How many RPMs?? |
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TRBenj
Grand Poobah Joined: June-29-2005 Location: NWCT Status: Offline Points: 21131 |
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Posted: August-07-2006 at 11:24am |
Sounds like you have the right idea. 43.5 MPH is about right for your boat. If it runs smooth and has enough holeshot for your taste, then I wouldnt bother changing anything. If you want a spare prop, you could upgrade to an Acme and gain some performance by turning a little more RPM.
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Mr.Green
Newbie Joined: May-21-2006 Location: Zimbabwe Status: Offline Points: 32 |
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OK, I just gave my boat a tune up, fluids, plugs, belts, etc and took her out for a full throttle assault. It looked like one of the plugs wasnt firing. It is running better now for sure, but I am still suspect of the prop. I dont have the correct wrench to pull the nut so I still cant read the numbers. Anyway, GPS said 43.5 MPH and the tack was reading around 4300 or so max. I still think I might have the wrong prop.
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duffnit
Senior Member Joined: October-12-2005 Location: United States Status: Offline Points: 235 |
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To the best of my knowlege there are no upgrades.
I am the third owner. The first owners didn't winterize the boat. The second owner purchased the boat and did a short block rebuild. From what I recall (minimal at best) he only replaced what was damaged. If there are any upgrades I guess I'll just have to deal with it...LOL.... But now you've peaked my curiosity. Guess I'll look over it next chance I have and see if something amiss. d |
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Danny
"no offense- but the rate at which you spread bad information is very impressive" |
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TRBenj
Grand Poobah Joined: June-29-2005 Location: NWCT Status: Offline Points: 21131 |
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Wow, those numbers are extremely impressive for a standard motor. Theyre actually pretty good for a ProBoss- our '92 with GT40 heads turned 4800 or so with an Acme 422. Youre sure it doesnt have heads on it?
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duffnit
Senior Member Joined: October-12-2005 Location: United States Status: Offline Points: 235 |
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TRBenj, believe it or not it's a standard 351 PCM with a recent carb rebuild unless the performance angel came while I was sleeping. d
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Danny
"no offense- but the rate at which you spread bad information is very impressive" |
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The Dude
Platinum Member Joined: October-19-2004 Location: Houston Status: Offline Points: 1334 |
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Jim in Houston is right. Ethanol is new in Texas, even though we're home to all the refineries.
Jim, I've lost a hair on top end...wondered if it was the ethanol too... |
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Mullet Free since 93
95 Sport |
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TRBenj
Grand Poobah Joined: June-29-2005 Location: NWCT Status: Offline Points: 21131 |
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Duffnit, judging by those numbers, you must have the ProBoss or GT40 motor, correct?
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duffnit
Senior Member Joined: October-12-2005 Location: United States Status: Offline Points: 235 |
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FWIW, the prop that came on my boat was a 4 blade 18 p OJ. The boat would run 46 WOT (GPS)but only turn around 4100 rpms.
Upon talking with Skidim I learned the prop I had was too large. After discussing several props with them, two were at the top of the list. The ACME #422, and the OJ #428 XMP. I bought the OJ because it was a little thicker and only a few $ more. Now the new prop turns just under 5000 rpms and pushes the boat to 47.8 GPS. Other noticable differences too as mentioned earlier, including the wake a little flatter on top. What I've learned is; props make a significant difference. We've all heard it, but know I know too!! Find the numbers on the prop and call Skidim or someone who deals with prop performance, if for no other reason than to eliminate the prop as your performance question. |
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Danny
"no offense- but the rate at which you spread bad information is very impressive" |
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p/allen
Gold Member Joined: March-14-2006 Location: Dixon Illinois Status: Offline Points: 942 |
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It is also feasable that the previous owner trashed the other prop and got this one cheap so he could sell the boat . It is also possible that he had a prop he wanted to keep and threw on this one. My .02
Pat |
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TRBenj
Grand Poobah Joined: June-29-2005 Location: NWCT Status: Offline Points: 21131 |
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The prop will have a slight effect on the wake, but not a huge one. Let us know if the prop has some dimensions on it. Joe/BKH/79/Hollywood/Mr Green- I wonder how many CCFan members are engineers? |
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Mr.Green
Newbie Joined: May-21-2006 Location: Zimbabwe Status: Offline Points: 32 |
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Is it possible that the previous owner was using this particular prop for wake effects? For wake boarding or surfing or something?? I do not know the previous owner and cant contact them. I will pull the nut and check the prop again. I will be out of town for a while so I will report back on monday.
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JoeinNY
Grand Poobah Joined: October-19-2005 Location: United States Status: Offline Points: 5695 |
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Not too far off BKH, MSME RIT. Although I think if I was smart I would have saved a few bucks and had a lot more female companionship down the road at the Suny U of B...
-Joe. |
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Hollywood
Moderator Group Joined: February-04-2004 Location: Twin Lakes, WI Status: Offline Points: 13512 |
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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.
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TRBenj
Grand Poobah Joined: June-29-2005 Location: NWCT Status: Offline Points: 21131 |
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Haha I like your reasoning! Ive hit stuff with my prop and I agree with your recommendation! |
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bkhallpass
Grand Poobah Joined: March-29-2005 Location: United States Status: Offline Points: 4723 |
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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 |
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Livin' the Dream
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JoeinNY
Grand Poobah Joined: October-19-2005 Location: United States Status: Offline Points: 5695 |
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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...
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TRBenj
Grand Poobah Joined: June-29-2005 Location: NWCT Status: Offline Points: 21131 |
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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. |
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JoeinNY
Grand Poobah Joined: October-19-2005 Location: United States Status: Offline Points: 5695 |
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Quote:
I would have thought that a prop with more pitch would be faster but with less hole shot and lower full throttle RPMs, like a higher gear in a car (lower numericaly) . This is somewhat true but as you alluded to its a bit more complicated than that. If you cannot reach the speed during which your engine produces its max horsepower because it takes more torque to get there than you have at your peak rpm then too steep a pitch prop will just all around suck, not good for top speed, not good for hole shot, not bad for gas mileage but the inevitable engine damage that will result from the overloading will wipe out that savings eventually.. |
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bkhallpass
Grand Poobah Joined: March-29-2005 Location: United States Status: Offline Points: 4723 |
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Yes, on my prop, the numbers are on the surface behind the nut. BKH
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Livin' the Dream
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Mr.Green
Newbie Joined: May-21-2006 Location: Zimbabwe Status: Offline Points: 32 |
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Thats kinda correct but not totally, you can bend metal with out yielding it.
Anyway, its not a stock prop, but it was confusing to me that it would have less RPMs AND less speed. I would have thought that a prop with more pitch would be faster but with less hole shot and lower full throttle RPMs, like a higher gear in a car (lower numericaly) . But I know there is more to defining a boat prop than there is a gear in a car. I am beginning to think it may be engine related, but still not sure. It starts and runs fine but I am beginning to wonder. Why cant I find the right numbers on my prop?? Maybe they are on the surface behind the nut...???... |
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79nautique
Grand Poobah Joined: January-27-2004 Location: United States Status: Offline Points: 7872 |
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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.
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TRBenj
Grand Poobah Joined: June-29-2005 Location: NWCT Status: Offline Points: 21131 |
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BKH is right, but a stock 240 horse 351w in good tune should turn 4400 RPM with a stock-ish (~14x16) prop.
79, I dont know if I agree with you. This was posted on PN about a year ago:
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bkhallpass
Grand Poobah Joined: March-29-2005 Location: United States Status: Offline Points: 4723 |
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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 |
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Livin' the Dream
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79nautique
Grand Poobah Joined: January-27-2004 Location: United States Status: Offline Points: 7872 |
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nibral isn't that much softer than SS they are almost the same.
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TRBenj
Grand Poobah Joined: June-29-2005 Location: NWCT Status: Offline Points: 21131 |
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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. |
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Mr.Green
Newbie Joined: May-21-2006 Location: Zimbabwe Status: Offline Points: 32 |
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Like I said, I was clocked by someone else that said we were doing 44 but I dont believe his speedo either. I will keep digging into this. Thanks for the info, keep it coming. I take it the prop info I posted isnt the right info??
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TRBenj
Grand Poobah Joined: June-29-2005 Location: NWCT Status: Offline Points: 21131 |
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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. |
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Mr.Green
Newbie Joined: May-21-2006 Location: Zimbabwe Status: Offline Points: 32 |
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OK, it is a 123:1 trans and the prop says 374?? It has like a backwards 7 in front of the 374, mabe a T not stamped in correctly?? Anyway, you would think I would know more about thes propellar thingys considering I used to work at a Nautique dealer way back in the day. The boat is only turning 4000 RPM and doing 37 (on my speedo) on a good day.
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Jim_In_Houston
Platinum Member Joined: September-06-2004 Location: United States Status: Offline Points: 1120 |
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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. |
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Happy owner of a '66 and a '68 Mustang
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Jim_In_Houston
Platinum Member Joined: September-06-2004 Location: United States Status: Offline Points: 1120 |
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BTW, while we're (I'm) discussing Texas, the state just raised our speed limit to 80.
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Happy owner of a '66 and a '68 Mustang
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