“>Math 355-01 – Pro ject – Pop-Pop Boat
• This assignment constitutes 10% of the course grade and will be graded out of 10 points.
• You should turn in a well-documented spreadsheet with appropriate graphs.
• Any student who builds a functional pop-pop boat will gain 2 bonus points, equivalent to 2% of the course
• The student who builds the fastest pop-pop boat will earn a special prize.
The goal of this project is to use numerical
methods to implement a simple model for the motion of a pop-pop boat, similar to the one depicted
at right. The pop-pop boat uses a simple steam
engine for locomotion, but has no moving parts.
Basic Principle of Operation
1. Heat from the candle eventually causes some of the water in the boiler to flash into a steam bubble, which
takes up significantly more volume than the water it replaces.
2. The creation of the bubble forces water out of the exhaust tube(s) and the boat is pushed forward in the
3. The steam bubble cools rapidly once outside the boiler and after a short amount of time it will condense,
creating a void in the exhaust tube(s).
4. A vacuum exists initially in the void and pulls water into the exhaust tubes while also pulling back
somewhat on the boat.
5. The cycle is now complete and will restart once the heat builds up enough to produce another steam
Mathematical Mod el
The mathematical model considered in this project ignores many aspects of the real system in favor
of a simple set of equations that captures the essential dynamics. The model focuses on two important
components of the pop-pop boat: (a) the oscillation of a water column in the exhaust and (b) the
propulsion of the boat due to the jet of expelled water from the exhaust. The system of differential
dt = v
dt = h − k 1x − k 2v dXdt = V dVdt = k 3(v +) 2 − k 4V,
where x(t) represents the displacement of the water column in the exhaust tube, v(t) is the oscillation
speed of the water column, X(t) is the distance travelled by the boat, and V(t) is the speed of the
boat. Note that v
+ is defined as follows,
v + = ( v v0 v <≥ 0.
The use of v
+ is related to the fact that water expelled from the exhaust produces a thrust force on
the boat, while there is a much smaller force pulling on the boat as water enters the exhaust tubes.
The role of the constants will now b e explained.
• The constant h > 0 represents heat added to the boiler, which has a tendency to produce steam
and, consequently, displace the water column.
• The constant k 1 > 0 accounts for two effects. First, as steam moves farther away from the boiler,
the rate of condensation will rise, forcing the water column to recede. Second, any air or steam
behind the water column will be compressible (i.e., springy) and, thus, resistant to displacement.
• The constant k 2 > 0 quantifies small frictional losses as the water column oscillates in the exhaust
• The thrust produced by the expelled water column is proportional to (v +) 2 and the constant of
proporationality is k 3 > 0.
• The constant k 4 > 0 represents drag forces on the hull of the boat as is slides through the water.
The following values will be assumed for the various constants for the remainder of this assignment.
h k 1 k 2 k 3 k 4
2 250 0.001 1.3 0.07
1. Set up a spreadsheet to obtain an approximate solution of the differential equations using the
improved Euler’s method over the time interval 0 ≤ t ≤ 20. The oscillations in the boiler occur
rapidly and this will necessitate a small step-size ∆t. Determine an appropriate value for ∆t
by starting with 0.1 and reducing it until the solution behaves nicely. Insert separate graphs
depicting each variable as a function of time.
2. Determine the maximum speed of the pop-pop boat (v is in units of meters per second) and
convert this to inches per second. What effect does increasing the heat constant by 25% have on
the maximum speed? Illustrate this with your spreadsheet.
3. Determine the oscillation frequency of the boiler using the given values for the constants. Is the
frequency affected by the heat constant h?
“>Math 355-01 – Pro ject – Pop-Pop Boat