View Full Version : Transient Cavitation

08-06-2012, 03:00 AM
I've tacked 70 hours on my 05 XStar w/ 8.1L since I purchased it in April w/ 360 hours. We primarily wake surf behind the boat and load the port side as heavy as I feel comfortable depending on the lake conditions on any given day. I pigbacked a #1100 ProX sack in the rear locker, always fill KGB, and usually have one adult passenger and one adult surfing.

Often (but not always), when I attempt to accelerate from 10 mph to 12 mph, I feel a distinct vibration and lack of power that I'm attributing to the effects of prop cavitation. I've been throwing a #240 sack in the bow walkway to "level" the boat out and sometimes it helps and sometimes it doesn't. Cavitation seems to be at its worst when I turn steering wheel even slightly to the right of center when boat is sacked out port side. If I take my hand(s) off the steering wheel at any speed above 3 mph, the boat goes into a hard left turn whether loaded up for surfing or just cruising back to dock with ballast empty.

I searched Cavitation on the forum tonight and saw a few pics of rudder damage from cavitation, but I'm 99% sure rudder is completely clean (will confirm next trip out to dock). Stainless prop will spin with 2 fingers of pressure and doesn't appear to have any dings. How do I determine what prop is on my boat to possibly identify that as the culprit?

Any thoughts or ideas as to what is going on and why it is not consistently producing cavitation effects?

08-06-2012, 09:22 AM
There are many things that cause cavitation but I'm willing to bet that you issues are cause by the extra ballast.

Water is a funny thing because you can pull it apart. Think of your prop as a screw. the blades are the threads of the screw. and the water the wood that you are screwing into and the boat the weight that you are trying to support. To get a screw to hold it has to be the right size for the load and you want t it to be parallel to the load it is trying to hold. if you put too much weight on a screw it will strip out the wood.

When you increase the load (ballast the heck out of the boat) but the wood (water) has not changed something else has to change. This means that the screw (prop) needs to be bigger or you need more screws. Since we cant change the number of screws you are left with needing a different screw. Contact ACME or OJ or decrease the weight.

The next best thing would be to try an level the boat out a bit and see if there is any change in performance.

Oh yes and inboards are supposed to turn if you let go of the steering wheel. This is called a dynamic instability which designed into the boat, and Motorcycles too, to take out all the backlash in the steering linkage so it is more responsive to the drivers inputs. Without this it would be like driving grandpas old Cadillac where you could give the steering wheel and 1/4 turn either direction before anything happened.

08-06-2012, 09:42 AM
What he said. Try a different prop more specifically geared towards extra torque.

08-08-2012, 03:59 PM
http://img855.imageshack.us/img855/4245/ruddero.jpg (http://imageshack.us/photo/my-images/855/ruddero.jpg/)

Called ACME and they recommended a more aggresive prop. Will update post after new screw is on.

08-08-2012, 04:15 PM
let me know how it works outs.