Up and down arrows to increase and decrease the size of blocks. Right arrow key to change the material. Click CLEAR or SAIL (make sure you are in Idle mode before you click sail) to clear or sail. Once you sail you cannot go back to editing your boat, so don't. Your progress will not save, unfortunately.
This is NOT supposed to be complicated. I was bored and made this. Do NOT critize me in the comments or complain about the project unless I say I will take suggestions to add on to this project. I know that the erased parts show up when you sail, but I really don't care.