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Comment by jcgrillo | Hacker News Reader

I'm not sure what configuration of props a ship like this has, but in my experience with a 40ft sailboat with a single propeller you have absolutely no rudder authority while reversing. I've read that some large ships also are direct drive--there's no transmission between the engine and the propeller, so "reversing" (if it's even possible) entails shutting down the engine and restarting it in reverse. This can be done with a two stroke engine. And yes, 8.5kt is not slow when you're displacing 100k tons, no correction will happen quickly.

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>entails shutting down the engine and restarting it in reverse. This can be done with a two stroke engine.

Funny, I was starting a 2 stroke chainsaw a couple of years ago. I yanked the cord, it kicked back, pulling my arm back down but the saw started up and ran anyway. But it would not cut at all. I killed it and restarted it and noticed the chain going the other (right) way and it was now cutting fine. It has started in reverse the first time!

This is actually common for a 2 stroke type engine and used in things like gas golf carts and snow machines. The direction the engine is started is the direction it runs.

So to accomplish this for propulsion, you'd add a reverse polarity switch for your starter and you're good to go.

You can even get a (2-stroke) Detroit Diesel to run in reverse.

You don’t want to do it very long, because they don’t make oil pressure in reverse.

> You don’t want to do it very long, because they don’t make oil pressure in reverse.

US energy policy has entered the chat.

Yeah, in theory, there's not a lot about an engines running direction that matters, its all the stuff that's hooked up to it.

As you start getting into more complex engines, where you have fuel injection, timed spark plugs, oil pumps, transmissions it becomes untenable, but not because a v6 can't run backwards but because you'd have to rebuild and time all the support systems.

A low speed marine diesel like you'll find in a large container ship is not your granddaddy's 6V71. They are complicated, yet simple. They are indeed direct drive (possibly through a reduction gear) and they do indeed operate both in forward and in reverse. See e.g. [1].

[1] https://chiefengineerlog.com/2022/06/24/main-engine-fixed-pi...

Yes, defiantly not saying that's not how these work, and I do know that large boats tend to be direct drive. I'm just pointing out that most engines could do this and the limitation is generally how it interfaces with the external components and not as much a function of internal combustion type engines.

For example, a transmission would expect the engine to always run in the same direction, but given correct fuel and ignition timings, it doesn't really matter which way the crankshaft is turning while things are running.

I accidentally installed an injector pump 180° out of time on a MB OM617 and it "ran" kinda.. with a massive white smoke screen. I was shocked it even started.

> my experience with a 40ft sailboat with a single propeller you have absolutely no rudder authority while reversing.

I have to protest here. Reversing and using the rudder on a 40ft boat works perfectly fine. I've done it on multiple sailing boats. You need to hold tight so that the rudder wont slap you if using a stick.

If you have enough way on to combat the prop walk, yes. But in a situation like backing down trying to come to a stop (at least in my boat, a 1962 Block Island 40) there's a very long "dead time" while transitioning from slowly moving forward to slowly moving in reverse where the rudder just doesn't do anything. The way I maneuver in these situations is to do all my heading corrections in forward gear, where prop wash over the rudder gives it authority. So it's a game of shots of reverse, corrections, rinse, repeat.

EDIT: also planning ahead is important, because if I do it right the prop walk in reverse can be used advantageously.

Also, with the BI-40's barn door rudder it'll slap you through the wheel if you're not careful. Almost broke my leg that way, not a lesson to forget.

Nice boat! Ye it does not look too nimble in harbours. Somewhere over 40ft with "light" boats is where I feel you get into the "you got one chance" harbour manouvers (unless there is some front sideways motor cheating).

It happens something that I really can't explain, but I guess it is weight related. Or maybe area. Dunno.

Yeah it's right around 20000lb displacement, so momentum is a real thing ;). The fiberglass is over 2" thick at the keel tapering to a mere 1" at the hull to deck joint. Decks are solid glass as well. At the time it was a newfangled material and they were scared of it so they used a lot.

Also, the bow seems to catch the wind really hard so you can get spun around if it's blowing and you head off the wind too much without enough way on. Leave room, plan ahead, have a backup plan, etc.

Isn’t there a pretty big difference in how much rudder authority you get between just making way astern, and having the engine in full reverse while still traveling forwards at seven knots?

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.

>I have to protest here. Reversing and using the rudder on a 40ft boat works perfectly fine. I've done it on multiple sailing boats. You need to hold tight so that the rudder wont slap you if using a stick.

this is a stupid argument from all users , comparing a 40ft sailboat to other 40ft sailboats is like making a broad assumption about cars when comparing a dragster to an SUV simply because they have a similar singular aspect.

ANY sailboat, regardless of size, can have poor reversing performance just simply due to its hull shape and rudder plan. Some boats famously cannot reverse with authority due to the turbulence created between the parts. Some boats use feathering propellers which collapse inwards when reverse is applied in order to make a more efficient sailing experience, which effectively makes propulsion reversing useless.

There is NO generalized '40 foot sailboat' in existence. They all act different, and there are some that are perfectly capable of doing what other 40 foot plans might consider IMPOSSIBLE.

This reminded me of an old physics book I read where author claimed that Titanic also lost rudder control and yawed because of full reverse.

Titanic was a “triple screw steamer,” meaning it had three propellers - port, center and starboard. The port and starboard propellers were capable of being reversed, while the center propeller could only propel forward. You can see this in the 1997 movie as they show the props when they throw the ship into reverse.

The center prop (that only goes forward) is the one directly behind the rudder, so the theory is partly that they lost some steering advantage when they lost the center prop.

> in my experience with a 40ft sailboat with a single propeller you have absolutely no rudder authority while reversing.

In general it depends on the rudders and the boat.

Longer keeled boats don't respond well in reverse at all but more modern boats (like mine, 1990) will do better but will still need some way to have steerage. I can certainly manouver around the marina in reverse, it's just harder than forwards and I need to be going a bit faster to get the control.

Long keeled boats don't respond well going forward either, right? Compared to flater boats with a modern keel.

All I really know about long keels is from what people have said. They tend to track well and don't tend to make as much leeway, but perhaps at the expense of speed due to the wetted area, and they are hard to steer backwards. Not being particularly manouverable forwards isn't really an issue if you're spending several hours going mostly in a straight line.

Modern flat boats (like the 2017 Dufour I learned on) are highly manouverable at slow speed, we practiced spinning the boat on the spot by using prop wash over the rudder forwards then ticking over in reverse. Could turn the boat in not much more space than the boat length, but may not track as well, may slam more, and make more leeway.

Going backwards in the marina I often steer on the engine rather than the rudder (though I keep the rudder aligned with the engine of course). Obviously that's in a tiny sailing boat with an external engine, but I thought large ships also often have a steerable front propeller to assist with steering and mooring. Although maybe these very large ships use tugboats for that.

an outboard? that is very very different because you control the direction of thrust also.

My boat has an inboard diesel so no ability to direct the prop. It does have a bow thruster, but it's only really used at slow speed, usually right at the point of docking and undocking in tight spaces, once you get the boat moving in forward or reverse you don't need it.

I have no idea about container ship sized boats, though I'd imagine a bow thruster of steerable prop might not be practical at that scale.

Well if you keep rudder aligned with the engine (i.e. parallel) you are really using both, not just the engine.

Since they were still moving forward while gunning it in reverse the rudder would still operate normally. They generally have bow thrusters too. I have no idea whether they could have been operational with the broader power/engine failures but if they were available I'm sure they were being used as well.

The question is what is the velocity of the water moving over the rudder? If forward velocity and the current due to a reversing prop cancel, then the rudder can't do anything.

I docked in reverse multiple times, same as parking a car in reverse. Just need water flowing along the rudder (from motion, not prop wash), but otherwise it's a great way to turn into tight spaces. Gotta be careful about prop walk, which will dominate the controllability until some reverse speed is established. So until you've decelerated to 0 and re-accelerated in reverse, you don't have much control beyond prop walk.

No variable-pitch propeller?

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