Water Kickback After Prop Change

[bloublasie]

Ive got a 15ft runabout with a 115 inline 6 Mercury hanging off the back.
I originally ran a 15p prop, and boat was porpiosing badly
I fitted a hydro stabiliser whiich solved the porpiosing but noticed that the RPMS ran too high.
Also had a rooster tail effect

I then fitted a 17p prop, and although it helped reduce the RPMS, I now have water blowing back onto the boat and splash well over 3000rpm

Is the motor mounted too high or too low?

 

[racerone]

Measure height of transom.----An inline 6 likely needs it to be 20 or 21" high.

 

[mr 88]

Generally speaking a 19" prop goes on a 15' boat that has a Tower on it . Do you have a 15" or 20" transom . Older pre 70s had 15" and unless your cavitation plate lines up with the bottom of your hull your probably dragging your drive and getting the prop wash back in the boat .

 

[airshot]

Or the foil is causing the spray to come back in, saw it before.

 

[JimS123]

Sounds like the motor is mounted too low. An optimum setup has the fin skimming the water, not dragging in it.

 

[Chris1956]

I would think a 19P prop would be pretty close. I ran a 21P prop on my 16 footer with a 1500 Tower.

The vast majority of the Towers are longshaft (20"). Your transom needs to match.

I doubt you will need a whale tail with that much power. With trim she should cruise at 50MPH, if your hull can go that fast.