Why salt spray beats every other ventilation problem
Salt water reaches your intake in three forms, and each requires a different defense:
| Droplet size | What it is | How you stop it |
|---|---|---|
| > 500 µm | Slugs, sheet spray, green water | Placement + knock-out plenum (gravity does the work) |
| 100–500 µm | Heavy wind-borne spray | Low air velocity in the box (impaction on first baffle) |
| 20–100 µm | Mist | Serpentine baffles + mesh/chevron demister pad |
| < 20 µm | Salt aerosol / haze | Passes almost everything — throttle the intake in AC mode, or accept it and wipe down |
The physics in one sentence: a 100 µm droplet falls at roughly 0.25 m/s and a 20 µm droplet at barely 0.01 m/s — so you cannot rely on gravity alone; you slow the air so big drops can't stay airborne, then impact the fine mist onto surfaces where it coalesces and runs to a drain.
Why you should care: even "invisible" salt accumulation corrodes
electronics, corrodes wiring terminals, coats fabrics, and — when AC coils see humid salt air — creates a
latent load and coil corrosion problem. A vent that passes a cup of seawater in a squall is a bad day; a
vent that passes a fine salt aerosol for weeks quietly destroys the interior.
Historical footnote: the dorade vent — cowl on a baffled box with a
drain — dates to the 1930 yacht Dorade (Olin & Rod Stephens). Dashew's contribution was scaling
the concept up for fast motorboats: engine-room and cabin intakes sized like small HVAC systems, placed in
the aerodynamic shadow of the house, with serviceable demisters.
Placement: the cheapest 80% of the solution
- Avoid the forward third of the boat and anything at deck level forward. A powerboat at 10–12 kn into a 20 kn breeze sees 28–32 kn of apparent wind loaded with spray exactly there. Forward-facing scoops in that zone are ram intakes for seawater.
- Best real estate: aft end of the coachroof/hardtop, or the aft face of the pilothouse — behind and below the roof edge, in the separated, decelerated airflow where most droplets have already been thrown clear or dropped out.
- Face openings aft (or downward), never forward. Aft-facing inlets sample slow, turbulent air and shed slugs of water. If you like traditional cowl vents, use rotatable ones on proper dorade boxes so you can tune them to conditions and point them aft.
- Install intakes in pairs, port and starboard, so you can use the cleaner leeward side as wind and sea state shift, and so rolling doesn't matter.
- Height is your friend: droplet flux falls off steeply with every increment of height above the deck and the waterline. Bonus: high-and-aft vents are also safe from downflooding (keep them closable from inside regardless).
- Keep clear of: engine exhaust, fuel/water fills, tank vents (fuel and holding), galley exhaust, and the anchor windlass wash-down zone.
Anatomy of a salt-trap vent box
- Aft-facing intake — rotatable cowl or fixed scoop on the hardtop/coachroof edge, ideally in pairs P/S. Opening faces the boat's wake, not the weather.
- Expansion plenum — cross-section at least 10× the duct area; target air speed under 100 fpm (0.5 m/s). Slugs and large drops hit the far wall or simply fall to the floor here.
- First baffle (knock-out) — takes the brunt of the spray load. Round the corners/edges: smooth bends keep pressure drop low so a small fan can still pull air through.
- Serpentine path — force at least two 90° turns before the cabin. The test: if you can see daylight through the vent from inside, water has a path too.
- Mist eliminator — chevron vanes or layered stainless/bronze mesh pad as the final stage; washable, drains to the sump. This is what catches the 20–100 µm mist that gravity won't.
- Sump and drains — every low point drains to a deck scupper aft or a seacock (never the bilge). Minimum ½″, works at heel both ways, protected by a grid or light flap so back-spray can't re-enter. Salt crusts plugs drains: make them inspectable.
- Outlet duct — sized for under ~600 fpm to keep it quiet, insulated on AC boats so it doesn't sweat. Fan: large-diameter, low-rpm 12 V DC axial — run continuously on solar.
- Damper + screen — close it from inside in storm/cold/security modes; removable bug screen on the cabin end. When the AC runs humid-weather mode, throttle to a trickle or shut.
The 10 rules
- Place high and aft, open aft. The spray shadow behind the house is worth more than any filter you can buy. Never forward-facing at deck level.
- Decelerate before you filter. Big plenum, small duct. Plenum area ≥ 10× duct area; box velocity < 100 fpm. Droplets that can't stay airborne don't need to be filtered.
- Force the turns. Two 90° turns minimum plus a demister pad. Smooth radii, not sharp corners — pressure drop is what forces air (and salt) through the leaks instead of the traps.
- Drain every low point, overboard or to deck — never to the bilge. Drains must work at heel and rolling. Check them; salt blocks weep holes quietly.
- Run the cabin slightly positive. Supply 10–20% more air than you exhaust. Exhaust-only fans depressurize the cabin and pull unfiltered salt air through every locker seam and cable pass-through. Galley/head exhausts need deliberate makeup-air paths and back-draft dampers.
- Two stages of filtration. Stage 1: washable demister (chevron or mesh) sized at ~300–500 fpm face velocity. Stage 2: fine/foam filter only in closed-boat AC mode, where flows are low. Fine filters in full salt spray load up fast and choke the system.
- Size fans for continuous, quiet, low-power operation. ~20 CFM per person of genuinely fresh air is the baseline; big slow DC fans use 5–15 W — trivial against a solar array, huge for interior air quality.
- Exhausts low, aft, opposite side from intakes, with dampers. Cross-ventilation path: high intake forward-ish on the chosen side → cabin → low exhaust aft on the other side.
- Materials and detailing: 316L stainless or matching aluminum (isolate dissimilar metals), no plated steel fasteners anywhere near salt air; seal duct joints — salt air leaking into hidden structure causes the worst corrosion; insulate ducts on AC boats.
- Maintain and monitor. Fresh-water-rinse demisters monthly in salt operation; verify drains; a $100 NDIR CO₂ sensor tells you when you actually need more air; a hygrometer near the vent outlet tells you when to stop letting humid salt air in.
Design targets (rules of thumb)
| Quantity | Target | Why |
|---|---|---|
| Fresh air per person | 15–25 CFM continuous | CO₂ and odor control (ASHRAE-derived) |
| Comfort ventilation, tropics, passive | 20–40 ACH equivalent | Hatches + wind scoops when conditions allow |
| Fan boost, closed boat | 6–10 ACH | Hot evenings, no breeze, hatches shut for rain/spray |
| Duct velocity | ≤ 400–600 fpm (2–3 m/s) | Quiet; low fan power; low pressure drop |
| Plenum velocity | ≤ 100 fpm (0.5 m/s) | Large-droplet fallout |
| Demister face velocity | 300–500 fpm (1.5–2.5 m/s) | Mist capture at low pressure drop |
| Cabin pressure balance | Supply 10–20% > exhaust | Keeps salt infiltrating out, not in |
| CO₂ (AC / closed mode) | < 1000 ppm | Trigger for makeup-air damper |
| Engine room | Per manufacturer | Combustion air plus several times that for heat extraction; same aft-facing/demister intake discipline, transom exhaust |
Rounding reality check: even the best dorade passes some sub-20 µm aerosol in sustained strong winds. Keep electronics out of the vent discharge path, wipe down near-vent surfaces as part of the routine, and switch to AC/closed mode when the spray machine is running hard.
Two-mode strategy: fresh-air-first with a big solar array
| Mode | Conditions | Intakes | Fans / AC | Notes |
|---|---|---|---|---|
| Vent mode | Mild anchorages, breeze, dry air | Open, dampers open | Low continuous DC fans; hatches/scoops for cross-flow; AC off | Fresh-air-only works across most of the trade-wind belt — this should be your default and it costs ~0.1–0.4 kWh/day |
| Boost | Hot, still evening | Open | Fans to 6–10 ACH with boat closed (rain, no-see-ums) | Still far cheaper than AC |
| AC mode | Hot & humid, or heavy spray | Throttled to a trickle (~10–20 CFM) or CO₂-triggered; fine filter in | AC carries the load | Trickle keeps CO₂ sane and holds slight positive pressure; humid salt air on AC coils is the enemy, so keep makeup small and condensate drains happy |
| Storm / cold / security | Passage, boarding seas, away from dock | Sealed from inside | AC, heat, or recirc fan only | All vent closures operable from inside; keep downflooding heights in mind |
AC integration notes
- A modest makeup trickle (10–20 CFM) through the demister keeps the boat positive and CO₂ reasonable; the latent load penalty is small at that flow, and a CO₂ sensor makes it automatic.
- Never let a big open vent sit in the discharge path of an AC return — you'll pull humid salt air straight across the coil.
- When switching AC → vent mode, give coils and condensate pans a fresh-water rinse periodically; salt + condensate is a corrosion cocktail.
- Insulate all ducts that carry cool air in a humid cabin — uninsulated ducts sweat onto everything.
Sizing calculator
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Continuous fresh air (CFM)
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Continuous ACH
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Boost flow (CFM)
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Duct dia., cont. @450 fpm
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Duct dia., boost @800 fpm
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Plenum @100 fpm (square)
—
Demister area @400 fpm
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Fan energy, 24 h (kWh)
Duct diameters are rounded up to common sizes. Plenum figure is one side of a square box at ≤100 fpm — a rectangular box with the same cross-sectional area is fine. Fan energy assumes an efficient DC axial (~0.1–0.2 W per CFM); quality counts more than quantity here.
Build & commissioning checklist
- Intakes high, aft, aft-facing; P/S pair; out of the forward spray fan
- Rotatable cowls (or adjustable scoops) so you can de-tune in weather
- Plenum ≥ 10× duct area; box velocity verified under 100 fpm at continuous flow
- No line of sight through the vent — two 90° turns minimum
- Demister pad: washable, hinged access, drains to sump
- All low points drained to deck scupper or seacock; drains flow at ±30° heel
- Dampers operable from inside the cabin; vent closures secure for boarding seas
- Supply exceeds exhaust 10–20%; galley/head exhausts have makeup paths + back-draft dampers
- Exhaust outlets low, aft, away from intakes and all tank/fuel vents and fills
- Sealed duct joints; insulated ducts on the AC boat; no ferrous fasteners
- Removable, washable bug screens at cabin ends
- Large slow 12 V DC fans, speed control, run continuously on solar
- NDIR CO₂ monitor in the sleeping cabin; hygrometer near a vent outlet
- Commissioning test: hose the intake with a deck wash at angles — zero water past the demister, drains gulp it happily
- Monthly: fresh-water rinse demisters, verify drains, wipe salt film near outlets
Disclaimer: these are design rules of thumb synthesized from offshore cruising practice (notably the Dashews' Offshore Cruising Encyclopedia and FPB design commentary), traditional dorade practice, and marine HVAC conventions — not a engineering specification. Vent closures, heights, and watertight integrity should be checked against ABYC/ISO downflooding requirements, and anything structural or through-hull deserves a naval architect's eye. Not affiliated with Dashew Offshore.