V20 with Black Max 150???

[randymbtech]

Hey guys, I'm new to the site, just bought a 1983 V20 cuddy with a 1989 Merc Black Max 150.

I'm trying to get the prop size right. The boat came with a beat up aluminum 19 pitch prop, and it performed terribly, so I bought a used Mercury Mirage 17 pitch stainless wheel. I am impressed with how it spins the Mirage. It comes out of the hole great, but the RPM's are too high at WOT (over 6000 RPM). It will do only about 36 MPH at WOT. I would like to be able to cruise at a lower RPM, so should I move up to a 19 pitch Mirage? I thought that the 19 Mirage would be way too big, but it seems to spin the 17 too easily.

Thanks in advance for any advice.

Randy

[THEFERMANATOR]

Sounds like you've got a 150 2.0L, if so it will have a 2:1 ratio in the lower unit which will need a higher pitch prop. A 19 MIRAGE for a 150 would be a good prop to try. Just remember that the MIRAGE is not a top speed prop. You might also need to reset your engine height for it, and make sure your tach is correct as propping with an innacurate tach is bad news.

[spareparts]

make sure you have a Mirage plus and not the original Mirage(it will say so on the prop). The original Mirage ws intended for I/os and can bog an OB, the Mirage plus was a better all around prop and works on OB's better. That 2:1 raito will help the smaller motor(2.0L) push a heavy boat liek a V, just take your time and get the right prop. And like Ferm said, make sure your tach is correct

[bigshrimpin]

Originally Posted by randymbtech

Hey guys, I'm new to the site, just bought a 1983 V20 cuddy with a 1986 Merc Black Max 150.

I'm trying to get the prop size right. The boat came with a beat up aluminum 19 pitch prop, and it performed terribly, so I bought a used Mercury Mirage 17 pitch stainless wheel. I am impressed with how it spins the Mirage. It comes out of the hole great, but the RPM's are too high at WOT (over 6000 RPM). It will do only about 36 MPH at WOT. I would like to be able to cruise at a lower RPM, so should I move up to a 19 pitch Mirage? I thought that the 19 Mirage would be way too big, but it seems to spin the 17 too easily.

Thanks in advance for any advice.

Randy

Randy - IMO the 17P mirage plus is probably a very good for you, but your tach has to be wrong . . . unless your are sucking a lot of air . . . your slip would have to be more than 20% to turn 6000 with 2:1 and 17P and only achieve 36mph. GPS's don't lie. You can run the numbers and see for yourself.

http://www.go-fast.com/Prop_Slip_Calculator.htm

[Destroyer]

Originally Posted by bigshrimpin

Randy - IMO the 17P mirage plus is probably a very good for you, but your tach has to be wrong . . . unless your are sucking a lot of air . . . your slip would have to be more than 20% to turn 6000 with 2:1 and 17P and only achieve 36mph. GPS's don't lie. You can run the numbers and see for yourself.

http://www.go-fast.com/Prop_Slip_Calculator.htm

I was going to comment that it sounded like he was cavatating. I think Bigshrimpin has his finger on the right button. Did you check your engine's cavatation plate with relation to your keel?.... Sounds like your engines up too high and you're sucking air.

[randymbtech]

Thanks for the advice guys. The cavitation plate is level with the very bottom of the transom, it looks right to me. I have to check and make sure the tach is set up right, the boat used to have an Evinrude, not it's a Merc, maybe the tach is wrong? I do think the prop is a bit too small, just because of the way the boat comes out of the water like a rocket. I need to try and find a Mirage Plus 19 to try out.

[bigshrimpin]

Randy - I know your tach is porked. You wouldn't be able to turn over 5800rpm with that motor unless you removed the spark advance module/idle stabilizer (or it's broken) . Assuming you have average prop slip (about 10%) . . . your RPM's right now are really about 5100/5200 with a 2:1 ratio lower unit to reach 36mph. (that's just simple math)

Don't bother with the 19 mirage plus. There is no way that 2.0L motor will turn the 19P and push the v20. The 19P will just lug the motor down (drop RPM's 400/500rpm) and give you poor performance and terrible fuel economy. That motor needs to be propped around 5500rpm.

I ran a seacraft 20MA with both a 19P and a 17P mirage plus with a 2.4L 225, 2.4L 200, and a 2.0L 150. I could turn the 19P about 5800 with the 200 and 225 (and just break 50mph) with the larger displacement motor. The seacraft is a lot lighter and faster hull than the v20. When I had a 2.0L 150hp motor on the exact same boat with a 17P mirage plus the boat could only hit 41mph (5500rpm)

You are getting decent performance with the 17P . . . Dial it in (raise the motor a hole or two) and fix your tach.

[randymbtech]

Got ya. I think your right. I am going to pick up a new tach this week, and start there.

[Airslot]

I owned a very similiar setup. 87v w/ 150 Black Max. Best prop for me was a 17p Rev 4 with the motor mounted one hole from the top. Loved that setup.

[reelapeelin]

What DIAMETER is your prop?...these Vs need a 15 1/4 to 15 1/2 inch diameter prop in my opinion...