Ventilation?

[dt4ever]

2008 Bayliner Discovery 215 with Mercruiser 5.0 alpha one leg with Apollo SS 17 x 141/4 and smart tabs. The boat gets on plane fully trim down quick and max out at 4400 RPM. At 3000 rpm cruise, if I trim out pass 1/4 from fully trim down the engine will start revving up slightly sometimes a bit violent but no loss in speed. Is this an example of prop ventilation? Prop and hub are brand new.

Any advice is greatly appreciated.

 

[cptrick3]

You said new prop. Did it do this with the old prop?

 

[dt4ever]

cptrick3,
Old prop was Mercury 19 pitch aluminum and the previous owner put hydrofoil on the alpha leg. WOT rpm did get 4800 with the old prop. I replaced them with this new prop and smart tabs. Never tried to trim out with old prop on so I have no idea.

 

[cptrick3]

I'm thinking that your gage may not be accurate and that you're trimming out more than you think you are. Are you beginning to porpoise at this point?

 

[dt4ever]

No Title

Boat never starts porpoising at this trim level when the engine revving up. Top speed was 46-47 mph gps @ 4400 rpm . I started to think maybe I mounted the smart tabs quite close to the leg so air being sucked down when the stern got lifted while on plane.

 

[cptrick3]

I'm at a loss, someone with more experience my give you additional advice.

 

[smokeonthewater]

Sooooo what's the problem???? Don't trim out so far....want more bow lift adjust the tabs for less pressure....

Cover the gauge up and adjust to best performance...

And yes rpm getting too high and spiking in turns is a sign of over trimming.... Called ventilation or blow out....
A different prop design can affect this too...

As I said what's the problem? Why are you trimming up higher? Plowing? Lower speed than expected? Etc....

 

[dt4ever]

Smoke,

As stated I never experience a blow out or prop ventilation before. I was playing with the trim to find the sweet spot while cruising. I'm just surprised that with such small trim adjustment would rev up the engine. Also I just want to make sure this is normal blow out not due to something else more serious. Thank for everyone input though.

 

[ondarvr]

The true function of a foil is to help prevent ventilation, so not having experienced it before would be normal now that its been removed. I'm not saying to put it back on or that you need it, just explaining why it may be different now.