Marine HVAC · Design Guide
Salt-Free Fresh Air: Ventilation Guidelines for Offshore Cabins
How to bring fresh air into a boat’s living space while leaving the salt outside — distilling the “sealed shell + conditioned air + a few very well-protected openings” approach popularized by Steve Dashew’s FPB powerboats, combined with long-standing commercial-ship and classic-ocean-racer practice.
The short version: no single gadget keeps salt out. You layer defenses:
- Site the intake high, sheltered, and out of the spray cone.
- Slow the air down (≤ 2 m/s) so droplets aren’t dragged through.
- Make the air turn — droplets can’t corner; they hit, stick, and drain.
- Coalesce and drain the fine mist through a tilted mesh pad with a sump and weeps.
- Filter the fines — dried salt crystals pass right through droplet separators.
- Push gently — keep the cabin at slight positive pressure so leaks flow outward.
1 · The Hybrid Strategy
The boats that handle this best don’t try to breathe freely all the time. They operate in three modes, and the ventilation system is designed to switch between them:
| Mode | Ventilation | Climate control |
|---|---|---|
| Tropical moorage / marina | Envelope sealed; small filtered fresh-air makeup only | AC on recirc + dehumidification (cheap on a big solar bank) |
| Temperate anchorage | Open dorades, hatches, and ports with screens; cross-flow | AC off; fresh air does the work |
| Offshore passage | Every passive opening sealed; filtered powered intake only | AC recirc + slight positive pressure |
Dashew’s published philosophy leans the same way: a well-insulated, tightly sealed shell with big glass for light, air conditioning doing the heavy lifting, and any opening to the weather built like a piece of engineering rather than an afterthought — because any vent exposed to wind-driven spray will admit salt aerosol, no matter how “waterproof” the cover claims to be.
Know your enemy — it’s two different contaminants:
- Spray droplets (roughly 20 µm to millimeters): removed by inertia — hoods, labyrinth vanes, mesh pads, and gravity.
- Dried salt aerosol (roughly 0.1–10 µm crystals, left behind when droplets evaporate): travels far past any baffle. Only a filter catches these, and they’re the ones that corrode electronics and coatings.
2 · Six Rules for a Salt-Excluding Intake
Rule 1 — Site it smart
- Mount intakes high on the superstructure, well above the deck-spray zone — on the pilothouse sides or top, not on the hull side or bulwarks.
- Keep them out of the bow-wave spray cone. What matters is the apparent wind and spray trajectory at cruising speed, not the dockside breeze — an intake that faces forward acts as a ram scoop for spray at 10 knots.
- Avoid re-ingestion zones: your own generator or engine exhaust, dinghy-tow splash, anchor-wash runoff, and (for CO safety) anything’s exhaust, including the neighbor’s at the dock.
- Make every intake accessible for cleaning. If rinsing the filter is a project, it won’t happen.
Rule 2 — Slow the air down
- Size ducts generously: target ≤ 2 m/s (≈ 400 fpm) in the supply duct, and follow the demister manufacturer’s face-velocity curve (knitted mesh typically wants ~1–2.5 m/s).
- Expand the duct into a plenum right after the intake — dropping the velocity lets droplets fall out instead of being carried deeper.
- Bonus: low velocity is quiet. High-velocity vents whistle, hiss, and broadcast salt.
Rule 3 — Make the air turn
- A droplet’s momentum carries it straight while the air changes direction. Force three or more sharp turns with a zig-zag vane pack (or a cyclonic swirl) and most droplets larger than a few tens of microns impinge on a blade, stick, and run down.
- This is the same physics behind ship engine-room intake vane separators and behind the classic Dorade box (Section 4).
Rule 4 — Coalesce and drain
- Downstream of the vanes, a knitted mesh demister pad (polypropylene or 316 stainless) intercepts fine droplets by impaction on fibers. Tilt it (~10–15°) so collected water migrates to a drip edge and gutter instead of being re-entrained downstream.
- Give the whole assembly a sump with reliable drains (two 8–10 mm weeps is a good starting point), routed overboard or to a drain path that stays dry — and always above the waterline, or with a proper trap if below.
- A horizontal pad with nowhere to drain is worse than no pad: it eventually dumps its accumulated brine downwind in one slug.
Rule 5 — Filter the fines
- Finish with a washable final filter (synthetic media or aluminum/stainless mesh, roughly G4/MERV-8 class) to catch dried salt crystals and dust. Paper media dies quickly in a marine environment.
- Fit pressure taps and a simple gauge across the filter. Note the clean pressure drop; wash or swap when it roughly doubles. Keep spare filters aboard.
Rule 6 — Push gently
- Run the supply fan so the cabin sits at slight positive pressure (a few pascals — practically: a tissue held at a door crack drifts outward). Then every leak in the envelope leaks outward, carrying salt and dust with it.
- This only works if exhaust appliances (galley, heads, dryers) have matched makeup air. A big exhaust fan with no makeup path drags the cabin negative and pulls salt through every gap — the single most common self-inflicted wound.
- Fit a backdraft damper so a stopped fan can’t become a spray inlet.
3 · Passive Ventilation at Anchor: The Modern Dorade
When the AC is off and the weather is kind, passive vents do the job — and the century-old Dorade box (developed for the yacht Dorade in the early 1930s) remains the benchmark, provided it’s built generously and drained honestly. The principle is identical to Rule 3 and 4 above: a tall gooseneck makes the air turn; spray and rainwater fall into an oversized box; a sump and weep drains send the water back outside; only air reaches the cabin.
- Oversize the box. A box cross-section of roughly 2× the pipe area and a depth of 1.5–2× the pipe diameter is a sane starting point; bigger is quieter and safer against splash-through.
- Two weep drains (8–10 mm) at the box bottom, routed overboard or to a dry drain path — never a single hole that a bit of debris can block.
- Insect screen at the cowl mouth; stainless, removable for cleaning.
- Install in pairs on opposite sides of the boat so wind on either tack drives cross-flow.
- Make them closable. Screw-in caps, plugs, or sliding covers so everything can be sealed before a passage or a squall — the modern refinement Dashew-style designs insist on.
- Pipe diameter: ≥ 75 mm (3”) per vent is a typical starting point for a livable cabin; several smaller vents beat one big noisy one.
4 · Working With AC and Solar
With a serious solar bank, running AC most of the time is entirely reasonable — and it changes the ventilation problem from “move lots of air” to “move a little controlled air.”
- Run recirculation as the default; add a small, continuously filtered fresh-air bleed through the Section-2 intake — on the order of 25–40 m³/h (15–25 cfm) per person keeps CO₂ comfortably under ~1,000 ppm without ever opening the envelope.
- The energy cost of fresh air is real but small. Illustrative numbers: 50 m³/h of 30 °C/80% RH tropical air versus a 24 °C/50% RH cabin is roughly 0.5–0.6 kg/h of condensate — call it ~400 W of latent load, or ~130 W electric at a COP of 3. Trivial next to a solar array; don’t starve the boat of fresh air to save it.
- Heat-recovery ventilators (ERVs) can trim that latent penalty further, but their cores foul in salt-laden air and they add maintenance. Many offshore boats sensibly skip them and accept the small AC cost.
- Targets: hold interior relative humidity around 45–60% (this also keeps deposited salt crystalline and far less corrosive) and CO₂ under ~1,000 ppm. Two inexpensive sensors tell you more than any rule of thumb.
- When away from the boat in the tropics: seal it and run a dehumidistat-controlled dehumidifier or AC cycle. An open, humid, salted boat grows mold; a sealed, dry one doesn’t.
- Fresh-air-only cruising (temperate waters): size the passive plant generously — paired dorades fore and aft, screened hatches with rain hoods, cross-flow through the accommodation — and enjoy free, silent ventilation whenever the forecast allows.
5 · Quick-Reference Sizing
Starting points for discussion with your naval architect — verify against manufacturer data and applicable standards.
| Parameter | Starting point | Notes |
|---|---|---|
| Supply duct velocity | ≤ 2 m/s (≈400 fpm) | Quiet; keeps droplets entrained less |
| Demister face velocity | ~1–2.5 m/s | Per manufacturer’s curve for the specific pad |
| Vane-pack direction changes | ≥ 3–4 | More turns = more droplet removal |
| Fresh-air makeup | 25–40 m³/h per person | Tune with a CO₂ sensor (<1,000 ppm) |
| Grille free-area factor | ×0.5–0.6 of gross | Size gross area accordingly |
| Plenum expansion | ≥ 2× duct cross-section | Right after the intake |
| Cabin pressure | +5 to +15 Pa | Tissue-at-the-crack test; needs makeup air discipline |
| Relative humidity | 45–60% | Also keeps salt deposits non-deliquescent |
| Weep drains | 2× 8–10 mm | Above waterline, or trapped if below |
| Dorade pipe | ≥ 75 mm ID, in pairs | Fore/aft, opposite sides |
| Dorade box | ≥ 2× pipe area; depth ≥ 1.5× pipe ID | Bigger = safer and quieter |
| Hood geometry | Slope ≥ 20°, outward drip lip | Sheds water clear of the opening plane |
6 · Materials & Detailing Checklist
Corrosion, drainage, and serviceability
- Metals: 316/L stainless for mesh, fasteners, and hardware; FRP or polypropylene for housings and pads; anodized aluminum only with proper isolation from stainless (crevice corrosion).
- No paper filter media anywhere in a wet airstream; washable synthetics or metal mesh only.
- Slope every surface that gets wet by 15–20° minimum; no horizontal ledges for salt ponds to form in.
- Gaskets: EPDM or neoprene; bed every deck penetration properly — a leaking intake frame defeats everything downstream of it.
- Access: quick-release filter frames, hinged demister trays, and a freshwater washdown tap within reach of every intake.
- Insulate ducts that pass through temperature boundaries to prevent condensation dripping inside; slope cold-side drains too.
- Fans: brushless DC/EC, marine-rated, speed-controllable, on a labeled breaker.
- CO safety: never site an intake near any exhaust; fit CO alarms in the accommodation regardless.
7 · Maintenance Schedule
| Interval | Task |
|---|---|
| Weekly (in salt use) | Freshwater rinse of exterior hoods, cowls, and grilles |
| Monthly | Pull and rinse final filters; confirm every weep drain runs free; glance at the ΔP gauge |
| Quarterly | Wash demister pads; verify sump cleanliness; check fan operation and damper seating |
| Annually | Full teardown: replace polypropylene pads as needed, renew gaskets, inspect for crevice corrosion, re-bed penetrations |
| Before every offshore leg | Seal all passive openings; verify backdraft dampers close; confirm positive pressure with the tissue test |
8 · Operating Playbook
Tropical moorage
- Sealed envelope; AC recirculating; small filtered makeup running continuously.
- Passive vents capped. Dehumidify when leaving the boat.
- Rinse intakes weekly — salt accumulation is fastest at rest in a salty breeze.
Temperate anchorage
- Caps off, screens in, hatches and dorades open for cross-flow; AC off.
- Watch the forecast: re-seal ahead of squalls rather than during them.
Offshore passage
- Everything passive sealed; only the engineered intake breathes, on positive pressure.
- Recirc + filtered makeup keeps CO₂ and humidity sane without exposing the interior to green water.
- After heavy spray days, rinse the exterior stages at the next opportunity.
9 · Pitfalls That Sink Good Intentions
- Forward-facing intakes that act as ram scoops for spray at speed.
- Horizontal demister pads with no drain — deferred failure, delivered as a slug of brine.
- Drains below the waterline without traps, or single weeps that clog.
- Negative cabin pressure from oversized exhaust fans with no makeup air — sucks salt through every seam.
- Trusting a “waterproof” vent cover to stop aerosol. Waterproof ≠ salt-proof.
- Skipping the final filter because the vanes “look like enough” — dried salt crystals sail straight through any droplet separator.
- Paper filters, which turn to pulp after the first wet blow.
- Unserviceable installations — if cleaning requires a contortionist, it won’t happen, and performance quietly dies.
10 · Further Reading
- Steve Dashew’s writings at SetSail.com and Dashew Offshore on FPB ventilation, insulation, and the sealed-shell approach to offshore comfort.
- Dave Gerr, Boat Mechanical Systems Handbook — solid chapters on marine ventilation sizing.
- Nigel Calder, Boatowner’s Mechanical and Electrical Manual — air conditioning, dehumidification, and corrosion management in practice.
- Olin J. Stephens II, All This and Sailing, Too — the origin story of the Dorade vent.
- Commercial-ship practice: engine-room intake mist eliminators and vane separators — marine HVAC suppliers’ catalogs are full of applicable selection curves.