Exhaust gases and cavitation question

[buckrogers]

My 19' Stingray ran fine last season. After winter storage, I am having a problem with cavitation (or ventilation). ... it was winterized and wraped.

I have the Mercruiser 3.0 with a stainless steel prop. Also have smart tabs on.

Last year I could get 47mph with the trim slightly up. This year I can bearly reach 30mph when it starts to porpoise. The only way to get along is with the trim about 50% down. At level trim I can hear the prop starting to slip at around 3000rpm.

My question: I don't see any gas escape through the prop when the motor is idling in the water. Outside the water I can feel gas blow out the prop. Even when I rev the motor, I still don't see gasses out the prop in the water. Everything blows out the side vents on the stern. I checked the exhaust bellows and it has no cuts and both clamps are tied down.

I also checked the lower unit drive by placing it in gear to see if the prop would slip. That looks fine. I even adjusted the tabs one level up. No difference.

My theory: To much gas blows out the stern vents and at 3000rpm it aerates the water in front of the prop so it starts ventilating. Is this possible? I'm grasping at straws here. Any ideas how to fix it?

 

[Fun Times]

Re: Exhaust gases and cavitation question

A few things come to mind first, Have you installed anything new on the out side of the boat that could aerate the path of the prop? Does the SS prop you are running have little black vent hole covers on the front side of the prop that help vent the prop to make it run better or wores in some cases? Maybe the vents are now missing. And also where you trying the boat with the same amount of weight this year as last year? One other test would be to get nail polish and Paint a line from the prop nut to the prop. Run the boat until you get a spin, then check the prop...if out of allignment it's the prop hub going bad.And yes exhaust gas can play in to this but I don't think that has happend in years But you never know. Because you say it now starts to porpoise, You also could have a problem with the bottom of the boats hull it's self. With either a hook or a rocker somewhere at the rear of the bottom of the hull Some times under the trailer bunks and some times not.Well just a few other thoughts to think about is all.Good luck with it.