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Fresh Air Without the Salt: Designing Marine Cabin Air Intakes
Boat Systems · Ventilation
Fresh Air Without the Salt: Designing Cabin Air Intakes for Cruising Powerboats
How designs like Steve Dashew’s keep salt spray out of the ventilation air — and practical guidelines for location, separation, sizing, and working alongside air conditioning on a solar-heavy electrical budget.
The boats people remember from the Dashew world (Beowulf, the FPB series) had a simple but powerful idea: don’t try to stop spray at the deck fitting — bring the air into a large, slow, baffled space where the water can fall out and drain away before it ever reaches the cabin. That concept scales to any cruising boat, and it works beautifully alongside modern AC: run recirculation when it’s hot and humid, and switch to cheap, solar-friendly DC-powered fresh air when the climate allows.
1. What you’re actually separating
“Salt spray” is really a spectrum of droplet sizes, and the good news is that most of the salt mass rides on droplets that are easy to remove:
Spume and sheet spray (roughly 100 μm – 2 mm) — torn off wave crests and the bow. These droplets have real inertia. A 100 μm droplet settles at roughly 0.25 m/s (about 50–60 ft/min), and anything larger falls faster. Simple baffles, turns, and slow air catch nearly all of it.
Fine aerosol (1–20 μm) — mist from bursting bubbles. These behave almost like a gas; a mesh pad will catch much of it, but expect a small amount of fine mist to reach the cabin in heavy conditions. It’s usually unnoticeable.
Spray ingestion also depends on conditions: it grows sharply with apparent wind and boat speed (noticeably worse above roughly 8–12 knots), is worst into head seas, and is concentrated near the bow and on the windward side. Which leads to the first rule: location beats hardware. An intake that never sees spray needs far less separating.
2. The Dashew-style approach
On several of Steve Dashew’s designs, the fresh-air story started with the engine room: large, aft-facing intake scoops mounted high on the superstructure fed a generous, low-velocity ventilation space, so incoming air slowed down and dropped its coarse water before it traveled anywhere. Cabin/HVAC makeup air was then taken from that conditioned air, through filtration, with the engine room kept at a slight negative pressure so engine odors stayed put. If the design you remember “kept the salt out of the cabin,” this is almost certainly the concept you saw.
A note on specifics: details varied from boat to boat across the Beowulf and FPB lines. If you can identify the exact design you’re remembering, the drawings and articles published by the Dashews (SetSail) are worth a close look. The guidelines below generalize the same underlying strategy, so they apply whether or not you share the intake with the engine room.
If you route cabin air through or near the engine room: keep the engine room at negative pressure relative to the cabin, never starve the engine of combustion air (engine-room airflow demands are large), and filter the cabin take-off high up and far from heat and oil mist. On many boats, a dedicated separate intake plenum is simpler and cleaner-smelling than sharing engine-room air.
3. Six principles that do the work
1. Locate it out of the spray
High, aft of the bow spray zone, and ideally facing away from the apparent wind. Twin port/starboard intakes let you pick the sheltered side.
2. Slow the air down
Open the duct into a big plenum so airspeed drops below about 350–400 ft/min. Slow air gives droplets time to fall; fast air carries them straight into your salon.
3. Force direction changes
Water droplets have inertia and can’t follow tight turns. A deflector plate under the deck fitting plus one or two 90°+ turns throws droplets onto surfaces, where they coalesce and run down.
4. Coalesce the fine stuff
A 100–150 mm pad of 316 stainless knit-mesh (or vane-pack) at 300–600 ft/min face velocity captures droplets down to about 5–10 μm. It’s the same technology used on gas-turbine and offshore-platform intakes, and it’s washable forever.
5. Drain everything
Every low point gets a drain to overboard (looped or duckbill so nothing blows back in). Captured water must never sit where airflow can re-entrain it.
6. Manage pressures
Keep the cabin at a slight positive pressure (a few pascals) so air leaks outward through hatches and gaps instead of pulling spray and dust in. Keep engine room and galley at negative pressure.
4. A reference arrangement
This is a compact version of the concept that fits a pilothouse or deck-saloon boat: a raised deck fitting over a baffled, drained plenum below decks, then mesh, fan, and duct to the cabin.
The deck fitting only keeps out green water and rain — the real separation happens in the baffled, drained plenum below decks. Arrows show airflow; blue dots show spray droplets falling out and draining overboard.
Component notes:
Deck fitting: 316 stainless mushroom or aft-facing scoop, raised as high as practical. Two fittings (port and starboard) with selector dampers are better than one.
Deflector plate: a plate 50–100 mm below the fitting outlet, slightly wider than the stem, so incoming air must turn sideways and coarse spray impinges immediately.
Plenum: sized for slow air (see table below); interior drained to the lowest corner. Painted light color for inspection; hinged or screwed lid so you can rinse it.
Demister panel: 100–150 mm deep 316 knit-mesh in a removable, slide-out frame for cleaning. Vane-pack (chevron) panels also work and drain even better.
Optional fine filter: a washable open-cell foam or pleated panel after the mesh if you spend time in dusty or polluted anchorages. Don’t put it before the mesh or it will load up with salt.
Fan: inline mixed-flow DC fan (12/24 V) with PWM or multi-speed control; brushless preferred. Size for the target flow at 100–200 Pa static pressure (mesh plus ducting).
Ducting: insulated (outside air run through an air-conditioned space will sweat), gentle bends, and weep/low-point drains where condensation can collect.
Backdraft damper: a light gravity or spring damper so you can seal the system when running fast into heavy spray or in driving rain.
5. Where to put the intake
Do
Mount as high as practical — pilothouse or flybridge top — well above bow spray and boarding seas.
Favor locations aft of the forward third of the vessel; spray concentration falls off sharply as you move aft.
Face openings aft or downward; prefer fittings shielded by the superstructure from head-sea spray.
Install port and starboard fittings with dampers so you can always select the sheltered (lee) side at anchor or on a beam reach.
Keep fittings accessible for rinsing and inspection.
Avoid
Bow pulpits, foredeck hatches, and anywhere in the anchor-wash / bow-spray zone.
Windward side locations when a sheltered alternative exists.
Near exhaust outlets (soot), generator wet exhaust, or galley vents.
Low spots where a boarding wave can submerge the fitting — if unavoidable, the plenum and drain become mandatory, not optional.
Long horizontal runs at high velocity before separation — fast air carries spray perfectly.
Remember that the “clean side” changes with conditions: running into a head sea, an aft-facing fitting is cleanest; at anchor in a steady breeze, use the lee fitting; in following seas, a forward-protected location may be best. Dampers and twin intakes are what make this practical.
6. Sizing and numbers
Parameter
Rule of thumb
Example: 200 CFM design point
Fresh air, per person
10–25 CFM per person (comfort level; ASHRAE residential minimums are lower)
4–6 crew → 40–150 CFM continuous
Fresh air, per volume
4–6 air changes per hour of salon/galley volume for “open-window” feel
3,000 ft³ space → 200–300 CFM peak
Duct velocity (quiet)
≤ 500–700 ft/min
8″ duct ≈ 575 ft/min at 200 CFM
Plenum cross-section
≤ 350–400 ft/min through-flow
≈ 0.5 ft² (e.g., 8″×9″), plus 2–3 duct-diameters of length
Demister face velocity
300–600 ft/min (1.5–3 m/s)
8″×12″ mesh ≈ 300 ft/min
System static pressure
50–200 Pa at design flow (mesh dominates)
Typical inline mixed-flow fan handles this easily
Fan power
≈ 0.05–0.15 W per CFM at low speed
10–30 W — comfortable on solar
Cabin pressurization
Slightly positive, ~2–5 Pa
Barely perceptible; enough to keep spray and insects out of gaps
Materials: 316 stainless (not 202 or “marine-grade” lookalikes), marine-grade aluminum, or UV-stabilized composites for fittings; 316 mesh; avoid galvanized steel anywhere in the system. Drains 12–16 mm with a loop or duckbill valve, discharged above the waterline.
7. Living with AC and a big solar array
Your instinct is right: with strong solar you can afford to run AC much of the time, but fresh air deserves its own strategy because it interacts with humidity. Four operating modes cover everything:
Mode 1 — Recirculate + fresh-air bleed (hot and humid)
Run the AC on recirculation with 10–15% outside air bled into the return through your intake system. CO2 and odors stay down while the dehumidifying coil does most of the work. This is the standard marine-HVAC compromise in the tropics.
Run high fresh air for 5–10 minutes every hour or two, then return to recirculation. You get a full air exchange without fighting latent humidity load all day. Easy to automate with a timer.
Mode 3 — Fresh-air only (temperate climates)
Compressor off; DC fan(s) moving 100–300 CFM of outside air through the cabin, exhausting through the galley, heads, and/or engine room. This is 10–40 W — trivially solar-powered — and in many locations it’s genuinely all you need, exactly as you suspected.
Mode 4 — The optional brain
A humidity/dew-point control that closes the fresh-air damper when outside dew point exceeds your cabin setpoint. Nice, but honestly a 3-speed fan switch and a manual damper deliver 90% of the value for 1% of the complexity.
Pressure note: whatever mode you’re in, watch the balance. A powerful galley range hood or engine-room exhaust with no makeup air will pull the cabin negative — and then hatches, dorades, and every gap become unfiltered intakes, pulling in spray, dust, and engine odor. Your dedicated intake is also your makeup-air system.
Heavy weather: at high speed into head seas, even a well-placed intake will see fine mist. That’s what the backdraft damper and recirculation mode are for — design the system so “sealed, recirculating” is a normal, supported state, not an emergency improvisation.
8. Passive and at-anchor options
For zero-watt ventilation, the classics still work — when done properly:
Dorade vents, done right: a true dorade is a cowl over a sealed box; air enters, turns, and water falls into the box and drains overboard. Many off-the-shelf “deck vents” are cosmetic and leak spray straight below. Look for a real internal baffle and a drain, or build your own box to the same principles above.
Hatch wind scoops: canvas scoops rigged to face aft (or to the breeze) work brilliantly at anchor in the trades and cost almost nothing. Add insect screens.
Solar vent fans: small solar-powered fans in dorade boxes or clamshell vents move modest air but are perfect for keeping a boat aired out while away.
Cross-ventilation planning: passive flow needs an inlet and an outlet on opposite sides of the pressure field; one open hatch mostly just burps. Intake high on one side, low exhaust or lee-side outlet on the other.
9. Maintenance
Rinse the mesh pad, deck fitting, and plenum with fresh water monthly (more often after long windward passages).
Verify drains flow freely; salt crystals and debris love the low corners of a plenum.
Inspect mesh for pitting or corrosion; keep one spare panel aboard.
Check fan operation and bearings annually; keep a spare fan — they’re inexpensive and the whole system depends on them.
Exercise selector and backdraft dampers so they don’t freeze with salt.
10. Build checklist
Intake located high, aft of bow spray zone, and/or paired port/starboard with selector dampers.
Raised 316 fitting (mushroom or aft-facing scoop) with backdraft damper.
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A couple of notes: the page is fully self-contained (no external fonts or scripts), so it can be dropped into any site as-is; the diagram is inline SVG, so it stays sharp at any size. If you later identify the exact Dashew design you're remembering, I can tighten the "Dashew-style approach" section to match its actual arrangement.