A sea trial is not a boat ride. It's the last hour you have to walk away clean.
Once you sign the acceptance of vessel, the boat's problems become your problems; and on a used boat, the expensive problems almost always live below the waterline or behind the engine hatch. This checklist covers what to watch, what to write down, and which numbers actually tell you whether you're buying a well-kept boat or somebody else's deferred maintenance.
Before You Leave the Dock
Get the contingency in writing first. Standard practice is an accepted offer subject to survey and sea trial, with your deposit in escrow. That language is what gives you the right to renegotiate or walk. If a seller wants you to trial a boat without it, that's information.
Do not let the seller pre-warm the engine. A cold start tells you more than anything else in the next two hours. Show up early. If you arrive and the engine is already at operating temperature, reschedule - a hard-starting engine, a smoky start, or a weak battery all disappear once it's warm.
Bring these:
- Your surveyor (and pay them to be there - it's the cheapest money you'll spend)
- An infrared thermometer
- A phone for video, especially of the tach and gauges at wide-open throttle
- A notepad, not your memory
- The engine manufacturer's spec sheet with the rated WOT RPM range for that specific model
Record the conditions. Fuel level, water tank level, number of people aboard, gear aboard, sea state. A boat trialed with 30 gallons of fuel and two people will perform very differently than the same boat loaded for a weekend. If you skip this, your numbers mean nothing.
The Cold Start
- Does it turn over strong, or does it labor?
- How many seconds to fire?
- Smoke color at startup and after 30 seconds. White that clears is usually condensation. Blue means oil. Black on a diesel under no load means fuel or air problems. Persistent smoke of any color is a conversation.
- Watch for blowby at the oil fill or breather.
- Oil pressure should come up promptly. Note how long.
- Check voltage, is the alternator charging?
Let it idle and go stand in the engine compartment. Listen. Look for weeping at hose clamps, exhaust elbows, and the raw water pump.
At the Dock, In Gear
Before you go anywhere, put it in forward at idle against the lines and then in reverse.
- Engagement should be smooth and immediate. A clunk, a delay, or grinding points at the transmission or shift cable.
- On an inboard, put a hand on the shaft log. Excessive vibration at idle in gear is an early warning of alignment, bearing, or prop problems.
- Check the stuffing box. A traditional packing gland should drip a few drops per minute while running and essentially stop when shut down. A dripless seal should not drip at all.
- Look at the bilge. Dry is what you want.
The Wide-Open Throttle Test
This is the single most informative thing you will do all day, and it's the one buyers most often skip because they feel rude pushing someone else's boat hard.
Push it hard. Run the boat to wide-open throttle, hold it long enough to stabilize, and read the tachometer against the engine manufacturer's rated WOT range.
Here's how to read the result:
RPM below the rated range. The engine can't reach its designed operating speed. That means either the boat is overpropped (too much pitch), it's carrying more weight or bottom growth than it should, or the engine is down on power. All three are worth knowing before you buy. An engine that lives its whole life below its WOT range runs hot, loads its bearings, and wears out early — so if the boat is chronically overpropped, ask what else that operating pattern has cost.
RPM above the rated range. Underpropped, or a slipping hub. Easy to fix with the right prop, but note that the boat has been giving away thrust and fuel economy.
RPM within range, but the boat feels slow. Look at bottom condition, prop condition, and load. A dinged or out-of-pitch prop will let the engine rev while producing less thrust than it should.
Video the tach at WOT. Sellers and brokers remember these numbers generously.
The Vibration Sweep
Run the boat up through the RPM range in increments; say 1000, 1500, 2000, 2500, 3000, and on up, and hold at each step for 15 to 20 seconds. Note anything you feel or hear at each one.
Vibration that appears at a specific RPM band and disappears above and below it is almost never "just how boats are." Common causes, roughly in order of how often they turn up:
- A bent or out-of-pitch blade
- Prop imbalance from a previous repair
- A worn cutless bearing (play in the shaft lets the prop wobble)
- Shaft runout, a bent shaft, often from a grounding
- Engine-to-shaft misalignment
- A failing motor mount
None of these are catastrophic on their own. All of them cost money, and several of them signal a past grounding that the seller may not have disclosed. Write down the exact RPM where it shows up, that number is what a prop shop or mechanic will ask you first.
Handling and Maneuvering
- Time to plane. Note it with a stopwatch. Compare against what the model should do.
- Planing attitude and trim response. Does the boat come up cleanly, or does it hang bow-high and plow?
- Hard-over turns, both directions. Listen for the prop ventilating or blowing out. Some of this is normal in a tight turn; a prop that blows out easily may be the wrong prop, mounted too high, or damaged.
- Steering. Play in the wheel, hard-over to hard-over turn count, any hydraulic weeping at the ram.
- Reverse under load. Back down hard. Chatter, shudder, or poor stopping power is worth chasing.
- Trim tabs, if fitted. Both sides, full travel, no groaning.
Systems Under Way
The dock demo isn't the test - running is. While underway, check:
- Chartplotter, radar, depth sounder, and autopilot with the engine running and charging
- VHF radio, including a radio check on an appropriate channel (not 16)
- Windlass under load, not just spinning free
- Air conditioning and generator if fitted, under actual load
- Head and freshwater pump
- Bilge pumps — every one of them, manually triggered
- Horn and all navigation lights
Electronics that work on shore power and fail underway usually mean a charging or grounding problem, which is a wiring job, which is expensive.
Immediately After Shutdown
Don't let everyone step off the boat. The five minutes right after a hard run are diagnostic.
- Pull the dipstick. Milky or coffee-colored oil means water intrusion. Stop there and call your surveyor over.
- Check transmission fluid the same way.
- Shoot the exhaust elbows, risers, and manifolds with the infrared thermometer. Big temperature differences between banks or between cylinders point at cooling restrictions.
- Look for new leaks in the engine compartment that weren't there cold.
- Feel the stuffing box. Warm is fine. Hot is not.
The Haul-Out: Where the Real Money Hides
If the sea trial goes well, the boat comes out of the water. This is where a prop shop's eyes are worth more than anyone's.
The propeller. Look at every blade edge for dings, curl, and erosion. Look for cavitation burn; a pitted, sandblasted-looking area on the blade face, which indicates the prop has been working wrong for a long time. Check that the blades match each other; a prior repair that pulled one blade out of pitch will vibrate forever until it's rebalanced.
Shaft play. Grab the prop and try to move it up, down, and side to side. Any perceptible play means a worn cutless bearing. That's a cheap part and a not-cheap haul-out, so it belongs on your negotiation list, not your surprise list next season.
Shaft straightness. A bent shaft usually shows up as the vibration you already noted at a specific RPM. Confirm it here.
Zinc anodes. Fully wasted zincs mean the boat sat neglected. Zincs that are gone while nearby bronze shows pink or the prop looks etched means active galvanic corrosion — a stray current problem in the marina or on the boat. That one's worth investigating hard.
Struts, rudder, and skeg. Play in the rudder, cracks or repairs at the strut, welded or ground-down skegs. A repaired skeg means the boat hit something. Ask what else got hit.
Bottom. Blisters, past repairs, and paint condition.
Before You Sign
Take everything you wrote down and sort it into three piles: deal-breakers, price adjustments, and things you'll handle yourself. Then negotiate against the first two before you sign acceptance - not after.
Almost none of what turns up on a good sea trial is fatal. A prop that's an inch off in pitch, a worn cutless bearing, tired zincs, a shaft that needs truing — these are ordinary, fixable items on an otherwise sound boat. But they're worth real money at the negotiating table, and they're worth nothing at all once you've signed.
Fixing What You Found
When the survey comes back with running gear items, Propeller Depot is where you get them handled without a dealer markup.
We stock inboard, outboard, and sterndrive propellers from Michigan Wheel, SOLAS, ACME, Volvo Penta, and more, plus cutless bearings, prop shafts, PYI and Tides Marine shaft seals, Zimar zinc anodes, and marine hardware - shipped worldwide.
If your WOT numbers came back outside the rated range, that's a propping problem, and it's the thing we do all day. Send us your engine specs or your OEM part number and we'll size it for you. Propeller Depot is here for you to ensure you don't go wasting a bunch of money on a purchase or boat parts.
Call 1-800-454-6960 or email sales@propellerdepot.com. Bring your sea trial notes. We'll tell you what they mean.