What prop to try next

AdamB

Petty Officer 1st Class
Joined
Aug 26, 2004
Messages
251
Hi, I have a 4,000 lbs 21 foot wellcraft cuddy with merc 260 (5.7) alpha one. Anyhow, the original prop that came with my boat (I bought used) was a cupped 14 X 19 aluminum. That will give me approx. 44 mph at 5200 rpms.<br /><br />I got a relatively good deal on a 14.4 X 22 (i know weird pitch) SS prop that the only marking on it says turbo. Anyhow, that drops my rpms down to 4200.<br /><br />I've heard 1 inch pitch drops 200 rpms right? So in my case, the 600 rpms were from pitch increase, was the other 400 rpms from changing to SS prop?<br /><br />Should I just try a 14X19 SS or 13.75 X 21 SS or?????
 

stevens

Senior Chief Petty Officer
Joined
May 4, 2005
Messages
799
Re: What prop to try next

Or, the last 400 drop in RPM came from the Turbo having a more aggressive cupping and a larger diameter as well. Changing to SS shouldn't in itself lower RPM, but the concurrent change in geometry would.<br /><br />If you bought the prop at a dealer, maybe you could exchange the 22P Turbo for a 20P or a 21P Stiletto and see where that would take you. Might still be too much, though.
 

TilliamWe

Banned
Joined
Dec 21, 2004
Messages
6,579
Re: What prop to try next

Adam, I am very familiar with that 22 Turbo. We have had one (my dad, actually) for years, and it's been practically a miracle prop. Sorry to hear it didn't work for you.<br />Is it possible that 5200 was not accurate? But even so, you lost 100 RPM by going to stainless. Then you lost up to 600 for the pitch increase. (and who knows if your 19 is really a 19?) When you lost those revs, you somehow pulled the motor down past it's power range, which is 4400 to about 4600.<br />That 22 Turbo that we have has always performed similiar to a 21 pitch mercury stainless.<br />So, confirm your tach. Then if it checks out, borrow a 21 pitch aluminum or stainless, and see how it does. If it still doesn't turn above 4200, get that 19 alum you have checked, and see what it really is!
 

AdamB

Petty Officer 1st Class
Joined
Aug 26, 2004
Messages
251
Re: What prop to try next

Sorry for the long delay. I just re-read my original post, the prop is a 14.5" dia not 14.4. Anyway, I have never checked the dash tach against anything else, probably should. The 44 mph comes from GPS. <br /><br />The turbo does have more forward rake (towards trim tab), so much in fact that I had to put on a skinnier trim tab.<br /><br />I was hoping to get the motor down into the 4600-4700 rpms range, that's why I thought the 22 pitch would work. <br /><br />I was having an ignition problem, will try it out again before selling it. Actually, I am switching the boat over to MPI fuel injection, which should also give more hp so hopefully it'll then swing the prop to a higher rpm. If not, then I'll drop back to 14 x 21 pitch.
 

AdamB

Petty Officer 1st Class
Joined
Aug 26, 2004
Messages
251
Re: What prop to try next

Okay, retested with 1/2 tank gas (20 gallons) and myself and 1 other guy. Got 4600-4800 rpms, with only 41 mph on the gps(38% slip according to program I'm using). The 14X19 does 5200 with 44 mph.( 29% slip which is decent for heavy cruiser)<br /><br />So, I'm getting some serious prop slip (if the tach is correct. The only thing I changed was to put on new spark plug wires, the old ones were arching badly.<br /><br />So, do I over-rev with aluminum prop to get more speed and better take off, or stay at a better rpm and get less top end? I thought if I switched to a higher pitch, I would get more top speed? <br /><br />Should I go down in dia.?, or would that give me more prop slip? Back to my original question, what prop to try next. I do have a torque shift, but it doesn't always upshift correctly, and my tach will pin itself if I'm not carefull.<br /><br />The turbo 22 pitch prop does have exhaust holes near the base, maybe this is causing the slip?
 

TilliamWe

Banned
Joined
Dec 21, 2004
Messages
6,579
Re: What prop to try next

The turbo 22 pitch prop does have exhaust holes near the base, maybe this is causing the slip?
It does at low speeds, that's why they are there. Not sure about higher speeds though.<br /><br />Your prop slippage % sounds way high. :confused: At 4600-4800 the higher pitch prop should have produced the same or better speed than the 19. <br /><br />At this point I am not sure what prop to tell you to try next. I will say, make sure you don't have anything in front of your drive, or the drive case itself is not damaged. Is it possible that something is disrupting the flow of water over the prop?
 

AdamB

Petty Officer 1st Class
Joined
Aug 26, 2004
Messages
251
Re: What prop to try next

I do run a hydro foil, which I have to remove to use the torque-shift. The torque shift won't shift with the foil on, maybe it is somehow disrupting the flow of water over the larger dia 14.5X22 turbo prop too.<br /><br />yeah, I was slightly worried that the rpms were that high, but the mphs weren't.
 
Top