The Battle of Riachuelo, on June 11, lasted about eight hours and was decisive, as it contributed to the isolation of Paraguay and its ships never again tried to attack the allied fleet. The inferiority of the Paraguayan ships was evident and discouraged fire combat from a distance. The ability of Paraguayan soldiers to fight with melee weapons was a factor confirmed throughout the war, so the option for boarding and hand-to-hand assault to capture Brazilian ships was more logical. However, the delay in the Paraguayan movement to the battle area was one of the decisive factors. Our hypothesis will basically test for the Paraguayans an advance on their operation, with the departure from Humaitá at least at 18:00 hours on the 10th. Then Paraguayan fleet would have arrived at the operation area with enough time to deploy. Then, we will prove if, even so, the victory will be Brazilian.