Floating in water

How can I make a rigidbody float up and down in water?

 

Lots of ways to do this, depends how you've created your water waves. Assuming it's a mesh that's being manipulated through script, take an invisible "surface object" (a plane or box with collider and rigid body) and attach your floating object to it with a spring joint. That way your floating object reacts in a very realistic manner: you can simulate heavier objects with looser springs, etc.

The other way is to do it with script, and I think Yoggy's boat demo a while back did this.

 

Another solution is to use the physics system to add buoyancy forces when the object enters a trigger. I'm using this method with a "buoyant object" script. Any rigidbody entering a trigger tagged with "fluid" has buoyant forces added based on the objects volume. This code is based on an example that was posted on the forums quite a while ago, though I've changed quite a bit after many revisions.

Code (csharp):

1.  
2.  
3. // density here is killograms per cubic meter. Unity's world units are meters and mass is commonly assumed to be killograms
4.   // Density of wood: 600 kg / cu.m
5.   // Density of water 1000 kg / cu meter
6.   // Density of iron 7000 kg / cu meter
7.   // densityOfBody = b.mass / curBodiesVolume[i];
8.  
9. var divider = 1.0;
10.  
11. var airborneDrag = -1.0; //If left at -1 will use the Rigidbody Drag
12. var fluidDrag = 0.0;
13.  
14. private var fluidDensity = 1000.0;
15. private var displacement = 0.0;
16.  
17. private var myRenderer : Renderer;
18. private var myRB : Rigidbody;
19. private var myColliders : Array;
20.  
21. private var timer = 0.0;
22.  
23. function Start(){
24.     var renderers = transform.root.gameObject.GetComponentsInChildren(Renderer);
25.     var rendMax = Vector3(0,0,0);
26.     for (var ren : Renderer in renderers){
27.         if(ren.bounds.size.sqrMagnitude>rendMax.sqrMagnitude){
28.             rendMax = ren.bounds.size;
29.             myRenderer=ren;
30.         }
31.     }
32.        
33.     if(myRB==null) myRB = GetComponent(Rigidbody);
34.     if(myColliders==null) myColliders = GetComponentsInChildren(Collider);
35.     for(var col : Collider in myColliders){
36.         displacement += col.bounds.size.x*col.bounds.size.y*col.bounds.size.z;
37.     }
38.     displacement = displacement/divider;
39.    
40.     if(airborneDrag<0)airborneDrag = myRB.drag;
41. }
42.  
43.  
44. function OnTriggerEnter(c : Collider){
45.     if(c.tag=="fluid") myRB.drag=fluidDrag;
46.    
47. }
48. function OnTriggerStay(c : Collider){
49.         if(myRenderer==null)return;
50.         var timetemp = Time.realtimeSinceStartup;
51.         if(c.tag=="fluid"){
52.             var surfaceYPos = c.transform.position.y;
53.  
54.             var bottom : Vector3 = transform.position;
55.  
56.             for(var col : Collider in myColliders){
57.                 if(col==null) continue;
58.                 var temp = col.ClosestPointOnBounds(transform.position-Vector3(0,100,0));
59.                 if(temp.y<bottom.y) bottom = temp;
60.             }
61.                        
62.             var bottomToSurface : float = (surfaceYPos-bottom.y);
63.             var bottomToCenter : float = (bottom - myRenderer.bounds.center).magnitude;
64.            
65.             var pctSubmerged : float = Mathf.Clamp01(bottomToSurface/(bottomToCenter*2));
66.             var force : Vector3 = displacement*fluidDensity*pctSubmerged*-1*Physics.gravity;
67.             bottom = Vector3.Lerp(bottom,transform.position,0.75);
68.             myRB.AddForceAtPosition(force,bottom);
69.         }
70.         timer += Time.realtimeSinceStartup-timetemp;
71. }
72.  
73. function OnTriggerExit(c : Collider){
74.     if(c.tag=="fluid")  myRB.drag=airborneDrag;
75. }
76.  
77.  
78.  

 

Thanks!