Boating on High-Altitude Lakes: Power Loss, Prop Changes, Cold Water and Mountain Weather

Before the trip, on your home water, note your wide-open-throttle RPM and top speed with a normal load. Check your owner's manual for the recommended WOT RPM range for your engine. This baseline tells you how much the altitude is actually costing you.

• Fuel injection (EFI), which Mercury says all new outboards and sterndrives have, automatically adjusts the fuel mixture for altitude and air temperature. It still can't make up the lost power, but it keeps the engine running cleanly.
• Carbureted engines don't adjust themselves. Mercury notes an untuned carbureted engine loses even more power than the 3% rule. Running rich up high can mean rough idle, smoke and fouled plugs.

If you boat at altitude regularly with a carbureted engine, have a dealer re-jet or adjust it for your typical elevation, and set it back before you return to low water. Don't guess at jet sizes yourself; the right change depends on the engine and elevation. See carburetor cleaning for general carb care.

On calm water with your normal load and fuel, trim for best speed and run up to wide-open throttle briefly. Note RPM and GPS speed. If RPM falls below your manual's recommended WOT range, the prop is effectively "too big" for the power you now have.

Running below the recommended range for long periods is hard on an engine, because it's laboring under load. Mercury's dealer note says occasionally topping out at 4,000 to 4,500 RPM won't hurt a modern outboard, which is why some anglers on short trips simply accept slower performance.

The Colorado dealer's experience is that simply changing to a lower-pitch propeller usually gives an acceptable improvement. A lower pitch lets the engine spin back up into its proper RPM range, improving hole shot and pulling power.

Rules of thumb:

• A day trip or a short vacation: many owners accept the slower boat.
• A week or more, heavy loads, watersports, or you live up high: a dealer-recommended lower-pitch prop is worth it.
• Some Midwest boaters buy the lower-pitch prop for the mountains and keep it as their spare at home, a tip from that dealer.

Ask a dealer or prop shop with altitude experience; many lend test props. See propeller for how pitch works and how to swap one.

On some sterndrive boats, a prop change alone isn't enough. The Colorado dealer describes sterndrives needing such a low prop pitch at altitude that the boat could barely get anywhere; changing the drive's gear ratio let them use a normal pitch with good hole shot and some speed. This is a dealer job, often ordered when the boat is built for a high-altitude dealer. Outboards from the factory may already come with suitable gearing.

Every pound counts more when you have less power. Leave extra gear at the dock, carry only the water and fuel you need, and spread weight evenly. Wakesurf boats may need less ballast to plane. See weight distribution and loading.

Mountain lakes are usually cool, but summer heat, long idle runs at no-wake speed, and silty or weedy intakes can still cause trouble. Check the telltale stream on outboards and keep an eye on the temperature gauge. See engine overheating if the alarm sounds.

UV is stronger at elevation. The EPA advises limiting midday sun (about 10 a.m. to 4 p.m.) even in winter and especially at higher altitudes, and notes that water reflects UV, raising your chance of sunburn. On a boat you get it from above and from the water. Wear hats, sunglasses, long sleeves and sunscreen, and reapply often. Drink plenty of water; thin, dry mountain air dehydrates people quickly.

Snow-fed lakes stay cold well into summer. The National Weather Service warns that warm air doesn't mean warm water, that cold shock can be just as severe in 50 to 60 °F water as in near-freezing water, and that gasping can be triggered in water as warm as 77 °F. The gasp reflex can cause drowning within the first minutes, even for strong swimmers.

• Everyone wears a life jacket, especially on cold water.
• Dress for the water temperature, not the air, in spring and fall.
• Watersports riders should consider a wetsuit.
• Know how to get someone back aboard quickly. See cold-weather boating.

The Colorado dealer in Mercury's article says weather along the Rockies is nearly impossible to predict: 88 °F one day, snow the next. Summer afternoons often bring fast-building thunderstorms with lightning and strong, gusty winds that kick up steep waves on big reservoirs.

• Check the forecast before you launch and plan to be off the water by early afternoon in storm season.
• Watch for towering clouds building over the peaks.
• At the first thunder, head for the ramp or a protected cove; don't wait. See lightning protection.

Many mountain lakes are remote, with no Coast Guard station and limited cell coverage. File a float plan with someone ashore, carry a handheld VHF if the lake has a marina or patrol on it, and save the local sheriff's or marine patrol number. Carry extra fuel; a heavier, slower boat burns more.

Budget time for the inspection station. Many western mountain states screen boats for invasive mussels before launch. Colorado, for example, is a mandatory inspection state: every trailered or motorized boat must be inspected by state-certified staff before it goes in the water, and motorized boats and sailboats also need an Aquatic Nuisance Species (ANS) stamp. Colorado Parks and Wildlife asks you to arrive with the drain plug out (and on hand), tarps and straps off, anchor and ropes out of their lockers, and your bait receipt ready. Arrive clean, drained and dry so you don't face a decontamination wait, and check the rules of each state and lake on your route.

Some lakes add their own rules. At Lake Tahoe, every motorized boat (even an eFoil or a kayak with a trolling motor) gets an inspection and a decontamination before launch, and the boat leaves the station with a sticker and a wire seal tying it to the trailer, which is removed at the ramp. The Tahoe program says arriving clean, drained and dry cuts the decontamination fee in half; that includes emptying ballast tanks, live wells and sea strainers and lowering the outdrive so its intakes drain. Bring any flush adapters your engine needs. Tahoe also bans most carbureted two-stroke outboards: direct-injected two-strokes and those certified to the newer California or EPA emission standards are allowed. Check your engine before you make the drive.

If you had a carbureted engine adjusted for altitude, have it set back for low elevation; an engine leaned for the mountains can run too lean at sea level. Swap back to your normal prop and check that WOT RPM is in range again.