Help me find a new prop. Results

rderenzy

Petty Officer 1st Class
Joined
Oct 5, 2015
Messages
333
1997
19.6 sylvan open bow
Merc 4.3 lx gen+
Current prop is old and I think pretty dinged up
14.25 x21
Aluminum

WOT 47 mph but at 4900 rpm
Water a little choppy but not too bad
320lbs worth of gear and people

I like to cruz but I'd like to tow tubes too
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Top of the page, Boat Parts and Accessories. Lots of props. Don't know where your rpms are with respect to what your WOT engine rating. If you are above it you need to add some pitch or below it drop the pitch. Rule of thumb is about 150-200 rpm per inch assuming that nothing else changed...prop brand, type, material, etc. and the amount of change depends on the rig.

For cruising and tubing, a 4 blade prop is recommended by most folks on here as you get better control when you are just above plane....boat doesn't want to fall back off plane and giving gas it comes back up and runs off so back it off again.......etc. The extra blade costs you some rpm so what I read is drop 1" of pitch for the 4 th blade if everything else is the same and ok to keep your engine rpms where they were....speed will fall off slightly but hole shot will improve significantly.
 

rderenzy

Petty Officer 1st Class
Joined
Oct 5, 2015
Messages
333
my RPM should be 4400-4800 at WOT
I'm worried because I was pretty much to 5k rpm

I did the prop selector, has me at 21 for sports, 24 for speed.

what way does the rpms go up...
so I'm at 21 now
if I go to a 19, that will do what? go to 5400 rpm? or go to4600 rpm?
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,464
what way does the rpms go up...
so I'm at 21 now
if I go to a 19, that will do what? go to 5400 rpm? or go to4600 rpm?

Ayuh,.... It sounds like ya need a 21" pitch, or 23" pitch,....

You'll have the best power for water sports with the 21", the 23" will labor the motor, pullin' up a skier or tube,...

Goin' down in pitch, raises the rpms,....

Goin' up in pitch, lowers the rpms,....
 

rderenzy

Petty Officer 1st Class
Joined
Oct 5, 2015
Messages
333
ahhhh. so I don't want to go lower, cause it will raise the RPM that is great advise, thank you. Bondo - your thoughts on my situation for a 3 blade vs 4 blade?
so my current 21 that hits 5000 rpm - should I just be very careful and not WOT it?
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
It appears to me your numbers may not be accurate. 47 @ 4900 indicates 20% slip 12% is more typical.
Is your speed by gps?Only way for truly accurate speed.As you can account for tides and current.
Also not unusual for a tach to not be accurate.
Perhaps you can compare to a shop tach.
 
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Maclin

Admiral
Joined
May 27, 2007
Messages
6,761
Hey Spike, what gear ratio did you plug in to the calculator? I was getting a lower slip%.
 

rderenzy

Petty Officer 1st Class
Joined
Oct 5, 2015
Messages
333
if I am truly getting 20% slip, what does that mean or what is wrong?
prop is a little dinged up

the speed 47 was from GPS - RPM from the dash, which could be inaccurate...

not sure really where I go from here, thanks for your help and input spike
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
20% is really high Could be the prop sometimes the tach is really high but seems unlikely in this case.
Being 19 years old it may be time for a new prop.I like the turning point Hustler Though it may need to be 1"lower pitch.
The solas Amita and Rubex have a good reputation.
 

Maclin

Admiral
Joined
May 27, 2007
Messages
6,761
Thanks Steel, I was using 1.78-1.81. I may be used to VP ranges for that engine :)
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
My 97 sales brochure on the lx is 190 hp, 44-4800 rpm, Alpha drive:

O = 1.62
P = 1.81
H = 1.94

No Bravo 1 with that engine, Bravo 2 and 3 for large boats but numbers are 2 and higher as you would expect.

Edit: I think you are getting distracted. I think you were where you want to be with the 4 blade alum, dropping 1 inch in pitch from your current prop. You want things that a lot of thrust gets you at lower rpms which is what shallow pitched, multi bladed (and larger diameter too but not necessary here) get for you. Once you get your "gear train" setup up for that, you can expect to over rev on light loads, trimmed to suit, and WOT.............................Welllllll cut the throttle back to where you are comfortable with the rpms.

Besides my arithmetic says that 5000/4800 is like 4.1% ovder rated...which is just a guess under usually estimated worse conditions. Heck you have more error than that in your instrumentation. On harming the engine if you do, heck, my 2011 little push rod, conventional GM V8, of 4.8 L is 302 HP rated at 5600 rpm and it's not a marine conversion, made to run wide open all day long!

I think you got lost in the fog. Back your scope up from 10 to something like 3 or 4 and get a larger field of view. Good luck!
 
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Leardriver

Petty Officer 1st Class
Joined
Oct 7, 2008
Messages
373
Your RPM's are perfect. With 3 or 4 people in the boat, more gas, and a 90 degree day, your RPM will slip down a couple hundred. If you want to pull a slalom skier, you want to be in the upper end of the allowable RPM range. No one runs their boat at the max rev range very often other than to verify that the prop is appropriate.
I would find a 4 blade 20 pitch and control over rev with my throttle control.
 
Joined
May 23, 2016
Messages
1
I would recommend going with a 20p 4 blade with between a 14 1/2 to 14 7/8"diameter if you are looking for something to give you a better cruise range and fuel economy as well as low range pulling torque. Keep in mind that going from a alum prop to a SST prop will normally lower your rpm's a little as well due to advanced blade design, less blade flex and more cup. A alum prop will normally have more slip due to the blade flex and less cup compared to a good SST prop.
 

rderenzy

Petty Officer 1st Class
Joined
Oct 5, 2015
Messages
333
all great info guys! thank you very much - good point about having more ppl in the boat and the RPMs going down. I think the 4 blade, 20p is the way to go.
I pulled some tubers yesterday with my current 21p and it handled GREAT! couldn't even feel the tuber with the hole shots and getting to plane.
but I forgot to check RPMs while pulling the tuber - DOH! but boat seemed to do VERY well

thanks all
 

turbocorrado

Petty Officer 1st Class
Joined
Aug 14, 2013
Messages
251
My 97 sales brochure on the lx is 190 hp, 44-4800 rpm, Alpha drive:

O = 1.62
P = 1.81
H = 1.94

No Bravo 1 with that engine, Bravo 2 and 3 for large boats but numbers are 2 and higher as you would expect.

slightly off topic here, but im trying to figure out my gear ratio for my alpha and the sticker is gone, i can see a serial number on the side of the casing that is punched into it. boat is also a 97. the O,P,H where would i find this info on the drive??
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Don't worry about it. Mark a white line on the crankshaft pulley where the timing needle is pointing, or some reliable reference point you can refer to.

Mark a line on the prop hub and slide on over to the gear box housing.

Put the shift/throttle lever on the remote in F gear.

Rotate the engine one complete revolution as determined by the crankshaft pulley mark. Wouldn't hurt to have someone monitor the mark on the prop.
--------------------
To get things into perspective, if it were a straight shaft, no gear ratio, it would be a 1:1 system and the prop would follow the mark on the crankshaft pulley. If the engine pulley revolves one full circle, the mark on the prop would revolve one full circle also.

If there was a gear ratio and it was 2.0 the engine would revolve twice for the prop to revolve once. So if you rolled the crankshaft a full revolution the mark on the prop would have only gone 0.5 revolution or half way around. We are operating between these extremes.
------------------

If your gear ratio is 1.62 the prop will move around 0.61 rev. Take a string and wrap it around the lower unit housing where you made your mark and measure the length. Assuming you are running a 4 ? diameter lower unit casting, it would be 4 ?" x pi. Pi = 22/7 or 3.1417 (relationship of the circumference of a circle to the diameter). Doing the math I have 13.35 inches or right at 13 ⅜" total. So 0.61 x 13.35 = 8.14" (8 ⅛" approx.)......the prop mark would have moved 8.14" around the circle.

For the 1.81, the ratio is .55 (larger ratio so less movement) rev so that would be 13.35 x .55 = 7.34" (7 ⅜" approx.) movement around the circle.

For the 1.94 ratio it would be right at half way around the circle...about 6 ⅝" travel.

I think I got all the arithmetic (algebra, naw, it's circles and lines so it's Plane Geometry....geez been a long time) right.

Try that.
 
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