Spring Commissioning Checklist — Getting Your Boat Ready After Winter

Every spring I see the same thing — guys pull their boat out of storage, dump in some fresh gas, crank it over, and then call me when it runs like garbage or won't start at all. Six months of sitting changes things. This checklist is how I work through a boat after winter, in the order I actually do it. Work through it once and you'll have a good season. Skip it and you're rolling the dice.

Spring commissioning isn't complicated, but it has to be done in the right order. You're not just "checking things" — you're reversing what winter did to rubber, fuel, electrical connections, and water passages that sat empty or full for months. Take an afternoon, go through this list, and your first trip out will be a good one.

I'm also going to stop you before you even open the oil fill cap and make sure you know why marine oil and car oil are not the same thing — because every spring I find out somebody swapped in whatever was on the shelf at the auto parts store, and it costs them dearly. More on that below.

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1. Visual Inspection — Before You Touch Anything Else

Walk around first. Find problems before they find you.

Check the hull for cracks, stress fractures, or soft spots Run your hand along the fiberglass, especially around the transom and below the waterline. Gelcoat spiderweb cracks are usually cosmetic. Deep cracks that go through the laminate are not — deal with those before the boat touches water.

Inspect the transom for softness Push firmly on the transom with your fist — it should feel solid, like a hardwood door. Any flex, sponginess, or give means water got into the transom core over winter. A soft transom is a real problem on older boats and needs to be addressed before it gets worse.

Look for rodent damage Mice, rats, and squirrels love stored boats. Check wiring, hoses, upholstery, and any foam areas. Chewed wires and gnawed fuel lines are fire hazards — find them now, not on the water.

Inspect all hoses and belts Look for cracks, brittleness, soft spots, or swelling on every hose — bilge, fuel, cooling. Belts on inboards and stern drives should show no cracking or fraying. Rubber gets stiff in the cold and doesn't always recover. Replace anything questionable now; hose replacement on the water is miserable.

Check the bilge for standing water or fuel smell Some water in the bilge is normal. Fuel smell is not — that means a leak somewhere. Find it before you turn the key. Run your bilge pump and make sure it's working before your first trip out.

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2. Fuel System

Old fuel causes more spring problems than anything else.

Smell and inspect your stored fuel Gasoline goes stale in as little as 30 days without stabilizer. After six months, untreated fuel has often degraded and separated, especially ethanol blends. Stale gas smells sour, not sharp. If it was stored without stabilizer, drain it and start fresh — do not try to run it.

Replace the fuel filter Inline fuel filters and canister filters are cheap. After a winter of sitting, sediment and varnish from degraded fuel settle at the bottom of the tank and go straight into the filter on first startup. Replace it every spring — this is not optional.

Inspect fuel lines for cracks and hardening — replace with marine-rated hose only Flex each fuel line by hand. Ethanol-blend fuel is hard on older rubber lines, and six months of cold makes it worse. Any line that cracks, shows surface checking, or feels rock-hard needs replacement. When you replace it, use marine-rated fuel hose — not automotive fuel hose from the auto parts store. Marine fuel hose is specifically engineered to prevent fuel permeation and vapor seepage in an enclosed bilge, and it is the only hose approved by the U.S. Coast Guard for use on boats. Automotive fuel hose is not Coast Guard approved and can allow gasoline vapor to seep through the hose wall into the bilge — a direct fire and explosion risk. Fuel line failure is a fire — take it seriously, and use the right hose.

Check primer bulb (outboards) Squeeze the primer bulb firmly — it should feel firm and hold pressure. If it stays collapsed, is cracked, or collapses back down immediately, replace it. A bad primer bulb will give you a no-start or lean condition from the dock.

Inspect the carburetor (if applicable) If the engine sat over winter with fuel in the carb and no stabilizer, expect varnish in the jets and passages. A rough idle or lean stumble on the first run is the carb telling you it needs to be cleaned. On older outboards especially, plan on a carb cleaning as part of spring commissioning.

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3. Engine and Powerhead

Don't start it until you've done these first.

Change the oil and filter (inboards / stern drives / 4-stroke outboards) If you didn't change the oil before storage, the oil sitting in your engine all winter has acids and combustion byproducts in it that have been attacking your bearings since October. Change it now. And use the right oil — see the section below on why marine oil matters.

Check gear lube in the lower unit (outboards / stern drives) Pull the lower unit drain plug. Gear lube should come out clean — amber to light brown. Mercury High Performance Gear Lube is greenish in color, which is normal — what you're looking for is consistency and clarity, not a specific shade. If the lube looks milky, white, or foamy regardless of color, water got in over winter through a worn seal. You need new seals and fresh gear lube before you run it. Running water-contaminated gear lube destroys the gears quickly.

Inspect and gap spark plugs — or replace them Pull every plug and read them (see the spark plug reading guide if you're not sure what you're looking at). If they're more than two years old or show carbon, oil, or corrosion, put new ones in. A fresh set of plugs is cheap insurance for a clean first start.

Check the propeller — repair or replace as necessary Take the prop off and look at it. Nicks and dings reduce efficiency and can cause vibration — have a prop shop straighten and balance anything that took a hit last season. More importantly, check for fishing line wrapped around the prop shaft — mono wraps tight and cuts seals. A damaged prop seal means water in your lower unit all season. Before reinstalling the prop, grease the propeller shaft with marine grease. This prevents the prop from seizing onto the shaft and makes removal much easier the next time.

Check the throttle and shift cables Move through the full range of throttle and shift by hand. They should operate smoothly with no binding, stiffness, or slack. Lubricate the pivot points and linkage. A sticky shift cable that hangs between gears will ruin your day and potentially your lower unit.

Grease all fittings Tilt and trim pivot points, steering components, throttle pivots — hit every grease fitting with fresh grease. Things that weren't greased before storage have been sitting dry and exposed to condensation all winter.

⚠️ Important — Read This Before You Add Oil

Why Marine Oil Is Not the Same as Car Oil

Every spring somebody tells me they used 10W-30 from the auto parts store because "oil is oil." It is not. I've seen this mistake kill engines, and I want to save you the repair bill — or the engine replacement.

The difference isn't marketing. It's chemistry. Marine engines live in a completely different environment than car engines, and the oil formulas reflect that. Here's what's actually different:

❌ Car Oil (What You Should Not Use)

Formulated for sealed crankcase ventilation — fumes recirculate through the engine
Contains friction modifiers designed for dry-sump engines with no water contact
Lower corrosion inhibitors — not built for saltwater or high-humidity environments
Additives can foam under the constant vibration and load profiles of marine engines
Does not meet TC-W3 or NMMA standards for marine use
May void your engine warranty

✅ Marine Oil (What Your Engine Needs)

Formulated for open crankcase ventilation — marine engines breathe differently
Higher anti-corrosion additive package — built for saltwater, humidity, and condensation
Anti-foam additives designed for the vibration and RPM profile of marine powerheads
2-stroke marine oil (TC-W3) burns cleanly — car 2-stroke oil leaves deposits
Designed for the longer run times and wide-open throttle conditions boats operate under
Meets NMMA (National Marine Manufacturers Association) certification standards

The 2-Stroke Issue Is Especially Critical

If you have an older 2-stroke outboard, using automotive 2-stroke oil is particularly damaging. 2-stroke marine oil is rated TC-W3, which means it's formulated to burn completely in a water-cooled engine. Car 2-stroke oil (for chainsaws, lawnmowers, etc.) does not burn cleanly in a marine environment — it leaves deposits on the pistons, rings, and power valves that cause carbon fouling, stuck rings, and eventually seized pistons. The engines I see with the most carbon buildup are almost always the ones that got the wrong oil at some point.

What Oil Should You Use?

Match the oil to your engine type. For 4-stroke inboards and stern drives, use a marine-certified 4-stroke oil from Quicksilver, Yamaha, Valvoline Marine, or Pennzoil Marine — look for the NMMA FC-W certification on the label. For 4-stroke outboards, most manufacturers specify their own branded oil or an NMMA FC-W certified equivalent. For 2-stroke outboards, use TC-W3 certified marine 2-stroke oil — full stop. No exceptions.

"The extra few dollars for the right oil is not a cost — it's cheap insurance. The engines I've seen damaged by the wrong oil cost thousands to repair or replace. Marine oil at $12 a quart is the best money you'll spend all season."

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4. Cooling System

This is what kills engines that seem to run fine at first.

Inspect the water pump impeller The impeller is the rubber vane inside your water pump that circulates cooling water through the engine. Rubber sets over winter — the blades flatten or crack from sitting in one position for months. On outboards, replace the impeller every two years, and always at the start of the season if it wasn't done last fall. A failed impeller overheats your engine within minutes. This is the most important item on this entire list.

Check the tell-tale stream (outboards) When you start the engine on muffs or in the water, look for the tell-tale stream immediately. It should be a steady, strong flow of water. A weak dribble, no stream at all, or water that cuts in and out means the impeller isn't pumping properly — shut the engine down and investigate. Don't run it hoping it gets better.

Inspect thermostat (inboards / stern drives) Thermostats can stick closed over winter. A stuck-closed thermostat will overheat your engine. A stuck-open one will cause the engine to run cold and never reach operating temperature — which leads to sludge buildup and fuel wash on the cylinder walls. If you're not sure, pull it and test it in a pan of hot water with a thermometer.

Flush and inspect raw water strainer (inboards) The raw water strainer is the last line of defense against debris entering your cooling system. Clean it out, inspect the housing for cracks, and make sure the sealing gasket is in good condition. If it leaks, you lose cooling water pressure and you'll have a wet bilge.

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5. Electrical and Battery

Cold kills batteries. Corrosion kills connections.

Test and charge the battery A battery that sat discharged all winter may have sulfated and won't take a proper charge. Put it on a smart charger, let it fully charge, and then load-test it. If it won't hold above 12.4 volts under load, replace it. Don't trust a weak battery on the water.

Clean and treat battery terminals Corrosion on battery terminals is a slow voltage killer. Clean them with a wire brush and baking soda solution, dry them, and coat them with terminal protector spray or petroleum jelly. Make sure the connections are tight — a loose terminal that arcs will drain the battery and can cause a fire.

Inspect wiring for rodent damage and corrosion Trace the main wiring harness as much as you can see. Look for chewed insulation, greenish corrosion on connections, or terminals that have pulled out of their housings. Pay attention to connections near the bilge where moisture collects.

Test navigation lights, bilge pump, and horn These are required by law and are safety equipment. Test every one before your first trip. Replace any burned bulbs. Make sure the bilge pump auto float switch activates correctly — it's your last line of defense if water gets in.

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6. Below the Waterline and Steering

What you can't see after launch is what sinks you.

Inspect and replace drain plug gasket if needed It sounds simple because it is, but a cracked or missing drain plug gasket is how boats sink. Inspect the rubber gasket — if it's cracked, flat, or doesn't seat firmly, replace it before launch. Always install the drain plug before the boat leaves the trailer. Every season someone forgets.

Inspect the hull bottom and running gear With the boat still on the trailer, look at the bottom. Inspect zinc anodes on outboards and stern drives — if they're more than 50% consumed, replace them. They're sacrificial by design, and if they're gone, the hardware they protect is corroding instead.

Check steering — helm to motor Turn the steering wheel lock to lock and watch the motor move. It should move smoothly and fully in both directions with no binding, stiffness, or play. Hydraulic steering: check fluid level and look for leaks at the ram. Cable steering: lubricate and check for fraying. Steering failure at speed is catastrophic.

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7. Safety Gear — Don't Skip This One

Coast Guard requirements. Non-negotiable.

Inspect life jackets (PFDs) for all aboard Check that you have the right number and type for your boat. Look for faded or deteriorated fabric, broken buckles, and missing or damaged inflator mechanisms on inflatable PFDs. Inflatable PFDs must be re-armed annually with a new CO2 cylinder if deployed, or inspected if not.

Check your fire extinguisher Look at the gauge — needle must be in the green. Check the pull pin and tamper seal. If the extinguisher is more than 12 years old or has been discharged, replace it. Mount it where it's accessible in an emergency, not buried under the seat cushion.

Check flares / visual distress signals Flares expire. Check the dates on every flare — expired flares don't count toward your legal requirement and may not work when you need them. Replace any that are expired or within 6 months of expiration.

Verify anchor and rode Check the rode (line or chain) for wear, fraying, or corrosion. Make sure the anchor is secure in its mount and the rode is cleated properly. An anchor that won't deploy quickly when you need it is not an anchor — it's ballast.

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8. Trailer — Don't Forget the Thing That Gets You There

A trailer failure on the highway is worse than a breakdown on the water.

Inspect tires — pressure, tread, and sidewalls Trailer tires are notorious for dry rot because they sit stationary for months and UV attacks the sidewall compound. Check both the tread and the sidewalls closely — look for cracks, checking, or crazing in the rubber, even if the tread looks fine. A sidewall blowout at highway speed with a loaded trailer is extremely dangerous. Check the inflation pressure cold against the tire's max PSI rating (trailer tires are typically run at or near max). If a tire is more than 5–6 years old, consider replacing it regardless of how it looks — the rubber compound degrades from the inside out.

Inspect and repack wheel bearings Wheel bearings are the single most common cause of trailer breakdowns. They sit submerged every time you launch and retrieve, then cook dry on the return trip home as the heat expands the hubs. Spin each wheel by hand — it should spin smoothly and quietly with no roughness, grinding, or wobble. Pull the dust cap and check the grease: it should be smooth and light tan or amber. Dark brown, gritty, or milky grease means the bearings need to be repacked or replaced now. If you have Bearing Buddy or similar pressurized hubs, pump fresh marine wheel bearing grease in slowly — you'll know you have enough when the spring-loaded front cap on the Bearing Buddy moves outward slightly and rocks back and forth with light finger pressure. That's the indicator that the hub is full. Do not try to purge grease out the rear seal — that's not how Bearing Buddies work and forcing too much grease can damage the rear seal. Repack or replace the bearings outright every 1–2 years, or immediately if there's any doubt. A bearing failure at speed throws the wheel off the trailer.

Check brakes (if equipped) Surge brakes and electric brakes both need annual inspection. For surge brakes, check the actuator coupler mechanism for free movement and corrosion — the hydraulic cylinder inside the actuator can corrode and seize over winter. Check the brake fluid level and look for any signs of leakage at the actuator or wheel cylinders. For electric brakes, test the breakaway battery — it should hold a charge and activate the brakes if the trailer separates. Check the breakaway cable is properly connected to the tow vehicle and hasn't corroded through. Inspect the brake pads or shoes for wear and the rotors or drums for surface rust, pitting, or scoring.

Inspect the frame and crossmembers for rust Get under the trailer and look closely at the frame, crossmembers, and any welds. Surface rust is normal and manageable — wire brush it and hit it with cold galvanizing spray or a rust-inhibiting paint. What you're looking for is rust-through: areas where the metal has corroded so deeply that it's pitted, flaking, or has become structurally thin. Pay special attention to the rear crossmember that supports the motor when the boat is on the trailer, the tongue, and any area around the drain holes. A frame that's rusting through is a serious structural failure waiting to happen — it needs professional repair or trailer replacement before it's loaded.

Check the hitch, coupler, and safety chains Make sure the coupler latch mechanism latches firmly and the locking pin or clip is in place. Verify that your ball size matches the coupler — a 2-inch coupler on a 1-7/8-inch ball is a rollover waiting to happen. Inspect the safety chains for kinks, links that are worn thin, or hooks that are bent or missing their keepers. The chains must be crossed under the tongue and have only enough slack to allow turns — not dragging on the ground. Inspect the hitch receiver and ball mount for cracks, and check that the hitch ball nut is torqued tight.

Test trailer lights and inspect wiring Plug in the trailer and have someone stand behind it while you work through all the lights: running lights, brake lights, left turn, right turn, and reverse if equipped. Trailer wiring lives a hard life — it gets dunked at the ramp, dragged across parking lots, and chewed by rodents over winter. Any light that's out, flickering, or working intermittently needs to be traced to the connection. Corrosion at the 4-pin or 7-pin connector is the most common culprit — clean it with electrical contact cleaner and apply dielectric grease before plugging in.

Inspect winch strap or cable, and winch mechanism Check the winch strap for fraying, mold, UV deterioration, or cuts — a failed strap means your boat slides off the trailer at the worst possible moment. If it's a cable, look for kinks, broken strands, or corrosion. Make sure the hook is in good shape and the keeper snap is working. Test the winch mechanism itself: it should ratchet smoothly in both directions and the brake should hold the load without slipping.

Check bunks or rollers and their hardware Bunk carpet wears down and gets compressed, reducing the support and grip that protect your hull. If the carpet is thin, hardened, or coming off the bunk boards, replace it — it's inexpensive and directly protects your fiberglass. Roller trailers: spin each roller and make sure they turn freely and aren't seized or cracked. Check all bunk and roller mounting bolts — they corrode and loosen over the season and can let a support shift under load.

💡 Dunk the Trailer Before You Launch Before you back the trailer into the water at the ramp, do a slow roll-around in the parking lot and watch the tires, check that the lights are still working, and pull the drain plug from the boat. At the ramp, back in slowly — don't submerge the trailer hubs any more than necessary. Hot hubs + cold water pulls water past the seals. The less time in the water, the better for your bearings.

The First Start — Do It on the Muffs, Not in the Water

After working through this checklist, do your first startup of the season on flushing muffs connected to a garden hose — not in the water at the ramp with a line of trailers behind you. You want to verify the tell-tale water flow, listen for any unusual noises, check for fuel or oil leaks, and let the engine reach full operating temperature before you launch. Any problems that show up on muffs are easy to deal with in your driveway. The same problems at the ramp with spectators watching are not.

💡 Blaine's Spring Rule Let the engine run on muffs until the thermostat opens and it reaches operating temperature — usually 10-15 minutes. Then advance to full throttle briefly (as much as the muffs allow) and make sure it responds cleanly. Run it back to idle and check the tell-tale one more time. If everything looks right, you're ready to launch.

⚠️ Never Run an Outboard Dry — Even for a Few Seconds Water pump impellers can be damaged in seconds without water flow. Never start your outboard unless you have either flushing muffs on a hose or the lower unit submerged in water. Even a 10-second test start without water can score or flatten the impeller blades.

🔧 What You'll Need for Spring Commissioning

🛢️ Marine 4-Stroke Engine Oil 🔩 Marine Oil Filter 🛢️ TC-W3 2-Stroke Marine Oil 💧 Water Pump Impeller Kit ⛽ Marine Fuel Filter ⚙️ Lower Unit Gear Lube 🔋 Marine Smart Battery Charger 🔌 Outboard Flushing Muffs 🔧 Marine Grease ⚡ Battery Terminal Cleaner & Protector 🔌 NGK Marine Spark Plugs ⚓ Zinc Anodes 🛒 Fuel Line & Primer Bulb (eBay) ⚙️ Marine Wheel Bearing Grease 🔩 Bearing Buddy Hub Protectors 🛞 Trailer Wheel Bearing Kit 💡 Trailer LED Light Kit 🪝 Trailer Winch Strap 🛡️ Cold Galvanizing / Rust Inhibitor Spray 📦 Trailer Bunk Carpet

* Affiliate links — I earn a small commission if you buy through these at no extra cost to you.

Something not right on the first startup?

Describe what it's doing — rough idle, no tell-tale, smoke, noise, anything — and I'll help you figure out where to look first.

Ask Blaine →

Technical References

Content in this article is based on hands-on experience and cross-referenced with the following industry sources. All written descriptions and recommendations are original to OldSchoolBoatGuy.com.