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The following is extracted from a report on the Miami Boat Show (from Vance Buhler <>) which appeared on the Internet Multihulls Email list:

"There were a few interesting boats. Among them Dr Sam Bradfield's lovely 25ft carbon Foiler Tri EIFO with it's 7' daggerboards and rudder to which were fastened 5' wide Tee foils. At the cost of some speed loss at the top end the foils are optimised to lift the boat at well under ten knots of windspeed! It also features a simple mechanical control system to trim the foils. It's all very nicely done and really ought to spin your crank if you want an extra 50% of speed and have 10kt winds to sail in. Top speed until then had been 27 kts. The pictures of it sailing at speed were impressive. No heeling, 2feet plus up out of the water, straight and level, relatively little spray. If you threw away the foils, etc., it would probably still do 27! Of course the crew would have to wear snorkels and masks with windshield wipers because they'd be under water most of the time. David Keiper of WILLIWAW fame says that we can expect a proper foiler on Ladder foils to break 50Kts. My guess is that the foils will have to be optimised for the high end and probably won't get up till the boat is doing at least 20."

We invited both Sam Bradfield and David Keiper to comment and expand on their different approaches. Dr Bradfield unfortunately was not available at the address we have, but we received the following from Mr Keiper:

"Sam Bradfield optimises his foil systems for racing around the buoys. Using a submerged foil system helps in typical race conditions of modest wind. One price paid by having submerged foil systems is that you can not optimise performance over a wide speed range. He sacrifices the high speed end in trying to get a high AVERAGE speed over a race course in typical conditions. If his beautifully crafted EIFO has a top speed of 27 knots, it is a tribute to his design abilities. That is about his top design speed. His boats should be judged on boatspeed/true windspeed. I know he is doing well in that category. Sam does lots of careful measurements of performance. I suppose it might be possible for a super-lightweight fragile catamaran to match Sam's boat on performance, but only because his foil tri EIFO is built to be durable, and weighs in substantially heavier.

Compared to Sam, I design for a higher take-off speed, usually around 12 knots. While that creates a moderate drag hump, there are tricks to aid in punching through the hump in marginal wind conditions.

I continue to design with ladder foils partly because ladder foils with high aspect-ratio elements can be designed to be reasonably optimum over a very wide speed range, probably the whole range, 8 to 50 knots. That requires careful attention to a lot of details. Not just any ladder foil will do. . . . In designing Williwaw, I optimised for the 10 to 20 knot speed range, since, in that day, no one even thought that sailing yachts could fly. In ideal conditions, strong wind and flat water, Williwaw got up to about 30 knots. Hydrofoil surfing out at sea, she probably had speed bursts to 25 knots.

Another reason I've stayed with ladder foils is that the elements can be built into truss arrangements that can withstand the worst punishment the sea can give. Submerged foils are necessarily cantilevered, and have a lower strength-to-weight ratio. Also, submerged foils require moving parts, which I don't like at sea.

There are foil sections that are free of cavitation up to about 50 knots , so such a sailing speed is a reasonable goal in ideal conditions. Just remember that Alexander Graham Bell pushed a power hydrofoil with ladder foils up to almost 62 knots in 1919. His foils were a bit crude- --flat plates, I believe.

Rather than just giving words or drawings, I would be happy to develop the ultimate sailing-speed machine that would do 50 knots. Unfortunately, I've never had the capital to build and campaign such a boat. I think I could do it with a small fraction of the capital that some other hydrofoil designers have had to play with."

We perhaps ought to mention that Dave Keiper's book "Hydrofoil Voyager" is available from: 1) Multihulls Magazine; 2) Multihull International magazine, which has the lowest price for UK buyers ( they also carry the video tape of Williwaw in action, in PAL format); 3) Philip Thompson, in Australia (e-mail:, who writes for Multihull World; and 4) from the publisher: Hinsdale Press, 123 South Pacific St., Cape Girardeau, MO 63703, USA.


Since then we have also received the following announcement of the availability of beachcat hydrofoil kits from Dave's company DAK Hydrofoils:

We can now report some good news. Our 3"-chord aluminium hydrofoil extrusions are being manufactured. With my assistant, who has a Hobie-18, we'll be testing the foils this summer. The test results will be applicable to most other beachcats.

We should be able to ship foil kits by late summer. We hope you continue to be interested. In the Multihulls magazine article (Nov/Dec '96, page 41), we said we could provide, in a do-it-yourself kit for under $600, the necessary hydrofoil hardware to get your boat flying on foils. We will stick to our promise on price, setting it at $595 plus shipping costs from Cape Girardeau. However, this price applies only to those who order the kit in the next 30 days or so, and also send us a deposit of $200 for the kit. [Note: We acknowledge receipt of all deposits. Deposits are fully refundable, if you change your mind about wanting a kit. Also, anyone who buys a kit and then changes their mind, we will help locate another purchaser so that you'll get your money back.] . . . At our low foil-kit prices, you won't need to take out a home equity loan to get flying on hydrofoils, as you might for buying a production hydrofoil sailboat on the market. You may even get some friends out sailing again with hydrofoils, because of the smoother ride, which gives a big-boat feel.

Our foils should be suitable for the Hobie 14, 16 and 18, Nacra 5.2, Prindle, Sea Spray, A-class, others, plus some home-built catamarans. Kits will differ in the attachment hardware. These kits are most suited to 14 to 21 foot catamarans that are lightweight. Heavy catamarans, usually of deeper draft and higher freeboard, are not well suited for foils. Our ladder foil unit also happens to be suitable for small powerboats.

The kit comes with step-by-step plans, and includes many construction tips. The most difficult part of construction has already been done--- the cutting and fitting together of the elements of the foil units at the proper angles, and the drilling of the pilot holes for screws. Thus , the builder does not have to do much metal working. The builder taps thread for the screws, roughs up the metal next to the joints to make the epoxy hold better, and epoxies the foil elements together. After the epoxy sets, there is some filing to streamline the joints that will be underwater. Additional epoxy filler will be added to make good fillets. Then you mount the main foils to the forward crossarm and deck edge with the hardware provided. Stainless-steel bolt hardware is provided. Stabilising fins will be attached to the bottoms of the rudder blades. On some boats, there may be some looseness in the rudder assembly, which needs to be corrected before you can go foiling. Kit construction will require a weekend or two of time.

We also offer a ready-to-install set of foils for a few of the most common beachcats. The price on that will be $1250, with a $400 deposit. The ready-to-install foils will already be coated for salt-water protection. (Those who build from our kit, and plan to sail in salt water, will want to take the foils to a local shop where they do "powder coating," which gives a slick, durable finish in the colour of your choice.)

We expect a catamaran with these foils to be able to get flying in a true wind of 9 to 10 knots, depending upon boat weight (number of crew), sail area, and the heading relative to the wind. In marginal winds, it is easiest to get flying while close reaching. However, the top speeds will be achieved with a stronger wind, and the true wind a bit aft of the beam. In ideal conditions, foils may double the speed. Mostly, foils will add 50% to speed when there is wind for flying. In light airs, one retracts the main foils, and sails the boat conventionally. The rudder stabilising fins are set to zero degrees angle of attack, and add only slightly to drag in light airs.

There is a smooth transition between hull buoyancy and hydrofoil lift. As the boat picks up speed and gains hydrofoil lift, the bow comes up, augmenting angles of attack and lift on the foils. Crew can also move aft to further augment lift off in marginal winds. Just before becoming fully foilborne, there is a modest drag hump. It is easiest to break through the drag hump while sailing 60 to 70 degrees from the true wind. If one turns the boat suddenly away from the wind by 10 or 15 degrees, suction on the lee hull stern is broken, and also the sail gets "pumped. " That usually gets the boat through the drag hump in marginal winds. There is a noticeable acceleration as soon as the boat becomes fully foilborne. As it accelerates to higher speeds, the bow comes back down to about level trim. If you find that the boat flies either bow up or bow down at high speed, you should change slightly the angle of attack of the rudder stabiliser fins.

The foils not only increase speed potential in moderate to strong winds, they increase stability markedly, making the boat much easier to manage. Foils reduce rolling, pitching, yawing and heaving. There is reduced hull pounding and rigging fatigue. Foils drastically reduce the likelihood of lee-bow burying or capsize. Not only that, crew comfort is increased, with a drier, smoother ride.

In case you are not familiar with our hydrofoil sailboat research and development, we should mention that our first creation, back in the late 1960's, was the awe-inspiring 31-foot hydrofoil trimaran "Williwaw," which successfully cruised 20,000 miles around the Pacific in all kinds of conditions. You can get an idea of the high hydrodynamic efficiency of our foil designs when you consider that a 10 to 12-knot wind is able to lift a yacht weighing 1.5 tons right out of the water with only 380 square feet of sail in a conventional sloop rig. . . . Later, in the early 1970's, we developed and test-marketed foils for the smaller beachcats, using 2"-chord foils. We stopped marketing during a recession, but continued research. Our new improved beachcat foil kits are the result. You can read about all our previous developments in the book, "Hydrofoil Voyager."

We invite you to visit our website if you haven't already. The website URL appears below.

We hope you are persuaded to get in on the great fun of hydrofoil sailing. It is definitely more fun to fly both hulls of your catamaran. Call or write if you have questions.


David A. Keiper DAK Hydrofoils, 123 South Pacific Street, Cape Girardeau, MO 63703, USA; phone: 573-651-6582; e-mail:;
Website URL:

Beach cat with Keiper foil kit

DAK Hydrofoils Beachcat Foil Kits

Further correspondence elicited the following:

The extrusion sections for the lifter is Clark-Y with thickened trailing edge. That has a rounded nose, and max. thickness almost 12% at 30% of chord from leading edge, and a flat surface underneath from 30% to 100% of chord. It is not suitable for reversing (shunting) proas. This section features a wide angle of attack range with good L/D, and can get fairly high lift without stalling.

For the strut, they are using the NACA 16-008 section with thickened trailing edge. A fairly thin strut, at 8% max. thickness, which minimises surface-piercing drag and ventilation. This strut section is suitable in the truss arrangement ladder-foils they design, where stresses are mainly tension and compression. It is not suited to cantilevered foils, where there may be a sizeable unsupported span subjected to bending stresses.

The foils retract on pivot axes running fore and aft. Thus the foil unit swings out to the side and then up. The main pivot axis is along the edge of the deck, at the apex of the foil unit, and the apex of two braces going forward from the foil unit. Looking at the drawing (above), you'll note a horizontal bar laying on top of the main crossarm. There are pivots (fore-and-aft axes) at each end of that bar. The middle of that bar clamps to the outer end of the crossarm.

As to the rudder and stabilising fin arrangement, the simplest is to have stabilising fins at the bottom of the rudder blades. They do not need to be retracted, as the fins have zero angle of attack. Keiper tells us he has also been trying to work out another more-complex arrangement, similar to that he had on his boat Stormy Petrel, in which the rudder and fin can be retracted (on a longitudinal axis), with a shallow rudder blade going in the water when the foil rudder is retracted.

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