Speedboat Scaredy-Cat

December 16, 2007 4:36 PM

Why didn't our speedboat capsize?

So there I was, out on a friend's motorboat in the middle of the Pacific when the waves started picking up. Being inexperienced at sea and particularly wimpy at high speeds, I was scared out of my mind. We were racing over waves, landing with a crash after each jump. When the captain swerved (chasing birds, he was), the whole boat tilted at an awfully severe angle, and it felt as though a tight grip on the railings was all that kept us from going overboard. I was told that this is perfectly safe due to the "physics of the boat", though nobody could be more specific than that. A good layman's explanation of how speedboats stay upright would help me to conquer my nerves and enjoy a relaxed day at sea when we go again.
I should say that nobody seemed the least bit concerned about the speed or the tipping of the boat. The ride was pretty standard; no crazy stunts, and no alcohol or bad driving involved. My fear is definitely rooted in my inability to understand how the boat can be safe when it feels so topsy-turvy.
Thanks!

posted by wetpaint to sports, hobbies, & recreation (9 comments total)

Two possibilities:

1) Your nervousness and inexperience made it feel a lot more topsy-turvy than it actually was, or
2) Your boat-owning friend is a bit of a daredevil, drives too fast in choppy seas, and wasn't very considerate of your nervousness and inexperience.

From your description, I'd guess we're dealing with C, all of the above.

As for the physics, it depends on the type of boat, but generally the two things keeping it rightside up are that it's heavier on the bottom; and the keel and to some extent the rest of the hull sticking down into the water adds stability (since for the boat to turn over sideways that keel would have to push a lot of water out of the way first, which can't happen very quickly.)
posted by ook at 5:37 PM on December 16, 2007

Scroll down to the "Boating Encyclopedia" page of this page for a full, scientific explanation. It starts:

Seaworthy powerboats are designed to resist capsize by careful calculation of their centers of gravity and buoyancy. As long as they maintain power and are correctly handled, they are relatively immune to capsize in anything less than full gale-force winds. Planing hulls gain stability at speed; displacement hulls often have passive or active stabilizers that reduce rolling as long as the boat is moving.But a powerboat that loses power [...]
posted by wackybrit at 6:07 PM on December 16, 2007

there is often more of the boat underwater than you might think and the weight of the water against it holds it down

that being said, i once watched a large speedboat going very fast in pretty choppy seas, after one particularly big wave it went up too high in the front, the back took water fast and the whole damn thing went under back first. this sent all the passengers into the water and left just the aft 1/3 of the boat visible as it bobbed up and down in the water

i was sitting on the deck of my parent's home at the time, which is on lake ontario, and called the coasties. a little while later the boat popped back out of the water!

so it is not impossible for bad things to happen out there! and when they do, it usually happens pretty damn fast....
posted by Salvatorparadise at 6:10 PM on December 16, 2007

What keeps the boat from tipping all the way over when the pilot swerves is (I think) the same force that keeps a speeding motorcycle from falling over in a curve, or keeps a race car on the track when it goes around a steeply banked corner. (Centripetal? IANAPhysicist.) Even though you get tilted at an angle, the bottom of the hull is still pushing down against the water rather than slicing sideways into it.

Also: an angle that might sink a boat if it were dipped straight down into the water will not sink the boat if the boat is moving forward, because the shape of the hull pushes water out of the way as it goes.

What angle do you estimate you were tipped at? Was holding onto the railing actually the only thing keeping you in the boat? Did your butt stay in your seat (or your feet on the deck), or were you dangling from the railing and screaming bloody murder?

On preview: it is possible that your pilot was doing Risky Dumb Stuff, as described by SalvatorParadise (it certainly is possible to push a boat's hull too far), but more likely that your perceptions of stability aren't attuned to the particular physics of boats, as described by ook.
posted by Orinda at 6:35 PM on December 16, 2007

People, please don't guess at science questions if you don't know!

wetpaint, a boat has a center of gravity, which depends on how the mass of the whole boat is distributed, and a center of buoyancy, which depends only on the shape of the hole the hull makes in the water. Boats' hulls are designed so that as they tip over, the center of buoyancy moves toward the boat's low side faster than the center of gravity does, so it works to push the boat back upright.

A sailboat, which has much of its mass aloft, will eventually tip over to an angle at which the center of gravity gets out past the center of buoyancy, and will then capsize. But it's not hard to design a powerboat, most of whose mass is in the engine near the waterline, so that it will always right itself. So don't be scared!
posted by nicwolff at 7:49 PM on December 16, 2007 [1 favorite has favorites]

Orinda, the railing was not really the only thing keeping me in the boat -- it just felt that way! I was bouncing up off my seat when we jumped waves, but probably not enough to fall into the ocean. The bouncing combined with the angles when we took turns (each side of the boat curving up at maybe a fifty-to-sixty degree angle) made it feel more dangerous.

Nicwolff, I believe you that a speedboat would always right itself. I just can't shake the feeling that it could very well tip over under the wrong circumstances and THEN right itself -- after depositing us passengers in the ocean! But your explanation answers my question and makes sense, so I'm (somewhat) reassured. Thank you everybody!!
posted by wetpaint at 8:47 PM on December 16, 2007

Nicwolff: People, please don't guess at science questions if you don't know!

You might reconsider this statement since what you are talking about is static equilibrium for displacement hulls and little to do with the question. The dynamic forces on a planing hull for a speed boat are much different and more akin to aerodynamics than buoyancy.

One of the most common accidents in speed boats is a person tumbling overboard when the driver makes a sudden swerve at high speed. A person who is standing can easily be thrown out if they are not expecting it or looking the other way, especially if alcohol is involved.

The boat itself is not in danger of overturning and passengers are not in danger if they are sitting down. You will simply bank around the turn much like when an airplane makes a turn. You may have noticed when flying that the plane will tilt at an angle when making a turn yet you don't fall out of your seat. Instead, the centrifugal force presses you down into your seat more securely. In fact the physics require the plane to tilt in order to make a turn. The same thing happens in the speed boat except the tilting may seem more severe if the radius of the turn is much tight and the speed is high. Nothing to worry about as long as the driver is not drinking.
posted by JackFlash at 9:56 PM on December 16, 2007

You may have noticed when flying that the plane will tilt at an angle when making a turn yet you don't fall out of your seat. Instead, the centrifugal force presses you down into your seat more securely. In fact the physics require the plane to tilt in order to make a turn.

Centrifugal force is a fictitious force.

What "pushes" you down in the seat in this case is your body's resistance to angular acceleration.
posted by de void at 11:27 AM on December 17, 2007

Centrifugal force is a fictitious force.

I almost hesitated to use the term centrifugal force because invariably someone always pops up to say it its fictitious. However, centrifugal force is very useful for explaining to the OP the forces that they feel without getting into a bunch of unnecessary mathematics. "Fictitious force" is a term of art in physics that does not have the same meaning as common usage. It does not mean that the force is imaginary. It means that it is a force that only appears in a non-inertial (accelerating) reference frame. To the person in that non-inertial frame the force is very real. Einstein used the Principle of Equivalence to state that "the gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is actually the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference." To the person in the speed boat, the centrifugal force is indistinguishable from gravity pushing you down in the seat. Centrifugal force is a very useful concept in physics when used appropriately and can lead to simpler answers in some cases.
posted by JackFlash at 12:28 PM on December 17, 2007 [1 favorite has favorites]