Prop Pitch

Rub-a-dub

Cadet
Joined
Feb 13, 2014
Messages
27
I have a 2017 Tracker with a 75hp 4stroke Merc. I am running full throttle 4800 rpm @ 38mph (gps) The specs say I should be 5200 full throttle. I am going to a SS prop to minimize scrapes and dings. My question..should I change my 17 pitch prop
to a different pitch to gain a couple hundred rpms?
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
If current prop is a 17 pitch go for same model one with 16 pitch whether alum or SS one. . Will up current wot rpm. BTW, how loaded do you like to go boating say, light, medium light or heavily loaded ?

Happy Boating
 

Rub-a-dub

Cadet
Joined
Feb 13, 2014
Messages
27
I usually am medium loaded (one passenger, full tank). I will try the 16p per recommendation...thanks!
 

Sea Rider

Supreme Mariner
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Sep 20, 2008
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You're welcome,

Report max wot rpm achieved as loaded. Would be great to know if the Rule of Thumb dials stated + 200 wot rpm or not going per one less pitch. I'm sure dials way more than that. Wild guessing + 350-400 rpm.. Only checkable with a wot run on flat calm water cond.

Happy Boating
 

harringtondav

Commander
Joined
May 26, 2018
Messages
2,438
Just a thought. If all you suffer is scratches and dings, SS is the way to go. I I/O in the sometimes trashy Miss. river. Dead heads (submerged, floating logs) are like enemy submarines. I've hit them with my SS prop. It came out fine. My prop shaft, not so good. So I stick to aluminum. They have taken one for the team, and saved my prop shaft many times.
 

WesNewell

Chief Petty Officer
Joined
Jan 3, 2018
Messages
497
2017 Tracker what? They have lots of models. Most of the ones i see at the dealership have the engine mounted too low to begin with, but no one can give you any good advice without knowing more than you provided.
 

Rub-a-dub

Cadet
Joined
Feb 13, 2014
Messages
27
Okay...…..Installed a 16 pitch SS. Had an increase of 400 rpm and 3 mph. This is just what I was looking for.
Thanks Sea Rider ! :thumb:
 

WesNewell

Chief Petty Officer
Joined
Jan 3, 2018
Messages
497
Theoretical speed at 4800 rpm with a 17p prop is 37 mph, yet you say you were getting 38 mph. That's faster than theoretically possible with a -3% slip. Theoretical speed at 5200 rpm with a 16p prop is 38 mph, yet you say you are getting 41 mph. That's faster than theoretically possible with a -8% slip. Somethings not right here.
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
Just a thought. If all you suffer is scratches and dings, SS is the way to go. I I/O in the sometimes trashy Miss. river. Dead heads (submerged, floating logs) are like enemy submarines. I've hit them with my SS prop. It came out fine. My prop shaft, not so good. So I stick to aluminum. They have taken one for the team, and saved my prop shaft many times.

That is something a lot of people pooh-bah as uncommon. Propshafts bend quite frequently when SS props are used. My SS and Propshaft both were badly wounded on rocks at about 21-22 mph. So much for the much praised Rubber Hub saving the prop
Click image for larger version  Name:	img579b.jpg Views:	4 Size:	142.1 KB ID:	10667618
 

harringtondav

Commander
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May 26, 2018
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That is something a lot of people pooh-bah as uncommon. Propshafts bend quite frequently when SS props are used.

I had two that looked just like that after hitting a Miss. river wing dam and a submerged old lock canal wall. Pilot error. A bent prop shaft with each. After hitting that dead head I had three bent shafts.

I worked with post heat treat shaft straightening in a trans. factory, so knew the process basics. I made a pair of oak support blocks for my 12T hyd. press. Using my bench centers and dial indicator I got all three w/in .0005" runout. 1/10th of the use limit. Magnaflux crack detector and developer verified the job. I put the last shaft back into service, with an aluminum prop. Haven't needed the spare shafts in 11+ yrs.
 

Sea Rider

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12,345
Okay...…..Installed a 16 pitch SS. Had an increase of 400 rpm and 3 mph. This is just what I was looking for.
Thanks Sea Rider ! :thumb:

Glad to read that one less pitch worked spot on for your application. This is another prime example that one less pitch doesn't dial the near cast on stone Rule of Thumb that states + 200 wot revs increase which everybody likes going for. Thanks for posting end results.

Happy Boating
 

wahlejim

Master Chief Petty Officer
Joined
Jul 23, 2015
Messages
884
Before we go casting doubts on the rule of thumb, or come to a decisive conclusion on numbers for this one, I think we need to verify some numbers. A 2017 75hp Merc has a gear ration of 2.07. IF the numbers are correct, we are currently seeing 5200 RPM, 16 pitch prop, 41mph. That is one magical prop since we have a prop slip of -8%. I am glad the OP is seeing the performance he is looking for here, hell I know I would be happy if my prop was magically spinning at -8% slip, but something is amiss here.
 

wahlejim

Master Chief Petty Officer
Joined
Jul 23, 2015
Messages
884
My biggest concern isn't that your speed is off, but rather, your tachometer. If you change the -8 to a +8, which is much more reasonable, you could be spinning as high as 6088 RPM, which is way over the 5500 max for that outboard. If you are only moving at 37mph, you are right at the top of the range. Regardless, you are flirting with this line and I think you are very close to where you need to be. The biggest concern is the RPM. Running over max is not good for that motor.
 

WesNewell

Chief Petty Officer
Joined
Jan 3, 2018
Messages
497
Before even considering prop pitch one should get the proper engine height. And that can very as much as several inches depending on whether the engine is mounted straight to the transom or on a jack plate. Mounted too low can cause up to a 25% reduction in top speed and also a reduction of rpm's because of the additional drag of the engine shaft. That's why I asked which Tracker he had. Should definitely check his tach setting for his engine too. My brothers ranger RT188 had the wrong setting for 2 years from the factory. Really screws you up when trying to determine prop pitch. And his engine was mounted ~2 inches too low with his 4" jack plate causing him ~ 10-15% in top speed. My new boat also had the engine mounted too low from the factory. Raising it just 1.5" gave me a 10% speed increase.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Before venturing in changing any installed prop for a different pitch one, must be 100% sure if current prop is revving right, if engine and transom are well height matched for lower leg to achieve the least possible drag and if tach reads acurate.

The issue with the latter is that near no one tachs current installed tach with another one to check if both reads same. If OP states that the max wot rpm range for that engine is 5200 he's at the max wot range already and very pleased with its performance.

Happy Boating
 
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