Boatbuilders turn to twin engines when their vessels reach about 25 feet. The hulls, especially those with deep-V designs, need more power to get out of the hole without sinking the stern, and to maintain a comfortable, fuel-efficient cruise speed. But with increased use of composite materials like PVC coring, larger boats can be built lighter. And boat manufacturers now have more powerful engines at their disposal, meaning they can stick with a single engine, which cuts down on initial purchase price and overall fuel consumption. …