New Prop - Here Are The Stats

[AguaSki]

I just tested my new prop and these are the stats:<br /><br />• Prop = 3 blade, Michigan Wheel Rapture SS, 13.25 X 15<br />• WOT RPM’s = 5,300<br />• Top Speed = 38.8 mph (GPS verified), but averaged closer to 37.5 mph<br />• Motor = 1978 Johnson V4, 140 hp<br />• Hull = 18 foot, 1978 Glastron SSV-188 fitted with Smart Tabs<br /><br />Something just does not seem correct with the stats. When I run the above numbers to calculate slip on the RBBI Prop Calculator I am told that “You have selected a speed that is faster than the prop can go at these conditions with no slip.” The calculator confirmed what I was thinking as I was driving the boat on the lake.<br /><br />I just had the spring service completed on the boat at an authorized Johnson/Evinrude dealership and service center. When I dropped the boat off I specifically asked to have the tachometer checked against a shop tachometer. When I picked the boat up last week, the service manager said that the boat tachometer appeared to be accurate to within 100 RPM. Despite what I was told, I am again second guessing my tachometer. If I assume 7% slip, 38.8 mph, 2:1 gear ratio, and a 15 pitch prop, the RBBI Prop Calculator says that my RPM’s should be 5,875. Although the manufacturer RPM range is 4,500-5,500, I was actually hoping that my RPM’s would be in the 5,800-5,900 neighborhood.<br /><br />I am looking for guidance from the wise participants of this forum. Do I have a tachometer problem? If my RPM’s are really 5,300, then I am potentially lugging my motor. I just want to get this right so that I can be confident that my motor is running as efficiently as possible.<br /><br />Once I get this worked out I may try altering the set-up to get 5,800-5,900 RPM’s out of a 17 pitch prop.<br /><br />Please provide your thoughts and opinions.

 

[ob]

Re: New Prop - Here Are The Stats

Check to insure that the tach setting is set to exactly 6 position.With the silencer cover removed,insure that carb flaps are opening fully horizontal with the throttle control lever fully advanced (engine not running)and the timer base at this full throttle position is advanced all the way to its stop screw.Make sure to have someone spin the prop while shifting from neutral into forward gear to advance throttle.I'd think that 140 should rap out more r's pushing that 18 ft rig with a 13 1/4 X 15 .Have you obtained any compression readings lately?Has the engine set out of use for a spell and possibly has restrictive deposits within any of the carbs fixed orifices?What height is the anti-ventilation plate of the lower unit in relation to the bottom of the boat hull with engine trimmed level?Even,above,below?Has the engine ever ran above the current WOT rpms ,as propped or otherwise?

 

[Silvertip]

Re: New Prop - Here Are The Stats

Using the numbers you provided, the calculator I used is "spot-on" (7% slip/37.3 MPH/5300 RPM). I agree with OB that something is amiss with the 5300 RPM. I think your tach is reasonably correct but the engine is not reving as high as it should. Perhaps you have a misidentified prop (higher pitch than stated). That's been known to happen. Hull has some drag that is not readily apparent. Tabs not set correctly. Most calculators indicate to add 1-inch of pitch if using a cupped prop. If you've not done that perhaps that accounts for some of the error (but not for the 5300 RPM). Last question is, did the engine ever run higher than 5300 with any other prop?

 

[AguaSki]

Re: New Prop - Here Are The Stats

Thanks ob.<br /><br />

I'd think that 140 should rap out more r's pushing that 18 ft rig with a 13 1/4 X 15 .

That’s just it, according to my slip calculations it is impossible for me to be going 38.8 mph at 5,300 RPM’s with a 15 pitch prop. Since I know my pitch (15), gear ratio (2:1 from my research), and I have a GPS verified speed (38.8 mph), the only variable that I think could be off is my tachometer. I am guessing that my RPM’s are actually higher than 5,300. I was actually expecting to be in the 34-35 mph range with the new prop, so 38.8 mph was a bit surprising.<br /><br />I will do the checks you mentioned and report back.<br /><br />As far as the motor status and history goes, I purchased the boat and motor last May from the same Johnson/Evinrude dealership that serviced the motor last week. The motor was completely rebuilt and came with a 1 year warranty. I had to break-in this rebuilt motor as if it were a new motor. The dealership completed the first several break-in steps in their test tank, and I did the last 10 hours of break-in at the lake. I am moderately mechanically inclined, but know very little about outboard motors. To compensate for my mechanical deficiencies, I rely on regular maintenance completed by experts. A compression check and carburetor adjustment was on the list of checks that are completed by the service center during the just completed annual service. While I don’t know the exact compression results, I expect that any out of the ordinary compression readings would have been reported back to me. Since I bought the boat last spring, it has been used most every weekend. With the old prop, the RPM’s also always seemed to be at about 5,300 (41 mph GPS verified speed) and this is what prompted me to find a prop that allows for higher RPM’s. The prop that came with the boat did not have any markings, and the service center was unable to measure the old prop for pitch, but guessed the old prop was about a 17 pitch. The service center recommended a 15 pitch prop as this seemed to be where the boat performed best in the test tank. The anti-ventilation plate seems to be even with the bottom of the hull.<br /><br />Hopefully this additional information is helpful.

 

[AguaSki]

Re: New Prop - Here Are The Stats

Using the numbers you provided, the calculator I used is "spot-on" (7% slip/37.3 MPH/5300 RPM).

What calculator did you use? Is it available on-line?

 

[AguaSki]

Re: New Prop - Here Are The Stats

Last question is, did the engine ever run higher than 5300 with any other prop?

I have never seen the motor rev past 5,300. These RPM's were typical before the Smart Tab installation. As I have learned more about outboard motors I have made it my spring mission to get the RPM's where they belong.

 

[AguaSki]

Re: New Prop - Here Are The Stats

I think your tach is reasonably correct

This means that the service center was correct when they said my tachometer was accurate to within 100 RPM.<br /><br />Now I am totally confused. Any help is appreciated.

 

[walleyehed]

Re: New Prop - Here Are The Stats

I calculate your slip at 5300 and 38.8MPH with the 15 to be about 3.34%...I don't think that's even possible with that boat.<br />I'm not sure 7% is possible..I'd guess 8-10% at best with the AV plate even with boat bottom and a real good SS prop.<br />I'd venture to guess the tach was not properly checked, and I'd bet money the tach is off quite a bit.<br />Any chance the tach is a Teleflex????

 

[AguaSki]

Re: New Prop - Here Are The Stats

Most calculators indicate to add 1-inch of pitch if using a cupped prop.

My prop is cupped. I forgot to add the one inch of pitch. Now when I plug-in the numbers (5,300 RPM, 2:1 gear ratio, 16 pitch, and 38.8 mph) I am told the slip is 3.366%. At least I am no longer doing the impossible, but I am still confused.

 

[AguaSki]

Re: New Prop - Here Are The Stats

Thanks Walleyehed.<br /><br />

Any chance the tach is a Teleflex????

I am not sure about the tach brand. The tach display does not mention a brand and I can't see any branding on the back of the tach. The display measures about 3.5 inches in diameter. I think all the gauges, tach included, are original equipment on this boat.<br /><br />Is a Teleflex tach good/bad? Teleflex seems to be the dominate brand in all the catalogs and websites I have visited. Do you have a tach brand recommendation that is historically accurate? Before I can get this RPM thing straitened out I need to be sure I am accurately measuring my RPM's.

 

[walleyehed]

Re: New Prop - Here Are The Stats

For Johnson/Evinrude, I recommend ONLY OMC/BRP tachs (OEM).<br />Tachs tend to take a beating and even though they tare the most important gauge in the dash to tell you what your engine is doing, they are also the most over-looked.<br />I've sold Teleflex for many years, as has my Dad in his years of service, but when we were OMC dealers, OMC tachs only...years went by, we no longer sell J/E and I went to Teleflex tachs...some I've found to be close, some way off, especially if they are more than 2 years old.<br />Dhadley suggested I run the BRP tach...now I do, and I'll never run Teleflex again.<br />You are so right in taking the time to insure your RPM before proceding...it can run you in circles if you don't.

 

[Dhadley]

Re: New Prop - Here Are The Stats

Ask your tech how he verified the tach. Unless he put the boat in the water and ran it he could only verify the tach at idle or on short revs in neutral. We're not worried about that. It could be dead on at idle and 300-400 off at wot.

 

[Silvertip]

Re: New Prop - Here Are The Stats

I used this prop calculator using your numbers. Add one inch to the pitch for cupped prop. (16 instead of 15 in your case)<br /><br /> http://www.rbbi.com/folders/prop/propcalc.htm

 

[AguaSki]

Re: New Prop - Here Are The Stats

Looks like a new tach is in order. I seem to find Teleflex everywhere I look. A few OMC/BRP tachs are on Ebay right now. Any thoughts on where I should be looking?

 

[walleyehed]

Re: New Prop - Here Are The Stats

JohnRude dealer or a NEW OMC/BRP on ebay...

 

[AguaSki]

Re: New Prop - Here Are The Stats

I just ordered a new OMC tach from an Ebay store. It will be about a week before I receive, and then I am leaving town to take my kids on spring break vacation. I will revive this post in about two weeks with new stats.

 

[AguaSki]

Re: New Prop - Here Are The Stats

It has been a while since I posted updated information. The OMC tachometer I purchased off Ebay has been installed for a few weeks, but it has been difficult to get lake time. I had the boat out yesterday and these are the new stats.<br /><br />• Same load and set-up as my original post on 03/04/2006.<br />• WOT RPM’s were 5,350.<br />• Maximum GPS verified speed was 39.1 mph, but when I was watching the display I saw between 38 mph and 38.5 mph.<br /><br />My new tachometer is showing approximately the same RPM’s as my old tachometer and the local Johnson/Evinrude dealer said my old tachometer matched his shop tach to within 100 rpm. It appears that I am getting fairly consistent RPM readings. <br /><br />My prop slip does not look right. Using the RBBI online propeller calculator I entered the following inputs:<br /><br />• Engine RPM = 5,350<br />• Gear Ratio = 2<br />• Propeller Pitch = 16 (Actually a 15 pitch cupped prop)<br />• Speed MPH = 39.1<br /><br />These inputs tell me my slip is 0.035289719626168226 or 3.5%. Does that sound right? I have always been told to plan on about 10% slip.<br /><br />What else should I be looking at? I am trying to get my WOT RPM’s in the 5,500 to 5,800 range.

 

[mmainelli]

Re: New Prop - Here Are The Stats

You don't want to be just in the WOT range, you want to be close to 5800 rpm. At this time you're 450 rpm away. Also, prop pitch rating already takes cupping into consideration, so don't make your 15P into 16P. Best advice would be to borrow (or buy used) a 13P, trying to pick up about 400 rpm. And you're going crazy with stats (slip, out to 18 decimal places!). By the way, a 13.25 diameter wheel sounds "under-classed" for a V-4 140 hp. That diameter wheel is typically 70-120 hp. Try a 3 x 14.25 x 13P. You've got to trial and error your way into performance. You'll never read your way in with calculations.

 

[walleyehed]

Re: New Prop - Here Are The Stats

By the way, a 13.25 diameter wheel sounds "under-classed" for a V-4 140 hp. That diameter wheel is typically 70-120 hp. Try a 3 x 14.25 x 13P.

These all use the same props...<br />The 13-1/4 is very common for 13 spline shafts.<br />The 13 pitch would be a 14" diameter in an OEM prop. Maybe this has been mentioned, but have you pulled the airbox off and verified the throttle plates are perfectly horizontal at WOT? (ENGINE OFF)

 

[AguaSki]

Re: New Prop - Here Are The Stats

Maybe this has been mentioned, but have you pulled the airbox off and verified the throttle plates are perfectly horizontal at WOT? (ENGINE OFF)

Still learning, but I think I did what was asked. I just pulled 6 bolts from a black plastic cover over the carburetor. With the motor off I moved my throttle control to the WOT position. What I think are the throttle plates moved to a near horizontal position, but they were definitely not in a perfectly horizontal position. What adjustments, if any, should I make to get the throttle plates perfectly horizontal?