Using slow-speed submersible mixers (typically used in wastewater treatment) for forward propulsion is highly unique. The massive 2.5-meter propellers operate at very low RPMs (often between 20 and 50 RPM). Therefore, you will not experience high-pitched cavitation or the "buzz" of a traditional boat outboard.
However, the 1/4" duplex stainless steel columns, pressurized to 10 psi, will act like tuned drums. While the pressurization increases structural rigidity (preventing the metal from buckling under thrust), it also creates a highly resonant chamber. The primary source of vibration will be the Blade Pass Frequency (BPF)—the rhythmic pressure wave created each time a massive propeller blade passes near the steel column.
| Speed Target | Motor / Prop Operation | Vibration Level | Acoustic Noise (Inside Living Area) |
|---|---|---|---|
| 0.5 MPH (Cruise / Loiter) |
Motors running at very low power. Due to the high drag coefficient of the platform and cables, minimal thrust is needed. Propellers spinning at bare minimum RPM. | Imperceptible. The 1-inch rubber pad will easily handle the minor high-frequency gear mesh vibrations. Low frequency thrust pulses are too weak to travel up the legs. |
Near Silent. You may hear a very faint, distant hum underwater if you put your ear to the floor, but ambient ocean sounds/waves will entirely mask it. |
| 1.0 MPH (Standard Transit) |
Drag is roughly 4 times higher than at 0.5 MPH. Motors are spooling up. Planetary gears are engaging with more torque. | Mild to Noticeable. You will feel a subtle, rhythmic "throbbing" or low-frequency pulsing feeling (1 Hz to 3 Hz) through the floorboards. The 1-inch rubber pad cannot absorb low-frequency waves, allowing the Blade Pass Frequency to transmit to the deck. |
Low Rumble. The pressurized steel legs will begin to resonate slightly. Inside the living area, it will sound comparable to a distant idling diesel truck or a large refrigerator hum. |
| 1.5 MPH (Sprint / High Current) |
Drag is roughly 9 times higher than at 0.5 MPH. Submersible mixers are reaching maximum torque output. At this speed, the water velocity starts pushing the limits of what a stationary mixer blade is designed to push against. | Significant. The thrust vectors will cause the 24-foot legs to flex slightly against the stabilizing cables. The entire platform will experience a noticeable, continuous rhythmic shaking (shudder). |
Moderate / Bothersome over time. The "drum effect" inside the 10-psi pressurized cylinders will be prominent. Expect a distinct, low-pitched droning noise. The stabilizing cables may also begin to "strum" due to vortex shedding, adding a harmonic hum. |
While the 1-inch tire-rubber layer between the floats and main body is excellent for blocking high-frequency electrical hums and metallic clanking, dense rubber does practically nothing to stop low-frequency, high-energy vibrations (under 10 Hz) generated by massive 2.5m blades. To achieve a quiet living space, implement the following at the mixer mounting point:
Instead of bolting the submersible mixers directly to the duplex stainless steel legs, mount them using heavy-duty stainless steel Wire Rope Isolators. These are loops of steel cable that compress and expand, absorbing massive low-frequency torque shifts and thrust pulses before they ever reach the steel resonant column.
Ensure that there is no metal-to-metal contact between the mixer bracket and the leg. Sleeving the mounting bolts in heavy polyurethane (used in automotive suspension) will effectively decouple the motor from the "drum."
Push the mixer mount as far practically outward from the steel leg as possible. The primary BPF vibration happens because the propeller pushes a wall of water against the cylinder wall as it rotates. Moving the prop slightly further away dramatically reduces this hydraulic pounding against the 1/4" steel.
If the inside of the floats are accessible before pressurization, spraying a layer of polyurea (like truck bed liner) or applying mastic acoustic tiles to the inside of the 1/4" steel will "deaden" the drum effect without adding significant weight.