So we finally got a chance to take the boat out for its maiden voyage and everything went good without any issues, except gps did not work so I have no idea on actual speed. Now this is my first boat and really have no experience with any large boat in general but we were cruising a small lake so speeds were below 35. The current prop is a HC-CL-24P Q-IV on a King Cobra outdrive (I am unsure of the ratio). Boat is a 1990 Advantage Offshore with 620 hp and 670 ft. lbs of torque. When cruising around 23-2500 and throttle steady I could trim the drive and feel when it felt the best or had some bite. Now when cruising at this rpm and giving it very slow and steady throttle it seemed to be ok but when you gave it some gas it seemed not to pull at all like I thought it would. Tomorrow we are taking it out on Tahoe so I am hoping to get a better feel for higher speeds. I am going to use my gps on my phone this time too. I have rebuilt the motor in this boat and I am guessing at least 200 more HP than the last motor that was in the boat. This is the prop the guy said he used for Lake Tahoe elevation.

OK finally got a good whole day out on the boat and here is what I have going. I cannot get the boat to go over 3800 rpm and that was with a lot of trim and did not pick up any speed. Cruised most of the day at 40 mph (GPS) at 3500 rpm and really made no difference between just over half throttle and full throttle and I did try adjusting the trim to see what would help or hurt. I found my ratio is 1:43 on the outdrive. The boat came with a 3 blade 20 pitch I was going to try but those are the only props I have. The motor has no issue revving and sounds great so do I have a prop issue? The boat is 30 ft long too.
thanks

30 feet long?
one engine?
if 1.43 ratio, it really is a low ratio imo
and a 3 blade 20 pitch does not seems right to me ... 19 with 3 blades ok, 20 with 4 blades maybe
but never seen a 3 blades prop with a pair number in pitch

i had a sunsation with twin 500 and 1.36 ratio spinning 23's with 3 blades

i have the feeling you have a bad set up, your single engine 30 feet boat is overloaded and cant spin this combo of ratio and prop! But im no pro just a guess...

My boat runs like a dawg at Tahoe as well- its jetted fat and for sea level.

Did the boat come from a lower elevation and needs some "tuning" to run at 6225 feet, or was that done when you rebuilt the engine?

I rebuilt the motor and we did dyno it in Reno before I put it in the boat. It was also dawg fat on the dyno and it still made 621 hp and 670 ft lbs. I have dropped from 80's to 76's all the way around on my jets. I might try a little leaner too. I thought my HP and torque would be plenty to push this thing, I guess I was wrong. This is my first boat and truly have no idea what I am doing besides this site and trial and error. I did look again and my 3 blade prop says 14 1/2 x 20. These are the 2 props it came with. I thought the guy said the 4 blade was for Tahoe, but that was last year and I can't remember anything anymore LOL. After reading more I would guess the 3 blade with a 20 pitch (I am guessing) would be easier to turn?

whatever you have, always try both or all props. you will find the 20 spins and runs boat way better. it may wind up to 6500 ad still be slower than you expect. you built the engine. so you bought the valvetrain. what is your practical rpm limit? make sure it doesn't rev past that. what engine? there were a lot of big fords paired with KC's.

I have a 548 BBC full roller valve train and cam but cam is smaller is is all done at 5500 rpm. I will definitely try the other prop next time I go out and report back. I am also assuming my drive ratio is what it states in the manual for this year and there is only one ratio.

cone clutch or big-geared dog clutch cobra? I don't think kings came in 1.5. I put an old king lower on a reg cobra upper and got 1.36 or so.

I have an 18 pitch stainless cobra prop. you want it, 25 and 25 shipping. been staring at it on the shelf for years. might work good dragging kids on tubes at 25 to 30...

It is a cone clutch model. Let me see what this 20 does and go from there.