How many RPMs??

The first thing I'd do is check to see what prop you have. I had a similar problem on my 89 SN and after replacing the existing prop, it was a night and day difference.

It is also possible that your speedo is off a tad.

mine turns 5500-6000 rpm depending on the prop I have on it.

He won't want a 542 if he's got the 1.23 tranny.

Mr. Green, it sounds like your prop has more pitch than your motor can turn. You should be closer to 4400RPM @ 42MPH for top speed with a stock 240 horse 351w.

Get some dimensions off your prop and the ID tag off the motor (on the intake manifold) to figure out which transmission you have and we can go from there.

ethanol has been in your gas for years at a 10% level if you are using the e-85 then that's a mistake and you should stop using it.

Ethanol has been in gas since the 80's and you don't have to have the vapor recovery systems for the gas to contain it. There are very few places I've been that have the vapor systems. California is about the only place that it is everyware and has been since the 80's

Yeh, we just got Ethanol locally. (Bummer) I don't think all of the stations have it. But it may be getting difficult to find gas without it. I haven't seen e85 yet but I've heard about it.

Gas milage on Ehtanol has definatly taken a nose dive, around 10% at least. "Everyone's" talking about it around these parts.

If all you've had is 10% Ehtanol gas since the 80s you wouldn't know what I am talking about.

Its tough to say for sure if the prop is the problem since we dont know much about it- but it certainly could be.

Id check your timing first. Make sure base timing is ~10 degrees and that it advances to ~30 by 3k RPM or so.

Id also check the carb- has it ever been rebuilt?

If youve got a little money to spend, a new prop wouldnt hurt. The new Acme's are a world better than the older props. Even if your old prop is not part of the problem, you will see much better performance and smoothness with the Acme.

For the record, I have the same motor/tranny combo as you. When I got the boat it ran 41 MPH @ 4200RPM with an OJ 4-blade. I replaced the ignition and set the timing and got 42 MPH @ 4400RPM. I switched to an Acme #470 and now run a strong 43 MPH @ 4600RPM. The holeshot is much improved as well.

It also wouldnt hurt to set your speedos- theres no way to tell how accurate they are until you do.

I wouldnt trust his speedo either. I dont think your boat is doing 44 when youre only turning 4000RPM. Most likely its 39-40.

I still dont know much about your prop, I couldnt find the part number searching the internet. As far as I know CC never put any SS props on their boats, so it is safe to assume its not stock. The exact dimensions are anyone's guess. Also, performance will vary from one brand of prop to another, even if they share the same dimensions. My guess is that your current prop is holding your boat back from performing its best.

FYI, if you ever hit the prop on anything in the water, a SS prop is much more likely to damage your strut, driveshaft, etc than a nibral prop. The nibral is much softer and will absorb some of the blow, saving costly repairs.

Mr. Green,

I've spent quite a bit of time in a couple of 1990 SNs, as well as my own 98SN and 2001 SAN. In their stock configurations, all ran at about 4600 to 4800 rpms on the top. I know some of these guys get more rpms, but some of the motors are older and others have made significant mods. Just a point of reference as you uncover this mystery. BKH

the yield is the big difference, that means if I remember correctly, that the SS will yield well before Nibral, tensil is when it breaks yield is when it moves.

Yes, on my prop, the numbers are on the surface behind the nut. BKH

BKH is right, check and see if there are any numbers on the prop.

Like Joe said, it is possible that your prop has more pitch than your motor can turn. A higher pitch prop will result in more speed at a given RPM... IF your motor has enough power to turn it that fast.

Your problem could still very well be engine-related. Most likely the prop was designed for use on our boats- there arent that many other ski boats turning RH props. It could just be a bad performing prop though.

79, youre right, as is Mr. Green. The yield strength is how much force it takes to deform the metal permanantly. The modulus of elasticity can be used to predict the load at which the metal will buckle under compression. I am not a mechanical engineer, so I cant say which property is more applicable to a prop hitting an underwater object.

JoeinNY wrote: I am a mechanical engineer and I can reasonably say your screwed either way don't hit stuff with your prop, and if you have to hit stuff with your prop hit soft stuff and hit it slowly...

Based upon that elegant and poignant analysis, I'm guessing maybe BSME S.U.N.Y.

BTW - I tell everyone that I let my engineering degree expire 20 years ago, but even I can understand this excellent engineering advice BKH

Modulus of elasticity tells you how much it will deform given a load. Yields aren't comparable because even though nibral his a higher yield, it will give a lot more than the stainless.