Hydros 101


A modern Unlimited Hydroplane is the world’s fastest racing boat, capable of speeds in excess of 200 mph. It represents the product of over 100 years of evolution in race boat design and incorporates the most powerful engines, most advanced construction techniques and the best safety systems available in boat racing today. All Unlimited Hydroplanes are a “three point” design, meaning they are designed to only touch the water at three points when racing: at the rear of the two front “sponsons” (the projections of the hull in front of the driver cockpit), and the propeller at the rear of the boat. “Runners” under the sponsons and “shoes” at the rear of the boat are generally all that touch the water during race conditions.

A modern Unlimited is made of aluminum, fiberglass, carbon fiber and graphite composites, and weighs a minimum of 6750 pounds in race trim. The boats are between 28 and 32 feet long, 12 to 14 1/2 feet wide and about seven feet from the bottom of the rudder to the top of the rear wing. Driver safety is paramount in the design and operation of an Unlimited, so the boats have a number of systems designed to keep the driver safe in the case of an accident. The fully enclosed cockpit is the primary safety feature, incorporating a full interior roll cage that is bonded to the cockpit shell to maintain the integrity of the driver area. The windows are cut from the canopy of F-16 fighter jets, giving the driver ultimate protection, yet allowing him to see clearly. Five point quick-release harnesses are used to keep the driver strapped into the seat and an escape hatch is built into the floor of the cockpit to allow the driver to escape if the hydroplane flips and lands upside down. Every driver is required to wear a certified helmet and a HANS device for head and neck protection. He also wears a mask that incorporates an on-board air supply that allows him to breathe in case the cockpit fills with water, but also lets him communicate via the onboard radio.

A “Rotor Burst protection System” (RBPS) is in place around the “hot end” of the engine to contain the fragments in case of a catastrophic failure of the turbine’s power rotor. All of today’s unlimited hydroplanes are powered by a single Lycoming T-55 L-7 turbine engine that once powered our military’s Chinook helicopters from as far back as the Vietnam War. The turbine is capable of outputs of around 3000 HP and runs on Jet-A (kerosene) fuel. The large tail pipe in the back of the boat is only to allow heat to exit; no thrust is created from the exhaust.

The engine’s output is hooked to a “gearbox” that has a single adjustable gear ratio that reduces the engine speed to the appropriate output shaft speed to make the propeller work most efficiently. A “long shaft” transfers the gearbox output through the bottom of the boat to the “strut” where the “short shaft” connects to it with a coupler. The short shaft has the propeller connected to the end and it allows the prop to be easily removed after every heat for inspection.

There is a fixed wing on the rear of the boat that can only be adjusted when in the pit area. This wing is only used for stabilizing and trimming the boat so that very little of the boat’s rear running surfaces touch the water. The second wing on the boat is located at the front of the “pickle fork” nose. Unlike the rear wing, this “canard” is actually controlled during racing by the driver via foot pedals. The canard allows the driver to more easily “fly” the boat, pushing the rear of the canard down to create lift and raise the nose of the boat, or raising it to lower the nose of the boat if it gets too high and is in danger of “blowing over”. The driver will also manipulate the canard in the turns or when rough water and windy conditions persist.

U-37 in San Diego - 2010

Salt buildup leads to pre-ignition

The large “cowling” behind the driver cockpit functions as an aerodynamic device and as a “scoop” to funnel intake air to the engine. The rear of the cowling is vented to allow for excess air to pass through so as not to trap air that would otherwise slow the boat down. During saltwater races, an additional extension to the cowling may be used to prevent saltwater from being ingested into the engine. Excessive saltwater ingestion will almost instantly reduce engine output due to the buildup of salt residue on the turbine blades. This buildup effectively changes the compression ratio of the engine, allowing for “pre-ignition” (like a backfire on an automotive engine) that will shoot large flames out of the back of the engine (see image at right), and can even damage an engine.

A single three blade, 16″ diameter propeller is allowed. Ideally, only one propeller blade should be touching the water when at racing speeds. Different pitch propellers are chosen for use based on course length, conditions and starting position. Race props can cost in excess of $15,000.The prop creates the distinctive “roostertail” behind the boat, raising literally tons of water into the air for up to 300 feet behind the boat.

The skidfin is a large metal fin that is attached to the area at the rear of the left sponson that allows the boat to “dig in” and make a turn without skipping across the water. The wall of water the skidfin throws up in the corner is one of the most spectacular sights in boat racing, reaching heights of nearly 50 feet and extending for 6-7 boat lengths behind the boat.

The rudder is a relatively small metal blade that is located in a bracket extending from the transom at the right rear of the boat that acts to steer the boat through the corners. The rudder only becomes truly effective at higher race speeds and makes maneuvering at low speeds somewhat challenging. A water pickup at the base of the rudder feeds onboard cooling and lubrication systems.

All Unlimiteds carry onboard digital recording devices that capture different streams of data, from engine speeds and fuel flow to wing angles and temperatures. All turbine engines are limited to 4.1 gallons per minute fuel flow and less than 115% “N2” speed. N2 speed is a pre-determined upper RPM limit that the engine cannot exceed for more than five seconds. The data recorders log these two critical data streams and are reviewed by officials at the end of every qualifying attempt and heat. Fuel flow and N2 violations will void a qualifying speed and will disqualify a boat from the Heat in which violation occurs.


Each Unlimited Hydroplane must qualify for the heat races at 130 mph or higher. The top qualifier receives 100 race points, runner-up receives 80 points, third highest qualifier 70 points, fourth 60, fifth 50, sixth 40, and the remaining boats get 30 points each. Boats that do not qualify or reach the 130 MPH threshold are awarded no points. The qualified teams are admitted into the flights of Heats. Because there are too many boats to run at once they are split into groups represented by an alphabetical letter. Traditionally, there are Heats 1-A, 1-B, 1-C. Once the first Heats are finished and points have been assigned, another drawing is made and the boats run 2-A, 2-B, 2-C. Finally, the third set of Heats is run. All qualifying Heats are three laps and the winner-take-all final is five laps. After each Heat, boats are awarded points based on how they finished: First place, 400 points; Second, 300 points; Third place, 225 points; Fourth place 169 points; Fifth place 127 points, Sixth place 95 points and Seventh place 71 points. The boats who have accumulated the most points during the preliminary heats make it to the winner-take-all final. There is no guarantee that the boat that wins the Fiinal Heat will come away with the most points, nor that the boat that wins the most races during the season will win the Championship. For instance, the 2014 season saw J. Michael Kelly in the U-1 Graham Trucking win four of the six races, but Jimmy Shane in the U-6 Oberto took the Championship at the second to last heat of the season by virtue of accumulating more points.


Prior to the start of each preliminary heat and winner-take all final, there is an official clock that will count backwards starting at “10 to the 5” – this means that there fifteen minutes to the start of the heat. Most boats will be lowered by crane into the water at this point. At “5 to the 5”, all drivers will be strapped into the boats with their safety gear in place. Most drivers will enter the race course when there are about five minutes remaining on the official clock. At the one minute mark or one minute “gun” all boats racing in the Heat or Final must be up and running. Since the shortest way around the course is on the inside, Lane 1 is the preferred lane to start in. All boats will be allowed to “fight for lanes” prior to the start, prioritizing pre-race strategy and boat setup. Once lanes are established at the entrance to the second turn prior to the start, boats will then position themselves so that they reach the start/finish line as the official clock strikes zero signifying the start of the race. Drivers do not want to “beat the clock” or they will be assessed a penalty, usually in the form of an addition lap or additional time added to their finishing time.


Speed is the key to winning, but speed alone may not get a boat to the checkered flag first. Strategy is the key to positioning the boat in the right lane to match the way the boat has been set up, and driving defensively can keep a faster boat behind a slower boat. At all times during a race, there are numerous things a driver must do: communicate via radio to their Crew Chief or “radio man”, manage their boat speed before the start and “fight” for the lane they feel will give them the best chance of success without jumping the gun, watch all the other boats to insure they maintain proper distance, watch out for the buoys on the inside of the course, and once the race is under way, “fly” their boats at the maximum speed they’re capable of without flipping! Roostertails and skidfin water are the most spectacular parts of hydroplane racing but present the most danger to other drivers on the course. Both lift TONS of water high in the air and both will lift other hydroplanes out of the water as well.

All boats are required to have “overlap” before a lane change can be made. This overlap is 7 boat lengths – one roostertail length – and it allows the roostertail to subside so as not to endanger the following driver. If a driver on the inside somehow drifts into a boat on the outside or is “pinched” by the boat on the outside, it is almost guaranteed the inside boat will be lifted out of the water and flipped by the wall of water coming off the skidfin. Kip Brown’s flip in Seattle is a perfect example of the power of this water:

Since the “lanes” that comprise a race course are about thirty feet wide, a boat running in lane six will travel much farther than a boat in lane one. However, the boat in lane one must make a much tighter arc to make it around the corner and scrubs off lots of speed to do it. The boat in an outside lane can make a much gentler turn allowing him to carry much more speed through to the exit of the corner. This means that the optimum combination of gear ratio and propeller pitch varies dramatically depending on which lane the boat races in. The correct choice of ratios and prop means a boat can win from the outside if the inside boat can’t put enough power down to get out of the corners fast enough.

Conversely, the leading boat is allowed to establish which portion of their 30 foot lane they choose to race in. By staying in the outside of their lane, a slower boat on the inside can force a faster boat on the outside to make a much wider arc. This will force the outside boat to run a longer course potentially slowing them down and allowing the slower boat on the inside to win.

Depending on the choices made during setup, almost every conceivable combination of faster and slower boats in different lanes can produce dramatic racing and very unexpected results. Every team on the water is capable of bringing home a race win – that’s what makes Unlimited Hydroplane racing one of the most amazing and spectacular forms of racing in the world!


By Dixon Smith

Question 1: Can a raceboat be made that will not flip?

Answer: Probably.

Question 2: Can that unflippable boat be a consistent winner?

Answer: Yes, but as one pundit said many years ago about another program, “when the Boeing Board of Directors changes the basic laws of Physics or gives us an endless budget, we can make this work.”

Real Answer: NO, not with current technology.

First a little history. Current raceboats as we know them are generally called 3 point hydroplanes or prop-riders. This is because they ride on a small area at the back of each sponson and on the lower half of the propeller. The most famous first 3 pointer is the Slo Mo Shun IV. The Slo Mo IV was not the first 3 pointer, although it is the best known. There were a number of limited hydroplanes that were prop-riders prior to the Slo Mo IV being built. The first famous flip of a raceboat occurred on Lake Washington in 1955 with the sister ship of the Slo Mo IV, the Slo Mo V, with Lou Fageol driving. The boat did a complete 360-degree flip and landed right side up, but with substantial damage. That happened over 50 years ago, and raceboats are still flipping.

There have been several rather spectacular and famous flips in the unlimited class. In Pasco many years ago, the Pay ‘n Pak did a 720-degree flip, where it went around 2 times before hitting the water. In San Diego in 1988, 2 boats, Circus Circus and the Miss Madison, did a side by side flip. A few years ago the Pico American Dream flipped in Seattle during a preliminary race heat, brought back to the pits upside down, repaired by the crew and ultimately won the final heat and the race. There have been occasions where a limited raceboat has done a 360-degree flip, landed right side up and continued running. Most flips cause substantial damage to the boat. Some flips have been disastrous; drivers killed and boats literally destroyed.

Now, back to the questions. First we need to understand a little about the physics of the problem. I will stay away from any equations, math or aerodynamic engineering stuff. I need to define a few things that will make this easier to understand.

Center of Gravity: The balance point of the boat. If you were to pick the boat up at only one point, and this was at the center of gravity, the boat would hang perfectly level. Without external forces, the boat will rotate around the center of gravity.

Center of Aerodynamic Lift: Some times called the center of pressure. If all of the aerodynamic lift were applied at the Center of Aerodynamic Lift, the effect on the boat would be the same as the real lift that the boat experiences.

Center of Hydrodynamic Lift: Same idea as the Center of Aerodynamic Lift, except the lift is from water, not air. These two are not in the same place.

Drag: All the stuff that keeps us from going fast. There is aerodynamic drag, drag from the air and hydrodynamic drag, drag from the water. An interesting and important point is that water is about 800 times denser than air, or another way to say that is a bucket of water weighs about 800 times more than a bucket of air. If you don’t think air weighs anything or drag from air is not significant, hold you hand out the window of a car going 70 mph palm down, and then rotate your hand about 90 degrees and see what happens. Now think about that force, but multiplied 800 times, and you get an idea of the drag force from water.

Lift: To demonstrate aerodynamic lift, do the same experiment as above, hand out the window with palm down. Now rotate your hand a small amount in each direction and feel the upward or downward force. This is the lift force from the air. The same ratio of hydrodynamic drag to aerodynamic drag applies to lift. For a given speed and area, the lift from water is about 800 times the lift from air. That is why the sponson area in the water is very small compared to the total area of the boat. Now, we need to have some stuff in the water, prop, rudder and skid fin. Anything else in the water is excess drag, and slows us down. So that is why to go fast, the boat needs to have as little sponson in the water as possible. The catch is how do we get as little sponson in the water as possible. Here is where those pesky laws of physics get in the way.

An arrow is a good example of something that works well aerodynamically. It has been developed over many thousands of years and has remained about the same for a long time, because it works. If you were to balance an arrow on your finger, you would find that the center of gravity is about 1/3 the distance back from the front. This is because the point on an arrow is somewhat heavy. The point being heavy is not only because it needs to be sharp and strong to penetrate a target or animal, but the weight up front makes the arrow more stable in flight. Also, note the feathers are at the very back of the arrow. Remember the center of gravity is near the front, but the aerodynamic center is just in front of the feathers, very far back. Also, remember, per the definition of center of gravity, that the arrow, if disturbed in flight, will rotate about the center of gravity. So if the arrow gets disturbed in flight and the front pitches up, it will rotate about the center of gravity, the tail will rotate down. When the tail rotates down, the feathers will increase lift and push the back up to re-level the arrow. If the nose of the arrow pitches down, the tail will rotate up, again around the center of gravity, and the feathers will push the tail down, again re-leveling the arrow. This is why an arrow flies straight and level. Arrows do not do 360-degree flips.

Well that sounds easy, let’s just design a boat like an arrow and make it stable. Here is where those pesky laws of physics and the practical world of boat racing don’t get along. All raceboats have to be propelled by a water propeller. So, we need to keep the prop in the water. To do this, we need some significant weight on the prop. Most boats have about 1/3 of their weight on the prop and about 2/3 of their weight on the sponsons. To do this, means the center of gravity needs to be significantly behind the sponsons. Remember that water drag is lots more than air drag, so we want to keep as much of the sponson out of the water as possible. The way this is done is to have the center of aerodynamic lift somewhat forward, to carry the sponsons, but not lift the prop out of the water. The result is the center of aerodynamic lift is forward of the center of gravity.

Two more pesky little problems rear their ugly heads about this time. The first is that lift goes up with speed, but much faster than speed. In fact, lift goes up with the square of speed. In other words, if speed increases 10% then lift increases 21%. The other little problem is the center of lift is not in a fixed position, it moves around. The boat attitude, nose up or level, and height above the water both affect the position of the center of lift. Here is the really bad news. As the boat pitches up, the overall lift on the boat increases. Remember the hand out the car window example and rotating your hand a small amount. Also, as the boat pitches up, the center of lift moves forward. Keep this in mind. Now, you say, but a lot of the boats now have canards that the driver can control.

For those who don’t know about canards, it is a movable wing, forward of the cockpit that the driver controls, typically with his left foot. With the canard, the driver has significant control of the overall aerodynamic lift on the boat. A few boats of what is called 2-wing design have aerodynamic flaps behind the front wing that serve the same purpose as a canard.

Lets put this all together and put the driver in the loop during a race. Remember, less sponson in the water makes us go faster. Also, both aerodynamic and hydrodynamic lift go up with speed, but faster than speed. One last thing, the aerodynamic center of pressure moves forward when the boat pitches up. Race water, a bit rough, accelerating down the straight, need to go faster so use the canard to lift the sponsons to get them just touching the water. Speed is increasing, all is good. Turn coming up, look for the other boats, and the sponsons hit a wave that the driver didn’t see. Because hydrodynamic lift is so strong and increases faster than boat speed the boat pitches nose up. Two critical things happen when the boat pitches nose up: (1) the total lift on the boat increases very fast with pitch up, and (2) the center of lift moves forward. If the driver is a little late changing the canard position, or the pitch up is so much that the canard cannot overcome the increased lift and center of pressure moving forward, then up we go. Initially the boat rotates around the prop, but as soon as the prop is out of the water, now the boat rotates around the center of gravity. This is why it looks like the boat hangs with the nose up for a short period of time, then quickly does some type of loop or roll.

Other things that can set the boat off and start the process of a flip are wind gusts and the boat entering a turn. The aerodynamic lift is a function of air speed, which is boat speed plus or minus wind. So on a gusty wind day, a gust of wind could increase the lift unexpectedly. This is a typical situation in San Diego, because of the local topography. Depending on boat design, entering a turn can significantly change the aerodynamic characteristics of a boat. Not all, but a significant percentages of flips occur at the entrance to a turn.

Back to the question of can a boat be designed to not flip and also be a winner? With the right stability control system, similar to what some fighter aircraft use, yes. For some classes of raceboats, these types of control systems are illegal at this time. Also, it will be very expensive to develop this type of system for a raceboat, well beyond the budget capabilities of most teams. In conclusion, a good boat design and an experienced driver are the best insurance against flipping a boat. But, so far, nobody has built a boat that will not flip.

© Dixon Smith

Dixon Smith was a long-time crew member on the Miss Budweiser hydroplane team. He began his racing career in the ‘60s on the crew of the Hawaii Kai III and Seattle Too. He refurbished the 1962-65 Miss Bardhal and drives it at vintage hydroplane exhibitions in Kennewick, Seattle and Chelan, WA.