It would be understandable for investors to approach the shipping industry with caution, especially after years of misery, but this time really might be different. Key industry metrics reported during first-quarter 2018 business updates, including the supply of available ships (at a multiyear low) and the number of ships on backlog (at a historic low), provide confidence that steadily rising rental rates aren't going to slide backwards anytime soon. That helps to explain why shares of Seaspan Corporation (NYSE: SSW) and Diana Shipping (NYSE: DSX) are up 36% and 28%, respectively, since the beginning of April.