[Polydamas]

Also remember that in the places where you really need to manoeuvre [approaches to and exits from harbours, busy rivers full of snags and sandbanks, narrow straits] you may have competing weather magic, and that the dangers of the Great Ocean include magical abnormalities. In my vision of the setting, if I allowed Essential Wood, a ship of very thin Essential Wood would start to develop mechanical problems as soon as it sailed into Low Mana or No Mana, and would attract nasty sea monsters from the Great Ocean hungry for the purity of its substance.