New Boats 2005

wave rider, re: cavitation.

I presume the air gaps on a rudder which you are refering two are when the waters surface drops down one side of the blade as you turn quickly.  This is not cavitation, what is happening (in simple terms) is that as you turn sharply the water cannot get to that side of the rudder fast enough to keep the surface at the same level, and so the surface drops down the foils length.  This is not good, partly because the rudder looses effectiveness as a result of the lost area in the water, it also puts higher loads on the blade and finally it creates a lot of drag which is not good for boat speed in a race. 

Cavitation is completely different an in actual fact has nothing to do with air.  Its a bit complicated so i will try and keep it as simple as i can. 

Cavitation is caused as a result of a sharp pressure drop in water flowing over a surface such as a hydrofoil or propeller.  As water flows past a foil the flow on one side speeds up, this increase in flow rate over the surface causes a drop in pressure on that side of the foil.  Now, as the pressure of a fluid (like water) decreases so too does the temperature at which it boils. 

Cavitation is a special case where the drop in pressure is so much that the water is able to boil at its current temperature, and does so instantly at the point of the pressure drop.  This creates the bubbles you see on the foil's surface.  The bubbles contain, in effect, steam.  The reson these bubbles dissapear is that the pressure drop is not as big over the whole foil and so the pressure of the liquid water around the bubble forces it to colapse and the steam condences back into the water around it. 

Below is a pic of a propeller cavitating.  the big bubble on the blade is created exactly how i described above, the bubbles on the shaft are created in the same way except that the presure drop is caused by the water flowing over the lip in the shaft from where the propeller is connected. 

The final bubbles are forming an arc coming off the tip of the blade.  These are again caviattion, but here the pressure drop cauing then is caused by a vortex creted by the blade.  In the middle of the vortex the water is also moving very fast and again the pressure drops here so much that the water cavitates.  The vortex is created because the water on the back of the blade has a higher pressure that the water infront of it.  The water 'wants' to try and even out the pressure and some of it tries to flow (leak) around the tip of the blade in order to do this.  Because of the water flow past the blade this casuses the 'leaking' water to form a vortex. 

I hope this is not to confusing.  if it is let me know and i'll try and clear things up.  If you are interested this is an interesting article on how people are trying to use cavitation to their advantage...  http://www.subsim.com/ssr/page33.html 

Yeah i guess it would slow yo down a fair bit would'nt it !

By the way is cavatation the air gaps the rudder makes?

If you try hard you can make a topper shine...................I am in the process of sanding the foils on my Topper to get ri of the scratches which someone said i should do !

I guess i don't want extra drag !

nah anything other than roto moulded doesnt count as plastic in my books