Went out on sunday on my Scarab (30', Twin 370s BBC with Bravo drives) and one engine would not grab. Engine revved perfectly, but the prop was not grabbing. At idle, the prop would grab, but trying to get on plane and the prop would not grab. Spun prop hub, or bad flex plate doughnut?

could be the drive

could be either one,check the prop first as it is much easier to look at.

Most times your symptoms turn out to be the coupler. You can look behind the engine when the engine in running in gear and see if the driveshaft is turning. If not your coupler is shot. If it is, your drive or prop hub are next to look at.

I second the coupler. Put the drive in gear, (motor off of course) and try to turn the prop. If it does turn, more than likely the coupler. Make sure to align your engine when you put the new in.

Agree...likley a coupler..particularly since it was sudden onset. Couplers don't usually fade slow...one minute they work...next they don't. Relatively cheap part...just a a$$ load of labor. As stated above....make sure you align correctly when reinstalling the motor.

I did put the drive in gear, and was unable to turn the prop either in forward or reverse. Hopefully it the prop. I'll check that when I get back after the holiday.

i'm betting its the prop hub. most of the time when a coupler goes you will smell burnt rubber and you can often see chunks of rubbery residue around the input shaft where it goes into the bell housing/coupler. if you didn't smell burnt rubber i think theres a good chance it is just the hub IMO.

second the rubber smell - I have lost more than a few couplers in the past and smelled rubber every time.

Every coupler I lost took out the aluminum splines, no smell.