I have confusion to estimate market size of the engine of the tug ship (global) from the BCG Munich casebook.

The solution posted in the book is confusing as I am not sure why we need to know #ports (Show below)

MS engine of tug = #countries x # ports in countries and average # tugs in countries x #engine/tug .

I am wondering whether there is other way to estimate number of ship produced globally. 

For more information, I posted the case and solution as per below.

Case:

Client is a manufacturer of marine engines - focus on small-engine ships (small tugs that help to maneuver and pull out large boats - Special propeller is necessary so that the ships can maneuver better).

The client is a supplier who only produces these rudder propellers and delivers them to the shipyard - these deliver to the end customer). Knowing nothing about the end customer (usage, problems, etc.) but would like to find out more about end customers. So is considering digitizing engines to find out more about end customers. 

I want to know from you if this is a good idea.

Solution:

Estimate the market: countries * number of ports * avg. Number of tugs per country * Number of engines from boat * Market share Total market are 130,000 ships

10% of which are tug ships (13k), 10% market share (1.3k), 2 engines per ship (2.6k)
2.6 motors is our market share, 15 years life expectancy> 2600/15 = 173 motors per year> rounding up to 200