outboard for slow cruising on big boat

Is it feasible to hang a small (say 20 hp) outboard on a heavy 40' power boat to use when one just wants to go slow? How long could such an engine be expected to last? The main engines are 454's and naturally, burn a lot of gas, make a lot of noise, generate a lot of heat, etc. Most times the boat is restricted to no-wake speeds.

 

No problem with service life , the biggest hassle will be deciding which OB swings a large Dia prop .

At reasonable throttle the boat will run at over std idle speeds.

For manuvering in a lock, the std engines will be needed, but not for those 10 klick speed zones.

FF