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Predictor Prop Help

Marlboro

Marlboro

Member
Ok, with my other problem solved, I went out with the wife last night and took along a handheld gps. Here are the details....

1989 Predictor
1/2 tank of fuel about 20L
one wife
1989 Johnson GT100 (j100stlcem)
Not sure on brand of prop, its aluminum, 3 blade, 14" dia. 17P.

Somewhere between 3/4 throttle and full (closer to 3/4) gps read 53km/h (roughly 33 mph) and tach read 5500 RPM. I started slowing down and the alarm started going off (overheat warning?) Not sure what the recommended RPM range at wot is for this motor yet but i'm assuming 5500 is too much. This obviously leads me to believe I need to step up in pitch.

Might be helpful to lay out my goals: for pulling a waterskier, the 17P will be good, its pretty snappy and gets the boat up on plane. For the other 95% of the time when just my wife and I are cruising around the islands, I would like something more efficient. More speed is great but my wife and I are putting the priority on good handling, easy planing and at times slower cruising speeds(I know this sounds crazy). I have read many articles about the effects of pitch and number of blades on props as they relate to RPM's. I'm leaning towards a four bladed prop for their quick planing, better planing at reduced speeds and good handling. The pitch needs to be increased but by how much? I know there are still variables out there. I haven't really had the boat at WOT so we don't know the actual WOT numbers. Also don't know the recommended RPM range for this engine.

People keep telling me to just get a 19P and go, but I know they aren't taking the math into account. I welcome any recommendations or ideas on how to proceed here.
 
Actually 5500 is your WOT max rpm. I'm guessing that a 19 will probably work just fine, even for pulling skiers and a 21 might even work for non-towing work. So you're not going to like this answer but...try a 19. If you are hitting 5500 at 3/4 throttle, you are definitely underpropped.

Incidentally, my Merc 115 has a 22 pitch on it and I run a 25 on my Mariner 175 (and probably could go up 1 or 2,) both of which I run on my Predictor.
 
Thanks for the response. So if 5500 is the engines max RPM, I really need to go up, especially being i'm only around 3/4 throttle. So if each 1" of pitch is approx. a drop of 200RPM, and leaving some room for comfort (alarm went off at 5500), maybe even more than a 19P would be the way to go. 21P would drop things 800 RPM approx, adding a 4th blade approx 200 RPM more (thought I read somewhere that a 4th blade would drop RPMs again). If I estimate that the engine would spin in the neighborhood of 6000RPM at WOT with that 17P then that would put me approx 5000RPM with a 4 bladed 21P? Does this sound reasonable?

I guess I need to find a friend around here with a prop I could try. Really need to know where that WOT number would be with a certain pitch.
 
Remember that you want to hit as close to that MAX rpm at WOT as you can, so propping for 5k will probably be a bit too large of a prop, you want to shoot for between 5250 and 5500 at WOT with the lightest load you will carry.
 
Looks like i'll be picking up a 4 bladed 19P aluminum. Should let me wind it out and see the numbers with the throttle all the way down.

I'll update after some testing....
 
Predicter prop help

Give us your 4 blade 19p results and I'll put them in the prop calculator.
Go to this page under outboard technical discussion here:
http://checkmate-boats.com/forums/showthread.php?t=22802
He shows 1.5% slip with a 4 blade and 10% with a 3 blade.
Don't know your ratio on the GT 100 but someone in Wisconsin had one on a Carlson and it ran high 50s. Sold now. Do you have a big lower unit? (15")
When I had a 120 it was 2.15:1. I've got a 13 x 21 Michigan SS for sale.
13 spline vented in perfect condition.
 
1.5% slip is a nearly impossible goal. Even on the high tech race boats with the most efficient set ups and exstreme high dollar props see a minimum of 5%. 90% of the daily user performance boats will be hard pressed to see less than 8%. I have spent countless hours on the phone with Bblades and Cutting Edge Propellars and they will both tell you that nothing pushes water anywhere close to 100%. As they told me, if you slip factor comes out to o, 1, 2, 3, or 4%, there is problem with the calculation. Either the tach reading is off a bit, the gear ratio is not what it should be, or most commonly, the prop pitch is stamped wrong. I thought I had 5% one time with a 3blade aluminum prop at 69mph on gps, until I found out I was doing the math wrong!
 
I tried the 19p prop. Unfortunately some other problems kept me from really being able to go full out. Crappy weather and idle issues kept me down for the remainder of the season up here. I got my hands on a service manual, definitely some things to try over the winter starting with rebuilding the carbs. Hopefully have her running like a top next year!


I will definitely post numbers. If I had to guess, its still a bit short, it did ride/plane better than the 17p that was on there but I think it will still max out the Rpms and then some. I'll know more with more testing but a 21p might be in my plans.....
 
If youve got the motor up high enough you should be able to turn a 19 pitch prop at 6000 RPM in the 54 MPH range. I could do that all day with the old johnson 90 on my Enticer. Dont worry about going over the high RPM number. There is still plenty of usable power up to 6000 RPM and under 6K you wont kill your motor.

I'm talking about a good 19 pitch and not a mystery aluminum prop.
 
wired said:
Dont worry about going over the high RPM number. There is still plenty of usable power up to 6000 RPM and under 6K you wont kill your motor.
Click to expand...

I hear ya, problem is, anytime I get near that 5500 RPM mark the damn alarm starts going off. I assume its temperature???
 
If your spline count is 15 and housing is 4" or larger you need to tell us your gear ratio. Turn the flywheel until the prop turns once and count the revolutions.
You should have a 13 spline unless it's a commercial motor. Here are some good props for the small LU in MN:
http://minneapolis.craigslist.org/hnp/boa/4201909504.html
http://minneapolis.craigslist.org/dak/boa/4183998526.html
If your overheat warning is sounding full check the heads for too hot (110-120)
Bad thermostat, water pump, or some get sand embedded around the cyl.
Might get help on MarineEngine.com forum.
 
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