Basic performance evaluation is crucial when developing a Mini Mortar Motorboat (MMMB). Below are simple methods for performance evaluation. Please refer to these, as groups may be required to provide this information during the competition entry stage.
Method for Measuring the Hull’s Saturated Weight: W (g)
The weight W is measured after the hull has been fully immersed in water for at least 24 hours.
Underwater Motor Specifications and Measurement of Submerged Weight with Battery: Wmf (g)
The motor is restricted to the Tamiya Underwater Motor (Yellow).
・Weight (excluding battery): 35.5–36.5g
・Battery (AA Alkaline): Approx. 23–25g depending on the manufacturer.
Therefore, the total dry weight of the underwater motor is approximately 58.5–61.5g.
When a motor weighing 58.5g (including a 23g battery) was measured using Archimedes’ principle to determine its submerged weight:
・Submerged weight of the motor with battery (Wmf): 15.5g
It is recommended to use this as the standard value.
Method for Measuring Loading Volume: Vc (cc)
To prevent the hull from tipping over, secure the external sides or bottom of the hull with fixing materials such as metal fittings, putty, clay, or tape, and then measure the weight Wc0. Next, pour water into the loading area and measure the weight Wc1 at the point where it holds the maximum amount of water without leaking.
The volume is obtained by: Vc = Wc1 – Wc0
Method for Measuring Displacement Volume (Vf) and Mean Draft (H) of the Hull Alone
- To ensure a boat floats, measuring buoyancy is necessary. The first step is measuring the displacement volume. First, place the “saturated hull” in a transparent bucket or container that is not too large for the hull, and measure the total weight Wf0 (bucket + hull).
- Gently pour in water and stop as soon as the hull begins to float. Mark the water level height (H (mm)) and measure the total weight Wf1 (bucket + hull) at that moment. This H represents the mean draft.
- Gently remove the hull, returning as many water droplets as possible to the bucket. Then, measure the total weight Wf2 (bucket + remaining water). If no water has left the system during steps 1–3, Wf1 – Wf0 = Wf2 should hold true. Check if there is a major discrepancy.
- Pour water back into the bucket up to the marking where the hull floated, and measure the total weight Wf3 including the bucket.
- From the above, the displacement volume Vf is calculated as:
Displacement Volume Vf = Wf3 – Wf2
Method for Measuring Displacement Volume (Vfm) and Mean Draft (Hm) of the Hull with Underwater Motor
By replacing the “hull” in the previous section with “a hull containing a weight (such as clay or pebbles) equivalent to the submerged weight of the motor (Wmf),” you can obtain the displacement volume (Vfm) and the mean draft (Hm) for the hull with the underwater motor attached.
Method for Measuring Maximum Displacement Volume of the Hull: Vfmax (cc)
Prepare a transparent container such as a bucket of appropriate size. While empty, place the hull upside down so that air is trapped inside. Then, place Weight A (measure its weight Wa beforehand; paperweights or iron blocks are ideal. If using stones, ensure they are fully saturated) to ensure the hull does not float due to trapped air when water is added. Before pouring water, measure:
Wfb = Container weight + Weight Wa + Hull weight W
(Wfb is not used in the calculation but should be noted for reference.)
Next, pour water into the container until the hull is completely submerged, and mark the water level. Measure:
Wfc = Container weight + Weight Wa + Hull weight W + Weight of water up to the mark (Wq1)
Finally, remove the hull from the container, leaving Weight A submerged. Pour water back to the previous mark and measure:
Wfd = Container weight + Weight Wa + Weight of water up to the mark (Wq2).
Calculate the volume as follows:
Vfmax = Wq2 – Wq1 = (Wfd + W) – Wfc
Design Guidelines
For a hull to be “capable of floating,” it is absolutely necessary to satisfy:
(Hull weight W) + (Submerged motor weight Wmf) < (Maximum displacement volume Vfmax) However, during operation, waves may cause water to accumulate inside the hull, making it impossible to satisfy the above condition. Therefore, for stable sailing, having a sufficiently shallow draft is prioritized. As a general guideline: (Hull weight W) + (Submerged motor weight Wmf) < (Loading volume Vc) It is considered desirable to have this degree of buoyancy margin. Of these three parameters, Wmf is a known constant, and the other two are easily measured, making them useful benchmarks during the development process.
