Prop wont go any faster.
 

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,

damn if you did the transom suprised no one told you to raise the x dimension at least an inch, or 6 knowing formulas lol

I think you just have a 54mph boat with that hull/engine combination.

The boat ran 54mph with a fresh transom, but all original 1979. Aluminum prop and all.I then rebuilt the drive, took it out and broke 4 pistons. The build is a stock bottom end with flat top teflon coated speed pro pistons, Vortec 906 heads, a comp 262 Xtreme marine flat cam, an Edelbrock air gap intake with an Edelbrock 1409 carb. The exhaust is a GLM setup with 3 inch spacers all exiting through Hardin slash tips. The boat still only goes 54, it gets there a whole bunch faster, but still 54.

Sprink says his 78 F18 did 65 with a 302 back in the day. That is where my crazy idea came from.

What RPM's do you turn?

at 54mph i can be anywhere from 4600rpm to 5500, i am scared to push the rpms higher. Even at 4600rpm and 54mph I am running 19% slip. Every prop I try gives my high slip.

Prop technology has changed a bunch in 35 years. Back then, you had to run the drives deep.

As in to get any bow lift with old props you needed a deep drive, but now we have bow lifting props. Is that kinda right?

I will say that when i am running "fast" I have to have a tiny bit of tab in it or it jumps out of the water. And also the spray comes out the side about 2 ft from the back of the boat.

Any idea on your compression ratio ? not picking on your build but it sounds like you might be running out of HP and just over trimming to get RPM.

actually I am not trimming at all. If I bring the drive out the boat porpoises bad. I just leave it tucked in. The bow lifts too much on its own. Needs tab down and drive in to keep it smooth. My compression is 9.75. I mirrored this build except i have an air gap intake and my cam has 3 degrees less exhaust and thiers made 412hp at 5600. Granted they had headers. http://www.chevyhiperformance.com/te...cam_355_build/

Any chance the hull has a weak spot and it's flexing

I dont think so, the transom and motor mounts are all new. The floor is rock solid. There is no stress cracks. My skeg was crooked and rough from where a chunk was missing and I half ass patched it. I fixed it and smoothed it all out with a belt sander so it should give the prop some cleaner water. Would a messed up skeg cause my issues? I will test it out tomorrow.

With the drive tucked in the motor is trying to bury the bow in the water, but you're running bow high. I could be wrong, I would check the hull for soft spots from the motor forward. Have someone video a fly by so you can see what it looks like from the side. I doubt the skeg would have anything to do with it.

just for kicks, I put a straight edge on the keel. The cavitation plate is 1/4 inch below the bottom of the boat. Maybe I need to set the angle of the boat a bit more nose down with the tabs so when I trim to neutral instead of down the bow will not rise more. I know I have tons of bow lift.

putting the nose down will not help increase the speed. you need to get the boat up and out of water. trim it up and put the tabs up.

This is a long shot but something might be hydrodynamically screwed up with the lower unit on the Alpha. Do you have access to another drive that you could pull the lower unit off of and try it? One other thing to consider would be to switch the drive to an Alpha Gen II. My brother did it on his F3 with good results using an old style Transom Assembly. He did a bit of grinding and shift cable modification but it worked well for him.

Another idea would be to move the tabs inboard towards the keel. This will for sure give you the ability to find the sweet spot angle without over trimming the drive.

Maybe I should try putting the tabs all the way up, and putting a little trim in it to keep the bow up. If I pull the tabs up and leave the drive tucked in the boat jumps up and down. As i am running currently though i am getting the spray to come out about 1-2 ft from the back. The spray is coming out behind the rear bench, I have a full sunpad.

Feels like i need to try one of those stern lifting props.

A cleaver style would give you stern lift.

Interesting

How deep is the prop centerline from the bottom of the boat?

9 inches

Well while it was on the trailer yesterday i hit the trim button all the way up until it stopped. I then rotated the trim limit switch 4 little lines to allow the trim switch to raise the drive a bit more. By doing that at max rpm I can now pull my tabs all the way up and maintain bow lift with the trim. I got 64 MPH on gps @ 5500rpm with a 23p Mirage. I still have a relatively high slip number (19%) but I picked up 10mph. I still have more throttle left but I didnt want to rev it any higher. I am thinking I could swing a 25 or possibly a 27p Mirage or Mirage Plus. What do you guys think?

Sounds like you are close. You could try a Mirage 25p but you likely won't get more and you will reduce the acceleration. Do you stop trimming when you get to 65 MPH? My thought is to with your 23P and get it behave more like a 24P vs the 25 Mirage. Have you talked about options with Brett at BBlades?

I have thought of calling Brett. With the 23p the boat is an animal at any speed, tons of power everywhere above 2000rpm. I would almost bet the farm I could pull a 26-27p prop.

As you go up in pitch acceleration will fall off. Planing time will also increase. A labbed 23 or a 25 is where you'll most likely end up. On boats with limit switches I usually trim up with the trailer button. The limit switch almost always stops before you hit the sweet spot.

I am spinning the mirage 23p with ease. I will just have to keep trying them out.