I have an 18ft Formula. I built a 355. No matter what prop I put on it, it wont go any faster than 54mph. I tried it back to back. 21 aluminum, 23 aluminum, 21 Mirage plus, and a 23 Mirage. Both of the stainless props are in perfect condition. The motor will rev faster but no speed is ever gained past 54 on gps. It feels like i am doing a burnout. Could a crooked and rough skeg cause this?

What is the condition of the bottom of the boat? Any bottom paint, growth, or other? It sounds like you have something robbing hydrodynamics; which could be the skeg. Does it track straight? Have you run a prop slip calculator with the different props and RPMs? Have you tried a 4 blade? Are you sure the engine coupler is in good shape?

Your drive is to deep

So an alpha SS would probably work wonders for me then? Are there any band aid fixes for a drive thats too deep? And why would Formula mount a drive so far down?

The bottom of the boat is perfect, coupler has been verified good. I consistantly get high slip numbers.

what ignition does the boat have,does it have a rev limiter?

It has an old school Mallory Mercruiser points setup with a points eliminator kit. I have also checked my advance curve and it is within spec. I can rev to 6k but get no gain in speed.

ok,that rules out what i was thinking,maybe something is causing the prop to loose it,s dynamic hookup with the water as others have said.

Ill be bummed if I rebuilt the transom, rebuilt the drive, built a 400hp motor, all to gain 0 mph. I really wanted to go for 60s, if the drive is too low it will be where my quest for speed ends til I can buy a 242.

How fast did it run before you built the motor ? what parts did you use, heads, cam, intake, carb, compression, Exhaust ? you may not be making the power where you need it,