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Tri-Foiler designer Greg Ketterman shares what he has learned about foils plus discussion on foil sensors

Friday March 12th 2004, Author: James Boyd, Location: United Kingdom
See last week's correspondence on this subject...

From France, Remi Laval-Jeantet, hydrodynamicist :

It's rather obvious that the completely foilborne state with 'T-type' foils in a 'bicycle-like configuration' is a rather impressive achievment.

It should not be forgotten that the Rohan Veal's Moth configuration can work only with the front dragging tiny rod which is a sensor of boat's state, mainly in elevation. This, Prof Sam Bradfield's concept which is a refinement of 1957 Hook's planing sensors principle, is the ONLY WAY to make T-foils stable on pitch axis. For heel control, Rohan must rely on his incredible skill ( this guy is a true acrobat !!! ).

Cam Elliott thoughts seem quite right...the irresistible raising of budgets which is driven by this innovation leads to progressive ruin of the Class by drastic crushing of the fleet's size.

I have worked since 1987 for Swiss Lakes Multihull's skippers and studied foils, sails, VPP's topics and I saw progressive fall in the number of competitors as the budget inflation became harder. Even for Swiss millionaires, too much is too much...It leads to a practical nonsense in 2001-2003 when Alinghi's cat won everything without serious contest.

The emergency process was to create in 2003 a monotypic Class with advanced, but standardized cats to stop "boat fitting's racing"...are the Moth's owners prepared to run on Industrially built "Clone Foilers" ?

Hobie Cat's Greg Ketterman, designer of Russell Long's record breaking speed sailer Longshot and subsequently the Hobie TriFoiler shares what he learned about tri-foiling:

Clearly for those that master foils they are the work of God and for those who do not they are the work of the devil. For Rohan Veal and anyone sailing a TriFoiler on a sunny day in 12 knots of breeze they are the work of God. People's reaction to foilers is completely understandable - people tend to not like what they do not understand and foilers are very difficult to understand completely. The potential of foils is clear - foils lift the boat free of the water and the drag on the boat is greatly reduced; however, hydrofoil sailboats are very complicated. In comparison to an airplane a hydrofoil sailboat is much more complicated for many reasons:

i) A foiler must fly within a very limited vertical dimension. In the case of the TriFoiler about +/- 8 inches. If it flies too low, the hull hits the water and defeats the purpose and if it flies too high the foil ventilates and loses lift.
ii) This vertical dimension goes up and down with each wave.
iii) The power source for a sailboat creates huge heeling, pitching and yawing moments on the boat as opposed to a plane where the thrust from the engine creates practically no moments on the plane.

A good analogy of a hydrofoil sailboat sailing over rough water is a plane flying through a low tunnel on a rough road with the engine spitting and sputtering and mounted high on one wing.

Considering these challenges it would not seem worthwhile or possible to achieve hydrofoil sailing, but it is. One of the great things about sailing from an engineering standpoint is that small improvements in drag or thrust can lead to dramatic improvements in performance. These improvements tend to have a snowballing effect. The better it works the faster itgoes. The faster it goes the better it works and so on. Sailors have always called it 'catching the apparent wind'. The effect is real, but to call it catching the apparent wind would not be accurate. Whether you are sailing a boat, riding a bike or driving a car, apparent wind is always the component of the wind that is in your face and is always drag. A better way to look at it and the cool thing about sailboats is that for a given wind speed they have nearly steady thrust no matter how fast they go.

As a sailboat goes faster it essentially generates more power the faster it goes - thus the snowballing effect. In contrast, most vehicles (motor vehicles & human powered vehicles) have a fixed amount of power and the thrust diminishes rapidly as they accelerate. With these vehicles improvements in drag or power equate to much less rewarding increases in speed.

In the case of the TriFoiler, drag has been greatly reduced even relative to other foilers. The main reason is the foils are very small. The actual lifting surface is considerably smaller than a rudder that you would typically find on a boat of this size. There are no flaps or moving parts below the water surface. The foils are elegantly tapered for high aspect ratio with a single elliptical tip. There is only one T-connection on the whole boat - on the rudder.

On the power side of the equation the TriFoiler can produce a lot of power because it is wide, the sails are very low and the weather foil will pull down if the boat tends to heel over.

A ride in a stock TriFoiler is likely to be the most thrilling and exhilarating ride you will have in a sailboat. The speed will allow you to use just about any other sailboat as a turning mark and you can literally do circles around them. In gybes you can experience 2 or 3 g's lateral acceleration which is about 2 or 3 times what a high performance sports car can produce excluding a Formula 1 race car that produces incredible amounts of down force. It is certainly possible to bruise your ribs, strain you neck and loose your cookies in a high speed gybe. Sailing Longshot is even faster primarily because the foils are smaller, the windage is much less and the sails are more rigid. It is like driving an extremely powerful car. It seems that for any number of reasons you can not open it up all the way - you feel it is just too fast, there is not enough space or it is time to slow down for a turn. When Longshot set the current world speed record in the A class of 43.55 knots the wind was blowing about 23 knots which makes efficiency less than 2, but it made literally dozens of runs around 40 knots where it was sailing 2.5 times wind speed.

An interesting thing to note is the TriFoiler does not accelerate just as the hull comes clear of the water. There are occasions when the hulls come clear of the water, but at low speeds the induced drag from the foils is enough drag to prevent it from really taking off. Once the boat accelerates a little more the induced drag goes down and then it really takes off. This is just more evidence of this snowballing effect.

Another interesting note about foilers is the amount of wind required to get on the foils is not very dependent on the size of the foils. I will admit that this seems very counterintuitive. We experimented with many different foils including very big and very small. On one occasion we reasoned that the weather foil of Longshot could be much smaller since the loads on the weather foil are much smaller and we figured it could return to the start line in displacement mode. We were very surprised when it got
on the foils on the wrong tack and took off. For the production foiler we were very interested in getting it to fly in as little wind as possible so we tried very large foils. It flew in a little less wind, but it was too much of a compromise to other aspects of performance. It is not like a plane. If a plane has a bigger wing, it needs less speed to take off and thus needs less power to take off. On a foiler the bigger foil is just more drag and does not let the boat go as fast which reduces the power generated. A smaller foil creates less drag lets the boat go faster and begins the snowball effect.

I see a lot of mis-information regarding ventilation and cavitation. Ventilation is when air from the surface is sucked down along a foil and totally disrupts the performance of the foil. The foil loses almost all of its lift and drag goes way up. The TriFoiler has minimised the problems with ventilation by keeping the foils piercing the water surface at 90 degrees and using fences on the foils. The fences on the TriFoiler are very small, but effective. The worst thing that happens is the foil will ventilate a few inches down to the next fence. This causes considerable drag, but it can be corrected by raising the boat up momentarily and letting the flow reattach. In our experience the foils rarely ventilate at higher speeds - over 30 knots. Ventilation is interesting because it can happen so suddenly and unpredictably and it can disrupt the foil so completely; however, when things are going good they go very good. It seems when the TriFoiler is well tuned and working well (sails are trimmed right, ride height is good, masts are rotated, and leading edges of the foils are good) it rarely ventilates.

Cavitation is completely different and appears to only be a problem over 42 knots. Pressure next to the foil goes so low that the water boils. It does not appear to affect lift, but apparently the drag goes up and I suspect this is why all of the speed records seem to have topped out around 43 -46 knots. Any sailboat with a centerboard or rudder will be affected by cavitation at these speeds and not just foilers.

The speed around the buoys is also very fast. We have raced a windward-leeward course against a fleet of Hobie Miracle 20s and beat them to the weather mark and easily beat them to the leeward mark in 15 to 20 knots of wind in a stock TriFoiler.

Straight line speed and speed around a course is easy to demonstrate. Rough water performance is not so easy, but I like to tell the story about Longshot in Brest, France during speed trials. The wind was very gusty and the direction was such that the end of the course was exposed to the open ocean and there was 6ft chop on the end of the course. This event was done by the Professional Board sailors Association PBA (Pascal Maka and all the best windsurfers) and they claimed it was too rough and nobody got any good speeds. Charante Maritime, the 60 foot slewed catamaran (see below), made several attempts. On one attempt I saw their beam hit a wave and put spray 30 feet into the air. They went in claiming the water was too rough. I was sailing TF3 and wasmaking a run, but bailed out thinking that the waves were just too big. Just after that I watched Russell Long in Longshot finish an amazing run and there had to be 6 feet between the hull and the water. He made the fastest run of the session of 32 knots. Later the wind shifted and Charante Maritime was able to go a little faster, but we had already packed up.

The key to rough water performance is to get the boat to platform over the small waves and contour with the big waves. In the beginning the problem was the TriFoiler would try and follow every little wave and the boat would shake violently so we added oil filled dampers and we increased the flexibility of the sensor arms. This acted as a high frequency filter and filtered out the smaller waves. The TriFoiler has no problem with bigger swells and it is impressive how quickly it can respond to waves to lift the boat up and over waves. On a rough day you can watch the mast going up and down with the waves. One advantage that the TriFoiler has is it senses the waves way out in front of the boat. It gives the boat time to react to the waves. I can think of only a few vehicles that can do this - Windsurfers, down hill skiers, BMX bike racers. These vehicles have the ability to anticipate bumps and stay in phase with the bumps. Most high speed boats have problems with waves because they get out of phase with the wave. A
ski boat for example will get launched off a face of wave and then crash into the face of the next wave. High performance multihulls will do the same thing to a less extent, but because they have little pitch dampening they have problems with certain wave conditions.

Obviously there will be wave conditions where foilers will be limited in speed and maybe knocked off the foils, but all boats are limited in rough water. It will probably require lots of research with height sensors and maybe deeper foils, but I think it is possible that it will be rough water where foilers may improve sailboat performance the most. One advantage that the TriFoiler has is it can turn and maneuver around rough areas very easily. In my experience there are always areas of rough and smooth water even in the roughest conditions. I once sailed under the Golden Gate bridge in San Francisco with the tide going out and there were areas where it was incredibly rough, but there were areas in between that were incredibly smooth. I had a blast sailing on that day. It was no problem to avoid the rough areas even if you were trying to maximise VMG upwind or
downwind - the TriFoiler is very tolerant of wind angles when racing upwind and downwind because it can accelerate so much when you bear away or heat it up. I do not know what is the best angle for racing up wind or down wind. It does not seem to matter much especially downwind.

It is interesting to speculate on what is the future of hydrofoil sailing or figure out what is the best application for hydrofoil sailing.

Commercially the TriFoiler was not a failure and it just missed being a hit. Making a few improvements and changing strategy a little bit maybe all it takes to be a grand slam. Of all the things that can be done with hydrofoil sailing, maybe creating a new class or a new recreational day sailor is not the easiest thing to do in fact it is incredibly difficult.
Setting records for the 500m course was obviously a logical thing to do and now I suspect that setting offshore records maybe the thing to do. It is conceivable to take a stock TriFoiler and go for the singlehanded 24 hour record or even the crewed 24 hour record. All you need is 24 hours, 15 knots of wind and a suitable length of runway. Now I am dreaming of a bigger TriFoiler that will perform like Longshot for some offshore records.

The hydrofoil Moths look very interesting because the boats are so simple that they will be able to test many different concepts. They are sure to make very rapid progress like that. It is also interesting as there seems to be a lot of room for improvement. Rohan Veal's performance upwind looks good, but it seems his speed reaching and downwind should be better. I suspect there will be some breakthroughs there.

There will always be people that will want to maintain the status quo. There is no doubt that hydrofoils will increase the engineering complexity of sailing considerably and there will be expensive failures. Foilers will be criticised for this, but since a small foiler has the potential to beat a Cheyenne, Castorama B&Q, an Orange 2 or a Geronimo that does not mean that costs have to go up - in fact it could be cheaper. The technical challenges are considerable, but for someone that wants records and sponsorship foilers should have promise.

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