Hovercraft

Hovercraft is a build event with a physics core. Before the tournament, each team designs, builds and calibrates a self-propelled vehicle that floats on a cushion of air, and for 2027 the vehicle also has to carry nickels supplied by the Event Supervisor down a timed 185 cm track. Three scores make up each run. The Distance Score rewards reaching the finish line, the Time Score rewards finishing as close as possible to a Target Time (TT) announced at the start of your competition time, and the new Mass Score rewards the number of nickels the vehicle carries the whole way. Speed alone wins nothing. A vehicle that repeats a known average speed with a known load beats one that is fast but unpredictable. Strong teams understand how a fan keeps a loaded craft off the floor, why a heavier craft needs more cushion pressure and a longer launch, how to hold a payload without tape, and why the score formula punishes an early finish exactly as much as a late one.

Event format

Impound changes how you prepare. Anything you want during competition time, including spare parts and your calibration sheets, must go into impound with the vehicle. Nothing new on paper may come into the competition area afterwards. Plan your calibration tables and your run log before you check in.

What the rules cover

The 2027 rules manual has the same Hovercraft text for Division B (pages B31 to B34) and Division C (pages C37 to C40) except for one number: the top of the Target Time range. It has seven numbered parts. The table below summarizes each part in plain words; read the manual itself for exact wording.

If you are working from an older guide or last season's notes, three changes matter most. First, the bonus run and Bonus Score are gone; the Final Score is simply your best Run Score. Second, the Distance Score and Time Score are now worth 40 points each, and the remaining 20 points come from the nickels. Third, competition time ends early once you have made two complete runs or two incomplete runs, so you get very few attempts.

The 2027 payload: ES-provided nickels

The Event Supervisor brings a fixed supply of U.S. nickels for every team:

A U.S. nickel has a specified mass of 5.000 g, a diameter of 21.21 mm and a thickness of 1.95 mm, so a full roll is a cylinder roughly 21 mm across and about 80 mm long once wrapped. Build your payload holder around those dimensions, but leave a little slack: bank wrappers vary, and the half roll is shorter and may be slightly irregular where the ends were trimmed.

The rules on carrying them are strict, and most of them exist so that every team works with the same unmodified load:

• You choose how much to carry, from nothing up to the full supply, and you may change the load from one run to the next.
• You may not unroll, break or otherwise alter the packaging of the rolls.
• You may not use tape or adhesive to fix rolls or nickels to the vehicle. A vehicle that needs tape or glue to hold the nickels may not carry any masses for the rest of its competition time.
• The nickel load goes on the vehicle before the run begins, when you place the craft behind the start line.
• Any roll or nickel that falls off before the Event Supervisor tells you to switch off at the end of the run does not count toward the Mass Score. Anything falling off (a vehicle part, a roll or a single nickel) also costs one construction penalty for that run, not one per item.

The practical consequence is that the holder is part of your vehicle design. You need a tray, cradle or set of slots that grips each roll and keeps loose coins from sliding out, using only its own shape, clips or covers. Because the holder must work without tape, test it with real bank rolls, and test it with a hard stop as well as a smooth run.

Vehicle physics: floating on an air cushion

A legal craft must levitate on a cushion of air while it moves down the track. The Event Supervisor may test this by pressing the craft down slightly: if it rises again, it is levitating. For 2027 the rules also say that touching the track surface and the inner faces of the side rails is allowed, while touching the tops or outer faces of the rails is not. So the goal is not zero contact. It is to keep enough air under the craft that contact friction stays small, and to keep the craft level and stable while it carries its load.

Here is the causal chain that holds a craft up:

↑ fan flow → ↑ pressure in the plenum (the space under the deck) → ↑ net upward force on the deck → the craft rises → the skirt gap opens → more air escapes under the skirt → the plenum pressure falls back until the lift force equals the weight.

The craft settles at the height where the air the fan supplies equals the air leaking out under the skirt. That equilibrium is why a skirt with a larger gap needs more fan flow, and why a skirt that is too tight makes the craft rub. The plenum is the enclosed region under the vehicle that the fan fills; it is the single word that connects the fan to the cushion.

Two design consequences follow. The first is the footprint. The craft must fit inside a 40.0 cm cube when it is levitated and ready to run, so the cushion can never be larger than 40 cm by 40 cm, which is 0.160 m². A larger cushion lowers the pressure needed for a given mass. The second is mass. With up to a kilogram of nickels available, the payload can easily outweigh the craft itself, so a light structure leaves more of the fan's capacity for the load.

The speed of air escaping under the skirt is set by the pressure difference. For an ideal, loss-free gap, Bernoulli's principle gives $v = \sqrt{2\,\Delta p / \rho}$, with $\rho \approx 1.2\ \text{kg/m}^3$ for air near sea level. Real leakage is slower because the flow contracts and loses energy at the gap, so treat the formula as an upper bound that shows the scaling.

How much harder the fan works with a full load

The ideal power the lift fan delivers to the cushion is the pressure times the volume of air per second that leaks out. Both rise when you add nickels: the pressure rises in proportion to the mass, and the leak speed rises with the square root of the pressure.

Where to put the load

The cushion pushes up over its whole area, so the craft balances best when its center of mass sits over the center of the cushion. If the nickels sit toward one side, that side sinks, its skirt gap closes and it drags on the track, while the opposite side lifts and leaks more air. The result is a craft that tilts, rubs and yaws toward the heavy side. Mount the payload holder near the middle of the deck, and if you plan to carry partial loads, arrange the slots so that every load you might choose stays balanced: for example, fill rolls in symmetric pairs, and place the half roll and loose nickels on the centerline.

Thrust, drag and motion

Many hovercraft use two air systems. A lift fan feeds the cushion and one or more propellers push the craft forward. Others use a single fan and split its flow between the cushion and a rear thrust duct. Either way, the 2027 rules require all lift and all thrust to come from air pressure. Wheels that drive the craft, rubber bands or springs that push it, and any other non-air propulsion are out. Non-electric energy may still power other functions, such as a timing mechanism.

Thrust works through Newton's third law. A propeller pushes air backward, and the air pushes the craft forward with an equal and opposite force. The craft's acceleration then follows Newton's second law.

A steady speed means zero net force, so drag equals thrust. Students who believe a moving object needs a continuing net force to keep moving make the classic error here. A net force changes velocity; it is not needed to hold a velocity once thrust balances drag and friction.

Average speed is what the Target Time measures

The run time runs from the moment the dowel crosses the start line until it crosses the finish line, 185.0 cm later. The Target Time is therefore a target for the average speed over the timed section:

$$\bar v = \frac{d}{t} = \frac{1.85\ \text{m}}{TT}$$

At the extremes, a 6.0 s target needs 1.85 / 6.0 = 0.308 m/s. The Division B maximum of 18.0 s needs only 0.103 m/s, and the Division C maximum of 26.0 s needs 0.071 m/s. A 10.0 s target needs 0.185 m/s. The slow end is hard in its own way: at 0.07 m/s, small amounts of friction or a slight slope in the track can stall the craft, and a stall of 3 seconds makes the run incomplete.

Using the start position: a running start

For 2027 you may place the craft anywhere behind the start line, held by the Event Supervisor's wood block. Because timing begins only when the dowel crosses the line, the launch acceleration can happen before the clock starts. If the craft has already reached its cruising speed when the dowel crosses, the timed section is run at nearly constant speed and $t \approx 1.85 / v$, which is far easier to calibrate.

There is a limit. If the dowel does not cross the start line within 3 seconds of launch, that attempt is not counted as a run. So you need to know how far back you can start.

Compare a standing start with the dowel on the line. The craft would spend its first 1.57 s at below-cruise speed, adding $v / (2a) = 0.185 / 0.236 = 0.78$ s to the run. That is 3.1 Time Score points lost at a 10.0 s target if you did not plan for it, and the loss changes with the load.

Kinetic energy, friction and drag at low speed

Air drag is far smaller at these speeds. With the drag equation $F_d = \tfrac{1}{2}\rho v^2 C_d A$, a rough $C_d \approx 1$ for a blunt shape, a frontal area of 0.040 m² and $v = 0.185$ m/s, $F_d \approx 0.5 \times 1.2 \times 0.185^2 \times 1 \times 0.040 \approx 0.0008$ N. Contact friction is the force that decides whether your run is repeatable, and it grows when a heavy load is badly placed or the cushion is underpowered. That is why the base surface rules matter: the track may sit on a table, a built-in base or the floor, small imperfections are allowed, and you may only dry-sweep it, not wet it or treat it.

Impulse, the barrier, and keeping the nickels aboard

The Event Supervisor supplies a cushioned barrier at least 5.0 cm past the finish line. For 2027 the barrier matters to your score, because any nickel that falls off before you are told to switch off does not count and triggers a construction penalty. The stop at the barrier is often the hardest moment for the payload.

The same reasoning applies to rolls. A roll lying lengthwise across the craft can roll forward at the stop, so cradles that hold each roll in a groove or under a retaining bar are safer than a flat deck. A soft bumper on the nose of the craft also lengthens the stopping time and reduces the force on the payload, the dowel and the frame.

Construction rules and why they exist

Division B and Division C share the construction section word for word. The table paraphrases Section 3 of the 2027 rules.

The safety parameters in 3.f are different from the rest. A vehicle that breaks any of them is not allowed to run until it is fixed, and if you cannot fix it during your competition time you receive participation points only. Other construction problems are scored as construction violations, each of which multiplies a run's scores by 0.8.

Speed control without a chip

Because integrated circuits are banned outside a commercial motor's own built-in electronics, a separate electronic speed controller is not allowed. That leaves a few legal ways to set thrust:

• Battery count. Changing the number of cells changes the voltage in steps (about 1.2 V per NiMH cell, 1.5 V per alkaline cell). Steps are coarse but repeatable.
• Series resistance. A rheostat or potentiometer in series with a motor lowers its current and speed. It wastes power as heat, and a small potentiometer can overheat at motor currents, so check its power rating.
• Propeller choice and pitch. A smaller or lower-pitch propeller gives less thrust at the same voltage.
• Thrust deflection. A vane that diverts part of the thrust air sideways or downward reduces forward thrust without touching the circuit.

Batteries, relays and timing mechanisms

With lithium banned, most teams use NiMH or alkaline cells. NiMH holds its voltage more steadily through a discharge, which helps repeatability, while alkaline voltage sags noticeably as the cell drains. Whatever you use, the label voltage decides the 12.0 V limit, not a meter reading, and the labels must be the manufacturer's originals.

Relays and mechanical switches open up simple automation. A relay lets one switch control a higher-current motor circuit. A mechanical switch tripped by another part of the vehicle, for example by a lever driven by a wind-up timer, could cut the thrust motor after a set time. Be careful with any shut-off scheme: a craft that stops moving for 3 seconds before the finish line makes the run incomplete, and the incomplete run loses its whole Time Score.

The run: from launch to finish

Knowing the sequence lets you plan the 8 minutes. The Event Supervisor checks the vehicle against the specifications before your competition time. If anything is out of spec, you are told as soon as possible and may fix it during your time. Then you receive the Target Time, and the 8-minute clock starts. Practice runs are not allowed.

• Prepare the track. You may dry-sweep the track and measure it and its surface. You may not wet it, put anything on it, or ask for it to be moved or re-leveled.
• Load and place. Put the vehicle, with its nickel load already on it, anywhere fully behind the start line. The Event Supervisor holds a wood block against the front of the craft. Switch on the motor or motors, then take your hands off the vehicle.
• Launch. Count down "3, 2, 1, launch". The Event Supervisor removes the block. Timing starts when the dowel crosses the start line and stops when it crosses the finish line.
• Hands off. From the moment your hands leave the vehicle at the start, you may not touch it or interfere with it in any way until the Event Supervisor says so. A touch or interference turns the run into a complete run with Distance, Time and Mass Scores of zero.
• Incomplete runs. A run is incomplete if the craft stops moving for 3 seconds before the finish line, or if the dowel has not crossed the finish within twice the Target Time. You cannot declare a run incomplete yourself. The Event Supervisor blocks the craft where the run was declared incomplete, you switch it off without moving it, and the distance from the finish line to the dowel is measured. Your clock pauses during the measurement.
• Not a run. If the dowel does not reach the start line within 3 seconds of launch, the attempt does not count as a run at all.
• Late starts. A run that begins before the 8 minutes are up is allowed to finish and is scored.
• Run limit. Competition time ends after two complete runs or two incomplete runs, or at 8 minutes, whichever comes first.
• Afterwards. The Event Supervisor reviews your scoresheet with you. To appeal, you must tell the Event Supervisor and leave the vehicle in impound.

Using the ramp is optional and may be decided run by run. The Event Supervisor places it in the same position for every team, and setting it up comes out of your 8 minutes. A ramp run has its Distance Score halved and its Mass Score set to zero, so the ramp is now a last resort for a craft that cannot start on the flat.

The track

The Event Supervisor supplies the track, so you build and calibrate against its stated dimensions. Build a faithful copy for practice.

The beams sit about 17 cm above the track and the top of your dowel must be at least 20 cm up, so the dowel always reaches beam height. What you must check is that nothing else on the craft, such as a propeller guard or a tall payload cover, reaches the beam before the dowel does. The tournament may run several tracks at once to keep teams moving.

Scoring

Scoring is the part teams most often misread, so this section works each formula with numbers. The formulas below are written in the manual's terms; check the scoring spreadsheet at soinc.org as well.

Notice what the Mass Score formula implies. A full roll of 40 nickels earns 4 points, which is 0.1 point per nickel, exactly the value of a loose nickel. So every nickel is worth 0.1 point however it is packaged, which is 0.02 point per gram, or 20 points per kilogram. Rolls are not worth more; they are simply far easier to hold securely than twenty loose coins.

The Time Score is symmetric: finishing 0.5 s early costs the same as 0.5 s late. Its sensitivity depends on the Target Time. At a 6.0 s target each 0.1 s of error costs $40 \times 0.1 / 6.0 = 0.67$ point. At 18.0 s it costs 0.22 point, and at 26.0 s only 0.15 point. Short targets are harsh and long targets forgiving, but long targets carry a greater risk of a stall.

Ties are broken in this order: the best Time Score from any run; then the fewest construction violations in the team's run that had the most of them (for a team with no runs, the count from the construction check); then the best Mass Score; then the second-best Run Score. The first tiebreaker means a precise Time Score matters even in a run that is not your best overall, and the third rewards carrying the load in every run.

Design principles for a repeatable loaded run

The design goal is a predictable average speed with the load you choose, using the power you can legally carry. Four principles drive most good designs.

Decide your load first. The full supply is worth 20 points, half a Distance Score. Because one dropped coin costs a 0.8 multiplier, the safest high-value loads are the ones you can hold mechanically. Four full rolls (16 points) plus the half roll (2 more) in fitted cradles is 18 points of low-risk mass. The last 2 points need twenty loose coins held in a closed tray or stacked slots. Design the vehicle around the load you will actually carry, then size the fan for it.

Keep the structure light and the cushion efficient. Lift pressure scales with total mass, and lift power with pressure to the 3/2 power. Every gram of frame you save is a gram of nickels you can carry for the same fan, or a larger lift margin for the same load. Use the largest cushion the 40 cm cube allows and place the load over its center.

Make thrust controllable, not large. The score rewards hitting a target average speed, so a stronger motor is not better. Excess thrust reaches speed early and makes the run harder to repeat. Pick a propeller, motor and battery combination whose output you can set repeatably with the legal controls (battery count, series resistance, propeller choice, thrust deflection), and calibrate it for each load.

Design the dowel and the payload cover together. The dowel must be within 3.0 cm of the front edge and lead the craft through the timing beam, and the payload holder must not stick up in front of it. A dowel that wobbles changes the timing, so make it a structural part of the frame. Make sure the propeller shields, which must stop a 3/8 inch dowel from reaching any blade, do not choke the airflow.

Testing logs and calibration

Practice runs are not allowed during competition time, and you may get as few as two runs, so all calibration happens before the tournament. Keep one row per practice run and compute the Run Score as you go. The table below is a sample log for a 10.0 s Target Time.

In run 1 the distance factor is $(185 - 37)/185 = 0.800$, applied to both the Distance and Mass Scores. Runs 3 and 4 show the loose-coin gamble: the best run of the day and a 20-point loss with the same configuration. A log like this tells you whether your holder is reliable enough to risk the last 2 points.

Record four things on every practice run: the load, the start position behind the line, the run time, and where any incomplete run stopped. Measure the lift margin at the start of each session. Repeatability is the real skill. If your complete runs with a given load are 9.2, 10.8 and 9.7 s, the average is close, but the spread is far too large to hit a half-second target at States.

Your impounded notes should include a calibration table for every Target Time your level can produce, for each load you might carry: the target average speed (1.85 m divided by the Target Time), the thrust setting, and the start position that gave that speed on your test track.

Strategy

Split the work by role. Both partners must be able to answer questions about the vehicle. A practical split is one student who loads, places and launches the craft and talks to the Event Supervisor, and one who keeps the run log, computes the score and checks the payload after each run. Swap roles in practice.

Plan for two runs. Competition time ends after two complete runs or two incomplete runs, so assume you will get two. Make the first run your safest scoring run: your best-calibrated load at the Target Time. Use the second either to repeat it or to add the riskier loose nickels if the first went cleanly.

Use the start position deliberately. A running start from 25 to 30 cm behind the line (for the loaded craft in the example above) removes launch lag from the timed section. Measure your own 3-second limit for each load and stay well inside it.

Check the track before your first run. Dry-sweep the lane, measure its width, and look for slopes or seams. You cannot ask for re-leveling, so if the surface slopes, adjust your thrust setting rather than the track.

Protect the impound and the safety check. A missed impound multiplies every run by 0.7, and a safety failure means participation points only. Before check-in, run a checklist from Section 3: the 40 cm cube with the load aboard, the dowel position and height, shielding on every propeller and impeller, original labels and no more than 12.0 V by label, no lithium or lead, no lasers, no integrated circuits outside a commercial motor, and an accessible manual switch on every motor. Impound your spare parts with the vehicle, and the batteries separately.

Know the appeal rule. To appeal you must tell the Event Supervisor and leave the vehicle in impound, so decide what you would dispute and how you would show it.

For the experimental discipline behind the logs, see the Experimental Design guide, which covers controls, repeated trials and variation. The Electric Vehicle guide covers another build event where the whole game is calibrating a run to a target, with the same logging and repeatability habits.

Common pitfalls

How tournaments test this

Hovercraft in 2027 has no written test. Your score comes entirely from your runs: the Distance, Time and Mass Scores of your best run, adjusted by penalty multipliers. The Event Supervisor also checks the vehicle against the construction and safety rules before your competition time and may ask either partner questions about how the vehicle was designed, built and operated. Being unable to explain your own vehicle is a problem under the building policy, so both partners should be able to describe the lift system, the thrust and speed control, the payload holder and the calibration method.

The question archive does contain written Hovercraft material from seasons when the event included a test, mostly the 2017-18 season. The archive holds 984 questions tagged to this event, 920 of them (93.5%) from 2017-18, and 763 (77.5%) from Division C tests. Those questions are general mechanics and fluids with a hovercraft in the story: forces and Newton's laws, kinetic energy, impulse, and cushion pressure. They are not part of the 2027 scoring, but the subset that matches the physics in this guide is a good way to check that you can explain your vehicle, so a selection appears below.

The recurring vocabulary in those questions mirrors the physics you need for the build: force, velocity, mass, acceleration, energy, kinetic energy, net force, constant velocity and air resistance. One archived question even asks for the blower pressure needed to carry a stack of coin rolls on a 40 cm footprint, the same calculation as the cushion-pressure example above.

Test yourself

These archived tournament questions cover the physics behind a loaded hovercraft. They come from seasons with a written test and are practice for understanding and explaining your vehicle, not a preview of a 2027 test. Answer each before you open the link, and check the official key against the explanation.

Test yourselfTest yourself: lift, cushion pressure and payload

0 / 0 correct

1. 1

   Your hovercraft has the maximum ground footprint of 40.0 cm x 40.0 cm. You have decided to maximize your mass score and therefore plan to have your hovercraft carry all 16 rolls of pennies, which have a total mass of 2.00 kg. The expected mass of your vehicle is 1.00 kg. Assuming the air pressure beneath your craft is uniform, what is the minimum static pressure that your blower must produce? Show your work.

   Show answer

   From: Iolani Invitational, 2017-18, Division C

Test yourselfTest yourself: thrust, motion and stopping

0 / 0 correct

1. 1

   A 750 kg hovercraft is traveling at a constant speed while its fan generates a continual thrust of 1500 Newtons. What is the sum of all the drag forces on the craft.

   Show answer

   From: Columbia Invitational, 2017-18, Division C

References and further reading

• Science Olympiad: the 2027 rules manual (Hovercraft B, pages B31 to B34; Hovercraft C, pages C37 to C40), the building policy, example track and ramp setups, and the scoring spreadsheet. The manual is the authority; this guide paraphrases it.
• U.S. Mint, coin specifications: mass, diameter and thickness of the nickel.
• SciOly Wiki: Hovercraft: the community page for the event. Link only; this guide does not reproduce its content.
• OpenStax, University Physics Volume 1: Newton's laws, impulse, kinetic energy, friction and fluid pressure.
• NASA Glenn Research Center, Bernoulli's principle: pressure and speed in a moving fluid.
• Figure credits appear beside each figure. All figures are from Wikimedia Commons under free licenses.