Another Prop Slip Question

[twschar]

Hi Guys (and Gals);<br />Is it possible to have too much prop pitch and have this cause slip?<br /><br />Here's my set-up and performance:<br />1994 Mirrocraft Aggressor (16ft Aluminun V-hull side consule)<br />1994 Mercury 60 HP 2-stroke (1.64:1 gear ratio)<br />WOT for motor is 5000-5500<br />The tachometer is a Medallion set on "3" (not sure of the model).<br />The combined weight of boat and motor is 1000-1100 lbs plus 2 people, 3 batteries, 15 gal gas, and another 100+ lbs of gear (roughly 1600-1800 lbs total)<br /><br />I bought the boat used with a 14" pitch stock(?) aluminum prop that looked like it may have been re-built.<br />With that prop, WOT was 6200 RPM and ~32 MPH (GPS) with the motor trim to get max MPH.<br /><br />Trying to get the RPM down, I went to a 15" pitch Turning Point Hustler figuring the higher pitch and cupping would help bring down the RPMs.<br /><br />Running with the new prop I'm still getting the same performance WOT performance (32 MPH @ 6200 RPM so I'm scratching my head. Calulated slip based on this is upper 30's @ WOT assuming the RPM is correct. The boat comes up on plane around 4000-4500 RPM.<br /><br />Did some research on the forums and looked at the following:<br />- Ventilation plate is even with the bottom of the boat<br />- bottom of the boat is "like new" condition - hardle a scratch<br />- Removing all transducers from back of the boat had negligible affect<br />- cutting payload down a couple hunderd pounds brings top end up to ~ 33.5 MPH (GPS) at the same max RPM (6200)<br />- If I throttle back to stay below Max WOT (5500 RPM), speed drops to 29 MPH @ 5500 RPM and 27 @ 5000, but it's hard to maintain constant speed under varying conditions.<br />- Still trying to figure out a good method of verifying the tach is correct<br />- Calculated Slip runs 60% below 3000 RPM then drops to ~39% @ 5000 RPM and holds there to WOT<br /><br />My objective is to get a set-up where I can keep the RPM below 5500 and still get decent performance. From what I've read, a 15" pitch prop is fairly high for this set-up and I'm concerned about going higher. Is it possible that the pitch is too high and this is causing cavitation problems?<br /><br />thanks.

 

[Boilermaker]

Re: Another Prop Slip Question

WELCOME ABOARD <br />I am not familure with Merc engines....but the setting on the back of the Teleflex tach is for how many poles your alternator has>>>not how many cylinders your engine has. It is how many pickup coils the alt. uses to produce the charging voltage.>>(just in case you didn't know that.)

 

[twschar]

Re: Another Prop Slip Question

Thanks BM. I knew about the pole setting and confirmed that it was set on 3 (60 HP Merc is a 3 cylinder). But I still want to run a test where I change the setting to make sure its responding properly when the setings are changed. <br />I'm also looking for suggestions on ways to check the tach's readings vs someother means of measuring RPM. I have an old car engine analyzer with leads that attach to the engine coil. But that's for a 4 stroke engine and only has settings for 4-, 6-, and 8-cylinder engines. I believe that setting it on 6-cylinder (4-stroke) would give the correct RPM for a 3-cylinder (2-stroke) if I knew were to hook up on the outboard.

 

[Purduebarry]

Re: Another Prop Slip Question

I looked at the Turning Points too. They run a bit smaller dimensions which may negate the 1" increase in pitch. Plus the efficiency of the Turning Point with cupping etc. may have kept the RPM's up.<br /><br />I would think you would have to go to a 17 or 19 to decrease the RPMs to the desired range. From what I've read you will get about 300 to 500 RPM drop with each increase in pitch. But these are in 2" increments (i.e 15, 17, 19, 21). You only went up 1", was the dimension the same or 1/4" smaller? Regardless, I still think the TP is more efficient. <br /><br />I think it would take at least a 19" to drop you 1000-1200 RPM's if you stuck with the TP. A Michigan Wheel Vortex or OEM Match 17" might do the trick, but if the tach is correct you need 1200 reduction in RPM. The 15" obviously didn't get you anywhere and I don't think hole shot is the issue with a 16' Alum, but top end and desired RPM is. Confirm tach and go for a higher pitch? Hopefully Kenny or Dehadley can clear this up.

 

[Texasmark]

Re: Another Prop Slip Question

tws, if you were cavitating, you'd be increasing rpm's, how much depends. I seriously doubt making the change you did caused cavitation; if anything, the opposite.<br /><br />What I think you are experiencing is a problem that I noticed with my Ballistic excursion.<br /><br />Just because the same manufacturer changes the pitch of a prop doesn't mean that he doesn't touch other things too. For example, some props in the same class by one mfgr have different diameters. You'd think that as you took pitch in one direction, diameter would go in one direction also.....but not necessarily.<br /><br />Since your speed and rpm's are the same for both props, and your load is the same, and you are going from a conventional prop to a higher performance one, I'm going to say that your 15" prop has less blade area than the original.<br /><br />It is more efficient and all, but there is less blade to grab water and hence instead of getting more speed with your increase in pitch you got nothing. <br /><br />Has to be something like that or one of your numbers would have changed....(Course the weather is warming up and that could be "the" negating factor.....cost me 2 mph in tests between Jan and May with the same set of conditions otherwise.)<br /><br />That's one reason I stuck with Ballistic.....the boat came with one and I didn't want to throw in any more unknowns.....well, these guys played with the blade area on me and I didn't get the performance on my engine I expected by going up 4" in pitch to a 21. Had to go up another 3" and I'm in pretty good shape now. (Mine is a 2.3:1 ratio so don't get too hung up on my numbers)<br /><br />So, looks like you have some more money to spend if you choose to continue in the pursuit.....check in here for a prop if you're going after another one. They have some real good deals....bought my last one here.<br /><br />Mark

 

[twschar]

Re: Another Prop Slip Question

Thanks for the comments and thoughts so far.<br /><br />I'm concerned about going much higher in pitch. From what I've read, 15" pitch should be more than adequate for this motor and boat. The high slip numbers indicate something else is wrong - either the tach or ?????.<br /><br />I also made a mistacke for the tach manufacturer - it's a Medallion. I didn't see a model, just a S/N 061698 and Rev A on the back. The setting on the back is currently on "3" (same as when I bought it). Anyone know what the currect setting should be for a 60HP 2-stroke Mercury?

 

[walleyehed]

Re: Another Prop Slip Question

I think you better look farther into the tach....<br />If your speed is a known GPS reading, we have the gear ratio and a known 15P prop, with the numbers you list that equates to over 43% slip, indicating to me the tach is either set wrong or WAY off.<br />I think your actual RPM is only 4200-4400...

 

[Boilermaker]

Re: Another Prop Slip Question

I think he is confusing number of poles, with Number of cylinders.<br /> How many poles on a 1994 60hp, 3 cyl. Merc, alternator???

 

[twschar]

Re: Another Prop Slip Question

Kenny, I agree, the slip sounds way to high. I'm tied up till next week, but I'll check into the Tach around Monday. Any thoughts on the best way to check it? I have an old automotive engine analyzer with a tachometer with leads that attach to the coil, but not sure how to attach to the 94 Merc. Also would need to figure out how to convert from a 4 stroke auto setting to a 2-stroke outboard.<br /><br />BM, got your comment on confusing alternator poles with # of cylinders and understand. I still need to understand where to set the tach since it's a Medallion, not a Teleflex (see my correction above). Seeing how Medallion is no longer in business, not sure where to get documentation on the tach.