🌊 Fresh Air, No Salt

Design guidelines for a low‑salt cabin intake — inspired by Dashew’s approach

Steve Dashew’s powerboat designs are legendary for their long‑range efficiency and thoughtful detailing. One feature that stands out is an air intake that dramatically reduces salt spray entering the cabin, even in rough conditions. With a large solar array you may run A/C most of the time, but there’s nothing like a breath of fresh ocean air — without the salt. Below we break down the principles, geometries, and practical steps to achieve the same result on your own boat.

🔍 How salt spray gets in

Wind and wave action create a fine mist of salt water droplets suspended in the air. When air enters a conventional scoop or open port, these droplets follow the airflow straight into the cabin. The key to stopping them is inertial separation — forcing the air to turn sharply while the heavier water droplets continue straight, where they are drained away.

            Core principle: Make the air change direction at least twice, with a gravity drain at
            the low point. Salt droplets, being ~800× denser than air, cannot make the turn and are flung onto
            a surface or into a drain.
        

🧱 Design geometry — the labyrinth intake

The most effective low‑salt intake uses a labyrinth or baffle path. This is a compact box with internal walls that force the air to travel up, down, and around. Here’s a schematic of the classic configuration:

Labyrinth intake — air travels a convoluted path while droplets hit the walls and drain out.

Key geometric guidelines

🌀 1. Multiple direction changes

At least 2–3 sharp turns (≥ 90°). Each turn strips out more than 80% of remaining droplets.

📏 2. Velocity reduction

Enlarge the cross‑section area 2–3× relative to the inlet duct. Lower speed lets drops fall out.

⬇️ 3. Low-point drain

A ½″–1″ drain tube at the lowest point of the box, led overboard. No standing water = no mold.

🧺 4. Demister pad (optional)

A coarse stainless steel or poly mesh pad captures the finest mist. Rinseable and reusable.

📐 Sizing & placement

Intake area should match your fresh‑air needs. For a 40–50 ft powerboat with occasional natural ventilation, a 4″–6″ diameter intake (12–28 sq in free area) is ample. If you plan to use it as a backup for A/C, size for about 100–150 CFM per person.

Boat length	Recommended inlet Ø	Min labyrinth volume	Drain size
30–40 ft	4″ (10 cm)	0.8–1.2 ft³	½″ ID
40–55 ft	5″–6″ (12–15 cm)	1.2–2.0 ft³	¾″ ID
55–70 ft	6″–8″ (15–20 cm)	2.0–3.5 ft³	1″ ID

Placement tips

High and center — Place the intake on the cabin top or hardtop, away from bow spray and low‑side green water.
Face aft or amidships — A rear‑facing scoop uses negative pressure to draw air; a forward‑facing one needs a dorade box to keep out rain and spray.
Avoid the engine room — Keep intakes separate from ER vents to avoid fumes and hot air.
Sun & solar — If you have a large solar array, mount the intake below or between panels to keep it shaded (cooler air is denser).

⚙️ The Dashew touch — centrifugal separator

Dashew refined the labyrinth into a compact centrifugal separator. Air enters tangentially into a cylindrical chamber, spins at high velocity, and droplets are flung to the outer wall where they coalesce and drain. Clean air exits from the center. This design is extremely efficient (99%+ salt removal) and very compact.

            DIY tip: You can approximate this with a 6″ PVC tee and a length of
            pipe as a cyclone body. Air enters the side, exits the top, and a drain at the bottom removes the brine.
            Dozens of cruising sailors have built effective versions for under $50.
        

🌬️ Integration with A/C and solar

Since you plan to run A/C most of the time from solar, the low‑salt intake serves two roles:

Natural ventilation mode — When conditions are mild, open the intake and a leeward exhaust port. The labyrinth keeps salt out even while moving.
A/C makeup air — Many marine A/C units require a small amount of fresh air. Route the clean intake to the A/C return plenum for a constant, filtered supply.

Because the labyrinth already removes salt and most particulates, you can use a simple washable filter (or none at all) before the A/C evaporator. This saves energy and maintenance.

📊 Quick sizing calculator

Estimate the intake area for natural ventilation:

Cabin volume (ft³): — recommended CFM (4 changes/hr)

Inlet velocity (ft/min): — required inlet area (in²)

Typical labyrinth intakes work well at 300–500 ft/min face velocity.

🧰 Materials & construction

Box: 3/8″ or 1/2″ marine plywood (epoxy‑coated), coosa board, or 3D‑printed ABS/PETG.
Internal baffles: Same material as box, sealed with marine adhesive.
Drain: Bronze or Marelon thru‑hull with a loop to prevent siphoning.
Demister media: Stainless steel scrubber pad (3M #86 or similar) or polypropylene mesh (Vand ‑air).
Gaskets: Closed‑cell foam for access panels.

✅ Final checklist

Inlet faces aft or is protected by a spray shield.
Air path contains at least two 90° direction changes.
Lowest point of the labyrinth has a dedicated drain ≥ ½″ ID.
Cross‑section area is 2–3× the inlet area to slow the air.
Access panel for cleaning and inspection.
All interior surfaces smooth and sloped toward drain.
Test with a garden sprayer — no droplets should pass through.

Inspired by Dashew’s FPB & Sundeer designs. For deeper reading, see Practical Seamanship or the Dashew Offshore blog. The principles above have been used successfully on many cruising boats, from 35′ sailboats to 80′ powercats.

💡 Remember: Even the best labyrinth can’t handle green water over the cabin top — close the intake in extreme conditions. Otherwise, enjoy fresh, salt‑free air wherever you cruise.