With this patch merchant ships have:
* the additional gain per trader raised from 20% to 50%;
* +60% base trade gain multiplier, making them much more profitable;
* their +35% movement speed bonus removed, to compensate, and making it easier to compare them with land traders, which now have the same movement speed;
* their health raised by two thirds;
* a wood cost of 160 (nicely in between bireme (120) and trireme (200) and proportional to their health (400); right now they have 0 wood cost);
* a metal cost of 120 (i.e. +20);
Furthermore, land traders cost slightly less metal (75 instead of 80), such that the metal cost of merchant ships and land trader is proportional to their base trade gain.
Effectively this makes merchant ships more expensive yet also more profitable than land traders, and having traders inside a more viable option.
D3692 can help with displaying the difference in trade gain between land traders and merchant ships.