CONTINUOUSWAVE

In spring of 2026 I re-powered my 1994 OUTRAGE 17 with a SUZUKI DF115BTL [later expanded to be a DF115BTLW], replacing the 2004 Mercury 115-HP FourStroke. The Mercury engine only had around 500 hours on it, but I was concerned with its age and reliability, as I had started using the boat for some remote boat-in camping, and in areas with no cellular connection and no boat towing service. Availability of repair parts was also a concern. Buying and installing a 5- to 10-HP auxiliary engine would have been a more economical choice for reliability, but I don't troll much, and so an auxiliary engine would feel useless except in case of emergency.

The Original Mercury 115-HP FourStroke
The 2004 Mercury 115 FourStroke weighed 386-lbs. The previous owner (who bought the boat new in 1994) had an Anti-Ventilation plate foil appendage installed, which I never removed. The propeller was an aluminum 17-pitch. The Mercury tachometer was not operational, so I am not sure what the engine’s maximum speed ever was.Top boat speed was 35-MPH from a GPS receiver.

The new engine setup is very different, which makes a direct engine to engine comparison difficult, but I'll go over what statistics and impressions I do have.

The New SUZUKI 115-HP
After much deliberation on engine brand and quotes from most of the local shops, I settled on a 2026 Suzuki DF115BTL as the replacement engine. This engine weighs 414-lbs, so there is now an extra 28-lbs on the transom. Total cost of the re-power was $13,900, and this included a top-mount remote shift and throttle control, labor, taxes, a Suzuki stainless steel propeller, and an SMG4 digital gauge.

The SMG4 gauge shows engine and related data that I didn't have access to with the old Mercury 115 Fourstroke, such as instantaneous fuel flow, total fuel used, fuel economy, trim setting, and engine temperature. There is a nifty feature to create a QR code which can be sent to a Suzuki repair shop or read via the phone app to interpret error codes and see engine run time in each RPM range.

After the initial three-hour break-in procedure I was allowed to bring the throttle to wide-open for brief periods. During initial testing the boat had a normal load: two people, one dog, one cooler with a bag of ice, and a full tank of fuel. Water conditions for testing were good, with 5-knot winds and no waves.

First, I assessed engine mounting height with the boat on plane, and found it was perfect, with the Anti-Ventilation plate visible and just skimming the top of the water. I did not note any porpoising or excessive bow rise during acceleration.

Next, the throttle was set to wide-open to test the propeller selection. The engine manual indicates the ideal WOT engine speed is 5000 to 6000-RPM, and the online community typically recommends the upper half of this range. The re-power shop chose a 21-pitch propeller, with a trade-in program if I needed to size up or down. In this test the engine was trimmed out until both engine speed and boat speed stopped increasing. The maximum engine speed was 5,950-RPM and maximum boat speed was 42-MPH. This produces a calculated SLIP of 8-percent, well within normal range.

At this point I was very satisfied with engine mounting, propeller selection, and the performance of the Suzuki 115. I had gained 7-MPH in boat speed over the original engine, which I would primarily attribute to less drag from proper engine mounting height, no A-V Plate foil appendage, and increased performance moving to a stainless steel propeller of the correct size.

Now that I had a working gauge with shiny new features I was eager to look at some statistics. I set the gauge to view instantaneous fuel economy, and tested various engine and boat speed combinations.



I recorded values at 500-RPM engine speed increments from 3000 to 6000-RPM. The boat gets on plane around 3,100-RPM, so there is a big spike in MPG at that point. The data was taken on the Mississippi River by averaging engine speeds and boat speeds from upstream and downstream test runs. There is probably some small variance in boat speed and MPG based on the river current at the specific spot the reading was taken. An engine speed of 4500-RPM seems to be a nice sweet spot for MPH and MPG: 30-MPH and 7.2-MPG.

One outcome of the re-power was reduced engine noise. The 2004 Mercury FourStroke engine was quite loud, although obviously quieter than a c.1994 original two-stroke-power-cycle engine would have been. The Suzuki runs with less engine noise [than the Mercury FourStroke] at all engine speeds, but the difference is especially noticeable when the boat is on plane. At engine speeds of 4000- to 5000-RPM I can hold a conversation with my co-pilot using a normal speaking voice, which was not possible before. The Suzuki does get louder (or just has noticeably [higher] pitch) starting at about 5300-RPM and going up to full throttle, but it's still possible to have a conversation while cruising, just with slightly increased speaking voice.

Summary
Overall I've been very pleased with the new SUZUKI DF115BTL engine: [compared to the Mercury 115 FourStroke] it is quieter, very fuel efficient, has plenty of power for my use, and gives me peace of mind when boating remote areas. And I hope it lasts another 20 years.

Q1: what engine mounting height (measured in holes from lowest possible) was used in this re-power?

Q2: what is the gear ratio of the SUZUKI DF115BTL?

Q3: what features does the model designation suffix “BTL” indicate?

Your measured fuel economy of about 8-MPG at moderate hydro-planing speeds is very impressive.

ASIDE ON ENGINE NOISE COMPARISON

Cala wrote:One outcome of the re-power was reduced engine noise. The 2004 Mercury FourStroke engine was quite loud, although obviously quieter than an c.1994 original two-stroke-power-cycle engine would have been. The Suzuki runs with less engine noise [than the Mercury FourStroke] at all engine speeds....

Regarding making comparison on sound loudness by human hearing, when one of two sounds is being recalled from memory and compared to a live sound source, making an accurate comparison of the relative loudness of the two is difficult to impossible, with the only case being if the difference is extraordinarily large, such as when one sound is 100-times louder than the other. The only really reliable method of sound comparison is by using a sound pressure level instrument and making live measurements of each sound in a controlled measurement process.

When making accurate comparison of sound loudness by human ear, even sounds presented with just a short time separation cannoy br accurately judged if the time lapse is more than 10-seconds or so.

Another significant influence in comparison of engine sounds is the base frequency of the noise, which in most engines is related to the occurence of a cylinder ignition of the fuel. In that realm the four-stroke-power-cycle engines produce an ignition or power stroke in each cylinder only once every two crankshaft rotations. In two-stroke-power-cycle engines there is a power stroke on every crankshaft rotation in every cylinder. This means the base noise frequency in a two-stroke-power-cycle engine will be at double the frequency of a four-stroke-power-cycle engine running at the same crankshaft rotation speed. Because human hearing sensitivity to low frequency declines as the frequency declines, there is a bias in human hearing to tend to judge the sound from a four-stoke-power-cycle engine as being less loud (because it has a lower base frequency) compared to a two-stroke-power-cycle engine running at the same crankshaft rotation speed (which produces a sound with double the base frequency).

jimh wrote:Q1: what engine mounting height (measured in holes from lowest possible) was used in this re-power?

The engine mounting height was one-hole-up from lowest possible.

jimh wrote:Q2: what is the gear ratio of the SUZUKI DF115BTL?

The gear ratio is 2.59:1.

jimh wrote:Q3: what features does the model designation suffix “BTL” indicate?

The full model description is DF115BTLW.

"B" is the engine generation. "A" models were the second generation engines from around 2010 to 2021, "B" is the third generation engine design from 2021 on, and apparently those engines have better power and fuel efficiency than the "A" version.

"T" is for mechanical controls, instead of "G" for digital shift-throttle controls.

"L" is for long shaft 20-inch, and I believe the alternatives are "S" for short [15-inch] and or "X" for XL [25-inch] shaft lengths.

"W" for white color.

Note on noise - I'm sure you're quite correct on being able to judge and compare [sound levels from memory and from live]. However, it is very easy to say "I couldn't hold a conversation for the 15 years I had my old engine, now I can".

I can assure you my hearing hasn't gotten better in the past 15 years. Whether [being able to converse at the helm] due to lower decibels or a different frequency doesn't really matter, I'm just happy to be able to talk to my copilot.

Thank you for the excellent summary of the engine model designation letters and the features they represent. That information is great to have.

ENGINE NOISE
To go on a bit more about engine noise, perhaps too much more, the first outboard engine that was specifically designed to have an improved sound signature--or what is now referred to as NVH for noise, vibration, and harshness--was the c.2004 Mercury VERADO inline six-cylinder engines. These engines were remarkably silent at idle speeds. The naturally balanced inline six configuration gave them perfect balance and absence of all vibration, and whatever noise they did make at idle speeds was masked with sound insulation and limited in high-frequency components to completely remove any "harshness."

Of course, this was the polar-opposite of the concurrent Mercury OptiMax engines, which were perhaps the most noisy and harsh sounding outboard engines ever made, due to using dual sets of direct injectors, one for fuel and one for air, and operating a small air compressor to create the pressurized air. In contrast to the OptiMax the VERADO was almost unbelievably quiet. The impression made on potential buyers was profound.

However, despite the almost fantastic absence of engine noise at idle, when the VERADO was operating at high-throttle settings, it produced a lot of engine noise, including significant noise contributed from the supercharger whine. This same characteristic is typical of all modern four-stroke-power-cycle engines. They all make much less noise at idle speeds than legacy outboard engines (in which engine noise was NEVER a consideration for design), but when operating at 5,000-RPM or higher, they make a lot of noise, which seems to be ignored by those praising them for being so quiet.

Cala wrote:...it is very easy to say "I couldn't hold a conversation for the 15 years I had my old engine, now I can.

That is a fair point.

With the new information of the engine gear ratio, and using the propeller pitch as 21, the SLIP can be computed from the graph data points for engine speed and boat speed:

RPM    MPH     SLIP
3000   12.3    46.6
3500   21.5    20.0
4000   26.0    15.3
4500   30.5    11.7
5000   32.7    14.8
5500   37.0    12.4
6000   42.0     8.8

With the SLIP value under 10 at maximum boat speed, the propeller is performing well. That the SLIP values increase above 4500 RPM is a bit odd. Perhaps the trim setting was adjusted to be further trimmed out at those higher engine speeds. This would account for the increase in SLIP.

Calculation of SLIP was done using the PROPELLER CALCULATOR at

https://continuouswave.com/calculators/propCalc.php

We enjoyed operating a Mercury VERADO 135-HP in-line four-cylinder, four-stroke-power-cycle, supercharged engine on our 190 Montauk. The VERADO 135-HP engine was incredibly quiet but perhaps slightly less quiet than the six-cylinder version. The supercharger whine at high RPM was almost undetectable due to the noise generated by the Montauk going through the water. We seldom operated the VERADO 135-HP engine at near maximum engine RPM so we found the VERADO to be an almost perfect engine. The boost provided by the supercharger was exhilarating.