How to Sea Trial Your Boat After Maintenance or Repair

With the boat still on the trailer or at the dock and the engine off:

• Look at every hose, clamp, fitting and wire near the work that was done. Tug gently.
• Check engine oil and, where you can, gear lube or drive oil levels, power trim and steering fluid.
• Make sure no tools, rags or caps were left in the engine compartment or under the cowl.
• Check the propeller: nut tight and its locking tab or cotter pin in place (Yamaha recommends a new cotter pin every time the prop goes back on).
• Confirm battery terminals are tight and the battery switch is on.
• On inboards, sterndrives and diesels, lay a clean absorbent pad or white rag under the engine (and generator) and write down the hour meter reading. Surveyor Frank Lanier suggests both in Practical Sailor: fresh drips stand out on a clean pad, and comparing hours before and after proves the meter works.
• Drain plug in.

Federal rules require boats with permanently installed gasoline engines to carry a label telling the operator to run the blower for four minutes and check the engine compartment bilge for gasoline vapors before starting. Do exactly that. Open the hatch and use your nose. Any fuel smell means stop.

As soon as the boat floats, open the hatches and look into the bilge with a flashlight before you go anywhere. Look for water coming in at the drain plug, through-hulls, transducers, shaft seal or stuffing box, and the drive bellows area on sterndrives. On a personal watercraft, open the seat and look in the hull. Stay tied up for a few minutes and look again. On a diesel or inboard with a stuffing box, note how the shaft seal behaves (see our stuffing box guide for what is normal for your seal type).

Start the engine and let it idle at the dock. Check right away:

• Outboards: the cooling water telltale stream should come out strong within a few seconds of starting.
• Sterndrives, inboards and diesels: water should come out with the exhaust. On a diesel, an exhaust with little or no water means shut down and check the raw-water seacock, strainer and pump.
• Personal watercraft: run it only in the water, following your owner's manual; check that the cooling outlet or "pee hole" flows if your model has one.

No cooling water within a few seconds means shut off immediately and find out why.

Let the engine warm up at idle while you watch temperature, oil pressure (if you have a gauge) and battery voltage, and listen for any alarm horn. Compare readings with what you normally see. Listen for new noises: rattles, squeals, knocking. With the engine still running and your hands well clear of belts and pulleys, look at the area that was serviced for drips of fuel, oil, coolant or water. A dry paper towel held near a fitting makes small leaks easy to see.

Still at the dock and tied securely:

• Steering: turn lock to lock. It should be smooth and even, with no binding, clunks or play. Hydraulic steering that feels spongy may have air in it.
• Kill switch: at idle, pull the lanyard. The engine should shut down immediately. Restart and clip it back on.
• Shifting: at idle, shift gently into forward, back to neutral, then reverse. Each shift should be clean, and the engine should not stall. Pause in neutral between directions.
• Trim and tilt: run the trim through its range and back down; listen for the pump laboring or the drive moving unevenly.
• Pumps and lights: bilge pump, blower and navigation lights work.

Idle away from the dock and run at low speed for several minutes. Then come back to neutral in open water and look in the bilge and engine compartment again (on outboards, a quick look under the cowl at idle, hands clear). Many leaks show up only once things are warm and under a little load. If the shaft seal, stuffing box or rudder area was worked on, look at it now and again at cruising speed with a helper at the helm: Practical Sailor notes these can be dry at the dock and leak only when the boat is moving.

Increase throttle in steps rather than going straight to full power. Hold each step for a minute or two and watch the gauges:

• Temperature should settle and stay steady at each step.
• Oil pressure should rise with RPM and hold.
• Voltage should show the charging system working.
• The boat should run smoothly with no new vibration.

If temperature climbs steadily, an alarm sounds or power drops, throttle back to idle and follow our overheat alarm response guide or the matching troubleshooting guide. BoatUS notes that a slow but steady temperature rise can point to a failing water pump impeller, and that you should not restart a hot engine until it has cooled. Come back slowly at low RPM if you must, watching the gauge, or call for a tow.

Mercury's advice on trim is a good baseline: trim the drive all the way in (down) before you accelerate so the boat climbs onto plane efficiently, then trim out (up) once on plane until the bow rises, steering lightens and speed picks up. Note how quickly the boat planes compared with before. Watch for the prop suddenly spinning up and losing grip (ventilation) when trimmed high or in turns (see our ventilation and cavitation guide). Steering should not pull hard to one side.

In open water with no traffic ahead, with your typical load aboard and trimmed for best speed, go to full throttle for a short run and note the highest steady RPM and the speed. Boating Magazine notes this is the number a prop shop will ask for. Compare the RPM with the full-throttle operating range in your owner's manual:

• Within the range: good. Your engine maker's manual says where in the range it should land with your load.
• Above the range: the engine is over-revving. Common causes are too little prop pitch, a prop that is ventilating, or a prop hub that is slipping (see our spun prop hub guide).
• Below the range: the engine is working too hard. Causes include too much prop pitch, a heavier load than usual, a fouled bottom, or an engine or fuel problem the work did not fix (or caused).

As a rough guide, Boating Magazine says each inch of prop pitch is worth about 200 RPM on recreational boats. Do not stay at full throttle longer than needed, and back off at the first sign of trouble. If there is current or a steady wind, Mercury's tip from its sea-trial checklist applies here too: make the same run in both directions and average the speeds, so your log number is fair to compare next season.

Come off plane smoothly, pause in neutral, and shift to reverse at idle. Practice a slow approach and a docking maneuver. A few checks from Mercury's sea-trial list are worth borrowing after steering, drive or prop work, with passengers seated and no boats nearby: at moderate speed with the drive trimmed down, make a tight turn each way and see whether the boat holds its RPM through the turn; back down at idle with the drive fully down and watch for water coming over the transom; and cross a few wakes at safe speed while listening for new rattles or bangs. Shifts should be crisp with no grinding, slipping or hard clunks; on sterndrives, a harsh or hard-to-shift drive after service is a reason to go back to the shop. Check that the throttle returns to idle normally and the engine idles smoothly in gear and in neutral.

Back at the dock, let the engine idle a minute, then shut it down. Then:

• Look in the bilge again for new water, fuel or oil.
• Look over the serviced area, hoses and fittings for drips now that everything is hot. Don't touch hot exhaust parts.
• On outboards and sterndrives, look at the gearcase and drive for oil streaks around seals and drain screws.
• On a personal watercraft, open the seat and check the hull for water after the ride.
• On a diesel, note the exhaust color during the run (see our diesel exhaust smoke guide) and check the shaft seal again.
• Recheck oil and fluid levels once the engine has cooled as your manual describes.

Write down in your BoatMend log:

• Date, engine hours and what work was done
• Water and air conditions, load and fuel level
• Idle and cruise temperatures, oil pressure and voltage
• RPM and speed at cruise, and at wide-open throttle (with trim position)
• Anything that felt, sounded or looked different

These numbers become your baseline. Next season, a lower WOT RPM or a hotter cruise temperature tells you something changed before it becomes a breakdown. Report any problem to the shop that did the work right away; most will want to see it while the job is fresh.