I use three transports with an explorer and make many small frogleaps from a port. Even discovering one sea zone, back to port. Then the following sea zone and back to port, etc.
Transports are cheaper than warship. I rather lose a transport than a warship.
There is a strange feature in naval attrition that goes like this : the bigger the fleet, the bigger your loss.
Ie, you lose 5% of your fleet every month.
If you have 20 ships in your fleet, you'll lose 20*5% = 1 ship the first month.
If you have only 5 ships, you'll lose 5*5% = 0,25 ship.
Conclusion after one month, you get from 20 to 19 ships or stay at 5 ships, as you haven't lost one 'full' ship yet. Therefore, make expeditions smaller.
Later in the game, when pirates appear, warships can replace transports.