Prop School Part 8: Propeller Finishes

The “finish” on a high-performance propeller refers to how the raw propeller casting is transformed into a ready-to-use propeller. The finish description can be confusing as different propeller shops and manufacturers apply them differently; some shops offer “blue printing” or “lab finishing” with the promise of increasing speed.

Mercury Marine produces a broad selection of Standard Finish propellers that deliver outstanding performance for recreational boating. To serve high-performance and racing customers, Mercury Racing offers its 20 propeller lines in two finish levels: Pro Finish and Lab Finish. The most popular Mercury Racing propeller models, the Bravo I and Revolution 4, are offered in both Pro and Lab Finishes. The Bravo I FS, LT, XS, XC, and Revolution 4 XP are all Pro Finish propellers.

Mercury Racing Rev 4 XP Pro Finish

Mercury Racing Pro Finish propellers retain the durability of a Standard Finish prop and the corrosion resistance of a highly polished surface while offering the benefit of a hand-finishing process. Mercury Racing propeller artisans start with a raw stainless steel propeller casting and finish and fine tune the propeller by hand. The end result is a blue-printed propeller that has perfectly matching cup heights across every blade and a custom blade thickness profile. The propeller is perfectly balanced, there will be consistency in RPM when running on a multi-engine setup, and in most applications the Pro Finish propeller will deliver higher top-end speed than a Standard Finish propeller. In most applications switching from a Standard to Pro Finish propeller will permit stepping up one inch of pitch due to the ability of the Pro Finish to generate higher RPM. The opportunity to run a propeller with more pitch can produce a 2- to 5-mph increase in top end-speed – simply from bolting on the propeller.

Mercury Racing Lab Finish Sterndrive CNC Cleaver

Mercury Racing Lab Finish propellers follow a similar journey but are taken to the next level with additional blade thinning, which creates a sharper wedge that cuts through the water with less power-robbing drag and, once again, an increase in top-end RPM and a step up in pitch by about one inch with a corresponding gain in top speed, compared to a Pro Finish model. Mercury Racing Lab Finish props are especially effective on lighter-weight high-performance boats capable of running above 80 mph. Lab Finish props are durable but the ultra-crisp blade edges must be maintained to retain peak performance.

Mercury Racing offers a diverse line of propellers that are continually tested and improved in the most demanding environments. Mercury Racing backs its props with a one-year factory warranty, which also covers any and all damage done to the engine upon propeller failure.

The Bravo I FS has taken the bass, walleye, and multi species market by storm, in part because of the top end speed gains from its Pro Finish.

The only propeller not built exclusively by hand is the CNC Cleaver, which features our Lab Finish specifications for maximum top end speed.

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In The Dyno Cell

“Trust but verify.”

Mercury Racing trusts technology and believes in the accuracy of all the cutting-edge computer-driven design tools at its disposal. The computer predicts the power potential of a new engine or an updated design. But that digital prediction is verified in the real world using a dynamometer, one of the most important tools in the Mercury Racing shop.

A dynamometer, or dyno, is a tool used to simultaneously measure torque against rotation and rotational speed (rpm), which is then used to calculate horsepower. The dyno can be used to generate a map of horsepower and torque curves, either through a transient sweep through the rpm range or through discrete steady-state points. For power development, Mercury Racing relies on steady-state, point-by-point power curves where the engine is held at wide-open throttle to saturate oil, air, and water temperatures. 

The Mercury Racing engineering department has six dyno cells. Two Schenck eddy-current dynamometers are dedicated to outboard engines.

Dynos at Mercury Racing are almost always in use. Mercury Racing technicians will often build the engine, instrument it, rig it on the dyno, run the dyno, and calibrate or develop the engine. The dyno is a primary development tool for Mercury Racing engine programs.

“Performance predictions, analysis in GT-Power, and computer simulations are qualified and refined by dyno data so that future predictions are more accurate,” said Mercury Racing Development Engineering Manager Chris Jenks. “A moderately instrumented engine for calibration will have about 40 thermocouples in air, exhaust, and water, and 20 pressure transducers reading everything from air-intake pressure at multiple points to exhaust back pressure to water pressures in the block and cylinder heads. Air, fuel, water, and blow-by flow rates are all measured. There are eight in-cylinder combustion pressure transducers that can be used for real-time monitoring of cylinder pressure in .10-degree increments, a useful tool for balancing cylinders and working to the edge of normal combustion. Typically, there are more than 400 raw or calculated channels of data are being recorded, reviewed, or monitored as shutdown limits as we work through development or are optimizing the calibration for an engine. The dyno allows us to run the engine consistently, week after week, at every operating point at which a customer can run the engine, and at some they can’t.”

Jenks explains that calibrating an engine is a circular process that is similar to painting a car, in which each layer of paint is followed by wet sanding and buffing to make the entire surface smoother. It’s a complex, time-consuming process. For example, there are 916 maps or required control inputs that build the calibration in the ECU (engine control unit) that operates the Mercury Racing 450R outboard. Sixty-three of those are considered base maps, whose foundation is relied on for the rest of the calibration. Those base maps contain 18,207 cells that represent roughly 7,000 discrete running conditions (rpm, load, engine temperature, ambient conditions), which are run on the dyno to rough map, check, and re-check for errors and interactions. Final calibration checks before production will go through each of those 18,000 cells point by point. 

In addition to engine development, the dynos are sometimes used for outboard exhaust emissions or power audits before they leave for customers. Warranty returns, although rare, are run on the dyno to re-create the complaint and diagnose the root cause. All sterndrive engines are run on the dynos for break-in and a power-verification run before shipping. 

Dyno development allows Mercury Racing to run an engine in a laboratory setting that captures all the extremes our customers are likely to dole out on the water. The result is a high-performance marine engine product that is powerful, reliable and refined.  

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