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How do Currents and Tides Affect the Maneuverability of a Boat?

Updated: Mar 12

While the Penobscot River flows to the ocean, its size and virtually no elevation change between Bangor and Penobscot Bay make it subject to seeing an ongoing, and outgoing tide. These tidal swings create moving sandbars, affect the maneuverability of a vessel, and create an interesting dynamic if a vessel loses power or has a person in the water or man overboard.

The Penobscot River changes tides roughly every 12 hours at the beginning and end of that 12-hour window is roughly 45 minutes of "slack" tide. This is when the water is undisturbed and not flowing in a direction. Because the Penobscot is a river, it really never experiences a true slack tide because there is constant water flow however, it is barely noticeable.

Ok, so why is this important? The difference between low and high tide is about 12 feet. Depending on the draft of a vessel (how much of the vessel is underwater) captains need to exercise caution understanding that water depth will be affected. Additionally, when the tides are moving, there is anywhere between a 6-8 knot current. (1 knot is 1.151 mph). So lets say the tide is outgoing, and I am on my boat motoring with the current. My speed over ground is going to be 6-8 knots faster than my speed over water. This means that if I stop my boat, I am still going to be moving with the current at a pretty decent speed. My maneuverability is also compromised at low speeds. Let's say I'm clutch ahead (in forward gear but not providing throttle) much like being in "drive" with a car but not stepping on the gas, visually looking at the shoreline, or stationary objects in the water, ill be moving at, you guessed it, about 8 knots. My vessel is moving "with" the water and not neccessarily "through" the water, which means I wont have water flowing under by boat. My rudder, or motor, needs water flowing by it to maneuver, the faster the water flows, the more maneuverability I have. So with little to no water flowing past my rudder, I wont be able to steer left or right of obstacle, even though it looks as though I'm traveling at a decent speed. Additionally, when I turn my vessel, I need to be prepared to be taken with the current when I am broadside ( current is coming towards the side instead of the bow or stern) a distance until I complete my turn.

Another factor to remember with tidal currents on the Penobscot is sandbars. Sandbars are areas where sand has built up to create a shallow bar. While usually marked on charts, thes bars can move or shift over time.

In an emergency situation, understanding currents and tides can be the difference saving a person in the water, or damaging your boat. If I was to loose power, the first, and probably most important thing to do is drop anchor and use plenty of scope. Scope is the amount of line let out, ideally, letting out 7 times the depth of water a vessel is in will provide the most holding power. Having too little line out pulls the anchor in an upward direction vs a lateral direction.

Lastly, when approaching a dock, or recovering a person in the water, Captains should approach from down current, or downstream. By making this approach, the Captain has more control of the vessel and thus, better maneuverability. Keep in mind the position of your props and if possible, have the vessel off or at a minumum, out of gear, at idle to prevent any injury to people in the water.

Hope you enjoyed the read, please send us a comment on what you would like us to write about next!

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