Boats crash/break and can kill their passengers when falling certain distances

Also breaks falling block 179 to 64.

again in 19w37a,
I have some details:
In a command block, you can summon a boat
summon minecraft:boat ~ ~y ~
where y = 12.5, 13.5, 49.5, 51.5, 111.5, 114.5, 198.5 or 202.5 (work with a tower)
(0.5 because a command block summon in the middle of a block)
Or you can run /data modify entity @e[type=boat,sort=nearest,limit=1] Motion[1] set value 1.0
(1.0 corresponds to 13 blocks, and 0.9 to 12)

Confirmed in 1.16.2

Notably, the boat breaks into 3 planks and 2 sticks like before 1.3.1. Might be somewhat related to MC-89954  - should boats even break like that at all?

The height appears to correspond to ((5x)²±x)/2

I tested this after seeing a reddit post on this bug.   I was able to reproduce it in 1.16.3 when falling from exactly 49 blocks, the player dies and the boat breaks into planks & sticks.

I saw the same Reddit post and decided to investigate. I skimmed MCP 1.12 because I still have it lying around and that version is affected.

In what it calls EntityBoat’s updateFallState, if the boat’s status is not ON_LAND, fall damage is nullified. That will be the case when falling from most heights, because the last time the boat’s status was updated was before it hit the ground. There is only a small exception because the boat checks blocks up to a thousandth of a block below it when updating its status. If it ends up just barely above the ground, its status becomes ON_LAND before hitting the ground, allowing fall damage to take effect.

This happens at the given heights because gravity subtracts 0.04F from the boat’s Y velocity. If it were exactly 0.04, the boat would have fallen an integral number of blocks every 2 ticks out of 25: 12, 13; 49, 51; 111, 114; 198, 202; and so on. The boat ends up falling very slightly less than that because 0.04F is rounded down as a 32-bit float, placing it just above the ground and allowing it to become ON_LAND before hitting it.

Can confirm in 21w03a.

Can confirm in 21w05b.

have done the math behind this and the numbers are matching up to a pattern, however for some reason 12 and 13 aren't breaking the boat (at least for me). If the World height increases the chance the next two numbers that will cause this are 310 and 315. I assume this is more of an issue with JAVA than with Minecraft, however you could possible make a "fix" by making the boats invulnerable in a way at these numbers.

Can confirm in 21w06a.

Can confirm in 21w07a.

Can confirm in 21w10a.

Can confirm in 21w20a

Can confirm in 1.17-pre1.

Can confirm in 1.17.1 Release Candidate 2.

works in 1.17.1

Can confirm in 1.17.1.

Can confirm in 21w41a.

Can confirm in 21w44a.

I can also confirm this behavior in 1.18.1.

Here's a code analysis of this issue. The following is based on a decompiled version of Minecraft 1.18.1 using MCP-Reborn. Please note that I'm quite tentative about this analysis, although I'm persuaded to believe that this is likely the cause of the problem. 🙂

Code Analysis (tentative):

net.minecraft.world.entity.vehicle.Boat.java

public class Boat extends Entity {
   ...
   protected void checkFallDamage(double p_38307_, boolean p_38308_, BlockState p_38309_, BlockPos p_38310_) {
      this.lastYd = this.getDeltaMovement().y;
      if (!this.isPassenger()) {
         if (p_38308_) {
            if (this.fallDistance > 3.0F) {
               if (this.status != Boat.Status.ON_LAND) {
                  this.resetFallDistance();
                  return;
               }

               this.causeFallDamage(this.fallDistance, 1.0F, DamageSource.FALL);
               if (!this.level.isClientSide && !this.isRemoved()) {
                  this.kill();
                  if (this.level.getGameRules().getBoolean(GameRules.RULE_DOENTITYDROPS)) {
                     for(int i = 0; i < 3; ++i) {
                        this.spawnAtLocation(this.getBoatType().getPlanks());
                     }

                     for(int j = 0; j < 2; ++j) {
                        this.spawnAtLocation(Items.STICK);
                     }
                  }
               }
            }

            this.resetFallDistance();
         } else if (!this.level.getFluidState(this.blockPosition().below()).is(FluidTags.WATER) && p_38307_ < 0.0D) {
            this.fallDistance = (float)((double)this.fallDistance - p_38307_);
         }

      }
   }
   ...

It's intended behavior that boats can prevent fall damage when riding them as stated in MC-105103, and the assigned "Mojang Priority" of this ticket (MC-119369) implies that boats should not occasionally be able to break into sticks and planks, however, if we look at the above class, we can see that checks are specifically carried out to make them behave this way, resulting in this problem occurring.

Can confirm in 22w12a. Additionally, if this issue affects a boat with a chest, the chest's contents disappear entirely from the world.

@unknown That might be worth a separate report.

Boats with chests was already added here, ampolive, and fixing this resolves that (and this has priority, so not intended to happen).

I will just add that to the description then.
Edit: well, that was reported as a separate issue anyway (MC-249501).

The presence of MC-249501 definitely makes this issue more important to fix. Boats breaking and players dying from fall damage are acceptable risks, but losing all items in a boat with chest is much more punishing in comparison, especially since MC-105103 is intended.

Boats can also break like this when driven out of the world border.

Decimal heights at which boats fall and break are given by the equation x(x+1)/50.

Can confirm in 1.19.

confirmed in 1.19, but doesnt work on 12 or 13 blocks. havent tested other heights except 49 blocks yet. Also, I dont die from the fall, as suggested in the description.

@railgun290 12 (and probably 13) blocks works fine for me in 1.19.2 if I'm not in the boat when it falls.

Can confirm in 1.19.2.

If a player is in the boat, it doesn't break for 12 and 13 blocks. I have tested 49 blocks and the boat break (the player die too).

For 12 or 13 blocks, the boat break when (no player in the boat) :

• breaking the block below (12 blocks high)

• pushing the boat with a piston horizontally (12 blocks 

• setting the y motion to certain values :

/execute as @e[type=boat,distance=..5] run data modify entity @s Motion[1] set value 0.6

Can confirm for 22w42a.

Also happens with rafts.

Can confirm this happening on 1.19.51 on Bedrock. I do not see any logical reason for this to be happening

@unknown, this is the Java Edition issue tracker used for exclusively tracking Java Edition bugs. Please head over to the MCPE (Bedrock Edition) tracker and address the problem accordingly over there if it hasn't already been reported yet. 🙂

Can confirm in 1.19.3

Can confirm in 23w03a

Can confirm in 1.19.4

I was intrigued by this. Adding to @Avoma's analysis, the problem is that there is a discrepancy between the 
onGround argument, which is true when the boat has landed, and this.status. The status is updated every tick, to either IN_AIR or ON_LAND (for the relevant problem; other values are possible otherwise).

So most of the time, we're entering checkFallDamage with onGround true, and the boat's status says it is in the air, so we reset the fall distance and do nothing else. But due to timing, in some situations the tick update has managed to set our status to ON_LAND, and we go on to do the "deal damage to passengers, replace boat with planks and sticks".

The correlation with heights is therefore more incidental; the real cause of when this happens is how the call to checkFallDamage correlates to how the last tick has set the status to ON_LAND.

The solution though, seems simple enough. The replacement of the boat with planks and sticks is a remnant from a very old behavior. There is no need to check anything else apart from the onGround argument, and then always reset the fall distance, regardless of the value of this.status.

One could hope that a Minecraft engineer gets to spend a few minutes on fixing this.

Can confirm in 1.20, just died to it 😞

Appears to be still an issue in 1.20.1, though my case specifically didn't have my boat (spruce boat with chest to be specific) fall as such. I rowed my boat at a high speed onto a "boat gate" which detects the boat using tripwire and pushes it up with a piston and a block of packed ice so it can move across the frozen river. Once the piston fired, the boat glitched into ice and broke into planks and sticks a few moments later.

Definitely still an issue in 1.20.1. I've tested it on a sandbox using pistons to push the boat off certain heights (tested with 12, 13, 49, 51 blocks). Curiously, the boat only breaks when a mob (tested on husks, pigs and chickens) is inside, but it doesn't when only boarded by the player. If both the player and a mob are inside, the boat breaks only after the player exits it. In all tested cases, the mob receives fall damage (unless it's unable to, as with chickens), but the player doesn't, even in Survival.

Also, the boat breaks and the mob inside breaks its limbs before it actually lands; for a 51-block fall it happens around 15-20 blocks above the ground.

UPD: If I recall correctly, when the boat glitched through a moved block (see my previous comment), I quickly dismounted the vehicle, and only then did it break. I've quickly tried pushing an empty boat, and it likewise broke. Once again, it didn't break when the player was inside, nor did it break once the player left it after landing.

It also happened in my world. Howerer the altitude difference was far lower than 12. Maybe that's a decimal.

A full mathematical analysis explaining why this happens is presented on the YT channel Stand-up Maths in this video.

This does not appear to be occurring on 24w13a for me…

Cannot reproduce as well in 24w13a

I am not able to reproduce in 24w13a if I'm inside the boat, but I am able to reproduce this by summoning a boat 12 blocks above the ground, so the core issue isn't fixed.

Hey ampolive! I helped Matt (who made the video) with testing this and understanding the decompiled code. As you mentioned, the easiest way to reproduce is using the summon command or placing a boat on a floating block and breaking it, without riding as a passenger. Vehicle mechanics when there’s a passenger seem to change things, but I could still cause player damage when riding the boat if it was dropped from 49 or 51 blocks - and then I moved forward a little on the ground or dismounted, depending on the version.

I haven’t tried in the latest snapshot but if nobody beats me (I’m sure they will!) I can try some of these variants and report back tomorrow.

Please someone explain the code to me , its driving me crazy.

in this line
this.status != Boat.Status.ON_LAND
this.status is referring to the player right ? what i dont understand is that when the player is on the ground it would continue and deal damage and when he is in the air it would reset the fall distance, why is the condition with boat.status.ON_LAND, why not just ON_LAND or boat.status? what is p_38308_ ?, why is it checking if the player is on the boat?
if (!this.isPassenger()

when is  CheckFallDamage called?

why does this even exist if boats or players shouldn't even take damage from falling.

just add a condition that if the player is on a boat he doesn't take damage from falling
 

Can reproduce in 1.20.4, from these heights:

49 blocks - inside and outside

12 blocks - outside, but not inside

I found it also works when boat is pushed up by piston into string connected to tripwire hook and then it slowly falls and breaks.

Just in case people havent seen yet, theres an in depth explanation on the youtube channel:
Stand-up Maths
I will shortly explain here as well. The problem occurs because of the way gravity works in minecraft, you fall 0.04 block/tick^2, which means every second your falling speed increases with 0.04 blocks, however, since values are stored differently when it comes to computers its actually 0.03999999910563 or something like that, i dont remember the exact number, however the point is that the part of the code that checks for fall damage is now seeing, that even though, to you, you are in the air, according to the code, you are very close to a block which the code sees as actually hitting a block and so you will take fall damage. I highly recommend watching Stand-up Maths video for a more visual and better explanation.
In short it might be smart to add a condition that checks if you are in a boat and then doesnt do checkfalldamage.

Else you need to rethink the way checkfalldamage works.

Or one more solution may be to change the gravity acceleration value to something that can be stored as its actual value, this wouldnt really be noticeable.

Also affects 24w14a.

As AlexHolmbrg commented, there's two easy solutions. This should be fixed quick: Don't check fall damage if on boat. Also change gravity value to a number that doesn't need to be rounded for better code and less problems like this in the future

I noticed that with this bug, not only do I get fall damage, but my boat breaks into planks and sticks. um... is this vanilla behavior? I know that this could happen on very old versions, but now this is impossible, the boat only can burn out in lava or drop as an boat item, but it cannot break into planks and sticks! It seems to me that this is not a bug, but the developers forgot to remove some old code...

Also affects in 1.21-pre1

How MC-249501 has been fixed without fix of this? Why did they take on that bug almost instantly, but didn’t even notice that STR was based on another bug? And why weren't both fixed?

I’m completely confused because I’m following what the developers are fixing for 1.21: they fixed two bugs related to The End dimension. I don't understand this at all. Why fix these things in The End is prioritized if players are more likely to use the boat than go to The End dimension?