Boat Slower Than It Used to Be? How to Find the Cause

Test with your usual load: same number of people, similar fuel level, same gear. Run the speed in two opposite directions and average them to cancel wind and current. A full fuel tank, a crowd aboard or a headwind can easily take a few mph off without anything being wrong.

Discover Boating has seen owners remove around 250 pounds of things they didn't need. Empty every locker and put back only the essentials. Remember liquids: gasoline weighs a little over 6 pounds per gallon and water about 8. Drain livewells, ballast tanks, freshwater and holding tanks if they are full, and move heavy gear off the bow.

Open the bilge and every hatch:

• Is the drain plug in? Is the bilge pump keeping up?
• Is water sitting in the bilge or in hidden compartments, a fishbox or a pontoon tube?
• Does the boat sit lower at the dock than it used to, with the bottom paint line under water? Water inside the boat is just heavy cargo you didn't mean to bring. If water keeps coming back, see the boat taking on water guide before you go out again.

Trim matters more than most owners think. Discover Boating's method: run up to full throttle, then bump the trim up in small taps while watching GPS speed. When RPM climbs but speed stops rising or drops, the prop is starting to ventilate; trim down a touch until it bites again. Make sure trim tabs are fully up for the test, and check that the trim gauge reads correctly. Over-trimming and tabs left down both cost speed.

Tilt the motor up (or look at the drive or running gear from a dinghy or with the boat on the trailer):

• Dings, bent or curled blade tips, missing chunks. Yamaha notes even small nicks can affect performance.
• Fishing line wrapped behind the prop, which can also damage the gearcase seal.
• The right prop? A spare or a "deal" prop of the wrong pitch or diameter changes everything. Read the size stamped on the hub and compare with what the boat had when it ran well.
• Worn edges and tips. Yamaha notes that sand, grit and silt slowly eat away blade edges, so a prop can look fine and still cost performance. BoatUS adds that tips worn down or rounded off shrink the diameter and blade area, which raises RPM and wastes fuel.
• Corrosion. On a bronze prop, a pinkish color and a dull "thud" instead of a ring when you tap a blade are signs of corrosion eating the alloy (BoatUS). That prop usually has to be replaced, and the cause found.
• Used boat? BoatUS warns that a freshly spray-painted prop can hide nicks and bends; look for the edge of the new paint. For dings and repair, see the propeller guide.

If the engine revs up but the boat doesn't accelerate to match, especially under hard load, the rubber or plastic hub inside the prop may have spun. Ventilation (air getting to the prop) looks similar: RPM flares and speed drops, often in turns or when trimmed high. See the spun hub and ventilation guides to tell them apart.

Haul the boat or have a diver look. BoatTEST describes how barnacles, grass, tube worms and slime quickly build up into a "fuzzy carpet" that slows the boat. Also check:

• Growth on the prop, gearcase, trim tabs, water intakes and the transducer.
• Peeling or rough bottom paint, or paint built up in thick layers over many seasons.
• In fresh water, zebra or quagga mussels on trailered boats left in the water for weeks. Clean the bottom (see the hull cleaning and bottom paint guides), then test again before chasing anything else.

With a clean bottom and normal load, run at wide-open throttle briefly and note the RPM:

• Inside the range, speed still low: look at prop slip (next step) and drag.
• Below the range: the engine is overloaded or down on power. Too much pitch, too much weight or growth, or an engine problem. Discover Boating explains that more pitch lowers WOT RPM and less pitch raises it. BoatUS's rule of thumb is that if a healthy engine with a clean hull can't get within about 100–200 RPM of its rated WOT, the prop is the likely issue, and that each inch of pitch changes WOT by very roughly 175–225 RPM (typical; your dealer or prop shop has the real numbers for your setup). Most props can be re-pitched at a prop shop.
• Above the range: the prop may be ventilating, slipping or have too little pitch, or the motor may be mounted too high. Running an engine below its WOT range for a long time is hard on it, so don't just live with it.

Slip is the difference between how far the prop would move the boat in theory and how far it actually goes. You need RPM, the gear ratio (owner's manual), the prop pitch in inches (stamped on the prop) and GPS speed in mph:

• Theoretical speed (mph) = (RPM ÷ gear ratio × pitch) ÷ 1056
• Slip % = (1 − actual speed ÷ theoretical speed) × 100 Mercury says calculated slip between about 5% and 25% is typical. Above 25%, something is likely wrong with the prop: it may be damaged, badly mismatched to the boat, or ventilating because the drive or outboard is mounted too high. Mercury's online slip calculator does the math for you.

If RPM is low even with a clean bottom, light load and the right prop, the engine may be sick. Work through the loss of power guide: fuel filters and water separator, old fuel, spark plugs, a clogged fuel vent, overheating or a warning light that limits power. Note any alarm or "guardian" mode message on your display. An engine that runs up fine and then slows itself down at high throttle, over and over, is often protecting itself (overheating or low oil pressure, for example) rather than "running out of power." Yamaha notes most outboards have an RPM reduction mode that limits engine speed when they detect overheating. Back off and check before running it hard again.

A boat that slowly gets heavier year after year, with no water in the bilge, may have water soaked into flotation foam or a wet core. Signs: it sits lower, feels sluggish and "heavy," the bilge never quite dries, or there are soft spots in the deck or transom. A trailer boat can be weighed at a certified truck scale and compared with the builder's dry weight plus engine, fuel and gear. See the waterlogged flotation foam guide, and ask a surveyor to confirm before you start cutting anything.

Think about what changed since the boat was fast: a new prop, a jack plate or engine height change, a new transducer or trolling motor bracket throwing spray, a T-top or tower, new bottom paint, more passengers than usual. Undo one change at a time and test.

Don't forget the air and the lake itself:

• Altitude. Mercury says engines lose about 3% of their horsepower for every 1,000 feet above sea level, so a boat that ran fine at a sea-level lake can feel tired on a mountain reservoir even when nothing is wrong. Fuel injection adjusts the mixture but can't make up the lost power; dealers in high country usually fix it with a lower-pitch prop, and sometimes a different gear ratio on sterndrives.
• Heat. Mercury also notes that cool spring and fall air is denser, so the engine makes more power than in the heat of summer. Some owners run a higher-pitch prop in cool months and a lower one in summer to stay inside the WOT RPM range. A small speed loss on a hot summer day is normal.

If you still can't find it, a dealer can run a compression test, read engine data, and sea-trial the boat with the right test props. Bring your log of speeds, RPM, load and what you have already checked; it saves diagnostic time.