1985 Starcraft porpoising issue. Prop??

derek4325

Petty Officer 2nd Class
Joined
Mar 7, 2015
Messages
167
I know its an I/O, i.e. not as much weight on the transom as an outboard, but old aluminum hulls are known to "hook". This is where there will be a bend in the hull at the back downward usually due to not being properly supported by the trailer at the stern. Put a long straight edge against the bottom and check. Doesn't take much to make a difference.
I'll look into that. I don't feel that's the issue since over half the engine is supported by the trailer but I'll rule out what I can. It's in the water now I'll check it next time it's out
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
13,782
What year is the Boat and Motor?
Regarding the differing WOT Ranges, Merc might have made some changes to the Motor, and needed the Higher Rpm to get the Rated HP.
 

cyclops222

Commander
Joined
Mar 21, 2024
Messages
2,397
Prop is too high up. Lower the motor a couple of inches. Your prop is sucking in air ? Very lightly loaded boats do that.
 

mr 88

Commander
Joined
Nov 3, 2010
Messages
2,230
I know its an I/O, i.e. not as much weight on the transom as an outboard, but old aluminum hulls are known to "hook". This is where there will be a bend in the hull at the back downward usually due to not being properly supported by the trailer at the stern. Put a long straight edge against the bottom and check. Doesn't take much to make a difference.
Actually a I/O is going to put a lot more weight in the stern than a outboard . 5-600 lbs vs 3-400 . The I/O is located about a foot from the transom . If trimming doesn't help you needs weight up front or some sort of tabs on the transom . I would stick with the 17" and if it's SS I might consider going to aluminum, that would up the rpms a bit . Or hit up a prop shop and have them bend you up a 16" prop .
 
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