Why Hydros Flip
Fifty years and still flipping
By Dixon Smith
Question 1: Can a raceboat be made that will not flip?
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
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.
© 1998 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 recently refurbished the 1962-65 Miss Bardhal and is currently employed at the Boeing Company in Seattle.