Complex sentence: At first the battle seemed to go to the Flemings as the larger French ships were grounding but when the tides changed the larger French ships came free and were able to join the fight and the battle turned to Grimaldi's advantage.
Split into simpler sentences: At first the battle seemed to go to the Flemings as the larger French ships were immobilized by grounding. But when the tides changed the larger French ships came free and joined the fight, turning the battle to Grimaldi's advantage.