question... gained rpm but no more speed?? UPDATE 5/24/07
 

I just put my boat in today, I have an 88 Chric Craft 245 Limited, SBC boat, I had issues last year witht the engine, I could not pull a 19p stainless past 4200 rpm, I built a better engine with more power and got 46-4700 RPM with the same 19P prop but no more speed? It will not plane for crap with a 21 Mirage amd could not get more then 4200 with the 21P prop and same speed (48GPS)
No matter what the boat stops at 48 MPH, I should have about 350-375HP with this new small block, boat is supposed to be 3750lbs dry, I will post a pic to get an idea of boat size.

Any suggestions?


http://i36.photobucket.com/albums/e4...g/102_2848.jpg

What was the old power and what is the new power?

Julie

Factory was 260hp with a 19P it went 45-47ish I cannot remember the rpm.

I did heads and cam intake I would say around 300-325hp with a 19P it went 4749 at best at 42-4300 rpm.

I did a new roller 350 shortblock with a little more cam and compression, dart heads, ZZ9 cam 112LCA,218/222 duration at .50 and 488/520 lift intake/exh. 9.5:1 compression. I would say around 350-375hp.
I want to know if going to a smaller diameter prop will plane easier and still be a 21P I am not sure what my 19 and 21 are now but I am pretty sure they measure 15-15.5 I need to measure them, yesterday I put on the 21P and it only went to 4200 rpm with a top speed of 47.

Kinda sounds like your boat hits a wall at 50 mph, and to get it over that point could take lots of horsepower.

Not sure why, stock BB ones will run 61-64 gps stock, I amn wondering if there is a hook in the hull... even if there is, will it kill it that bad? I have never heard of a hook in these but that would be my luck...

I'd try a 19p Mirage + and trim it to the moon.
To make 350HP from a 350cube you need to spin it 5200+rpm.
You have a big boat to push with a small block. I have bud with a 245 Formula SS with a 330HP/454 and she runs 52mph on her best days.

I hear ya, it is a big boat for a small block, I thought I would be able to get it to move, that is getting difficult, the 19P I have is a raptor I think by quicksilver, it is a stainless prop, would a mirage be better? how much does diameter effect it?

I don't know what a raptor is. The Quicksilver props I'm familiar with are similar to the a Merc Mirage.
I'm suggesting a Merc Mirage plus. I think the diameter is 14-5/8" on the 19p.
The Mirage plus has more cup and I believe more blade area than the standard Mirage or Quicksilver.
The Mirage plus will lift the bow and get her to run with less drag.

I was looking at mirage props online and different offerings, it said the plus was more for bigger power boats,think it would be OK? I saw a laserII also, it seemed like it would be good too, I am going to do alot of researche before starting to buy, props get expensive so I want to do good!! I plan to go back up to the boat this weekend, I will do more playing and logging things.

I think that the 14.5+ inch diameter prop is just too much for a lot of small blocks. I really like the laser II series which are 13.75 inches - 13.5 inches. I will show my experience:

21ft boat
406 small block - aprox 450hp @ 5600 rpm
Volvo SX outdrive - 1.66 gear ratio

Prop #1 - Mirage 25 x 14.5 = 67 mph @ 5000 rpm

Prop #2 - Rapture 25 x 14.25 = 69 mph @ 5400 rpm

Prop #3 - Laser II 25 x 13.75 = 74 mph @ 5650 rpm

Prop #4 - Laser II 26 x 13.5 = 76 mph @ 5600 rpm

I read in a magazine article that to get the fastest prop, you should run the smallest diameter possible with the most pitch possible to attain your desired engine rpm. Think of it this way. How easy would it be to spin a 25 pitch prop that was 30 ft in diameter as opposed to one that was a foot in diameter. Too small and the slip would be too great. Too big, and you could not rotate it.

Just my two cents.

Chris