Why two engines in a hull

I am naval architecture student, i am doing my final year project high speed catamaran ferry.

I noticed during my premilinary studies that a lot of cats are now fitted with two engines in a single hull and in some cases 3 in a single hull.

i have seen such arrangments in www.austal.com, major cat buliders of australia.

Can anyone tell me the advantages of such an arrangment??

Currently i have also selected such type of arrangment, with two engines in a hull and 4 engines and 4 waterjets in all..

 

Well, any multi-engine design benefits from increased safety due to redundant engines and also sees greater manueverability. Also, several small engines are easier to distibute weight versus one big engine (I can get you pics from the last ship I sailed engine side on if you want to see big engine mounts). Several engines also allows greater control over speed since one or more engines can be idled. Critical speeds are less of a problem. Each engine can be brought up past critical individually and the other engines off-harmonic revs can suppress the one critical. Less destructive shaking and discomfort for passengers. Fuel efficiency can be another... Ferries spend alot of time at low speed docking... one engine (out of two or three in the hull) can better do the job with less fuel oil consumed than the one big engine sitting alone in the hull.

These are just my observations from the commercial shipping side of the industry and knowledge passed to me by fellow officers and designers. I hope this helps...

 

I agree with steam freak, redundancy and maneuvarability are at the top of the list for two engines in a single hull. "Weight:total power" ratio is usually higher with 2 plants, so on smaller boats (inherently more maneuvarable anyway, and not as likely to be in blue water) maybe it doesn't make as much sense. On the down side: heavier, greater expense, greater likelihood for a breakdown of one of the engines, more maintenance time and $$ and more below decks space used. For any given application it's up to the end-user to decide if the trade-offs are worth it.
I'm in the planking stage of building a 78' schooner in Thailand to round-the-world in, and I'll have 2 engines for these reasons, despite various "experts'" opinions on the matter (can you guys hear me sticking my tongue out at you?)

 

how are the 2 waterjets fitted in 1 demi hull of a catamaran without having any interference effects between them. how is the engines coupled with waterjets in the same case.