More than 80% of international trade by volume and 70% by value is transported by sea. Few investors can participate directly in shipping but investment trusts make it convenient and cost-effective to do so. Step forward Tufton Assets (stock market ticker: SHPP for sterling shares, SHIP for dollar-denominated stock), a second-hand marine leasing specialist that has delivered a high and rising income, earning its place as the most valuable asset in my Isa.