Fresh Air Without the Salt: Intake Design Guidelines for Cruising Boats

Yes — there are well-established guidelines, and the approach used on Steve Dashew's FPB powerboats is a good template: treat salt spray as a droplet problem, not an air problem. Slow the air down, make it change direction several times, give droplets a surface to hit, and give the collected water an easy way back overboard.

1. The Core Principle

Salt spray travels as water droplets with mass. Air can turn sharp corners; droplets can't — they continue straight and hit a surface, where they coalesce and can be drained. Every good marine intake (Dorade box, gooseneck, ship's mist eliminator, the FPB plenums) combines four levers:

Low velocity — droplet carry-over rises steeply with airspeed.
Sharp direction changes — at least 2–4 turns of 90° or more before air reaches the cabin.
A coalescing surface — baffles, chevron vanes, or knitted-mesh demister.
Generous drainage + shutoff — every low point drains overboard, and the whole thing can be closed watertight in heavy weather.

The Dashew pattern: published descriptions of Wind Horse and the FPB 64/78 series describe large, high-mounted intake plenums with down- or aft-facing openings, internal baffles and water traps, oversized drains, and positive closures for offshore. Their earlier sailboats used classic Dorade boxes. The recurring theme: big cross-section, low velocity, big drains, and a way to button up completely.

2. Design Guidelines

2.1 Location and orientation

Mount the intake high and inboard, ideally in the aft half of the boat — spray is worst at the bow.
Face the opening away from the apparent wind and spray: aft-facing cowls or a downward-facing gooseneck ingest far less than forward-facing scoops underway.
A lee-side intake with a small fan works very well: spray impingement on the lee side is dramatically lower.
Keep well clear of engine/generator exhaust, head vents, and holding-tank vents (CO and odor safety).

2.2 Keep velocities low

Target a face velocity through louvers and separators of ≤ 2–3 m/s (400–600 fpm) in spray-exposed locations; lower is better.
This is why good marine plenums look oversized — the area is cheap, and it is what lets the physics work.

2.3 Tortuous path (gooseneck, Dorade, labyrinth)

Gooseneck: the opening faces straight down; air must make a 180° turn to enter. Simple, robust, classic ship practice.
Dorade box: a cowl feeds a box; air rises over an internal standpipe into the cabin while water collects in the box and drains overboard. Rule of thumb: box cross-section ≥ 2× the vent-pipe area, top of the standpipe above the bottom of the cowl opening.
Baffled plenum / chevron vanes: 2–3 zig-zag passes with drain hooks; effective down to roughly 10–20 µm droplets at design velocity.

2.4 Demisters and rain-rated louvers

Knitted-mesh demister pad (polypropylene or 316L stainless), 100–150 mm thick, at 2–4 m/s face velocity: captures droplets ≳ 5–10 µm with negligible pressure drop.
Weather louvers rated to EN 13030 Class A/B reject roughly 99%/95% of wind-driven rain (tested at 75 mm/h rain in 13 m/s wind) — a good first stage.
Avoid fine insect screens at the exposed face: they wet-out, hold a water film, and re-entrain droplets. Keep face mesh coarse.
Typical order: louver → baffles → demister → filter → fan → cabin.

2.5 Drainage

Every low point gets a drain overboard, minimum Ø20–25 mm, at least two per box, discharging above the waterline.
Provide an air break or loop so the fan can't suck air (and water) back up the drain.
Size the box for the "green-water gulp": it must swallow and drain a boarding wave without passing water inboard.
Commission with a washdown hose aimed at the intake — verify the drains keep up and the interior stays dry.

2.6 Heavy-weather shutoff

Fit a positive watertight closure at the deck penetration, operable from inside. Offshore standards (ISO 12217 downflooding criteria, EU RCD Category A, ABYC guidance) treat any ventilator as a potential downflooding opening — its height matters and it must be closable.

2.7 Fan, filter, and positive pressure

Use a small variable-speed brushless marine-rated fan (sealed/IP55+; consumer computer fans corrode quickly in salt air).
Run a slight positive cabin pressure (a few pascals is enough): air leaks outward, so spray and dust don't find their own way in.
Put a replaceable synthetic pre-filter (~MERV 8) after the demister to catch residual salt before it reaches your interior and electronics.

3. Schematic: Spray-Separating Intake Plenum

Schematic cross-section. Air rises into a down-facing cowl, weaves through baffles (droplets impact and drain overboard), passes a demister pad, and is pushed into the cabin by a variable-speed fan, holding slight positive pressure.

4. Rules of Thumb

Parameter	Guideline
Louver / intake face velocity	≤ 2–3 m/s (≈ 400–600 fpm) in spray-exposed locations
Direction changes before cabin	≥ 2–4 turns of 90° or more
Chevron/vane separator	2–3 passes with drain hooks; effective to ~10–20 µm droplets
Knitted-mesh demister	100–150 mm thick; 2–4 m/s face velocity; captures ≳ 5–10 µm
Weather louver rating	EN 13030 Class A/B (≈ 99% / 95% driven-rain rejection)
Drains	≥ 2 × Ø20–25 mm per box, overboard above waterline, air break or loop
Continuous fresh air	10–15 CFM (17–25 m³/h) per person
Free-cooling boost	6–20 air changes per hour for a perceptible breeze
Mounting	High, inboard, aft- or down-facing; clear of exhaust and holding-tank vents
Closure	Watertight shutoff at the penetration, operable from inside
Pressure	Slight positive pressure (a few Pa) maintained by the fan
Filter	Replaceable synthetic ~MERV 8, downstream of the demister

5. Worked Sizing Example

Two crew, continuous trickle of 30 CFM (≈ 51 m³/h), boost to 200 CFM for free cooling:

Area (cm²) ≈ 4.7 × Q (CFM) ÷ v (m/s)

Demister for trickle at 2 m/s: 4.7 × 30 ÷ 2 ≈ 70 cm² — palm-sized (≈ 9 × 8 cm).
Face louver at 1.5 m/s for trickle: ≈ 95 cm² (≈ 10 × 10 cm).
Demister for 200 CFM boost at 2.5 m/s: ≈ 380 cm² (≈ 25 × 15 cm).

The areas are modest — generous sizing costs almost nothing and is what makes the separation work. For a ~40 m³ interior, 6–20 ACH free cooling means roughly 140–470 CFM of boost capacity.

6. Two Operating Modes

At anchor / in harbor: spray separation barely matters. Open hatches, use wind scoops and solar vents, and move lots of air. This is your "free cooling" mode.
Offshore / underway: button up and run the powered trickle through the separator. Slight positive pressure keeps salt out of every other crack, and the A/C handles temperature and humidity.

Designing for both modes means you rarely compromise: big openings for the anchorage, one small well-engineered opening for sea.

7. Pairing with a Solar-Powered A/C

Mini-splits and most marine A/C units recirculate only — they provide no fresh air. Without a dedicated makeup path, CO₂ and humidity climb even with the A/C running.
Feed the 10–15 CFM/person trickle into the A/C return so incoming air is cooled and dehumidified before it reaches you.
A CO₂ monitor is a cheap way to dial fan speed: aim to stay under ~1,000 ppm.
Optionally add a small HRV/ERV (through-wall units such as the Lunos e², popular in vans and tiny homes) downstream of the demister to recover cooling energy. In the humid tropics choose an ERV, which transfers moisture out of the incoming air.
With abundant solar, the simplest strategy is to over-ventilate slightly and let the A/C absorb the extra load.

8. Materials and Maintenance

316L stainless, HDPE, or PVC in the wet path; isolate dissimilar metals; avoid bare aluminum (crevice/pitting corrosion in salt mist).
Provide fresh-water rinse access to the cowl, baffles, and demister; salt crystals are your early-warning telltale.
Inspect the demister annually; replace the pre-filter on a schedule; check drains for blockages.
Expect some salt over years — design interior surfaces below the intake to tolerate occasional wipe-down.

9. Off-the-Shelf Starting Points

Dorade boxes and cowl vents: Vetus, Beckson, Sea Dog — proven, cheap, and easy to retrofit.
Solar mushroom vents (Nicro/Marinco) for anchor-mode exhaust.
EN 13030-rated weather louvers and knitted-mesh demister pads (AMACS, Koch-Glitsch) for a custom plenum.
Marine HVAC mist eliminators (Delta T Systems, Munters, Novenco) if you're building a larger trunked system.
12 V brushless sealed blowers (IP55 or better) for the fan stage.

Safety notes: Treat every intake as a potential downflooding point — its height and its closure figure into stability/downflooding criteria (ISO 12217, RCD category, ABYC guidance). Keep intakes away from engine and generator exhaust, and fit a CO alarm in the cabin.