1978 Starcraft I/O Super Sport 18ft Prop Size Question

[StarCraft78SS]

Good afternoon,

Quick question, I'm currently running a 13 3/4 x 15 prop. I'm wondering if this is the correct prop for this boat. At WOT around 3900 - 4300 RPM I'm getting porpoising. I'm wondering if I'm running out of prop because it will start around 28-30 mph but top speed is about 32-33mph and no matter the trim or tilt it continues to porpoise.

Let me know your thoughts.

 

[alldodge]

Not seeing the prop as the cause of porpoising, in most cases this is caused by load placement. Need more weight in the stern and less in the bow

 

[Alumarine]

Are you sure the drive is able to trim in all the way towards the transom?
Also, your prop, speed and rpm numbers seem odd.
With a 15" pitch prop you should be over revving by a lot at WOT.
A 14.25" x 21 or a 14.5" x 19 should work well for that boat.

 

[Gibbles]

My experience with that is with a boat that's overpowered (outboard).
Too much weight in the rear of the boat.
Solved by adding weight to the front, like moving the battery up there, or the fuel tank.

for a while I ran with a couple sand bags inside of some trash bags to prevent sand from making a mess.

Once that worked, I figured moving a large marine battery to the front should do the same thing.