Why boats suffer more than cars

Cars burn a tank in a week; boats store fuel for months in vented tanks in humid air. E10's ethanol is hygroscopic — it pulls water out of the air until the fuel phase separates: a corrosive water/ethanol layer drops to the tank bottom, exactly where the pickup lives, and the gasoline above it loses octane.

What you'll see in the field

• Gummy varnish in carb bowls and VST tanks; clogged idle jets
• Deteriorated fuel lines and primer bulbs (pre-ethanol-rated rubber turns to mush and sheds black debris into filters)
• Water in the separator filter; whitish layer in a drained sample
• Corroded aluminum tank surfaces and green fuzz on fittings
• Dissolved resin in very old fiberglass tanks (a repower/retank conversation)

Treatment and prevention

1. Stabilizer every fill in the off-season months, and always before storage — run treated fuel through the engine so it reaches carbs/injectors.
2. Water-separating fuel filter on every boat you service. If there isn't one, quote it — it's cheap insurance and your best diagnostic window.
3. Keep tanks either near-full (less air space, less condensation) or near-empty and treated — the worst is half-full and untreated for six months.
4. Prefer ethanol-free fuel where available for small carbureted engines and anything stored long.
5. Replace any fuel hose not marked with a current marine rating; while you're there, check hose clamps and the anti-siphon valve.

The phase-separation call

No additive un-separates fuel. If a sample shows a distinct bottom layer:

1. Pump the tank dry (into proper containers — ventilate, no ignition sources, bond containers to prevent static).
2. Dispose of it properly; do not "filter it and run it."
3. Clean or replace filters, drain the carb bowls/VST, refill with fresh treated fuel.

Common mistakes

• Selling a tune-up when the tank fuel is the disease — it will come right back
• Trusting stabilizer poured in at the dock but never run through the engine
• Using automotive-grade additives that don't address water in a marine context
• Ignoring the fill and vent hoses — they rot too and leak into the bilge

When to walk away

Fuel visibly leaking from a built-in tank, or gasoline smell in the bilge, is a stop-work safety issue. Explain, document, and do not start the engine until the leak is found.