# Why do ships lean to the outside, but boats lean to the inside of a turn?

I have a coworker who was once stationed on a Navy aircraft carrier. While there, he noticed that the carrier leaned to the outside of turns. He asked me why a large boat leans to the outside but a small boat (like a ski boat) leans to the inside of a turn. My theory is that a small boat does in fact lean to the outside of a turn when it is not planning. It just leans so little you don’t usually notice it. I also theorize that the planning hulls of ski boats are designed to lean to the inside so that they provide a “coordinated” turn. What is the true answer?

I figure it is the relation of a vessel’s center of gravity versus it’s center of geometry. A top heavy ship would lean outside, but a bottom heavy speedboat would bank into a turn.

Total WAG here, but…eh. It seems logical.

Smaller boats have to lean in because they’re smaller than the waves. If they lean outward while they’re turning, a wave will capsize them.
Larger boats don’t capsize as easily and leaning outward won’t harm them.
In other words, it’s not a trick of physics, it’s done because of physics.

Center of gravity plays a role, but not nearly the final role.

On smaller vessels, other than “ships”, even say 32 foot fishig boats that are somewhat top heavy, the top heavy-ness can be over ridden by the power of the engine(s).

Here’s why:

If it is V-hulled, and you turn your boat left, you require power to do so, and the engine is pushing the boat around from behind, and the power point (prop) is actually on the left inside of the rear of the boat (that’s where the props wind up because they are connected to said motor) pushing the bottom of the vee hull around AND up, so that the boat leans to the inside (under power) and the lean accentuates the turn. The boat is rolling over on it’s hull while the motor continues to push up and around. What is key here is that the boat has good power relative to it’s weight, and its steering is done by the motor itself, whereas on a “ship” the rudders turn the boat while the props propel forward. This means all the weight is going to want to follow it’s original direction, the rudders turn the boat and the boat leans over away from the turn.

A ship is somewhat under powered and the lean to the outside can’t be over-ridden by engine power like in smaller craft.

To exaggerate the role the motor plays, take a jet powered craft, like jet boats: the whole motor and nozzle turn and create powerful forces that push the bottomm of the v hull up and around and create the inside lean - even with top heavy people on board…these jet boats have ultimate turning ability above and beyond other boats because of engine power/thrust. Top Heaviness has a dimished role.

Power, engines that turn and hull design play a role to easily out-do the top heavy issue.

First, I think everybody here understands why the ship leans to the outside. It has a rudder and keel that force the ship to track in the direction that the rudder allows. However, on a ship, there is a massive amount of mass out of the water. This mass has nothing to make it turn except the fact that its welded to the mass that is in the water. Therefore, Newton’s 1st Law says that the top half wants to keep going and lean to the outside of the turn.

Anyway, the perplexing issue has more to do with why the small boat does lean to the inside. Philster, you had some very good observations about how the vectored thrust of an outboard motor or jetski can cause this. But, I regret to say that vectored thrust is not the overriding factor. Cases in point:

1. PT-boat – 4 inboard engines with direct-drive props (no steerable lower unit), and seperate rudders. It still leans to the inside.
2. Mastercraft skiboat – 1 inboard engine with direct drive prop, seperate rudder, leans to the inside.

Any other theories?

Philster is on the right track, but maybe didn’t explain it as well as he might have. I’m not sure I can do better. Imagine a small motorboat, turning left, looking at it from the rear at the waterline. To turn left, there is a force being applied to the right, either on the rudder or on the prop (if outboard). This force makes the stern go to the right, swinging it around and making the turn.

This turning force is applied below the center of bouyancy, somewhere under the water line. In effect, the turning force is exherting a counterclockwise twist on the boat, along the center of bouyancy, thus making it lean
If you have a model and think about the forces in play, it makes sense. Kinda like a pinwheel.

Think about what would happen on an outboard ski boat if you tied it up so that it could not move front to rear, then cranked the motor up and turned hard to port or starboard. It would twist in the direction of the turn.

In larger vessels, the very same forces are at work as well, but as the center of gravity is much higher than the center of bouyancy, the vessel leans outward from the centripital force. And as Philster says, it is a power issue: Big displacement boats such as trawlers will also lean outwards. If your boat doesn’t have the power to get up and plane, you probably won’t notice this effect much.

I’m inclined to believe that story.

So, the direction and amount that a boat leans when turning is the ratio of the roll-axis force that can be imposed by the rudder, to the centrifugal force.

I guess this means that an aircraft carrier with a big enough rudder would lean to the inside of a turn.

I think everyone is on the right track. But allow me to elaborate on my points and address the follow-up posts:

Take an extreme like an aircaraft carrier. It has none of the ingredients that could create an inside lean in a turn.

>First, it is under powered (relative to other smaller craft)

>Second, hull design is v-shaped, but it is an CONCAVE V, whereas jet boats (popular in Navy ops), PT boats, sport boats and smaller fishing boats have CONVEX V hulls that encourage planing.

>Third, the props point forward and don’t turn with the motor to create up ‘n’ around twist power in a turn.

>Also, let’s keep in mind that Carriers and many other boats aren’t as top heavy as you’d think. Much mass and weight counter the weight above the water line. In turns the wall of water against the outside of the boat creates or helps define the axis (fulcrum?) of the vessel so that the top is inclined to lean outward. Top heaviness doesn’t get us to the answer.

>On the PT boat point: You don’t need all the ingredients to create inside lean when turning. In the PT boat case, the hull design and power to create planing are enough, despite the fact that it’s straight props/rudders. The Navy has found much increased turning performance by using more jet powered boats in their aresenal with nozzles that turn. By combining the power to plane, the turning jet, and the hull design, maximum turning performance is achieved.