Formula 242 SS sbc Props?
 

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.

I would appreciate any insight that anyone may have.

What is model of 4 blade do you have? Is it stainless?

You would drop rpms going to a 19 mirage. Is the bottom of the boat clean? Boat not loaded with gear and extra weight? Are you trimming the drive up properly? If none of that is the case then your engine is down on power because a stock 260 horse 350 should push that boat to close to formulas number of 46, but maybe 42-43 realistically.

Mercury off shore is what is says on the prop
If I trim up I lose MPH
boat is bottom painted but is clean
Engine is freshly built great compression and very healthy

by no means should you be losing mph when trimming up... how much seat time do you have with boats?

To give you a point of reference...I had a 1986 242 SS with a 260 Mercruiser/Alpha 1. Bone stock with a 19" SS 3 blade it would run 46 MPH at 4800 RPM with full fuel and 2 adults. I bought the boat new and it had no bottom paint. The best I ever saw from it was 48 MPH with a light fuel load and myself only.

Not knowing the history of your boat it's tough to make a direct comparison. These boats weigh around 4200# dry...it's possible that you are "wet" in the transom and or stringers and that could add several hundred lbs. If possible run your rig over a scale and subtract the weight of fuel and trailer to rule this out. Otherwise, get your hands on a 19" SS 3 blade prop and run it...you may be pleasantly surprised.

Good luck with this and let us know what you discover...with your build you should be seeing 50 after 10 hrs of break in.

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.

It sounds like you have found your "Sweet Spot". As your engine breaks in after about 10 hours you will probably see a bit more as the engine frees up some.

To be on the safe side, pull a spark plug or two from each bank and "read' them for mixture. You want to make sure that you don't run too lean...especially during this "break in" period. What carb are you running?

I broke the motor in awhile ago. Motor has about 75hr since the rebuild. Seems as the boat does not like the smaller diameter. Was able to squeeze max 4200 rpm out of it with both enertias. I may try a 15p mirage.

I just don't understand how people are achieving what they are with a 19P.

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.