Sizing your prop
 

Many of you may know this, but for those that don't.....


The best propeller size for your boat and engine combination is based on the recommended operating range at wide open throttle (w.o.t.) for your engine, which you will find in your operator's manual. This will be expressed in terms of a certain horsepower at a certain RPM (revolutions per minute).

The goal in prop selection is to determine what propeller style and size will maximize performance for your boat, while allowing your engine to operate in the recommended RPM range. The correct propeller will prevent the engine from over-revving, yet allow it to reach the minimum RPM where maximum horsepower is produced.

Run the boat/motor at w.o.t. under normal operating load to determine the maximum RPM you are able to obtain. A tachometer is necessary for this test. Adjust the motor trim angle for the optimum performance. If during this test, you begin to exceed the maximum rated RPM of the engine, reduce throttle setting to a position where maximum RPM is not exceeded.

If your test results in your being able to over-rev the engine, you need to increase the pitch of the propeller. Increasing the pitch increment by 1" will result in approximately 200 RPM drop. If your testing shows, however, that you are only able to obtain a RPM somewhat lower than the maximum rating given by your engine manufacturer, you would need to decrease pitch. Decreasing pitch would increase your RPM.

Example:

Operating Range =


5000-5600 RPM

Top End of Operating Range =


5600 RPM

Tachometer Reading =


4800 RPM

Difference =


800 RPM

For every 1" of pitch size, the effect will be approximately 200 RPM. Knowing this, take the difference in the above example at 800 and divide it by 200. The result is 4. The prop to use will be 4" in pitch less than the prop that was used.


Switching from an uncupped to a cupped propeller will also reduce your RPM. The cupped propeller of the same pitch and diameter will typically reduce your RPM by approximately 200.

Once your wide open throttle RPM falls within the recommended range of the engine manufacturer, you have a propeller that is suited correctly for your boat with respect to RPM. If you use your boat for fishing, cruising and skiing, one prop probably won't do all three things equally well. It is best in circumstances like this to have two propellers. One to accommodate one set of circumstances and the other to perform best under the different load. It is imperative that the wide open throttle RPM fall within the range specified by your engine manufacturer.

Re: Sizing your prop
 

;DNICE TO KNOW THANKS

Re: Sizing your prop
 

Good Post!! ;)

Re: Sizing your prop
 

Well put, Tin...only thing I'd add to that is ''trim that motor at WOT til she SUCKIN' DUCKS!!" ;) ...

Re: Sizing your prop
 

How would that apply to a kicker? Any rule of thumb there....<aside from keep it away from a spinning prop>

Re: Sizing your prop
 

Thanks for the info.

Something that has me confused though is WOT (as it relates to prop choice).
Best performance, gas mileage and least wear and tear are obtained when running at WOT, so does that mean you should try to always run at WOT?

Example, with setup perfect prop so motor runs at recommended 5800 RPM WOT:
Is it better to run 5800 RPM all the time? (best for engine and fuel usage?)
4800 RPM most of the time?
3000 RPM if it gets you on a plane?
Run lower than WOT, such as 4200 RPM most of the time, but make sure you run WOT (5800RPM) several times during outing?

Lower cruise speed cause more wear and tear and reduced mileage?

Re: Sizing your prop
 

You will definately NOT be getting best gas mileage at WOT. MORE wear and tear at WOT too. Only thing you will get at WOT is top speed.

Re: Sizing your prop
 

Rob, I understand your confusion here...we're not sayin' best performance/ gas, etc at WOT...no, that's when you're gonna be using MOST fuel...

a prop that allows the motor the obtain it's max mfrs rpms at WOT is the correct prop and throughout the operating range, particularly at ''cruise'', will usually deliver better fuel economy than a prop that doesn't allow max rpms...hope that clears it up for ya ;) ...

Re: Sizing your prop
 

Thanks, makes sense.
"Tune" with prop that allows your motor to run WOT at it's max specified WOT RPM, then it will be properly tuned to run it's best at all RPM's.

Re: Sizing your prop
 

"Loading" your engine within the recommended RPM range will also ultimately determine the life of the motor. Following the guidelines that Tin posted (excellent post BTW Tin), you're looking to obtain the Mfg suggested range. If you are below that range at WOT, you're working the motor too hard, burning excess gas, and poor performance. Above the range, and you're not working the motor correctly and will over-rev the motor. Not good.

Re: Sizing your prop
 

If you compare the performance tests of a large number of outboard powered boats you will find that best fuel economy is almost always @4000rpm (cruising) or 1000rpm (trolling). 3000rpm is most often the worst for fuel economy. This all is effected by the fact that most boats are struggling to be on a plane @3000rpm and not in the engines designed power band.

Re: Sizing your prop
 

I'm running a 165 HP 4.3L v6 chevy i/o. Anybody know the max operating rpm's at WOT?

Re: Sizing your prop
 

I know as a rule of thumb your RPMs won't be as high as a 2cycle outboard +- 5000 to 6000....I think you should be somewhere around 42-4500 MAX, but you can't quote me on that

Re: Sizing your prop
 

Sounds about right phester.

Re: Sizing your prop
 

I'm in the ballpark, turning a 141/2 x 21 prop.
Thanks,
Rev

Re: Sizing your prop
 

Rev, didja ever find out the max RPMS for your motor?...

Re: Sizing your prop
 

No, not officially. I was hoping someone here would know. I'll do some research and post what I find.

Re: Sizing your prop
 

http://boards.trailerboats.com/cgi-b...c;f=6;t=000851

Looks like 4400-4800

Re: Sizing your prop
 

And w/ a light load, good conditions, trimmed up, etc, will she turn those numbers w/ that prop?...if you can try a Merc Mirage in that size, I think you'l keep it...

Re: Sizing your prop
 

I'm not sure of WOT rpm's but I know I'm over 4000.
I'm a little gun shy of full throttle but next time I'm out I'll check it out.

Re: Sizing your prop
 

Went out last night and hit 4600-4700 rpm's at WOT with that 21 pitch prop according to my engine tach.I think the I/O's can spin a bigger pitch than OB's however no where near the speed. Only 32 mph. But I still love her.

Re: Sizing your prop
 

Rev, if she's tachin' out at those numbers, sounds like right prop ;) ...ENJOY!! ;D 8) ;D 8) ...

, Re: Sizing your prop
 

RP,
Thanx for the feedback, by the way I'm jealous of your boats' name. That's a winner!

Re: Sizing your prop
 

Thanks Rev...my daughter helped come up w/ that... ;) ...

Re: Sizing your prop
 

Not to beat a dead horse or thread, but I'm replacing my prop. The size i have now is a 14x19.
I went to west Marine and the have a 141/2 x 19 inch prop. Will this work ?? or cause problems ??
Running a 200 Merc.
Thanks. ;)

Re: Sizing your prop
 

Hammer, I'm thinkin a 15 1/2" dia x 17 pitch is gonna be THE prop for ya ;)...if you can try a Merc Mirage in that size, I'll bet you'll be smilin' like a mule eatin' briars ;D ;D ...

Re: Sizing your prop
 

Thanks Reel,
Should I replace the hub as well. Is it required ??

Re: Sizing your prop
 

RAP, is that right for a 200hp motor??? I thought a 19 pitch is for a 200. I'm running a 14x19, only I have not replaced my tach yet so I can't tell what I'm running WOT.

hammer, what are you toppin out at (rpm's)?

Re: Sizing your prop
 

Was doing 39 mph on a dinged up prop and pushing about 5000 rpms.

Re: Sizing your prop
 

Although a different application, my bud's cat w/ twin 200 Optis has THAT size Mirage on both...gets ALL the RPMs there are to get and boy is she FAST :o ...and fuel efficient...

Hammer, those numbers sound like you can get some more outta that motor...and it's not about goin' as fast as you can...it's about havin' the most efficient prop in terms of performance, gas economy and your motor's life-time... ;) 8) ...

Re: Sizing your prop
 

hmmm... i really need to replace my tach. The only time I did a speed check at WOT, I hit 49mph. I thought that was pretty good considering the massive T-top I have. Boat seems to like around 32 mph for cruise.

Re: Sizing your prop
 

After scootin' down the ditch in labii's CC w/ a 150, that top-end sounds about right w/ a 200...those CCs are fast... ;) ...