from 1-8 props efficiency & other considerations

My general understanding is that one prop is always the most efficient way to propel a boat (or they wouldnt try to design single screws on container ships), doubles by definition gives you redundancy and potentially some steering, but i'm curious what other effects are had when you start going north of two - having seen videos of up to EIGHT outboards on some of those go-fast boats I was curious how much (if any) efficiency they might be giving up...

Whether there are performance considerations ever favoring more instead of less...

Whether cost for more props is ever less, because of certain high HP drives costing more (per HP transmitted)

At what point of spacing/being too close props interfere with one another in a bad way...

If there is ever a "good" way (counterrotating props real close together maybe?)

Etc.

 

Smugglers use multiple engines for damage redundancy, the same reason most warships are twin shafts. 8 engines really won't drive you faster than 3, but you still will be going fast if 2 or 3 take a round and die.

Once above 2 shafts for redundancy, it is generally a power density issue. There is only so much power you can put into the water with a given sized wheel. While efficiency can be increased by going to a bigger wheel or a true counter-rotating prop set (i.e. LH and RH prop on a single shaft), there are practical limitations in fabrication and draft. Thus large high speed ships (warships and true liners) with more than ~ 100,000 shp tend to multiple shafts, generally 4 these days. And yes there are spacing and rotation issues that have to be accounted for, but these are different than true counter-rotating propeller sets.