Last reviewed: 2026-05-10 The Mercury 115 HP FourStroke is the motor Mercury keeps headlining for one specific reason: it's the lightest 115 HP outboard available. That sounds like marketing until you watch a customer try to repower a 19 ft aluminum fishing boat that was...
Last reviewed: 2026-05-10
The Mercury 115 HP FourStroke is the motor Mercury keeps headlining for one specific reason: it's the lightest 115 HP outboard available.
That sounds like marketing until you watch a customer try to repower a 19 ft aluminum fishing boat that was rigged for a heavy 115 two-stroke and discover that switching to a 359 lb modern FourStroke changes how the boat sits in the water. Less stern weight. Better trim. The boat planes faster on less throttle. The customer thinks the new motor is more powerful. It's actually just lighter.
That's the 115 FourStroke story. Same 2.1L block as the 75, the 90, and the 100. Tuned to run higher in the RPM band. Wrapped in the lightest 115 HP cowling on the market. For 17 to 20 ft aluminum boats, mid-to-large pontoons, and family runabouts that want headroom without crossing into V6 territory, the 115 is the answer.
Pricing scope: This article is about the standard 115 ELPT FourStroke and the 115 ELPT Command Thrust. There's also a 115 Pro XS (different motor, tuned for performance hole-shot) and a counter-rotation 115 (twin engine setups). Different SKUs, different prices, different recommendations. If you're shopping a specific configuration, build a quote so we're comparing the right one.
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Quick answer
The Mercury 115 ELPT FourStroke is a 2.1L inline-4 outboard with electric start, power trim and tilt, a 20" long shaft, electronic fuel injection, SmartCraft digital integration, and a dry weight of 359 lb (363 lb in the Command Thrust version). Mercury rates it for 5,000 to 6,000 RPM at full throttle and backs it with the 6-year warranty (3 factory + 3 promotional, when active).
It's the right motor for 17 to 20 ft aluminum fishing boats, mid-to-large pontoons (especially with the Command Thrust gearcase), family runabouts wanting watersports headroom, and most repowers stepping out of older 115 two-strokes or smaller V6s.
It's the wrong motor if you need bass-boat hole-shot (look at the 115 Pro XS), you're powering a 24+ ft heavy tri-toon (look at the 150 V6), or your hull is rated for under 90 HP.
Why "lightest 115 HP available" actually matters
Mercury's marketing department leads with the weight number, and most buyers' eyes glaze over. They shouldn't.
A modern Mercury 115 FourStroke at 359 lb is roughly 80 to 100 lb lighter than the older Mercury 115 OptiMax it commonly replaces (425 lb-ish), and significantly lighter than equivalent two-stroke V6 motors that used to sit in this size class. On a 17 to 19 ft aluminum hull, that weight difference shows up everywhere:
- The boat sits flatter at rest. Less stern squat means less water in the bilge area on a wet day, easier loading, better balance.
- The boat planes earlier. Less weight to lift means the hull breaks free with less throttle. Customers report the boat feels "more responsive" without realizing the motor is just lighter.
- Fuel economy improves. Same horsepower, less hull drag fighting weight, lower fuel burn at cruise.
- Trailer towing is easier. 80 lb less rear-of-boat weight matters when you're towing 90 minutes from Toronto to Rice Lake.
"Lightest 115 HP available" is the spec line that does the most actual work for an Ontario boater. It's not just a number on the brochure.
What the 2.1L family shares
The 75, 80, 90, 100, and 115 HP FourStrokes are all built on the same 2.1L inline-4 block. The 115 is the highest tuning of that family, runs to 6,000 RPM, makes peak HP later in the curve, optimized for boats that want headroom.
What that means in practice:
- Same maintenance-free valve train as the smaller motors in the family (and the same architecture as Mercury's V6 and V8 motors). No regular cam service. No valve lash adjustments for the life of the motor.
- Same 17,000 hours of factory testing before this generation shipped.
- Same 3 + 3 = 6 year warranty (3 factory + 3 promotional bonus when active).
- Same alternator output (35 amp, 441 watts), enough to run modern fishing electronics, lights, livewell pumps, accessories.
- Same SmartCraft digital integration, gauges, VesselView mobile, Active Trim, all available.
You're getting the family durability story with the family's highest-tuning motor.
What "ELPT" and "CT" mean
Same model code system as the 75 and 90:
- E is electric start
- L is long shaft (20")
- PT is power trim and tilt
- CT is Command Thrust gearcase (larger, bigger prop swing)
A "115 ELPT FourStroke" is electric start, long shaft, power trim, standard gearcase. A "115 ELPT CT FourStroke" is the same motor with the larger gearcase. The Command Thrust upgrade is where 115 buyers most often get tripped up.
When to take the Command Thrust upgrade
For pontoons: almost always.
For aluminum fishing boats: rarely.
For glass runabouts: case-by-case.
The Command Thrust gearcase is physically larger and swings a bigger prop (usually 14 or 14.5" diameter instead of 13"). The bigger prop trades top-end speed for low-end pulling power. On a pontoon, that translates directly into hole-shot, planing speed, and load-carrying ability. On an aluminum fishing boat that's already light and slick, you don't need it, the standard gearcase is more efficient.
Rule at HBW: if it's a pontoon, default to CT. If it's not, default to standard. We adjust based on the specific boat and load.
Where the 115 HP FourStroke is the right answer
The 115 ELPT lands cleanly on:
- 17 to 20 ft aluminum fishing boats running heavier loads (four people, full gear, livewell, two batteries, kicker, trolling motor). Boats from Lund, Princecraft, Crestliner, Legend in this size range. Cruise around 30 to 35 mph, top out around 42 to 48 mph depending on prop and load.
- Mid-to-large pontoons (20 to 24 ft) with normal cottage loads. The CT version is the right call here. The bigger gearcase makes a meaningful difference on a flat-bottom hull.
- Family runabouts (17 to 20 ft fibreglass) doing mixed-use cottage life. Cruise, fish, tow a tube, watersports occasionally. The 115 has the headroom for tubing and skiing without straining.
- Repowers replacing 115 to 150 HP older two-strokes. Most boats originally rigged with a 115 OptiMax, 115-150 OMC, 90-115 Yamaha two-stroke can step into a 115 FourStroke and run better than they did when new. Lighter, quieter, cleaner, more reliable. The boat usually feels reborn.
- Repowers stepping down from a 150 V6 that was always too much motor. Some boats were originally over-powered. A modern 115 FourStroke does the job the original buyer thought they needed the V6 for.
Where the 115 HP FourStroke is the wrong call
Skip the standard 115 ELPT if:
- You need bass-boat hole-shot. The 115 ELPT FourStroke is tuned for fuel economy, smooth cruising, and reliability. The 115 Pro XS is the same powerhead tuned hot, with a sport gearcase, designed for hole-shot. If you're chasing acceleration off the dock for tournament fishing, the Pro XS is the conversation.
- You're powering a heavy tri-toon, a 24+ ft pontoon with full load, or a houseboat. That's 150 V6 or higher. The 115 will push it, but it'll work harder than you want for the life of the motor.
- Your hull is rated for under 90 HP. Capacity plate is the ceiling. Don't argue with it.
- You want to step into a V6 anyway. If you're already considering 150 because the boat needs it (tubing four kids, hauling a fully-loaded fishing tournament rig), don't compromise to a 115 FourStroke. Get the V6.
A common pattern: customer comes in with a 19 ft glass runabout, a tired 150 OMC two-stroke from 1998, and the assumption that they need to repower with a 150. Once we look at how they actually use the boat (cruise, fish, occasional tubing), the 115 ELPT FourStroke is sometimes the right answer. Lighter than the V6 they're considering, cheaper to run, easier to insure, and the boat is rated for it. They keep the headroom they need without paying for headroom they don't.
How the 115 compares to the 90 and the 150
|
90 ELPT |
115 ELPT |
150 ELPT (V6) |
| Engine block |
2.1L inline-4 |
2.1L inline-4 |
3.0L V6 |
| Full-throttle RPM |
5,000-6,000 |
5,000-6,000 |
5,200-6,000 |
| Dry weight |
359 lb |
359 lb (363 CT) |
455 lb |
| Best hull range |
16-18 ft aluminum, mid pontoons |
17-20 ft aluminum, mid-large pontoons |
19-22 ft aluminum, large pontoons, runabouts |
| Repower for |
Old 75-90 two-strokes |
Old 90-150 two-strokes |
Old 150-200 two-strokes |
The 90 to 115 step is tuning, not a different motor. Same physical 2.1L block. Mercury squeezes more peak HP out of the 115 by pushing the RPM ceiling higher and tuning the prop for more aggressive use. There's no weight penalty going from 90 to 115. The decision is hull size, load, and use case.
The 115 to 150 step is a different family entirely. The 150 is a V6 with a different block, different cylinder count, different weight class (~100 lb heavier), different price. Most boats that genuinely need 150 HP don't make sense with a 115. Most boats that work fine with a 115 don't benefit from a 150.
What HBW checks before recommending the 115
- Capacity plate (always first)
- Hull condition and load patterns
- Transom height (20" or 25", the 115 is most commonly ELPT 20" but EXLPT 25" exists)
- Existing rigging (cables, controls, gauges, fuel system)
- Use case (fishing, cruising, watersports, mixed) which informs prop and gearcase
- Whether the customer should be looking at the Pro XS instead (if hole-shot matters)
- Whether the customer should be looking at the 150 V6 instead (if load is genuinely heavy)
The 115 is in a spot where it can be the right answer or the wrong answer depending on the boat. We don't quote a 115 without asking how it'll get used.
What we'd actually recommend
The 115 ELPT FourStroke is the motor we'd put on a 17 to 20 ft aluminum fishing boat or a mid-to-large pontoon (with CT) for an Ontario boater who wants real headroom without crossing into V6 territory. It's where the 2.1L family delivers its best balance of power, weight, and fuel economy.
We've been selling and servicing Mercury outboards on Rice Lake since 1947. Most 115 FourStroke repowers we do replace older 115 OptiMax or 115-150 two-strokes that have hit the end of their service life. The boat invariably feels lighter, runs quieter, and uses 25-35% less fuel for the same day on the water. Customers who were nervous about giving up V6 sound and "feel" usually report they don't miss it after a season.
Would we recommend it for every 17-20 ft boat? No. Some buyers genuinely want the Pro XS for performance, and some genuinely need the 150 V6 for load. The 115 ELPT is the default for boaters who want enough motor and don't want to over-buy.
If you buy from us, we're also the ones servicing it. Mercury and MerCruiser only. The 115 FourStroke is the motor we know cold because it's on so many of the boats that come through our service bay every spring.
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