What you are listening to is the result of many billions of £, $ and many other currencies investment into the internal combustion engine for transportation over 100 years. Each succeding design making small improvements. Improved tolerances, vibration dampers, mounting systems, insulation
, better combustion etc etc.
Even though makers of boat
engines cannot afford that level of investment, the spin-off from the car and truck world has made boat
engines much smoother, but even the best boat engines sound far more agricultural than my car turbo diesel
. Part of the problem is that typically, boat structures are dynamically less stiff which limits the effectiveness of the mounting system, the engine mounts are pretty crude and rigid connections to a propeller
shaft do not help. Your car design will go through a complex and expensive dynamic modelling design to reduce vibration and noise justified by amortising the cost over millions of cars sold
, boat builders cannot afford that. Only the most sophisticated naval warships can justify the cost and time involved.