Buyer due diligence · All · 11 min read

Sea Trial Checklist for Heavy Fiberglass Boats

Run engines to WOT, log trim-tab response, and capture fuel burn on a $350,000+ sea trial so survey findings match real operating data.

Last reviewed November 4, 2026

Cold start and dock checks

Record ambient temp and cold-start smoke—blue smoke over 90 seconds on diesels suggests oil control issues ($4,000–$9,000 top-end work).

Verify trim tabs full cycle and bow thruster load at dock—weak thruster may indicate battery or solenoid issues $800–$2,200.

Check engine oil and transmission fluid color before launch—milky transmission fluid signals cooler failure $2,500–$4,500.

Confirm fuel filters recently changed if boat sat 6+ months—air-bound fuel systems cause false running faults on trial.

Time 0–30 mph (or plane) seconds—express cruiser that planes in 28 seconds versus seller claim 22 seconds indicates bottom growth or prop issue.

Hold WOT 3–5 minutes logging RPM—twin 350 HP diesels rated 3,600 RPM should reach 3,550–3,650 loaded; 3,420 RPM suggests pitch or fuel restriction.

Capture fuel burn at cruise and WOT—GPS fuel flow meter versus tank vent sounding validates seller economy claims ($2,000/year fuel swing).

Note vibration bands through RPM range—harmonic vibration at 2,800 RPM may indicate cutlass bearing wear $1,200–$2,800 per shaft.

IR scan exhaust risers after WOT—hot spot 40°F above paired engine indicates restricted flow or raw-water pump impeller failure.

Stuffing box temperatures above 140°F suggest alignment or packing issues—infrared gun $50 tool saves $6,000 shaft log damage.

Run both AC units at cruise load—weak chilling at 2,500 RPM may show belt slip or seacock restriction not seen dockside.

Generator load test 50% nameplate for 20 minutes underway—voltage sag under load flags AVR or exhaust issues $1,500–$3,000.

Who pays for sea trial fuel?

Usually buyer—budget $200–$400 diesel for twin-engine trial plus captain if required by marina.

Can sea trial fail but survey pass?

Yes—purchase agreements should allow separate sea trial rejection with defined RPM and smoke standards.

Is IR scan required?

Not required, but a $150–$300 thermal survey add-on is a small cost next to a missed cooling failure on a $400k boat.