How do I increase my protection of the sea lanes?

[Extreme Unction]

I have 51 ships in my fleet.

When I mouseover the "Tariffs" line of my budget, it tells me that I only have 23 ships protecting 26 sea lanes, and my tariffs are thus reduced.

What do I need to do to get more ships protecting my sea lanes?

EDIT: Perhaps another way to ask the same question: What exactly are sea lanes? If I knew the answer to that question, the solution to my problem would probably be self-evident.

 

[brifbates]

You have to build big or light ships. Galleys and transports don't count for sea lane protection.

You need 1 big/light ship for each province that is on a different continent AND not directly connected by land to your capital to get 100% of the tariff value.

 

[The Blood Eagle]

The 'sea lanes' are just the imaginary shuttling of goods back to your homeland. After all, if your treasury is in your capital, you could have all the colonies in the world but they wouldn't be worth anything to the king himself unless you could manage to get the goods home without incident.

Sort of like how England was constantly raiding Spanish galleons - took the benefit of Spain having colonies and dumped the money into Elizabeth's coffers.