeled well over (and some all the way) but Stormy was flying level. Isn’t this how sail boats should be?
When the foils are rotated up and out of the water the boat acts and sails like any other small trimaran…fast but sometimes with a quick, bouncey motion. I use it this way when the winds are light because the foils would create drag Another nice feature when the foils are set is that they act as “flopper stoppers,” even when the boat is not moving.
A short explanation of the hydrofoils may be needed at this point. Dave Keiper designed and built the first (and only so far) ocean going trimaran hydrofoil, Williwaw, a 31 footer hack in the early 1970s. His experience of sailing to New Zealand and back led to refinements and improvements on his next boat, Stormy. Extruded aluminum foils are screwed and epoxied together to form a ladderlike unit which is permanently attached to each float (ama). The foil section is in the shape of an airplane wing to provide lift in the water, and since water is approximately 800 times denser than air, the foil can be quite small compared to an airplane wing.
I am definitely spoiled now with my Stormy’s level sailing and am glad that this was my first sailboat. A few recent sails on small monohull sailboats had me climbing a vertical wall to stay in the boat. A lot of people enjoy this though and to them this is sailing. I’ll stay with my maximum 10% heel and sitting in the boat, thank you!
