And, now, here's the solution...

Well...

The age of the ship plus the age of the boiler is 42. Let's represent this as:

We know that the difference in their ages can, of course, be expressed as:

That's pretty simple.

So, the expression

is how old the boiler was, when the ship was the boiler's current age. For example, if the ship is now 14 years old and the boiler is now 10 years old, how old would the boiler have been when the ship was only 10? Well, it would have to be four years younger, or six.

Knowing this, the phrase: "The ship is twice as old as the boiler was when the ship was as old as the boiler is now," can be represented by this expression:

Or,

Or,

Or,

Let's add 2S to each side, and you get

Which is the same as saying

We can then easily solve the equation, by plugging this into

and we get:

DONE!

Photo is the actual ferry from the Puzzler. (We think.) Thanks from Long Island Ferry
The Cape Henlopen Ferry was built as a World War II landing craft (LST 510) in Jeffersonville, Indiana and participated in the D-day invasion at Normandy. In 1966, it was converted to a passenger and auto ferry and served on the Lewes, Delaware - Cape May, New Jersey Route. It was purchased in 1983 by Cross Sound Ferry and underwent a total refurbishment before entering service. The ferry was repowered during the winter of 1995 with EMD 12-645 diesel engines, ensuring a long future of trouble free operation.