silly question ?

[fcfc]

Why do marine gearboxes are integral with the engine ?

In the typical case, this mean that engine silentblocks will have to handle all the prop thrust and torque.

With an aquadrive or similar, this thrust absorbing unit will have to be dimensionned for prop torque. and the engine mounts will still have to handle that prop torque.

Why not engine on soft silent blocks (engine torque and weight only). flexible shaft to a separate gearbox. flex shaft will only have to handle engine torque.

Separate gearbox on hard silentblocks to handle thrust, like an aquadrive unit.

BTW some cars had a front engine and a rear gearbox. (alfaromeo, porsche transaxle).

Thanks.

Francois.

[jehardiman]

One simple answer is harmonics. You want the shaft to be as tortionally stiff and have the largest mass possible on the end of the shaft to de-couple prop torque fluctuations. The last thing you want is a drive train that can be excited by the prop rpm, it can rip the engines right out of the beds.

Other reason include ease of alignment so as to provide an economical gearbox. Steam turbines, which are very succeptable to vibration and have large thermal movement, are de-coupled from the main reduction gearbox by tuned quill shafts. This type of gearbox is so expensive that no-one could afford to buy a replacement on failure, so they are leased from the manufacturer who is responsible for the setup of the whole train. The same is true for co-generation plants where multiple prime movers are linked through a common gearbox.

While it is possible to de-couple the shaft from the engine by various hydraulic methods, these methods are less efficient, mechanically more complicated, and heavier which limits their appeal.