Comment by efitz | Hacker News Reader

Comment by efitz | Hacker News Reader

Parent Full thread

It's complicated.

The rudder is a wing, it's just vertically oriented and underwater.

The rudder is capable of stalling, just like any wing. The rudder only produces lift related to the flow of water over the rudder. The lift produced by the rudder is what is experienced as turning force. The tiller or wheel changes the angle of attack.

I used to helm a racing sailboat with a high aspect (long & narrow) rudder. It could provide a lot of turning force but stalled easily. It didn't work as well under power as it did under sail; I suspect this was due to the turbulent flow off of the propeller, which was forward of the rudder.

On the Dali, the rudder should have been providing some turning force due to the 7+ knot flow of water over the rudder. Full reverse propeller might have impacted that; I can't comment because I've never helmed a ship that large.

Additionally, a single-propeller vessel like the Dali, will have "prop walk" - asymmetric thrust that pushes the stern of the craft one way or the other while the propeller is rotating.

I'm only a minimally experienced (coastal cruising) sailor so there's plenty of things I don't know, but this is the first time I've heard the rudder as a wing (lifting surface) rather than as a neutral control surface.

It sort of makes high-level sense that a lifting bias could in theory work as a counteraction to propwalk. But the terminology is a bit confusing because aerodynamic lift is a byproduct of air being a compressible medium, whereas water is not. Maybe lift means something different when we're talking about water?

At any rate in scenarios where the prop is not engaged, which in a sailboat is most of them, I don't think I've ever noticed a tendency for heading to track predominantly one way or the other, in circumstances where it seems that would be very pronounced and hard to miss, like extended running downwind. Is the lifting body rudder mainly a performance boat thing? Or perhaps am I just so used to trimming this bias out that I don't recognize where it's coming from?

> But the terminology is a bit confusing because aerodynamic lift is a byproduct of air being a compressible medium, whereas water is not. Maybe lift means something different when we're talking about water?

A https://en.wikipedia.org/wiki/Hydrofoil is a wing that produces lift in water.

Compressibility is only relevant in aircraft once they start getting near the speed of sound.

How about ground effect for helicopters (or hovercraft)?

Works just the same underwater. (Inviscid, incompressible). Low Mach number means compressibility is not significant. High Reynolds number means viscosity is not significant. Same for other dimensionless numbers and physical phenomena (Froude number etc).

Though note: compressibility is not the same as "exerting pressure". A hydrofoil or marine propeller works on pressure differences, even though the fluid is effectively incompressible.

For laypeople and casual sailors, thinking of the rudder as just redirecting water is good enough.

But water and air are both fluids and the same aerodynamic/hydrodynamic rules apply.

A rudder on a boat or airplane is symmetrical in cross section; the chord on both sides is equal. Wings and hydrofoils are asymmetrical; usually the “top” has a deeper chord than the “bottom”. But rudders are a still a kind of wing in that they generate a useful force by redirecting a fluid and thereby inducing a pressure differential. The pressure differential between the two sides is what causes lift - vertically with wings/foils and horizontally with rudders. If you think about it, this makes perfect sense – it would be silly to think that rudders and ailerons and elevators obey different laws than wings. In fact, one of the first things you learn as a racing sailor is that the sails themselves are wings - they’re not “parachutes” as commonly believed.

Anyway, my point was that you can stall a rudder just like an airplane can stall its wings- if the angle of attack is too high.

Stalling, the rudder, most commonly occurs during a round up, if you’re familiar with sailing with spinnakers. The rudder in that case is no longer able to generate enough lift (turning force) To counter the turning force, imparted by the force on the spinnaker forward of the center of mass of the boat, and the boat turns uncontrollably.

I am just speculating, but if the rudder became very misaligned compared to the direction of the ship, then when power was restored, the rudder might not be able to establish laminar flow and therefore would be stalled and unable to provide turning force.

It might be confusing to call the rudder a wing though, since there is no lift at 0 attack angle.

True. The main reason I keep calling it a wing is to reinforce the concept that it works by producing lift.

To your last point - sailboats move in response to the sum of the forces on them. Most sloops are designed to make it easy to balance the forces of the jib and the main, so that the center of effort is near the center of mass. If the sails are trimmed in this fashion you need very little rudder input - in fact it is easy to steer the sailboat with just the sails, if they are well trimmed.t If you let the jib a little out of trim, the force it imparts on the boat will decrease, and since the force on the main is constant, you’ll turn to windward. Likewise you can ease the main to turn to leeward if the sails are balanced and the rudder neutral. Handy trick if your rudder gets damaged.

An interesting note is that well-designed sailboats are designed to round up in the case where the headsail gets overpowered. As the sail gets overpowered, the boat will naturally try to turn away from the wind and also will heel further. The heeling of the boat will cause the headsail to lose lift and spill air before the main sail does, and the keel, which normally generates lift and drag to counter sideslip, combined with the now-more-powered mainsail, Generate a strong leeward yawing force aft of the center of mass which causes the boat to turn sharply into the wind. Rounding up only happens when you are losing control of the boat so turning into the wind ends up putting you in a safe mode where you can recover. There are a few boats out there where the keel is too far forward and/or the main spills air before the headsail; these boats round down in uncontrolled situations and can jibe unexpectedly – very dangerous.

>aerodynamic lift is a byproduct of air being a compressible medium

This is not correct.

I wonder if the "prop walk" effect might be stronger in shallow water.

Reply on news.ycombinator.com