last time when i was in phuket, the boat there also had similar engines. all big NA diesel engines, probably >10,000 cc & taken from lorries or buses. and with NO air filter, intake resonator or any exhaust bullet/muffler at all, these things were FREAKING LOUD! probably most of the boat operators are partly-deaf from long-term exposure to such horrible noise... poor fellas, sacrifice their hearing to make a living...