A sea trial is the only part of a used-boat purchase where the boat has to prove it works. Surveys catch structure and osmosis. Paperwork tells you who owned it. But the engine under load, the way the hull tracks in a chop, the wandering autopilot, the alternator that never quite tops the bank : those only show up when you leave the pontoon. Treat the sea trial as a structured test, not a joyride, and you will save yourself the arguments that start three months after signing.

Before the lines come off

Half of a good sea trial happens with the boat still tied up. Get on board an hour before departure. That hour is where you check the things the seller cannot hide once you are watching.

  • Cold start. Insist the engine is cold when you arrive. A used diesel that has been warmed up before you see it can hide hard starting, glow-plug problems, injector leaks and blue smoke on the first crank. If you find a warm engine, that alone is a data point.
  • Fluids and belts. Pull the dipstick. Milky oil means water ingress. Black, thin oil with a fuel smell means injector or pump issues. Check coolant colour and level in the expansion tank, and inspect belts for glaze and cracking.
  • Fuel and filters. Open the primary filter bowl if you can. Water at the bottom, black sludge or a brown biofilm is a warning. Diesel bug can be dormant for months and wake up on the first rough passage : we cover the tells in how to spot and treat diesel bug.
  • Battery banks. Note resting voltage on the house and start banks before anything is switched on. Under 12.4 V on a bank that is supposedly healthy tells you something.
  • Bilges. Dry, or wet? Fresh water, salt water or oily water? Lift the sole boards you are allowed to lift and photograph what you find.

Then start the engine and let it idle while you note the exhaust colour, the sound of the raw-water pump, and whether cooling water is coming out at a steady rate. A weak or intermittent stream on a warm morning is worth investigating before you leave the harbour.

Engine and drivetrain under load

Once you are clear of the harbour, the engine test is the single most valuable part of the trial. Hours on the counter tell you a little. Hours under load tell you almost everything, and the two numbers are often at odds : more on that in how many engine hours are too many on a used boat.

Run through this sequence, and take notes as you go:

  1. Idle for five minutes. Watch coolant temperature stabilise. On most marine diesels you should see 75 to 85 °C. A stubbornly cold engine can mean a stuck thermostat. A creeping temperature at idle is worse.
  2. Cruise RPM for at least 20 minutes. This is where the engine reveals itself. Check exhaust colour under load. White at temperature means water in a cylinder. Blue means oil. Persistent black under normal load means fuel it cannot burn : injectors, turbo, or a fouled hull.
  3. Full throttle for two to three minutes. Compare the RPM you reach against the manufacturer's rated maximum. An engine that will not reach its rated WOT is telling you something : prop, transmission, fuel delivery or compression. This is one of the most diagnostic tests you will do all day.
  4. Back to cruise, then idle. Watch temperature recovery. Listen for injector chatter as things settle.
  5. Astern under load. Motor astern hard for 30 seconds. Cutlass bearing rumble, shaft vibration, transmission clunks or a spongy gear engagement all show up here.

Throughout, put a hand on the engine block near the head at several points, watch the tachometer for hunting, and check the raw-water outflow whenever you can. On a sailboat, note the vibration signature on the wheel or tiller at cruise RPM. On a motor boat, note trim behaviour as you come onto the plane and how long it takes to get there fully loaded.

Handling, steering and the hull

Once the engine has proven itself, the boat has to prove itself. Pick a heading that gives you real sea state if you can, even if it means asking the seller to change the plan. Flat water in the lee of a breakwater will hide almost everything you want to find.

Steering. On a wheel, check for play at the helm before the rudder moves. More than a few degrees of dead zone means a worn quadrant, cable stretch or a tired hydraulic ram. Turn hard lock to lock and count the turns : compare against the boat's spec if you have it. Listen for groans in the steering pedestal.

Autopilot. Engage it on a settled heading. A healthy pilot will hold within a couple of degrees. One that hunts across ten degrees is either badly tuned, undersized or fighting a mechanical problem in the drive. Bring the boat onto a beam sea and see whether the pilot can keep up.

Sailing behaviour, if applicable. Hoist everything. Do the halyards run cleanly? Do the sails set without obvious blown-out shapes? On the wind, note the angle of heel at which the boat wants to sit, and whether she has weather helm you can live with. Tack. Gybe. Reef under way, because that is when reefing lines seize up on you.

Sea-keeping. On a motor boat, run beam-on to the swell at cruise. Then quartering, then head sea. Note water on the screen, slamming, and how the trim tabs behave. On any boat, listen. Locker doors that swing, floorboards that squeak, bulkheads that creak in a way that stops when you slow down : all worth writing down.

Electrics, electronics and systems

Systems are where sellers most often say "yeah, that's just a small thing." Test everything, and test it under way, because vibration and heel change how equipment behaves.

  • Charging. With the engine at cruise, check that the alternator is actually charging the house bank, not just the start bank. Note the voltage at the batteries, not just at the panel display.
  • Instruments. Compare depth, log and wind readings against what you can see and feel. A log stuck at zero is a paddlewheel that has not turned in months. Wind angles that disagree with the sails by 20 degrees mean a misaligned masthead unit.
  • Radar and AIS. Turn them on. Radar should paint targets you can see. AIS should be transmitting, not just receiving : ask a shore contact or a nearby boat to confirm.
  • VHF. Do a radio check on a working channel. Test the DSC distress button cover is intact and the MMSI is programmed.
  • Bilge pumps. Manual and automatic. Pour water in and watch them cycle. Note the amp draw if you can.
  • Domestic systems. Fridge, freshwater pump, water heater on engine loop, gas solenoid, heads. On a charter-heritage boat especially, these are the systems that tend to have been patched rather than fixed. If that is the boat you are considering, our notes on whether an ex-charter boat is worth buying are worth a read before you make an offer.

Bring a clamp meter if you own one. Voltage at rest lies. Current under load does not.

What to log, and what to do after

A good sea trial produces evidence, not impressions. Keep a running log with timestamps : engine start, first cruise RPM, WOT attempt, temperatures, any alarms, sail changes, autopilot behaviour, anything odd. Photograph the instrument cluster at cruise and at WOT. If you have permission, take a short video of the exhaust at full throttle.

Cross-reference what you saw against the boat's paperwork. Engine hours on the counter should match the service book. Recent invoices should match the parts you can see. If they do not, ask why. A proper history check before the trial makes these questions sharper, because you already know what has and has not been done.

Finally, put a real number on what the trial revealed. A rebuilt injector pump is four figures. A new autopilot drive is four figures. A cutlass bearing plus haul-out is not free. These add to the buying price and to the running costs that people already underestimate : see our summary of the hidden costs of a boat if you need to frame that conversation with a partner or a bank.

Sea trials are short, and memory is unreliable. If the boat you buy carries an Oria Box afterwards, you will have engine hours, RPM curves, coolant temperatures, voltages and tracks logged automatically from day one, which makes the next annual service and the next resale easier to defend with numbers. But the trial itself is still on you : the more you write down that morning, the fewer surprises you inherit.