Smaller pitch now less RPM, speed and blow out.

RobsTV · May 15, 2006

Boat and motor in sig. Sorry for long detailed post. Original prop numbers were not readable. Measured and it was 13" diameter. Specs in OEM manual suggested standard equipped prop was 13x19. Prop is not stainless steel or aluminum, as a magnet easily sticks to prop. Test with original prop had two of us, plus 35 gallons fuel, at max WOT of 4800RPM (new Teleflex) and a max speed of 38MPH GPS. Fuel was old and treated, so original test was really just to use up all the old gas, and how the boat/motor were after being stored for 15 years. (carbs and maintenance was done prior to test). Based on what I read here, we need to get RPM's up near 5800. After testing, noticed some good size stress cracks on prop. Replaced with Michigan 13 1/4 x 17 aluminum (11003), to help bring RPM's up. Guessed that new gas, new prop, plus doing a link and sync would get us to our target RPM's. Now with new prop, 3 people and 40 gallons new gas, but link and sync not yet performed, best we could get was 4400 RPM and 31 MPH. Also with new prop, sometimes if I wasn't carefully accelerating, prop would blow out or wash out or cavitation (or what ever the term is when RPM's suddenly rise, yet boat slows when powering). It was like motor was now to high on transom (fully trimed down)?. Looking at motor, and it was about 3/4" above top of transom, so thought we could lower it down. Another possibility of cause was new transducer that we also mounted about 12" starboard of center line. On return trip we raised transducer, and the cavitation appeared to be gone. We also changed Power pack right before test, so if the new PP4 (yet looked like someone else had opened and possibly installed it) was defective, it may have caused lower RPM's? We plan on putting old prop on for one more test, just to confirm PP4 is good, and see how it works with transducer. Transducer needs moved anyway, as it fails about 10MPH. Of course we know link and sync will need to be done. But, that would not have caused the issues. Lower RPM, speed and cavitation. Could transducer be whole cavitation problem? Is motor too high? Should we lower it? Old prop never cavitated Is old steel(?) prop worth fixing, or obsolete, plus too large (or small)? Thanks in advance, Rob

ezeke · May 15, 2006

Re: Smaller pitch now less RPM, speed and blow out.

At 4400 Engine RPM the most you can do on a 17 pitch is 36.83 MPH with your motor, so I think your motor is doing well at 31 MPH with a 20' boat. A prop with a lower pitch will close the gap between optimum and actual by some amount, but to get to 5,800 RPM you will probably have to go to a 13 pitch.

RobsTV · May 15, 2006

Re: Smaller pitch now less RPM, speed and blow out.

Thanks. What's up with the old prop being metal that a magnet can stick too? Was that common? Is it worthless nowadays? Should we spend the money on getting old prop repaired, or stick with the new prop and keep old as-is for emergency use?? Since old prop did 4800 RPM and 38MPH, does that sound like it was around 13x19? Motor height still seems odd as well.

ezeke · May 15, 2006

Re: Smaller pitch now less RPM, speed and blow out.

I'm thinking that the stainless prop was [EDIT] 19 Pitch. Transducer might cause ventilation (air in front of the blades). How high is the cavitation plate above the bottom of the boat?

RobsTV · May 15, 2006

Re: Smaller pitch now less RPM, speed and blow out.

Thought stainless steel was non-magnetic? (to confirm, just tested a bunch of SS silverware, and none of it would stick to magnet). Edited after checking: Cavitation plate is lined up about 1/4" to 1/8" below bottom of hull. Engine was raised up to it's current position before I got it. Old prop did not cavitate during 7 hours of high speed fuel wasting tests. Another thought. Is it possible rubber in standard hub in the new Michigan prop is slipping that much under heavy load, yet grabs fine other times? Or is that either a good or bad item? After measuring this, I am thinking height is about right (or at least I don't want to go any lower)?.?

Dave Abrahamson · May 15, 2006

Re: Smaller pitch now less RPM, speed and blow out.

You can scribe a mark on the new prop hub and on the prop itself. Then run the prop, take it off and see if the marks still line up. If no, the hub is slipping. And not all stainless is the same. Depends on the nickel content among other things. A magnet sticks to my stainless prop too. Good Luck! Dave

RobsTV · May 15, 2006

Re: Smaller pitch now less RPM, speed and blow out.

Thanks. Confirms old prop was SS. New prop is cupped, which if old prop was not cupped, may explain lower RPM. But still, going from 19 to 17 seems like that should have compensated for cupped prop?.? When a prop is older and warn, does it change to a lower pitch? I'll link & sync this week during next trip out, as well as mark prop and hub to check for slipping, plus will remove transponder during tests. Will report back with results.

budsbud66 · May 17, 2006

Re: Smaller pitch now less RPM, speed and blow out.

i think the aluminum prop is hurting your performance. I just won an ebay auction for a 13 3/8 x 17p in SS. If it doesnt do what i want it to do on my setup i will be selling it.. would you be interested? wont be more then 70$ incl shipping..

RobsTV · May 17, 2006

Re: Smaller pitch now less RPM, speed and blow out.

Thanks team_starcraft, Yes, let me know if you don't use it.

budsbud66 · May 17, 2006

Re: Smaller pitch now less RPM, speed and blow out.

I think it will be too much pitch for me.. but i cant find a happy medium between 15 and 17 pitch. 15 = too much RPM's 17 = too bad of hole shot.. but ive only compared them in aluminum. so now ill try SS.. The prop game is kinda fun.. i dont have the funds to try the real good ones thoe.

walleyehed · May 17, 2006

Re: Smaller pitch now less RPM, speed and blow out.

15P in a quality SS would be a good bet..it'll pull harder than a 15 alum, yet be more efficient.

RobsTV · May 20, 2006

Re: Smaller pitch now less RPM, speed and blow out.

Today's testing is done. Transducer location caused previous cavitation/ventilation/blowout thing. Moved and corrected. Checked link and sync, and all was correct. Had a possible bad coil (crack) and another with a bad wire. Only one new coil was in stock, which we replaced, and ordered other three. Ran with old SS prop to confirm previous numbers with new Powerpack and we were back up to 4900RPM, but at only 35MPH. Close enough. Must have been a tail wind last time when we hit 38MPH at 4800RPM. Still need more RPM. Will work on prop after the rest of the coils come in. New 13.25x17 aluminum prop would only pull 4600 RPM @ 31 MPH in today's tests I thought old prop was stock 13x19 but now I'm thinking it is also a 13x17 except it was SS. Will take to prop shop this week to see exactly what it is and probably get it rebuilt. New prop is cupped. Old one?.?. I don't really want to lose any more top end speed by going to a 15 pitch, but if wear and tear is too much by running this setup with WOT at 4900 (maybe the other coils will help a little), then perhaps a 15 pitch is better? It jumps out of the hole now with the old SS prop and the new 17, so needs no improvement there. Perhaps tach is not accurate as well, although it is brand new Teleflex? Finally did a compression check and was shocked to see all cylinders 145 to 152 PSI! Not bad for an original 1975.

walleyehed · May 20, 2006

Re: Smaller pitch now less RPM, speed and blow out.

I'm shocked to see 145-152 as well.. about 130-135 is all I've ever seen..usually less. You're getting hung up on this speed thing when your engine is nowhere near the power band in which it should run. You're 800-1000 RPM short of where it should be, so I would check with a shop on the tach. By the way, what in the world makes you believe a 15P will give too much RPM????? If it DID give a full 400 more, you're still short of where it should be.

RobsTV · May 21, 2006

Re: Smaller pitch now less RPM, speed and blow out.

Think you might be confusing my post with those of team_starcraft. I don't think 15P will be too much RPM, (still far from it), but I do think it will be too much loss of top speed going with 15P. Am I thinking wrong? My thought "example": If a motor was tuned properly and with a specific prop was running 5800 RPM WOT (13P), yet only ran 28MPH. Raising pitch 4 (17P) might lower WOT RPM's down to 5000 RPM, but speed might increase to 32MPH. Again, just an example, not real numbers, but shows that you can not maintain optimal specified WOT if you desire either more hole shot or more top end speed. Does that mean motor and prop combo is not correct? Along the same train of thought, if you had a motor that was not properly tuned, or had a problem, and WOT was 1000 RPM less than it should be, by lowering prop pitch 4, that would bring motor RPM's back up close to where it should be, which may lead you to think all is well, but in reality, it is still in bad shape and screwed up. Point is, running proper "spec" WOT ability alone does not have anything to do with wear and tear of motor, and is not an indicator of a properly running or tuned motor. Wrong? The speed thing is somewhat important because the other boats we hang with all have at least 200HP that they like to run at above 30MPH. Here on the central west coast of Florida, we need to go out at least 20 to 30 miles miles just to hit 45ft of water.

Texasmark · May 21, 2006

Re: Smaller pitch now less RPM, speed and blow out.

On the magnet, the amount of nickle determines how well it will stick, as mentioned, which also determines the grade (number) of the steel. The more iron in the mix, the more readily it will stick and the faster the prop will rust....yes SS props rusting. BTDT Your silverware probably has much more nickle in it than your prop since it is eating utensils. Hey Robs, did I read you right on old gas. You said the boat had not been run for 15 years and you did a "burn off the old gas run". Was the fuel 15 years old? Mark Mark

walleyehed · May 21, 2006

Re: Smaller pitch now less RPM, speed and blow out.

Point is, running proper "spec" WOT ability alone does not have anything to do with wear and tear of motor, and is not an indicator of a properly running or tuned motor. Wrong?

You don't seem to understand that you DO NOT increase pitch to increase speed...speed is a Direct result of efficiency, of which you have very little at 4900. RPM is the most important factor in the set-up, speed is only a result. If you continue to run at 4900-5000 WOT, all you will have left will be a worn and torn engine.

If a motor was tuned properly and with a specific prop was running 5800 RPM WOT (13P), yet only ran 28MPH. Raising pitch 4 (17P) might lower WOT RPM's down to 5000 RPM, but speed might increase to 32MPH. Again, just an example, not real numbers, but shows that you can not maintain optimal specified WOT if you desire either more hole shot or more top end speed. Does that mean motor and prop combo is not correct?

If you cannot maintain the speed you want and keep the RPM up, you need a bigger engine...you simply do not increase pitch to gain speed unless you are already over the top of 6000RPM(best hole-shot). It takes 17HP to gain 1MPH in general...you can gain more speed by optimizing your set-up, than you will by just changing props. You do not change props to suit the speed, you change after optimizing to suit RPM.

RobsTV · May 22, 2006

Re: Smaller pitch now less RPM, speed and blow out.

Texasmark, don't know age of old fuel. Was stored 15 years, bought, carbs rebuilt, tweaked and tested/towed back in (primer bulb kept collapsing), and then sat for 2 or 3 years before I bought it. Fuel was probably 2 or 3 years old. I treated it with Seafoam and replaced 20% with fresh before testing and using it all up. Filled with fresh, added back the fuel removed from first test, and used that up as well. Now it's all fresh (45 gallons). Ran pretty good though during every test. Changed fuel filter 4 times as a precaution. walleyehed, yes I still do not understand speed and pitch relationship. If you throw out speed, and prop motor to run specific RPM, then wouldn't all the boats of the same class run the same prop? Why would we need 7 prop choices for a motor? Sunday's tests showed we were turning 27-28MPH at 3900RPM's cruising speed (1000RPM less than WOT), and motor sounded and seemed to run great at those numbers. If I dropped from a 17 pitch to a 15 pitch, I would need to turn 5600RPM to go the same WOT speed the 17 pitch prop does at 4900RPM's (with slip being equal). That would be fine, but I doubt it would pick up 700RPM's. Plus, the real kicker to the deal is I just bought the 17 pitch mail order, and can not exchange it, so would need to sell at a loss if I tested another pitch. I understand less efficient and more wear and tear will result with above. The 1975 Evinrude factory service manual states that the standard equipped prop is 13x19. Sure, it must be changed to something better suited to the hull and use motor will be run in, but how many owners never bother, and just run it as shipped? If it tore up motor too soon, yet was the prop designated by manufacturer, wouldn't they suggest a lower pitch to begin with as standard? The 135HP was the largest HP motor Evinrude made in 1975, so I would think it was designed primarily for use with the larger heavier hulls that would drop RPM's the most. Thanks for the input. I'm sure it will kick in my brain shortly.

Texasmark · May 22, 2006

Re: Smaller pitch now less RPM, speed and blow out.

One way to measure the efficiency Kenny was addressing (in my opinion) is "prop slip". I have proven it several times on my current boat. The lower the slip the higher the efficiency and the higher the proportional speed. I have a padded boat. At 4500 rpm's my pad is not working and I am only doing somewhere around 35 mph (don't remember exactly but the principle is where we are headed). When I increase rpm's to 5600 my speed goes to 52 (on a cold day). So I get a disproportional increase in speed (24% rpm increase and 48% speed increase), in hard earned mph....faster you go the harder they are to get.....because I get on the pad, the hull drag decreases drastically which decreases the slip drastically. To prove it you can to to the prop calculator and play with actual and theoretical speeds and see your slip for yourself. HTH Mark

RobsTV · May 23, 2006

Re: Smaller pitch now less RPM, speed and blow out.

Have steplift V20 hull. The 135HP was 1.92 gear ratio. (I think it is important to note again that this was the most powerful motor you could get in 1975) Looking like about 15% slip here at both 4900 RPM WOT (35MPH) and 3900 RPM (28MPH) cruising speed. I did not record RPM and speed before getting up on plane, but I think it took about 3200 RPM's. 15% slip when on plane doesn't seem like a bad number, but of course depends on accurate tach, which I haven't had accuracy confirmed on the new Teleflex. If it was off, and let's say I was really turning 5500 RPM WOT, then slip would shoot up to 24%, or be less efficient.?.? Tach seems correct at 800 RPM idle, and stalls if RPM is lowered to 600 RPM (in the water). 1000 - 1050 RPM cruise down no wake river equals about 4.6 to 5 MPH(GPS). BTW, unlike other posts I've read about idle on these 135's, this thing idles perfectly smooth. Some results with prop slip calculator if slip stayed as constant as it is now (15%). With 17P at recommended 5800 RPM speed = 42MPH or great! (I wish) Cruising speed would stay "where I am now", or 28MPH @ 3900RPM. If I went to 15P, and "if" WOT at 5800RPM would drop speed to 37MPH, or close to what I am at now running 900 less RPM. I would need to raise cruising RPM's to 4500 to maintain 28MPH. It would also take more RPM's to get up to plane speed. Confused on how that would be better? If I dropped to 13P, which has a better chance of bringing me up to 5800 RPM, my new top end speed would only be 31MPH, and I would need to maintain 5200 RPM's to cruise at 28MPH. Might take 4200 RPM just to get up on a plane. EDIT: Just took a look at on-line manual for the 1975 135HP to see what recommended cruising RPM speed was. Could not find, but did see that WOT for this motor is only 5000 RPM. Later year Evinrudes did show up to 6000 RPM WOT, but this 1975 showed only 5000 RPM's. I read in these forums that with today's junk gas you need to turn more, like 5800 RPM, but was that recommendation in reference to a motor designed originally to turn 5500 RPM? Heck, if my tach was off by only 5%, I would then be turning close to 5200 RPM's WOT as it is now setup. What if I had a 1975 75HP Evinrude speced at WOT of 5500 RPM? How about 1981 200HP rated at WOT of 5750 RPM? Seems like 5800 as a general rule is a bad rule. Add 800? So those with '81 200HP will have issues unless they run at 6550 RPM's?.?.?. See how I am confused? One online prop website (turningpointpropellers) selection tool stated that the WOT for prop choice with the 135HP needed to be 4750-5250RPM's.

Texasmark · May 23, 2006

Re: Smaller pitch now less RPM, speed and blow out.

I think as you slow the boat to get your rpm's up you are increasing your hull wetted surface increasing your drag and increasing your slip so your numbers may be off when you change pitch. Don't know what to tell you about WOT rpm recommendations, past to present. Maybe Dhaley can jump in here as he seems to have a lot of experience with OMC's and performance from those days. I run my engine higher rpm's than recommended and don't personally have a problem with it. Matter of fact, I get better performance and response when it is 500 or so above the max recommendation. Mark

Smaller pitch now less RPM, speed and blow out.

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