Hang tender off the boom, secure fore and aft, fill with water untill your boat lists and puts the rail almost in the water (make sure all ports are shut tight), if possible, have a spotter on the bridge with radio
, you can measure the hight of the mast using a line
from the top of the mast to the water. I'm sure you already have a view of those. steer close to side of the bridge away from the tender. You can practice at anchor
. Let the bridge tender know what you are up to before heading under.