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Things That Go Bump on the Hull


Avoiding shallows, stumps, shoals, rocks, and other (mostly) underwater obstructions

In some areas, crab and lobster pots are everywhere, even in shallow water. A spinning propeller can snag one and cause internal damage to the outdrive (claim #0211595).

Claim #0208762: On a beautiful spring morning, after several days of rain, the owner of a 45-foot sailboat and his wife left their slip on the Potomac River in Maryland, en route to a new marina on the Chesapeake Bay. The water, they noted, was muddy and there were lots of sticks, leaves, and other flotsam on the river that the two of them were constantly dodging. After several monotonous hours, there was a loud thump on the hull, followed a few seconds later by a loud bang! The wife, who was at the helm, throttled back, but it was too late; they looked back and saw an enormous refrigerator-sized log pop up behind the boat. The two had been busy talking and their few seconds of inattention had resulted in the loss of an expensive folding prop, bent prop shaft, and ruined shaft log. After being towed to a boatyard, the couple had to wait several weeks to get their boat repaired while the yards finished painting and launching boats for the season. Had they waited a day or two for the debris to wash away or kept a better lookout, they wouldn’t have wasted the best part of the sailing season while their boat was repaired.

Striking a submerged object, which includes running aground, is the most common damage claim, according to the BoatUS Marine Insurance claim files, accounting for almost a quarter of all claims. There is a mistaken belief that an occasional grounding or bump on the hull is no big deal, a part of being on the water. While it’s true that damage typically is nothing more than a bent prop, there have been plenty of claims for boats that were severely damaged or even sunk.

Floating Debris

This boat struck a submerged rock at high speed, tearing the outdrive off of the boat, though the hull was undamaged. Lake levels can change year to year; check state and local agencies if you’re unfamiliar with the waters (Claim #1008843).

After a heavy rain, most bodies of water end up with lots of stuff that was washed from shore. Also, boaters in many areas of the country have to negotiate “minefields” of crab pots and lobster pots, as well as logs (Claim #9989083), old tires (Claim #0288733), floating lines (Claim #8903748), kelp (Claim #0212421), and even an old canoe (Claim #0576388). When something is snagged, the damage is rarely to the hull; it’s the underwater running gear that’s most likely to suffer. An engine that is stopped abruptly can be severely damaged, even breaking internal components. If you see (or hear) that you’ve just run over something, cut the power immediately and you might get lucky. If you do get a line wrapped around your prop and can’t dislodge it, don’t be tempted to try to power it off; a giant ball of rope wrapped around a shaft can pull the strut out of the hull, or worse, pull the shaft out, leaving a gaping hole.

Even in good visibility, debris like logs and branches are hard to see. Keep an extra sharp lookout after heavy rains. Watch for irregularities on the surface that might signify debris and be prepared to make a sharp turn or cut power if you spot something. If you find yourself in a field of crab or lobster buoys, slow down. Some floats may be partially submerged and even harder to see; it helps to have a spotter up forward directing, but avoid turning sharply and swinging the stern into the buoy you were trying to avoid. If you do get a line wrapped around your prop, it may be possible to unwind the line by turning the shaft by hand (make sure the engine is off). Diving over the side with a sharp knife is a last resort and should only be attempted if there is someone else on the boat.


The most important tools for avoiding a grounding are a chart, a depth sounder, and a watchful eye. Charts show depths at the lowest tide you’re likely to encounter, though the official depths can be iffy after a few days of strong directional winds or during prolonged seasonal droughts. Use your depth sounder to make sure you’re in channels marked on charts or away from known hazards and use your eyes to confirm what the chart says; wind or current could sweep you off course. If something doesn’t look right, slow way down until you’ve ascertained your position. Be aware of how much (if any) offset your depth sounder has. Offset is the difference between where your transducer is mounted and the bottom of the boat, sometimes as much as two or three feet on a sailboat. You might think you have five feet under the keel, but if the transducer is mounted two feet up from the bottom of the keel, you only have three feet. Keep in mind that even recent charts can’t show new shoals formed by currents, such as in the Intracoastal Waterway, or by recent storms.

Inspect the Bilge!

A hard grounding can tear a hole in the bottom of a keel and fill the boat with water. If your boat hits a rock or reef the first thing you should do (after looking after your crew) is check the bilge for rising water. (Claim #1010521).

Let’s suppose that despite your best efforts, your boat strikes a submerged object or runs aground. Hitting the bottom in some parts of the country, like the Chesapeake Bay where the bottom is typically muddy, usually isn’t too bad. The same is true of hitting a small branch. If you hit rocks or a tree trunk, there could be serious damage.

No matter what you hit, bilges should be checked immediately for rising water.

Especially when you go aground, it is important not to get angry; shoving the throttle forward to power off will either run the boat farther aground or, worse, kick up debris that gets sucked into the engine. Mud and sand can easily bypass intake strainers and end up inside the engine’s cooling passages or the heat exchanger. Either way, the gunk has to pass through the intake pump, which is equipped with rubber impellers that can break apart and get sucked into the engine. The engine overheats and the situation goes from bad to worse.

If you should ever find yourself aground or with a bent prop, take a deep breath, think nice thoughts, and evaluate the situation. If everything is OK, you can start the engine (keep an eye on the temperature gauge) and try to back off. If your prop is kicking up debris, shut it down quickly. You may be able to wait for the tide. If not, stay put and call TowBoatUS for assistance.

Note that many boats have sunk several hours after striking a submerged object or running aground. The owners assumed the boat was fine, only to find out that a strut or through-hull was dislodged and began leaking. In fact, a study by Seaworthy found that four percent of boats that sank at the dock did so after striking a submerged object while underway. Even if you checked the bilge after initially striking the submerged object or the grounding, once back at the dock, re-inspect for leaks; any signs of flooding mean a haul-out is needed right away.

Bumps with Stumps

Claim #900383: The owner of a bass boat was, as he said, zipping along at a good clip on a lake in Alabama he was unfamiliar with, when he heard a tremendous crash and the boat suddenly stopped. As he cut the engine, he looked back but didn’t see anything. When the engine wouldn’t restart, he went to the stern to find that the entire outboard leg had been sheared off and there were cracks in the transom. Using the trolling motor, he limped back to his marina. When he related his story to the marina manager, he was told that the lake level had dropped a couple of feet since the previous year and many old stumps were now close enough to the surface to snag an unwary boater. During drought conditions, some lakes and rivers can drop several feet, and boaters have hit stumps (Claim #9909821), old dams (Claim #0108765), pipelines (Claim #9908973), and even an old car (Claim #0107636).

In inland areas, before you launch in an unfamiliar place, ask the locals about the lake level and any well known obstructions. Usually state, federal, or local agencies keep tabs on lake levels and an Internet search can provide details. In larger bodies of water that are heavily traveled, charts generally show submerged pipelines, rocks, ledges, and other permanent obstructions, though sometimes with a buoy that may not be at the exact position of the danger; give it a wide berth. Be careful not to get into trouble by passing on the wrong side of a marker that warns of an underwater obstruction. Once again, your eyes are one of your best tools. Be aware that a floating branch might be connected to a sunken tree. To prevent riverbank erosion, some large rivers have wing dams near shore that are completely underwater (and invisible).

The presence of wing dams in rivers, like other underwater obstructions, may be indicated by ripples, eddies, or swirls.

BoatUS Editors

Contributor, BoatUS Magazine