The engine idles smooth w/o a clutch. It accelerates the same with or without a clutch. A clutch is used only to make initialy start-up smooth, or in the case of a centrifugal clutch to allow the engine to idle w/o prop thrust on the water. W/o a clutch, as you turn on the key you need to get a feel for opening the throttle in synch with revving the motor through the initial turns to get it to about about 1500 RPMS, then you can throttle back to normal idle just fine.
When i started doing this in 2010-2011 we all thought a clutch was necessary, but it turned out that if you build in the intake and exhaust correctly (using the stock intake plenum, and stock length for the headers, e.g., ) and after 2015 and extensive testing I started making and selling YG4 and YG4i power plants abd conversion kits without clutches - which my customers loved, since it eliminated the single-point-of-failure that is the clutch.
The internal rubber damper is thought to be sufficient for damping torsional vibration when a prop is hard coupled to the PTO countershaft through a conventional gearbox such as the AK7 Mohawk sells, Rotax C, and the Hy-Vo chain Mohawk "Silent Drive" - all of which have been extensively tested and flown for many years with no soft start clutch. But I still use a BMW flex shaft coupler with M10x 78mm on-center steel sleeved holes. You can't use the Rotax donut, it will shred in under ten hours of normal flying.
If anyone is considering re-gearing the countershaft internally, it simply cannot be done within the confines of the stock block, and any changes to the casting will significantly compromise the integrity of the crankcase leading to imminent and catastrophic failure. You would have to start from scratch and make your own block. It aint' worth it.
Skytrax "All-In-One" with sprag clutch is $3500.00, Mohawk Aero adapter with conventional gearbox (there are several choices) costs $2995.00 and comes with either an RK400 clutch (especially desired by float planes and air boat guys) or a PTO flange coupling with a BMW heavy-duty rubber flex shaft coupling (a much beefier "rubber donut"). Gearbox can be mounted upright, or down.
Someone once built a successful belt drive Apex on a FW tractor before the options that are available today were on the market. It is actually the first successful Apex, but it was never replicated after i started making gearbox adapters for the Apex, as it is impractical. Matter-of-fact, he provided Steve Henry with the first wiring info to successfully run the Apex, and Steve gave that info to me in 2017, then to others starting around early 2019. That was a one-off that no one to my knowledge has ever done since. These days a belt-drive PSRU on a YG4 makes about much sense as sticking a Ferrari engine in a VW bus. It is heavy, it creates a lot of harmonic vibration, and belts wear out quickly. Most belt systems are only good for about 6800 RPM, hence they work well on say a Subaru or a 351 c.i. Ford airboat, but not so great for a Yamaha Genesis 4-cylinder aircraft. Or a 3 cylinder, though this has been several times with limited success by individual builders (none for repeat general sales). Both the YG3 and the YG4 turn the same relative PTO shaft speeds, up to 8700 RPMs+, well beyond reasonable cog belt speeds.
If you desire to re-invent the wheel, be aware that it will cost you a ton of money more than either Skytrax or Mohawk, and set you back at least another year - and that's if you already know what you are doing, have excellent CAD skills and software, and know where to get gears, hobs, castings, CNC work, raw materials, all the hardware bits and pieces...then there's the testing.
If anyone is curious to see the Silent Drive, go to:
Mohawk Aero Silent Drive Video,
and also this one:
Silent Drive Flight Demo
it has about 50 hours on it, no soft-start clutch, and does have a prop-strike protection "slipper" (break-away) clutch integral to the new 49-tooth Arctic Cat sled top gear. That part only cost me $400, whereas the same **** thing from Rotax in their 912 series gearbox is $1200. That is exactly the same slipper clutch Bambardier uses on their Ski Doos, folks. Why do they charge 3x as much as they are actually worth? Because it is Rotax, that's all.
Talk to ya later, have a good day. Time to get out to the shop and finish cutting the keyway for the sprag clutch drum that's on the mill for a YG2 customer.