Propellers 101

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Bob Bourget2

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I get asked almost everyday about "props"...so I figured it wouldn't hurt to spew out a little rhetoric....

Let's start with the basics..

-1] EXAMPLE: 13 x 19. Signifies 13" diameter, with a 19" pitch. Pitch is measured in inches, and the number represents the THEORETICAL distance it will travel in ONE revolution. You must factor in SLIP, as it is necessary to produce thrust. The only time you have "zero slip" is when you're running around wide-open, and you throttle back quickly, and the prop free-wheels through the water. Most bass boats {Nitros} will run anywhere from 12-18% slip.

-2] Engine RPM. Simply the most crucial aspect of what prop you are using, buying, trying, etc. No matter what, always prop at or near the maximum recommended rpm range your motor was designed to run in. If your motor has an "operating range" of 5000-5500 rpm, that means your motor will develop peak power within those parameters...BUT....what it doesn't tell you is how much of a "load" is being placed on the motor at the bottom end of that scale, with a prop that has too much pitch. Honestly, the majority of boaters do not know how to properly "prop" a motor.

Here's a little guide....1/2 tank fuel, 2 people, best trim for max speed. Do not OVER-TRIM and get "false" rpm, stop trimming when speed stops increasing. Now...if you take that same set-up out by yourself, with an empty tank, you might find yourself on the rev-limiter. That's NOT a bad thing. RPM will not hurt the motors, but lugging them WILL. Pre-ignition, detonation, piston damage, and at times, complete powerhead failure can and could result. Let it spin....with todays junk fuels, it is just another reason for "proper props".

-3] "I want a good hole-shot, don't care too much about top end". There simply is no better prop for this application than the HIGH-FIVE. Tremendous hole-shot, stern lift, ski application. Tourny skiers use non other.

-4] "I want top-end, don't care about hole-shot". Fury, Tempest, and Trophy. Rake, cup, blade design all contribute to solid performers. The Tempest props are my fav because it seems the majority of bass boaters run tourny loads, and the boats need the extra diameter to carry the bow.

-5] "I want IT ALL. Good hole-shot, and the most top end". Come on, who doesn't! This is where YOU come in. Boat set-up, and prop choice is paramount. I've made the comment to some {and they don't like it}, that all being equal in terms of boat, motor and horsepower, MY boat will run 5mph faster than yours. Why? Because I will take the time to make sure the bottom is straight, the motor tuned to peak efficiency, weight dsitribution, set-back and the prop itself worked or "tuned" for my set-up, and I can drive it to the limit. Not eating for a couple days helps too. You know...getting +5mph on a 65mph boat is the equivalent of going from a 150 to a 200. That's right....and a "worked" prop is a lot less money than a repower.

-6] Prop condition. Back to square one. Dings, nicks, bent or missing blades makes me CRINGE. More fuel, less speed, damage to g'case is all happening right before your eyes {and wallet}. FIX IT. Have your high-dollar prop repaired by a reputable rebuilder and not a hack shop. Customers wonder why they lost 2-3 mph after there prop was rebuilt...why....the rebuilder did not do his job. Trailing edges should be crisp, sharp and not buffed off. Have a close look at a new Tempest, Trophy or Fury, Can cut yourself on it can't 'ya.......?

-7] PVS. These are the adjustable plugs on most Merc props. They are in place to assist in the hole-shot. Getting modern, stern heavy bass-boats out of the hole is quite a chore. You have to remember that outboards do not have trannies. Try getting a motorcycle to get off the line in third gear. Ain't gonna happen. The "PVS" or "positive ventilation system" is in place to "fool" the outboard into thinking it has a transmission. It allows exhaust gas to escape out through the holes which induces "slip", higher rpm, and get the motor into the meat of the powerband, where it develops more power. The props design allows it to be used in this fashion and the result is quicker acceleration. Experiment to suit your set-up. With my 2.5, I played with exhaust block-off, and diversion to achieve what I needed to do a 10 second quarter, at nearly 10,500rpm. You want bite and slip at the same time....hard to grasp, but true.

-8] Here's a quick formula for determining actual speed:

RPM X GEAR RATIO X PITCH DIVIDED BY 1056 - SLIP% = ACTUAL SPEED.

-9] For the Fast crowd...there are the true "over-hub" design. ALL exhaust gas passes over the hub. Choppers, Mazco RE, Lightning ET, are a few of the "fast-of the-fastest" props built. These props allow a considerable amount of "slip" but once they grab hold, are the most tenacious "biting" props available, and used for most VERY light air-entrapment v-hull, or tunnel boats. We did a prop test for Bass-N-Walleye Mag a few years back, and "even" on a Nitro, a Savage 896 with a Pro-Max 200 posted the highest top-end with one of these wheels. Most of the Allisons, Strokers, high end Bullets and the like, have to use these these props. Reasons? They are a true surface piercing blade design, with upwards of 20 degree + rake angles, that allow mega-bow lift with less trim. The "tubeless" or no-hub design gives less tail lift then the hubbed props, and assist in reducing slip even under "one-blade-in-at-a-time" operation. Negatives? On some stern heavy boats, they simply will NOT plane. :rolleyes:

Have fun!!!:rolleyes:
 
All great advice. As a matter of fact I sliced myself last week walking by my Trophy that was worked by Mark Croxton last winter.
 
My DAH worked tempest is the same way, its like a razor its so sharp.



I learned a long time ago that the prop is the most important peice of the setup puzzle and its something a lot of guys tend to overlook. The general thought is a stock prop is good enough and its just not. When i had my 882 i tried 5 different stock 23 tempests and they all ran differently. I saw the light once i had mine worked and it was prefect. I didnt experiment with props too much on my Z7, my 24 tempest thats been worked does very well.
 
Mark Croxton lives a short drive from me. This guy knows how to rework props for top performance. His skills are unbelieveable. Many brand new props out of the box are horribly balanced and out of pitch.
 
Hey bob b. I should be getting my tach in just a day or 2 so i dont know how many rpms im turning. I have a 2000 nx750 with a 125 elpto. It has a 26 pitch lazer2 prop. The question is it has 1 large pvs vent plug then 2 with nothing. Boat has a terrible hole shot without a hydrofoil but i want to take it off from what all ive heard. Thanks bob b and awesome post i cant believe how much of a difference a prop makes.
 
Best down and dirty prop explanation ever. Any way to pin this to the top of the board? As soon as it slips to the second page the questions will start again! :lol:;)
 
Chris....5250 MAX. The motor you have has a harmonic "imbalance" at rpm's north of the 5250. What if you run it there you say?? Crankshaft breakage! Hydrofoils....do what your boat likes PERIOD. I have one on my Grizzly. Love it....world of difference. ;)
 
Bob great info, all compiled in one place. After our previous discussion my Tempest is going to get worked soon, need to save the bucks for that. I am also going to raise it a bit and see how it does.



Thanks!
 
Ok thank you bob i appreciate you. I cant wait til i get my new tach so i can finally see what im actually running
 

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