Opposite forces aren't canceling out

Development Programming & Scripting
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1

I’m trying to figure out an odd bug that’s occuring on my project. It seems like opposite forces aren’t canceling out like they should, causing the agent to move around in sporadic cyclic patterns. The goal is to use AddForceAtLocation to simulate buoyancy by calculating the buoyant force, and applying it to the center of volume, and gravity to the center of mass. This was working in unreal 4.27 but when we moved to UE5.3 we saw this bug begin to occur. I can’t find any reason from the documenation, but assume something with the changed physics engine and our code is causing this to break.
I verified with log statements that the force vectors are equal and opposite.

 FVector ActorLocation = GetActorLocation();
	FRotator ActorRotation = GetActorRotation();

	// Check to see how underwater we are
	FVector* points = SurfacePoints.GetData();
	int count = 0;
	for(int i=0;i<NumSurfacePoints;i++){	
		FVector p_world = ActorLocation + ActorRotation.RotateVector(points[i]);
		if(p_world.Z < SurfaceLevel)
			count++;
	}
	float ratio = count*1.0 / NumSurfacePoints;

    // Get and apply Buoyant Force
	float BuoyantForce = Volume * Gravity * WaterDensity * ratio;
	FVector BuoyantVector = FVector(0, 0, BuoyantForce);
	BuoyantVector = ConvertLinearVector(BuoyantVector, ClientToUE);
	// BuoyantVector = GetActorRotation().RotateVector(BuoyantVector);

    FVector COB_World = ActorLocation + ActorRotation.RotateVector(CenterBuoyancy + OffsetToOrigin);
	// DrawDebugLine(GetWorld(), COB_World, COB_World + BuoyantVector, FColor::Green, false, 1./30, ECC_WorldStatic, 3);
	RootMesh->AddForceAtLocation(BuoyantVector, COB_World);
	UE_LOG(LogHolodeck, Warning, TEXT("Buoyant Force x: %f y: %f z: %f"), BuoyantVector.X, BuoyantVector.Y, BuoyantVector.Z);

	FVector GravityVector = ConvertLinearVector(FVector(0, 0, -Gravity*MassInKG), ClientToUE);
	GravityVector = GetActorRotation().RotateVector(GravityVector);
	RootMesh->AddForceAtLocation(GravityVector, RootMesh->GetCenterOfMass());
	UE_LOG(LogHolodeck, Warning, TEXT("Gravity Force x: %f y: %f z: %f"), GravityVector.X, GravityVector.Y, GravityVector.Z);
2

Update: I’ve now hit a point where if I start it below water, and don’t apply any other forces to try and move the vehicle it sits stably, but once I add another force to try and drive the vehicle it once again unstably bounces around extremely rapidly in unexpected ways.

this is current version of that function.

// ShowBoundingBox(.1);
	// ShowSurfacePoints(.1);
    //Get all the values we need once
    FVector ActorLocation = GetActorLocation();
	FRotator ActorRotation = GetActorRotation();

	// Check to see how underwater we are
	FVector* points = SurfacePoints.GetData();
	int count = 0;
	for(int i=0;i<NumSurfacePoints;i++){	
		FVector p_world = ActorLocation + ActorRotation.RotateVector(points[i]);
		if(p_world.Z < SurfaceLevel)
			count++;
	}
	float ratio = count*1.0 / NumSurfacePoints;

    // Get and apply Buoyant Force
	float BuoyantForce = Volume * Gravity * WaterDensity * ratio;
	FVector BuoyantVector = FVector(0, 0, BuoyantForce);
	BuoyantVector = ConvertLinearVector(BuoyantVector, ClientToUE);
	BuoyantVector = GetActorRotation().RotateVector(BuoyantVector);

    // FVector COB_World = ActorLocation + ActorRotation.RotateVector(CenterBuoyancy + OffsetToOrigin);
	// DrawDebugLine(GetWorld(), COB_World, COB_World + BuoyantVector, FColor::Green, false, 1./30, ECC_WorldStatic, 3);
	RootMesh->AddForceAtLocationLocal(BuoyantVector, CenterBuoyancy+OffsetToOrigin, "None");
	// UE_LOG(LogHolodeck, Warning, TEXT("Mass of agent: %f"), RootMesh->GetMass());
	UE_LOG(LogHolodeck, Warning, TEXT("Buoyant Force x: %f y: %f z: %f"), BuoyantVector.X, BuoyantVector.Y, BuoyantVector.Z);

	FVector GravityVector = ConvertLinearVector(FVector(0, 0, -Gravity*MassInKG), ClientToUE);
	GravityVector = GetActorRotation().RotateVector(GravityVector);
	RootMesh->AddForceAtLocationLocal(GravityVector, CenterMass+OffsetToOrigin,"None");
	UE_LOG(LogHolodeck, Warning, TEXT("Gravity Force x: %f y: %f z: %f"), GravityVector.X, GravityVector.Y, GravityVector.Z);
3

Upon further debugging, it seems like it’s an issue with forces applied off center in specific, and the torque messing things up.
This also doesn’t occur in packaged builds, just when testing in the run in standalone mode.