Hole Shot Blowout

[1FASTLASER]

Originally Posted by nofearengineer

I simply asked "if the prop is blowing out at takeoff, where the motor is trimmed down, why doesn't it blow out on the top end, when it's trimmed up and sitting higher in the water?"

Trust me, "I don't know why, but years of experience have taught me it is so" is a very legitimate answer. I tell people "I don't know" all the time.

Ok let me try and explain or clarify. Actually upon holeshot what you is happening is called cavitation.........cavitation is simply air that has entered the props path be it naturally or forced, thus creating high rpms with little or no forward movement. Now for a correct holeshot there HAS to be a certain amount of cavitation to allow the engine to spool up (rpms raise) so that the engine can get the hull moving more effeciently. BUT the amount of cavitation needs to be controlled.........for example mercury has the removable and different size PVS plugs for sebveral of there props.
Ok now when you are up and running the whole picture has changed. Now you are no longer dealing with the factor that exhaust has introduced air into the props path because you are "out running " the exhaust.......what happens now especially more prevalent at elevated engine heights is "blowout" .....same high rpms with no more forward boat movement but has happened for a different reason. Know before I go any further let me say this ......cavitation can cause the higher rpms at speed but it is not exhaust related now...now its air being introduced because of a hull imperfection,a skimmer transducer improperly adjusted,a flatbottom boat that has rivets,,,,so forth etc etc......This is where figuring out WHAT is the cause and fixing it comes into play. After reading magicians post and getting all the pertinent info I was able to deduce that he had changed props and gone to a larger diameter prop. His engine height was previously adjusted for the smaller prop. Now when you are up and running and have optimized your setup ...in his case with a 3 blade prop.......what is going on at that time is 2 ears are in the water while one is out of the water thus allowing the engine to comeup on the rpms more efficiently but not to much. If it is too much...."Blowout occurs"....Now maybe you can see what I am referring to. Same scenario cause by a different circumstance. After reading what he posted I was able to figure out that the larger diameter prop was actually allowing to much prop out of the water at running speed....hence the raise in pms and no more mph.