By Michael Carr – “Come more to the left, now straighten out,” Steve directed. “On this wreck, the current comes in from the east, so you need to compensate for that extra drift before you drop the divers.”
I was driving the twin-screw dive boat south, stemming the Gulf Stream’s western edge, alternating my glance between the chart plotter, sonar, compass, and vessel traffic.
On our stern platform & rear deck stood a dozen SCUBA divers, regulators in their mouths, ready to drop off into the deep blue water. With a 2-knot north-flowing current we had to drop our divers south of the wreck, which lay on the sandy bottom at 130 ft.
It would be difficult to anchor or grapple at the wreck in 130 feet with a 2-knot current. And even if we did grab the wreck, a SCUBA diver would not be able to swim down a line, against such a current to reach the wreck. US Navy diving guidelines for SCUBA dives is one (1) knot maximum, just for this reason. You cannot swim against currents stronger than a knot, and if you could your mask and regulator would be torn from your face.
Getting onto wrecks with strong current requires entering the water upstream from the wreck and drifting down onto its superstructure. Once on a wreck, it’s hull and deckhouse provides shelter.
Similar to an aircraft coming in for a landing, a diver must descend expeditiously while drifting towards the wreck, ensuring they are on the bottom before reaching the wreck and don’t overshoot.
SCUBA divers normally use a descent rate of 60 feet per minute, and so the “lead” for dropping onto a wreck is calculated using inputs of current speed and time it will take to descend. A one-knot current moves a diver horizontally at 1.7 feet per second, so for every second a diver is descending they are being carried 1.7 feet. A two-knot current moves a diver 3.4 feet per second. Doing some math reveals that for a wreck at 130 feet a diver needs 2.2 minutes, or 132 seconds to descend, and during that descent time, they will be swept horizontally 449 feet.
Setting up the dive boat for the divers on this dive requires passing over the wreck, observing its profile on the sonar, and then continuing another 449 feet to the south before launching the divers by yelling “DIVE DIVE DIVE!”
When you yell DIVE DIVE DIVE you also place the boat’s engines in neutral. Immediately upon hitting the water divers orient themselves to the current and start a fast descent. No time is spent on the surface. Divers enter the water “heavy”, no air in their buoyancy compensators, and often carrying extra lead weight. Constantly clearing their ears on the way down. Like an eagle diving to catch a fish, divers head straight to the bottom, letting the current sweep them towards the intended wreck.
Getting the boat in position is a challenge. Wind and seas often set you off course, and boat speed needs to be fast enough to maintain steerage, but not so fast you that you shoot over the diver drop location.
GPS vectors on the chart plotter assist in determining boat speed and direction, but the closer you are to the wreck location the more the vector direction changes. As you first approach a wreck the chart scale might be set to one mile, and then reduced to 0.5 miles, to 0.25 miles, and then down to 50 feet. As you zoom in it’s imperative to maintain a steady heading and consistent speed.
Once you drive over the wreck the chart plotter will start showing distance back to the wreck, and on this day we want to be 449 feet south of the wreck. Wait, wait, wait. Bingo, we are passing over the wreck, it’s profile shows prominently on the sonar. Just as quickly the profile disappears off the screen. The distance back to the wreck now shows on the screen’s readout, slowly increasing…50, 150, 200, etc.
At 400 feet I turn around and yell down to the aft deck, STANDBY! Our speed over the ground, stemming the current and heading south, away from the wreck, is 3.5 knots. Our distance south of wreck soon shows 450 ft. I pull the throttles back to neutral, while simultaneously turning to the stern and yelling, loud and clear, “DIVE DIVE DIVE!!
As soon as the first “DIVE” is out my mouth divers are leaping off the stern, there is no pause, or hesitation, and within seconds a dozen divers are off the boat and have disappeared below the surface. Only bubbles are briefly visible, but they too soon disappear in the current.
“Nice drop,” Steve says. “I think you nailed it, but we will see”. You never know if you really nailed it until several minutes have passed. If your drop was wrong divers will miss the wreck and soon return to the surface to be picked up and re-dropped. Each passing minute without a diver on the surface is a good sign.
I turn the bezel on my watch to mark the drop time and write the time in my pocket notebook. Drop time 10:42. We will now drift with the current, but also adjust our position to stay near the wreck location and wait for divers to surface.
When the divers complete their dive and depart the wreck they will immediately start drifting north. At the approved ascent rate of 30 feet per minute, and with safety stops at 40 feet and 20 feet, it will take several minutes for divers to reach the surface. During this time they will drift north at 3.4 feet per second. Steve and I scan the horizon, as does the Divemaster on the lower deck. After 45 minutes we see a float pop to the surface, first one, and then a second, and a third. We are now half a mile north of the wreck. As each diver’s head appears they give the “OK” sign, an arm making an “O” to the top of their head.
I am relieved, it appears we dropped them on the wreck and they are coming up as a group. We maneuver our boat over to each diver and back up to them. Dive ladders are dropped, and after taking off their fins, climb aboard. Our first recovered group drops onto the side benches, pull off their masks, and look up at Steve and me on the bridge. “Great drop!” they shout, “You put us right on the wreck, perfect lead distance.”
I look at Steve with relief. “Whew,” I say out loud.
Steve looks back making a quirky smile, “Yea, a little different than driving boats in the Army?”