Formula 242 SS sbc Prop?
 

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.

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.

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

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

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.

What is the drive ratio?

Like SB said, we need more info.

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

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.

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.

What Diameter was that 17"prop?

It was a 17p 14.75" IIRC.

I am going to try to compare to the 17p Mirage:
17P 14.5" Enertia
19P 14" Enertia

Seems as the boat does not like the smaller diameter. Was able to squeeze max 4200 rpm out of it. May try a 15p mirage.

Call B Blades. They will consult with you over the phone. They helped guide me in the right direction. They actually recommended a 4 blade prop for mine, but that was before the re power.

Check out my old thread.....

http://www.offshoreonly.com/forums/p...lection-2.html

Thanks so much @Jonesyfxr

So I found a winner. I tried the 17p enertia Eco. I was able to achieve 46mph @4600 rpm and a nice cruise speed of 34mph @3400rpm, I don't think it's going to get better than that. I was very impressed with that prop. I was able to get on plane with no issues with 3 people and 60 gallons of fuel. I gained 1.3mph on top end and about 2mph at 3400 and only lost 50rpm from the mirage plus 17p. I also tried a 15p Mirage plus and was able to get 5k rpms but at a lousy 41mph.

Man that boat must be heavy! I'd think a 19p would be better, but what do I know? Lol

If you need any help send me a PM, I'm not far from you.

Thanks man I really appreciate it. The 19 did very poorly. It's about 4200lbs dry.

Consider it a pay it forward. It's the least I can do after all the halo that was given to me here by OSO members.