Before the file, three quick flags on assumptions I had to make (all called out inside the HTML too):
1. **Leg count:** Your description says 3 legs everywhere except the mixer sentence ("each of the 4 legs"). I assumed **3 mixers**, matching the trimaran layout. If it's really 4, per-mixer thrust drops ~25%, rpm drops ~13%, and noise drops another ~1–2 dB.
2. **Mixer class unknown:** "Slow-speed submersible mixer" reads like the wastewater-treatment class (large prop, low rpm, e.g., Flygt 4600-type). I ran two brackets: **Class A (large/slow, recommended)** and **Class B (compact/high-speed)** so you can see the design lever.
3. **Why the numbers look low:** At 0.5–1.5 mph your hull drag is only ~45–390 N total. That's 1–8% of a typical mixer's rated thrust, so the mixers loaf along at 30–100 rpm — the noise is dominated by motor hum, not hydraulics.
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Seastead — Submersible Mixer Noise & Vibration Estimate
Submersible Mixer Noise & Vibration Estimate
Seastead concept · 3 foil legs · mixers as low-speed auxiliary propulsion · speeds 0.5 / 1.0 / 1.5 mph
Scope note: The description specifies three legs/floats throughout, but the mixer
paragraph says “each of the 4 legs.” Estimates below assume 3 mixers (one per leg), all running and
load-sharing. If a 4th float exists, per-unit thrust falls ~25%, rpm falls ~13%, and levels drop a
further ~1–2 dB — the conclusions do not change.
Bottom Line Up Front
0.5 mph
≈ 45 N total thrust · mixers ≈ 33 rpm
Imperceptible
Below ambient sea noise. Nothing felt or heard indoors.
1.0 mph
≈ 175 N total thrust · mixers ≈ 67 rpm
Barely perceptible
Possible faint hum outdoors near a leg; masked indoors.
1.5 mph
≈ 390 N total thrust · mixers ≈ 100 rpm
Faint low hum
Audible outdoors in dead-calm conditions; indoors generally inaudible.
Headline reason: at these speeds the hull needs only 1–8% of a typical mixer’s rated thrust,
so the props turn at 30–100 rpm. Noise is dominated by motor/VFD hum, not hydraulics, and blade-pass
frequencies fall in the infrasonic range (felt as gentle pulsation at worst, not heard).
1 · Assumptions
Mixer Class A (baseline): wastewater-style low-speed unit — ~0.9 m (3 ft) prop,
3 blades, 350 rpm max, ~3 kW, ~1,600 N rated thrust.
Mixer Class B (comparison): compact high-speed unit — ~0.45 m (18 in) prop,
3 blades, 1,450 rpm max, ~2.2 kW, ~700 N rated thrust.
All mixers run simultaneously and share load equally (best practice — see §7).
Mixer depth ≈ 2 m (near bottom of half-submerged legs); salt water; non-cavitating operation.
Thrust demand from hull drag estimate (§2), accurate to roughly ±50%.
1″ rubber joint between legs and main body in place; resilient mixer mounts recommended (§7).
All levels are concept-stage engineering estimates (±5–10 dB), to be confirmed with
vendor data and sea trials.
2 · Step 1 — Thrust Required at Each Speed
Drag of the three foil legs (friction + form on the blunt leading edges + appendage allowance) scales
approximately with speed squared:
Vessel speed
m/s
Est. total thrust (3 legs)
Per mixer (3 load-sharing)
0.5 mph
0.22
≈ 45 N (10 lbf)
≈ 15 N
1.0 mph
0.45
≈ 175 N (39 lbf)
≈ 58 N
1.5 mph
0.67
≈ 390 N (88 lbf)
≈ 130 N
3 · Step 2 — Resulting Mixer Duty Point
In near-bollard conditions (vessel speed ≪ jet velocity), thrust scales with rpm² and power with rpm³.
Solving n = nmax·√(T/Trated):
Speed
Mixer rpm (A)
% of rated thrust
Prop power (A)
Tip speed (A)
Cavitation margin
0.5 mph
33 rpm
~1%
~3 W
1.6 m/s
Very large
1.0 mph
67 rpm
~4%
~17 W
3.2 m/s
Very large
1.5 mph
100 rpm
~8%
~70 W
4.7 m/s
Large
Even at 1.5 mph the Class A mixers draw under 100 W each and run at ~29% of rated rpm —
deep in their quiet, non-cavitating regime.
4 · Predicted Noise & Vibration — Class A (Large / Slow — Recommended)
Speed
Mixer rpm
Blade-pass freq.
Airborne, exterior (~3 m, calm sea)
Airborne, interior (living area)
Floor vibration (near leg roots)
Verdict
0.5 mph
33
1.7 Hz
38–44 dBA
25–33 dBA
< 0.05 mm/s
Imperceptible
1.0 mph
67
3.4 Hz
41–47 dBA
28–36 dBA
0.02–0.10 mm/s
Barely perceptible
1.5 mph
100
5.0 Hz
44–50 dBA
31–40 dBA
0.05–0.20 mm/s
Faint low hum outdoors
Open-water wind/wave ambient is typically 40–55 dBA on deck — i.e., in most conditions the mixers are
masked by the environment itself. Underwater radiated noise is estimated at
~100–120 dB re 1 µPa @ 1 m (broadband), decaying to near ocean ambient within ~100 m.
Design takeaway: mixer choice is the single biggest noise lever — roughly a
15–20 dB difference between classes. A large, slow prop producing the same thrust is far
quieter than a small, fast one. Class B at 1.5 mph also puts blade-pass energy (21–31 Hz) right where
floor panels resonate, and tip speed (~14.7 m/s) approaches cavitation inception at 2 m depth.
6 · Frequency Content
Source
Class A (33 / 67 / 100 rpm)
Character
Shaft rate (1×)
0.6 / 1.1 / 1.7 Hz
Infrasonic — never heard; imperceptible
Blade passing (3×)
1.7 / 3.4 / 5.0 Hz
Infrasonic — at most a gentle surge/pulsation; too low to excite typical floor panels (>15 Hz)
Motor magnetic (2× electrical)
~2 / ~4 / ~7 Hz
Infrasonic with VFD; higher-order slot harmonics faint
Bearing tones
> 1 kHz
Very low energy; absorbed by water and structure
VFD switching
2–16 kHz
Negligible once submerged and filtered
With Class A, essentially all mixer excitation is below the audible band. What little
reaches the living area arrives structure-borne through the leg → 1″ rubber joint → frame path, and is
attenuated further by the joint (estimated 10–25 dB above ~60–100 Hz; low frequencies pass more readily,
but they are infrasonic anyway).
7 · Sensitivities, Risks & Mitigations
Run all mixers together, load-shared. Running a single mixer triples its rpm
(+~5 dB, tip speed ×1.7). Shared operation is the quiet mode.
Buy the largest-diameter, slowest mixer that fits. This is worth more than every
other mitigation combined.
Resilient mixer mounts (elastomer isolators rated for submergence): saves ~5–10 dB
indoors. Size isolator natural frequency well below the lowest excitation you care about.
Don’t short-circuit the 1″ rubber joint: any rigid conduit, cable tray, or pipe
bridging the leg-to-body joint transmits vibration directly. Route mixer and thruster cables with
flexible drip-loops across the joint.
Keep props clean. Marine growth unbalances props quickly — imbalance vibration scales
with rpm². Schedule dive inspections; specify factory dynamic balancing (ISO 21940 G6.3 or better).
VFD tuning: program skip frequencies around any measured structural resonance;
use sine/dv-dt filters to limit motor whine.
Cavitation guard-band: maintain ≥ 1.5–2 m immersion and keep tip speed ≲ 15 m/s.
Class A never approaches this; Class B at 1.5 mph is marginal.
RIM drives will usually be louder than these mixers at equivalent thrust (smaller
diameter, higher rpm) — if quiet operation matters, shift slow-speed duty to the mixers.
8 · Reference Scales
Sound pressure level
Level
Anchor
30 dBA
Whisper, quiet bedroom
40 dBA
Quiet library
50 dBA
Refrigerator hum, light rain
60 dBA
Normal conversation
Vibration (continuous exposure)
Level
Perception
< 0.1 mm/s
Imperceptible
0.1–0.5 mm/s
Barely perceptible
0.5–2 mm/s
Clearly perceptible
2–5 mm/s
Annoying if continuous
9 · Verification Plan (Sea Trials)
Instrument one leg root and the floor above it with a triaxial accelerometer (a phone-based app is
adequate for screening).
Log Class-1/2 SPL at deck center, at each door, and inside the living area at all three speeds,
with mixers-only (RIM drives off).
Sweep mixer rpm slowly to find structural resonances; record them as VFD skip bands.
Repeat with one mixer failed (load redistributed) to capture the degraded-mode noise case.
Re-check after 3–6 months of growth accumulation to validate the cleaning interval.
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**Key takeaways:**
- **At all three speeds, mixer noise/vibration should be a non-issue** — roughly 25–40 dBA indoors and 0.05–0.2 mm/s floor vibration at 1.5 mph, i.e., below sea-state ambient and well under annoyance thresholds. The mixers only need 1–8% of rated thrust.
- **The big design decision is mixer selection, not mounting:** a large slow prop (Class A) vs. compact fast prop (Class B) is worth ~15–20 dB. Choose Class A.
- **Watch the details that erode the margin:** rigid conduits bridging the rubber joint, single-mixer operation, prop fouling, and VFD resonances.
If you can share the actual mixer model (or intended purpose — circulation vs. propulsion), I can tighten the duty-point math considerably.