Most of the rivers in South America and a few in the United States such as the Missouri River require very shallow draft in the order of 6 feet rather than 8 to 10 feet which is normal in the Mississippi and the ICWW.  Depending upon the total horsepower, triple and even quad screw is normal. The overall size of the boat is larger than conventional towboats of similar horsepower to carry the required fuel and float at a significantly lower draft.