Glen-L.com

I just went through most of this . . . First a couple of guys are mixing up ventilation and cavitation. Ventilation is from big air pockets, cavitation is the little teeny tiny air bubbles due to negative pressure, they pop and cause pitting. Ventilation is that big rev deal and the boat will go nowhere. You are not experiencing that.
The ONLY way I can break the prop loose from the water, or ventilation, is HARD throttle in reverse, and hard throttle at that

You want the bow up, even with an I/B shaft angle. Boats are more typically affected by wetted surface than shaft angle, mo stuff in da wawa is bad.
I tend to believe my bow rides wetted on plane, bilge water collects under the engine on plane. Matter of fact, bilge water stays forward of garboard plugs on plane. This may be a tell-tale point of interest here.

I think you need to start over with some spec stuff.

Engine age? 3.0 right? 135 Mercruiser from an I/O?
1986 engine by seriel number, fresh rings, bearings, valve job for this build though. Already cut the element out of a oil filter looking for metal flakes, looks clean.

I saw that you said you were getting poorer fuel efficiency at 3000 RPM than you should wide open throttle or something similar. Exactly what is your fuel consumption at 3000 RPM , exactly in GPH. Do you have a flow meter? Also at WOT RPM. Fuel use is a very good indicator of horsepower made. Unless the timing is way off, or it is smoking, or you have a ton of blowby, if you burn fuel you make horsepower . . .
Timing is good. Fuel flow @ 3000 is 3 gal in 35minutes, and 21mph. My target for the project build was 18kts and 3gal/hr. I may have to accept 3gal in 50 minutes, but I am right now at 1gallon in 10 minutes=6gal/hr at cruise. That seems awful high.
If you do a math formula for fuel burned=HP; then how much HP am I making at 3gal/35 minutes?

Also, one very important thing, are you absolutely 100% positive that your tach is correct? Absolutely, 100% positive? No doubt whatsoever? Checked with a shop tach or photo tach?
Tach checked against a optical tach @3600.

Driveline issues would usually present other symptoms, for example if it was the bearing in your shaft log it would be screaming bloody murder. You could take that out and try it with just the strut's bearing too. If it was the strut bearing, it would've failed by now. Anything that can take that much load out (assuming that you are not getting full power now) is gonna be telling you about it . . .

G,Day Kens. Firstly I think 28mph. is pretty reasonable from your set up.
Yeah, sort of. 28mph aint bad for a 25 footer, but, it is bad at merely 2480lbs though. My light weight should allow better fuel consumption.
If I can believe your tacho and GPS your combination is most efficient when the pitch to diameter ratio is roughly equal at 16x16 and 16x15, and I dont believe your 135hp. engine has anything left to give.
I am starting to believe my 135hp engine in only making 110hp.
To achieve 22mph at 3000 rpm you will need to compromise a bit on acceleration by giving up some of the propeller efficiency. my best bet would be as follows 1.5:1 gear ratio 12.5x14.5 prop should give 22 mph at 3000 rpm and 32 mph at 4400 rpm.
I am looking strictly for a good cruise, I can care less about acceleration or a hole shot.
Fixing boats is easy Fixing women hard.
thanks for reminding me; the wife wants me to name the boat after her! And after all this, it may be appropriate.
Regds Ron