Tremendous effort goes into good boat set-up. My conversations about set-up too often turn to power consumed by a drive train,or generated by an engine, and always… propellers. The goal is efficiency – accepting some sacrifice for boat control. Really, your goal is the euphoric joy and adrenalin rush of high performance boating! My goal is to help you get there.
Here, I’ll focus on drives. (We’ll cover engine power and props later.) Between engine crankshaft (drive input) and prop shaft (drive output), basic functions are required: gear reduction (so props are efficient); offset of input vs. output shafts (so they’re wet) and ability to change direction (steering and trim are good).
Mercury Racing employs several sterndrives for those functions. Each occupies a unique performance envelope and capacity. Unfortunately, each has parasitic losses: clutch slippage; gear efficiency; number and nature of gear interfaces; U-joint friction; bearing drag; gear oil (quantity, temperature, viscosity and lubricity); and oil windage/pumping losses. So here, drive by drive, are the results of those parasites…
We package the Bravo One XR drive with our 525 EFI and 600 SCi engines. XR has an internal cone clutch transmission with positive engagement: No slip. It has two forged, straight bevel gear sets. The lower gears run wet; uppers are oiled by a groove in the vertical shaft. At 5,200 rpm input, a Bravo One XR consumes 25 hp or 4% of a 600 SCi’s output.
The , NXT1 with an external cone clutch transmission, is one tough, efficient machine. This drive/clutch combination was developed for EU662 SCi and 700 SCi engines. Both transmission and drive are dry-sump designs. Drive gears are forged straight bevel. Like the Bravo, this transmission is a positive engagement design. There is a noticeable efficiency gain using a cone clutch, dry-sump oiling and following lubricant recommendations: At 5,200 rpm input, an NXT1drive consumes 10 hp. Its transmission takes 5 hp; however, at Racing we dyno engines after the transmission (662SCi and up). Therefore, only 10 hp is the loss to the prop, or 1.4% of a 700 SCi’s rated power.
Our engineers package the dry-sump Six drive with 850 SCi, 1025 SCi, 1075 SCi and 1200 SCi engines. It employs three forged, straight bevel gear sets, two spur “splitter” gears and twin pinions to reduce its hydrodynamic profile. At 6,000 rpm input, a dry-sump Six consumes 12 hp. The hydraulic transmission consumes 15 hp. Only 12 hp is the loss to the prop, or 1.0% of a 1200 SCi.
The dry-sump M8 employs a dry-sump hydraulic transmission, too. Both were designed for our 1350 engine. It uses a similar gear configuration and oiling approach as the Six – only scaled up for the Big Fat Monster Torque of QC4v. The transmission has 35% more friction surface than a Six transmission. At 6,500 rpm input, an M8 consumes 22 hp and its transmission consumes 17 hp. But, only 22 hp is the loss to the prop, or 1.6% with a 1350. While that is 10 hp more than a Six, it’s almost entirely due to higher rpm with bigger gears — the price of higher power capacity and longer life.
In pursuit of performance, I believe there is more to gain from set up, prop selection and drive height optimization than by tweaking or over-powering drives: From M8 to Bravo, drive parasitic loss ranges from only 1.0 to 4.0 percent. And losses go up exponentially, not linearly, with input speed. So, choose the right power package for your fun, run it at no more than rated maximum power and rpm and please use our recommended lubricants, properly filled (both very important, more later). You’ll be rewarded with top performance and robustness.