# Why do some boats lean into turns?

In summary: That's why they lean into the turn. In summary, a sailing boat will lean outward due to centrifugal force, while a power boat will lean inward due to the lack of inertia.
Hi, I do quite a lot of sailing, and I have recently noticed that when sailing boats make a sharp turn, they heel towards the outside of the turn. However, power boats lean towards the inside of the turn.

I assume it is something to do with the fact that they have differnt types of hull; sailing boats have displacement hulls, while power boats have planing hulls, and perhaps something to do with the fact that sailing boats have keels, while power boats do not.

I am only a keen GCSE student so please try and keep answers simple :D

The outward lean might be due to the ordinary inertia (centrifugal force), which is indeed increased with the presence of a deep long keel on sailboats. But for the inward lean of power boats, a little thinking must be done. Since powerboats have no keels, they experience less inertia but they do not experience 0 inertia, since it must be compensated for the speed the have!

I thought it was because the thrust and/or rudder is below the center of mass of the boat.

The keel keeps the sailboat on an even heading and also without a keel any slight side wind would tip the sailboat over.

rcgldr said:
I thought it was because the thrust and/or rudder is below the center of mass of the boat.
Yes, it's just that. Typical power boat will have its prop bellow its center of mass and turn with the rudder. When you make a turn, this turns into a torque turning the boat inward. Simple as that.

Reason for it is also pretty straight forward. Power boat will ride pretty high at speed, so center of mass is above the surface. If it wasn't for leaning inwards, it would basically tip over due to centrifugal effect.

ok, thanks that definately clears it up for me :)

I think the two examples in the OP are probably extremes. A sailing dinghy will have its rudder hung on the transom and a large proportion of its area will be above the centre of pressure of the water on the hull. (I think this centre of pressure is at least as important as the CM, already mentioned) Hence there will be a little roll induced and this could well be in a sense to roll 'out of' the turn. I sail a keel boat with a deep rudder which is all below the hull. This could well cause roll in the other (the powerboat) direction but I must say I have never noticed this. I must make a note when I remember and I'm next sailing but generally things are less dramatic on a sailing cruiser and she tends to turn elegantly on the proverbial sixpence rather than executing the flashy curved path with all that wash you get from the fast boys with their hundreds of kW engines.
A "power boat" will often have an outboard drive (or steerable propellors) which acts well below centre of effort of the hull (when the prop is not actually tipped up to avoid grounding), giving the expected 'roll into' the turn. I wonder whether large motor vessels (such as fast warships) would also lean into turns as their props are not steered and they are not below the hull.

There is an extreme example of "high center of mass" and that is the aircraft carrier. When she is in a hard turn she leans outward dramatically...you need to "hold on" during this maneuver!

That's the beauty of PF. There is such a wealth of experience here. That's brilliant info. Not just the Isle of Wight Ferry - a frickin Aircraft Carrier!

I have just re-thought this Aircraft Carrier situation. No ship can have a centre of Mass above it's centre of flotation - or it will capsize. Rather like a double decker bus, an Aircraft Carrier appears to have a high CM but it just can't have. I suspect that an aircraft carrier needs to have a deep draught (keel) to reduce its roll when traveling in a straight line (to help keep it level for landing planes on it). Its propellor will not be far below the water line and so the deflected water from the rudder will probably be high up relative to the centre of pressure on the keel - causing it to roll out of the turn - possibly worse than a sailing dinghy.

A turn is an acceleration, and for that the sum of the forces must add up to a vector in the direction of acceleration. Draw a free body diagram, and you will see that the direction of that resulting force underwater is in the opposite direction for sail and power boats.

That applies to the typical planning hull power boat. For a displacement hull power boat, the forces will add up in the same way as the sailboat, which is also a displacement hull.

A more interesting contemplation is why an airboat also leans toward the inside of the turn.

sophiecentaur said:
[..] I wonder whether large motor vessels (such as fast warships) would also lean into turns as their props are not steered and they are not below the hull.
Indeed not! I recently was on a cruise ship, and I noticed that when it made a sharp turn, it leaned outward (very noticeable on the top deck!).

Yes. That ship is a displacement vessel.

When I tilt my kayak to the left, it turns right. It is a matter of hull design detail.
Here is the explanation, with diagram:

If the paddler leans the boat towards the left, the left edge of the kayak digs into the water and causes the kayak to travel along the curved path that is actually the left side of the kayak. The kayak starts to turn to the right.
http://www.outer-island.com/Articles.html

Respectfully submitted,
Steve

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sophiecentaur said:
I have just re-thought this Aircraft Carrier situation. No ship can have a centre of Mass above it's centre of flotation - or it will capsize. Rather like a double decker bus, an Aircraft Carrier appears to have a high CM but it just can't have.
Thats not true for either vehicle: leaning away from the turn is what cells you the com is above the support. Why don't they tip over?

If a bus leans in one direction, the outboard wheels apply a larger force and inboard ones a smaller force and the resulting torque keeps it from flipping over.

For a flat-bottom ship, the outboard side of the hull goes deeper under the water in a turn, which increases the buoyant force on that side (and vice vera on the inboard side). This would not work on a boat with a round bottom.

rcgldr said:
I thought it was because the thrust and/or rudder is below the center of mass of the boat.

You can begin turning on a jet ski from a dead stop because the thrust is vectored. If you do this, you begin to lean into the turn almost immediately due to the trust from the jet.

russ_watters said:
Thats not true for either vehicle: leaning away from the turn is what cells you the com is above the support. Why don't they tip over?

If a bus leans in one direction, the outboard wheels apply a larger force and inboard ones a smaller force and the resulting torque keeps it from flipping over.

For a flat-bottom ship, the outboard side of the hull goes deeper under the water in a turn, which increases the buoyant force on that side (and vice vera on the inboard side). This would not work on a boat with a round bottom.

My point in comparing a bus with an aircraft carrier was that the outward appearance tells you little about the position of the CM and the real stability of the thing.

If you take a flat bottomed power boat - like a dory or RIB, for instance, and drive it into a turn, it will always tend to roll into the turn. This must be because of the action of the steering mechanism (the propeller pushing water directly in the direction of the centre of the curve to point the hull where you want it to go) and the prop is low down in the water (beneath the hull by a significant amount) - tipping the boat inwards. This effect happens even with the boat stationary - so there is no centripetal / centrifugal effect at all, at that stage. There is no rudder at all is such boats. However stable the hull is, it will still tend to roll out of the turn when not under power.
Boats with a rudder and fixed propellers all, either from the simple rudder's motion through the water (sailing boat) or with the propelled water deflected by the rudder at the same level as the hull - will have water deflected definitely not below the level of the hull. So there will be little or no compensating couple to roll the hull into the turn. The amount of roll will, of course, depend upon the hull shape but I can't see how it can be ever 'into' the turn because the speed is low, the radius is large and CM will not be far above the centre of bouyancy. A rounded hull, lightly loaded will be worse than a flat hull with a full load when turning. (That is why they ballast ships with no cargo).
Plenty of boats have a 'rounded hull' but the stability depends upon the relative position of the CM and the centre of bouyancy (which shifts as the boat heels). Even a barrel can be stable as long as it has sufficient fixed ballast at a low enough point (beneath the CB). I was looking at the Cruiser HMS Belfast today (on the Thames near London Bridge) and she draws 20ft, which compares with a similar amount of freeboard (largely hollow for crew quarters). There is obviously a huge proportion of her mass well below the surface - engines,boilers, fuel and magazine - although the appearance is that most of her is above the water. Warships are not pleasant to be in in bad weather, I am told.

When a boat is under sail, the effect of the wind on the sail will dominate the roll, whatever point of sailing, and I don't think that anything much can be said about the likely direction of roll. When running (down-wind), a keel boat will roll badly (effect of the wind) but a dinghy will behave much better if you raise the centre board because you are much less likely to 'screw' round into the wind and the boat just 'skids' along and can be controlled much easier with the rudder (castor effect).

flatmaster said:
You can begin turning on a jet ski from a dead stop because the thrust is vectored. If you do this, you begin to lean into the turn almost immediately due to the trust from the jet.

That is interesting. I guess the jet nozzle is near the bottom of the hull. Does is point up or down? They have a very high power/weight ratio and the bows have quite a noticeable 'scallop', too, which would make the hull ride up on the outside of the curve as soon as it is moving at all. We don't get many of those craft where I sail but I must keep my eyes peeled to see one in action.

sophiecentaur, You are certainly correct that an Aircraft Carrier looks top-heavy from outward appearances. I learned a lot of new information while searching out this “Aircraft Carrier Stability” question.

Here is an excellent explanation of general hull design for stability:
http://www.rcwarships.com/rcwarships/nwc/stability.html

The British Royal Navy’s newest design:
Future Aircraft Carrier (CVF) Queen Elizabeth Class
http://navy-matters.beedall.com/cvf1-14.htm

The newest American Navy aircraft carrier design:
http://www.naval-technology.com/projects/george-h-w-bush/

Finally, this seems to be the main reason carriers behave the way they do…the “bilge keel” and it is used on the new GHWBush:

“When a ship rocks back and forth, it can make people seasick. Even worse, it can make it dangerous for jets to land on aircraft carriers. For these reasons and many others, it's important for engineers to design bilge keels (or fins) to keep boats from rolling back and forth. Become an engineer for a day and discover the best way to keep from rocking the boat in this engineering science fair project!

A bilge keel is usually a simple piece of steel on the bottom of a ship that is aligned with the direction that the water passes over the hull (see Figure 2, below, for a diagram of bilge keels and other types of keels). By aligning it this way, the bilge keel doesn't slow the ship down very much when it is traveling straight ahead. But when the ship begins to roll due to wind and waves, the bilge keels act like a plow through the water. The plowing action of the bilge keels generates a force that acts in a direction opposite to the rolling motion, and this force slows down the roll. The bilge keel only works when the ship is rolling and doesn't hurt the straight-ahead speed very much. Since it is just a simple piece of steel and doesn't have any moving parts, it is fairly inexpensive to build, too. Combine those benefits and you have an efficient way to design a better ship!”
http://www.sciencebuddies.org/science-fair-projects/project_ideas/Aero_p038.shtml

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sophiecentaur, where CoM is located relative to support is irrelevant for question of stability. The question is whether CoM rises or falls when the boat is tilted. If it falls, the boat is unstable and will capsize. If CoM rises, when boat is tilted, the boat will be stable. It's not difficult to construct a ship whose CoM is above the water line, yet is entirely stable. I don't know if aircraft carrier is an example of such a ship, but it's entirely possible. A trivial example is pretty much any catamaran.

The COM doesn't move, the center of buoyancy moves -- laterally -- when the ship rolls. If the ship rolls right, the center of buoyancy moves to the right, providing a restoring torqure. That's what keeps the ship from flipping over.

See the wiki link I posted above for good diagrams.

where CoM is located relative to support is irrelevant for question of stability.

K^2, the RYA yachtmaster theory course explains the basic physics of stability, and it is essentially affected by the relative positions of the centre of mass and the centre of boyancy; the distance between them is the righting lever.

I see many are getting very close to understanding the solution to the original question, but not quite getting there. Or perhaps maybe they are there, but are having trouble communicating it such that all can understand clearly.

Many and probably most vessels have a center of mass higher than the center of buoyancy. The ballasted displacement hull of a small vessel is the one big exception. Many sailboats fit this category, though certainly many displacement vessels have a high CG.

A sailboat works through the interaction of two foils or wings, each producing lift. You have a hydrofoil under water, which is the keel and rudder. It produces a lift vector. You have a sail above, which produces a lift vector like an airplane wing. The trick is to trim the vessel such that these two vectors add up and point in the direction of travel.

These vectors also cause motion in another plane, causing the boat to lean one way or the other. But now you must also add in the weight vector and the buoyancy vector. In a turn, a fifth vector is added, which is the acceleration vector.

But in a turn, when coming about, the sail vector goes to zero, and the remaining vectors add up and cause the boat to lean away from the turn. You see the same thing in a displacement power boat or ship in which the sail vector is always zero. In these vessels, the thrust vector from the propeller is much smaller than all the other vectors, so it has little influence in this current discussion.

A planing hull power boat is much different. The thrust vector is very large such that it becomes a dominant factor. The hydrofoil vector is zero or near enough to zero, and the aero vector is very small, except on very fast boats. In a turn, the boat will slip sideways over the surface of the water, which produces a lift vector at the leading edge of the slip that tends to cause the boat to lean into the turn. The thrust vector acts in the same direction, except in the case of an airboat. But the airboat also leans into the turn because the hydrodynamic lift produces by sliding sideways overwhelms the thrust vector.

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russ_watters said:
The COM doesn't move, the center of buoyancy moves -- laterally -- when the ship rolls. If the ship rolls right, the center of buoyancy moves to the right, providing a restoring torqure. That's what keeps the ship from flipping over.

See the wiki link I posted above for good diagrams.

COM moves in some cases.

http://en.wikipedia.org/wiki/Icebreaker

The optimal shape for moving through ice makes icebreakers uncomfortable in open water and gives them poor fuel efficiency. In open-water travel, icebreakers tend to roll side to side to the discomfort of the crew. Some new icebreakers, such as the USCGC Healy, make use of anti-roll tanks, incompletely filled ballast tanks which span the beam of the vessel. Ballast water in these tanks is allowed to move side to side, or slosh, as a free surface. Retarding baffles inside the anti-roll tank slow the side-to-side flow of water. By varying the water level inside the anti-roll tank, the natural frequency of the slosh is used to counteract the rolling of the vessel. Anti-roll tanks by their nature decrease a ship's stability and must always be used with caution. Use of computer-controlled valves allow for better control of these anti-roll tanks. A greater concern is how well a ship cuts through waves. The ability of a ship to cut through waves can greatly affect its fuel efficiency and even its safety in a storm. Most ships use a sharp or bulbous bow to cut through waves and help prevent waves from slamming the bow of the ship. However, icebreakers have a round sled-like bow. They tend to slam into waves, which can be risky in high seas.

@Pkruse
Yes, the OP was asking more about sailboats and we focused more on power boats. Life gets very complicated for dealing with this issue on a sailboat...

The sailboat's heel (when under sail) is dominated by the wind, so it will always heel away from the wind, unless of course, the wind is coming directly from the front or back. But what that means for how the heel looks in a turn is complicated by which direction and how far you are turning into the wind.

Consider a large, keeled, sailboat with the wind 90 degrees off to the left, therefore heeling right (I'm going to use a mix of nautical and non-nautical terms here...). There are four types of turns that can be made:

1. "Falling off" means turning away from the wind while keeping it on the same side (less than a 90 degree turn, to the right in this case). The sailboat remains heeled to the right, into the turn. This type of turn is fairly common, but you don't turn far and generally don't turn fast.

2. "Heading up" is turning toward the wind, while again keeping it on the same side. In our case, that's up to about 45 degrees to the left. The heel is thus out of the turn. This is also fairly common, but again you don't turn far or fast.

3. Jibing is turning across the wind, with it behind you. A single maneuver can be a full 270 degree turn, but is really cut into pieces, with a fall-off, a jibe, then a head-up. The jibe is what happens in the instant that a fall-off turns into a head-up: With the sail out 90 degrees to the right (wind to the left at the stat, but moving behind and then to the right), a little bit of wind gets behind the sail and it flips all the way from one side of the mast to the other in an instant. This is thus dangerous to both people and to the boat if allowed to happen uncontrolled. In a jibe, since you're switching between falling off and heading up, the boat starts off leaning into the turn and finishes leaning away from the turn. This maneuver is fairly rare.

4. Tacking/coming about. This is the most common large turn and in many cases is even done 270+ degrees in order to avoid jibing, even though it means a longer turn. But it also includes elements of both falling off and heading up. Since it starts with heading up, then becomes falling off, it starts with heeling out of the turn and ends with heeling into the turn.

And as can be seen, none of that has anything to do with inertial/centripetal/gravitational forces, unlike with a large displacement hull ship.

A postscript/caveat to the sailboat: for a small, non-ballasted sailboat, you're the ballast so the the direction of heel depends more on where you are sitting and how strong the wind is than anything else.

I networked at great length with a ship design engineer who specialized in minimizing sea sickness to passengers and crew, a very sophisticated specialty. Some effort is given to minimizing roll, but far more effort is put into making the roll period far outside the range that normally promotes sea sickness.

He had never used anti roll tanks in a design, preferring instead to use active roll stabilization with fins under the boat and an inertial control system. You can install an off the shelf system in a 60 ft trawler for about \$60,000.

He was helping me with a catamaran design that I was working on at the time. I could not do much with that because a catamaran will always have a very snappy roll period.

russ_watters said:
The COM doesn't move, the center of buoyancy moves -- laterally -- when the ship rolls. If the ship rolls right, the center of buoyancy moves to the right, providing a restoring torqure. That's what keeps the ship from flipping over.

See the wiki link I posted above for good diagrams.
CoM movement is the definition of stability. Yes, on a flat-bottom vessel or catamaran, the movement of center of buoyancy is what ensures it.

Catamaran is the best example. It's center of buoyancy can be approximated to simply jump from one float to the other. The CoM position as function of tilt angle can then be plotted as connection of two arcs about each of the floats. The cusp in the middle, providing a local minimum, is what ensures the stability. Movement of center of buoyancy is what ensures such a minimum exists despite high CoM.

K^2 said:
sophiecentaur, where CoM is located relative to support is irrelevant for question of stability. The question is whether CoM rises or falls when the boat is tilted. If it falls, the boat is unstable and will capsize. If CoM rises, when boat is tilted, the boat will be stable. It's not difficult to construct a ship whose CoM is above the water line, yet is entirely stable. I don't know if aircraft carrier is an example of such a ship, but it's entirely possible. A trivial example is pretty much any catamaran.
I agree with most of that about rising CM but in boats the range of angle for which CM rises, before it falls, is very relevant as a good boat design will require a large angle of limiting stability (I think that is the term). They have to be able to recover after a knock down.

angle of limiting stability (I think that is the term).

actually known as the angle of vanishing stability (AVS), the angle at which the righting moment becomes 0 and the boat will capsize.

Yes, of course. And with higher CoM, that angle is decreased, so low CoM is still preferable. No question about that.

Note: My mother tongue being French, I apologize for the quality of my english

I have aluminium boat with almost a flat bottom hull (very small ‘’V’’ shape.At speed over 28 mph the boat turn flat and slide. So the radius of the turn is very long. At speed under 28 mph the boat lean and turn shaply. To solve this sliding reaction I am wondering if a fin installed center of the boat could help to turn shaply? Yes if the boat height is to high the boat could flip over. For ex: the water ski specialize boats have fin…Correctcraft boats, mastercraft boats.

What do you think of this solution?

Good idea. I think a (small) skeg about half way along could help you a lot. I think that, above 28mph, the hull starts planing so high that the only contact with the water must be right at the back, so there is very little turning moment (steering effect) of the prop and the hull. The skeg will maintain a 'pivot' well in front of the outboard.
btw, if you have a heavy load at the front (keeping the nose down), does the boat turn better at speed? Certainly, a towed water skier would tend to lift the bow out of the water unless the tow is attached very low to the transom.

Thank you for the info.

Now I am going to visit dealerships with boat equip with small skeg and mesure the hight, length and the position (middle, forward or backward). Than with different boat builders there will be a ''tipic'' place for the skeg.

I think personnaly that the skeg should be in the middle of the 19 footer boat?? Just in front of the pylon were the tow is attach now. I will make sure that the tow is lower as possible just to clear the outboard motor.

For your info the boat nose stay low during skiing. I am not worried about boat flip over the width is 96 inches.

For me it is interesting to use physics to solve daily problem and experiment.

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