Noise and vibration analysis for submersible mixer propulsion at operational speeds. Based on 2.5m diameter propellers, duplex stainless steel structure, and rubber isolation mounting.
Analysis based on thrust requirements, propeller blade passing frequency, cavitation onset thresholds, and structural transmission through stainless steel legs.
The dominant frequencies shift with speed. At low RPM, the blade-pass frequency dominates. As speed increases, harmonics and cavitation noise add higher-frequency content.
Blade passing creates pressure pulses at 2-12 Hz depending on RPM. Thrust fluctuations add broadband content.
Duplex SS efficiently transmits vibration (low damping). The 45-degree angle and 24ft length create structural resonances.
Critical damping element. The 1" tire-like rubber provides 60-92% isolation depending on frequency. Most effective above 5 Hz.
Install a rubber or polyurethane bushing between the mixer housing and leg attachment point. Expected reduction: 15-25%.
Use speed ramping instead of steady-state. Avoid resonance frequencies. Brief high-speed bursts are better than sustained operation.
Add tuned mass dampers near the top of each leg, or increase rubber isolation thickness to 2" for better low-frequency performance.
Use 5+ blade propellers instead of 3-4. More blades = lower amplitude per blade = smoother operation. Consider skew/sweep blade design.
Run mixers during daytime only. Use 0.5 MPH for station-keeping, 1+ MPH only when actively relocating. Plan routes using eddies.
Install accelerometers on the living area floor to measure actual vibration levels. This helps validate predictions and detect issues early.
| Parameter | 0.5 MPH | 1.0 MPH | 1.5 MPH |
|---|---|---|---|
| Propeller RPM | 35-50 | 70-95 | 100-140 |
| Thrust per prop | 120-180 lbs | 450-650 lbs | 1,000-1,400 lbs |
| Blade pass freq | 2-4 Hz | 5-7 Hz | 8-12 Hz |
| Airborne noise | 45-50 dB | 55-62 dB | 65-75 dB |
| Underwater noise | 120-130 dB | 140-150 dB | 155-165 dB |
| Vibration amplitude | <0.01 mm | 0.02-0.05 mm | 0.08-0.15 mm |
| Cavitation | None | Slight tip | Moderate |
| Living comfort | Excellent | Good | Fair |
Vibration is below human perception threshold. The rubber isolation effectively absorbs the low-frequency content. Acoustic noise comparable to a quiet library. This speed is ideal for station-keeping and nighttime operation.
Vibration becomes barely perceptible, similar to a refrigerator running. Most people will acclimate quickly. Suitable for extended daytime operation. The slight tip cavitation adds some high-frequency content but remains manageable.
Vibration is clearly noticeable through floors and furniture. Comparable to a washing machine on spin cycle. Light sleepers may be disturbed. Recommend limiting to short bursts for position adjustments or emergency maneuvering.
The duplex stainless steel legs efficiently transmit vibration due to their low damping properties. The 1" rubber isolation layer is critical and provides 60-92% reduction depending on frequency. Consider increasing to 2" for better low-frequency isolation.