javi1782

So I have been doing a lot of reading and I am a bit confused when it comes to my prop.

I currently have a 89 242ss
sbc roller with pistons bored .30, a slightly more aggressive cam and forged crank
Alpha Drive
4 blade 17P Mercury Offshore Mercury Marine Series Prop
@ WOT 4200rpm at 38 MPH

The mechanic was telling me that the motor should turn 5200rpm and that the power band is between 4500 - 5200.

I contacted Formula and they said that when they sold these boats they were achieving 46mph on the sbc with a Mirage 19P.

Here is where I am confused, even though I would be gaining rpm's going from a 4 blade to a 3 blade wouldn't I lose rpm's due to the larger pitch and diameter.

I would think I would have to go to a smaller pitch and a 3 blade to achieve more rpm's, faster top end and faster cruise speed.

So what do you guys think I should get in order to achieve 5200rpm?
I would appreciate any insight that anyone may have.

Jonesyfxr

iTrader: (1)

I'd say more pitch and less diameter will get you in your RPM range.

I'm building a roller SBC with Vortec heads, XM Cam, Airgap intake, Corsa diverters and a 750 Holley for my 21 foot BR. I Bought a Mirage 21 P to start off with.

javi1782

But the 19P mirage is a larger Diameter than what I currently have

Jonesyfxr

iTrader: (1)

Maybe try giving B blades a call. They helped me get on the right track with my stock 260hp 5.7.

SB

iTrader: (1)

You've given us absoltuely no information on your powerplant and rigging.

What speed, rpm, and prop did your boat run with the stock 260hp engine ? Again, yours.

Trash

What is the drive ratio?

Like SB said, we need more info.

cooper_361

I have a 89 242 ls, my previous engine was a 377 mpi 320 hp, it ran about 51 mph with an enertia 17p at 5150 rpm. It was the best handling and top speed prop on that boat.
I now have a scorpion 377 with 377 hp and now runs about 59mph at 5300 rpm with an enertia 22p

javi1782

Update- I rented a 17p mirage plus and was able to achieve 45.3mph on gps at 4600rpm. I may try a 17p or 16p enertia and see how it does.

Jonesyfxr

iTrader: (1)

My 21 foot Cobia Allante with a stock 260hp 350, an Alpha 1.5 ratio drive and a Solus Amita 3 aluminum prop went right about 50 on the speedo. I was being paced by a friend's boat that has a GPS speedo and his was reading 47 and I was pulling away from him.

I think you should start with a 19p prop with less diameter to get your RPM's up a tad.

javi1782

What Diameter was that 17"prop?