While cruising along in a narrow channel if the boat leans or pulls toward the bank your boat is not possessed! You are experiencing “bank suction”. This occurs when the vessel’s propellers are too close the shallow shoreline. The propellers can actually start to pull the boat towards the shallow bank. This is common to all boats when cruising too close to the edge.
What edge? You may think that you are a good distance away from the actual shoreline and all is well. But you may not realize that there is another edge beneath the water. The actual deep water channel or cut is created from a high velocity tidal current eroding the soft mud and leaving the harder clay as a distinctive underwater ledge. If your propellers get too close to the this ledge they will suck the stern of your boat closer to the edge of this underwater channel. This can cause the boat to lean noticeably toward the shallow edge.
Best immediate solution is to pull back the throttle to reduce propeller suction thus reducing bank suction. Now steer away from the bank. Another advantage of reducing speed is the bow will go down and more important the stern and props will raise, making a lesser draft for the stern and lessening any bank suction.
Prevention is to stay in the channel! But you need to know where it is.
Bottom suction can also occur when running in shallow water with a deep draft boat. The props are hunting for water to to pull through the blades and can not get enough water because the sea floor is too close to the bottom of the boat. The boat’s propellers then pull the boat down giving you a sinking feeling. The stern is digging a hole in the water. Solution? Just back off the throttle and the stern will lift. Now you can idle away from the shallow area. Grounding avoided!
Ever see a large vessel wobbling down the ICW? It is safe to assume the stern is being pulled from one side of the channel to the other hunting for deep water. The vessel helmsman is having a rough time steering the boat. You guessed it- the only way to correct this erratic steering is to reduce speed. The props will get more water and the boat will steer better. Slowing down solves a few issues. No grounding and it becomes easier to steer.
Don’t blame the autopilot. It is doing the job it was designed to do. The autopilot is trying to keep the boat on course but the bottom suction is causing the stern to move. All the hunting for deeper water causes the autopilot to constantly correct the course line. Turn off the autopilot, reduce your speed, raise the stern and manually drive with ease until you find deeper water.
Bank and bottom suction can occur to any size boat traveling at displacement speed. Yes, dinner boats, tugs, shrimpers and your own pleasure boat can experience the “suction” issue. Think it through, reduce your speed and steer into deeper water for a suction free cruise.
Captains Chris & Alyse Caldwell are USCG 100 ton Masters and Cruising Coaches who offer Personal Boat Training Online or Onboard your boat anywhere! The Caldwell’s help build your cruising confidence with hands-on training and with their AskCaptainChris.com training videos filled with tons of tips for the boater who loves learning. If you have additional questions for Captain Chris or Captain Alyse, please email them at [email protected]