A sea trial tests the boat and its machinery under way: how the engines start, whether they reach their rated rpm under load, how they run at each speed, and what the engine room looks like straight after. It's usually part of a pre-purchase survey, and it's often where engine problems first show.
Before leaving the dock
Start cold. BoatUS says the engines shouldn't be run for 24 hours before the survey, and Practical Sailor tells buyers to ask the owner not to warm them up. Soundings explains why: slow cranking points to the batteries or connections, and hard starting to the ignition or fuel system.
Before the start:
- Fluids. Engine oil, coolant and gearbox oil levels, to compare after the run.
- The bilge. Note the water level and any oil. Soundings suggests dipping a white card: green or light orange means coolant, brown engine oil, pink gearbox fluid.
- Drip cloths under the engine and gearbox, so leaks show after a hard run.
- Hour meters. Record them before and after, which also proves they work.
- Alarms. Soundings says they should sound when the key is turned.
Under way
Full throttle and rated rpm. Practical Sailor says rpm at full throttle should be within 100 or so of the maker's figure. Soundings gives typical figures: 4,000 to 4,400 rpm for most V8 gas inboards, low-speed diesels governed to 2,500 rpm and within 50 of it, and high-speed diesels at 2,800 to 3,400 rpm, some newer ones up to 4,000, within 100 of their specification. Practical Sailor notes that rpm above the rating can mean the propeller is too small, and rpm below it a propeller too large in pitch or diameter.
Readings at each speed. Practical Sailor records oil pressure, temperature and voltage at slow, half, cruising and full throttle. Soundings adds an infrared scan of the heat exchangers, oil coolers, risers and manifolds at full throttle.
Temperatures. Steve D'Antonio, in Cruising World, says no exposed exhaust part should exceed 200°F, the water-cooled exhaust and its hose typically shouldn't exceed 150°F, and the engine compartment shouldn't run more than 30°F above the cabin. See engine overheating.
Smoke. Soundings notes black smoke at start-up suggests a rich mixture, blue smoke that hangs on the water suggests oil getting past the valve guides or rings, and white smoke can mean coolant loss. See exhaust smoke.
Vibration. Soundings says every boat vibrates through some part of its range but shouldn't constantly. Causes include shaft run-out, an unbalanced propeller or a worn cutless bearing. See shaft misalignment.
Steering and gearbox. Wandering suggests play in the steering linkage or a misaligned rudder. Note noisy or abrupt shifting. Practical Sailor adds a back-down test to watch the engine mounts, and a check of the shaft seal and rudder gland under way.
After the run
Check the fluids again, look at the drip cloths and the bilge, and look for leaks around the exhaust and the shaft seal. Soundings says a rising oil level with a milky look means water in the oil, and coolant above its normal level can mean seawater getting through a failed heat exchanger. D'Antonio recommends oil, gearbox fluid and coolant samples for lab analysis, at about $30 each.
Write it up as tables
Readings are easiest to compare in one table per engine, one row per engine speed. On twin engines, a difference between the two often says more than any single figure.
The sea trial report template has the tables and checks in Word. With Axsail you say the readings under way and it builds one table per engine, with the conditions and the rpm for each row.