```html Estimated Noise & Vibration From Submersible Mixers (Thrusters)

Estimated Noise & Vibration From Submersible Mixers/Thrusters at 0.5–1.5 mph

Important: With the information provided (no RPM, motor power, prop geometry, ducting, control method, mounting details, distance to living spaces, and no drag estimate), any numerical noise/vibration values can only be order-of-magnitude ranges. The speed in mph does not directly set noise; the thruster RPM, blade loading, and cavitation margin do. The most useful thing we can do now is (1) give realistic ranges and (2) show how noise scales with speed if drag is roughly quadratic.

1) Key scaling (why noise tends to rise fast with speed)

For a platform-like structure (not a streamlined hull), drag is often approximated as:

Drag ∝ V² and required propulsion power roughly P ∝ Drag · V ∝ V³

So going from 0.5 mph to 1.0 mph can require ~8× the mechanical power, and 0.5 mph to 1.5 mph can require ~27×. Acoustic noise often increases with mechanical power (very roughly), so a common first-cut estimate is:

0.5 → 1.0 mph: ~8× power ⇒ ~+9 dB (because 10·log10(8) ≈ 9)
0.5 → 1.5 mph: ~27× power ⇒ ~+14 dB (10·log10(27) ≈ 14)

These dB changes are “source power” style estimates. Actual perceived onboard noise may increase less (or more) depending on structure-borne paths, resonances, and isolation.

2) What dominates noise & vibration for large slow propellers

Non-cavitating operation: tonal components at blade-pass frequency, plus motor/drive noise. Usually “low-frequency hum.”
Approaching cavitation: noise rises steeply and becomes broadband (“hiss/roar”), often the single biggest jump.
Structure-borne vibration: depends heavily on how thrust loads are reacted into the structure (hard-mounted thruster = much higher transmission).

With a 2.5 m diameter propeller, it is possible to stay very quiet if you keep tip speed and blade loading low enough to avoid cavitation. Most “bad” outcomes come from (a) too much thrust per prop, (b) too high RPM, (c) inflow turbulence near structure, or (d) rigid thrust paths into the platform.

3) Estimated ranges at 0.5 / 1.0 / 1.5 mph (4 thrusters total)

Below are practical, broad ranges assuming: (1) four identical thrusters sharing load, (2) large, slow propellers intended to avoid cavitation, (3) decent but not perfect vibration isolation (you mentioned a 1-inch rubber layer plus potential mixer mounting isolation), (4) living area above water with structural paths through the legs/columns.

Speed	Relative thrust / power trend	Underwater noise (source level) ballpark*	Onboard (living space) noise & vibration ballpark*	What you’re likely to notice
0.5 mph (0.22 m/s)	Lowest thrust demand. If control is smooth and RPM is low, likely non-cavitating.	~145–160 dB re 1 µPa @ 1 m (combined from multiple thrusters can add ~+6 dB vs one)	~30–40 dBA in living area (quiet room to light HVAC level), vibration often <0.2 mm/s RMS if isolation is effective.	Low-frequency hum; possible faint tonal “thrum” at blade-pass frequency.
1.0 mph (0.45 m/s)	~4× drag and ~8× power vs 0.5 mph (typical). Noise commonly rises by ~+6 to +12 dB if still non-cavitating.	~150–170 dB re 1 µPa @ 1 m (wide range; strongly dependent on RPM and inflow turbulence)	~35–50 dBA in living area, vibration often ~0.2–0.6 mm/s RMS if thrust is not hard-coupled into the main structure.	Hum becomes clearly audible indoors at night; structure-borne “buzz” may appear if any resonance is excited.
1.5 mph (0.67 m/s)	~9× drag and ~27× power vs 0.5 mph (typical). Risk of cavitation increases if RPM or blade loading rises too far.	~155–175 dB if non-cavitating, but can jump to ~170–185 dB if cavitation starts.	~40–60 dBA in living area (noticeable/possibly annoying), vibration often ~0.5–1.5 mm/s RMS if any hard structural path exists or if cavitation occurs.	If cavitation: broadband “roar/hiss” and noticeably higher vibration. Without cavitation: stronger tonal components.

*These are not guarantees—just plausible bands. Real outcomes can be quieter with excellent prop selection + isolation, or much louder if cavitation or structural resonance occurs. Underwater “dB re 1 µPa @ 1 m” is the common acoustic unit for marine source level; onboard values are approximate A-weighted airborne levels in the living space.

4) How to reduce vibration transmission (practical mounting guidance)

A 1-inch rubber layer between floats and the main body helps, but the critical detail is: where does the thruster thrust load go? If thrust is reacted through stiff members directly into the living platform, rubber layers elsewhere may not help much.

Use a dedicated “thruster subframe” at each leg bottom with resilient mounts to the leg shell/frame.
Avoid rigid thrust paths (e.g., a hard strut directly into the main deck structure).
Use elastomer/isolator mounts sized for static + dynamic loads (with proper shear/compression design, not just “soft rubber”).
Keep the prop inflow clean: mounting too close to columns, cables, or corners increases turbulence → vibration + noise.
Direct drive / low gear noise: gearboxes can add tonal noise; if gearing is needed, isolate it and keep mesh frequencies away from structural resonances.
Prevent cavitation: keep prop tip speed modest, increase blade area if needed, consider ducted props if appropriate, and avoid operating at high thrust in aerated water near the surface.

5) What I would need to produce a tighter estimate

Propeller diameter (you gave 2.5 m), blade count, pitch, and whether it is ducted or open.
Max and typical RPM at each target speed and the motor/drive type.
Estimated total drag at 0.5/1.0/1.5 mph (or enough geometry to approximate Cd·A).
Thruster distance to living area and the structural path (hard points, frames, bulkheads).
Mounting concept drawing (even a simple sketch): where isolators go and how thrust is reacted.

6) Quick sanity-check: expected change with speed (relative)

Speed	Relative power (if P ∝ V³)	Expected noise change vs 0.5 mph (rule-of-thumb)
0.5 mph	1×	Baseline
1.0 mph	8×	~ +9 dB (often observed as “about twice as loud” subjectively, depending on frequency)
1.5 mph	27×	~ +14 dB (unless cavitation starts, in which case the increase can be larger)

If you want, I can convert your geometry into a rough drag estimate and back-calculate approximate thrust per mixer and RPM ranges that keep tip speed low and avoid cavitation—then the noise/vibration estimates can be narrowed substantially.

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