There isn't any way to cavitate a 24P SST Laser II with 150 hp Merc on the Ranger boat trimmed all the way down and not be operator or set up error. The only other possibility is the prop hub is spun, meaning the rubber clutch is bad. The rpm's go up with the boat moving faster trimmed down. This isn't cavitation. 

As I recall does the owner have a problem getting the boat to perform? Some boat/engine combos can be a little tricky and need to be finessed by using thr throttle backing off power until the boat is on plane then adding trim up and power as needed.

Tom

 

I thought once a hub was spun it was always spun. No?

Does it just slip from time to time. Again, unless there is something wrong with the trim that is not allowing it to be trimmed all the way down, I trimmed it until it stopped and never touched it when the cavitation began. Now, whether it's set up error or not is what I'm trying to figure out.

Check and see it the steel bar like they use on manually tilted motors is in and  it's in the very bottom holes.

If you know anyone with prop that will work on your motor, see if you can borrow it and try it, even if it a pitch or two off, all your wanting to do is test the hole shot.

As stated before, maybe the prior owner had extra weight in the boat or the weight not distributed. How many people/weight/ were in the boat on your test run? Weight can change everything, I know it makes a huge difference on my boat. Next time out, clean out the boat and take just the essentials to see if there is a difference. Good luck, always fun to have a new to you toy to play with.