You can transport 150 iron per trip given you can carry 450 carry weight since 30 iron will weigh around 300lbs and the karve can carry 120. Once you make 1 trip you can build a longship which can carry so much iron you won’t fill it up to capacity ever. I don’t see why you need to make a ton of trips, just get a load of iron over time, and then once you have enough load all of it up in the ship and sail back