Prop Help

[5 Dollar Fishing Game]

I want to upgrade my prop.  Johnson 70hp 2 stroke.  From what I researched online for 1989, it is a 13 spline.  The various sites list several diameters like 13, 13.5, 13.7/8, etc.  How do I know what I have?  Existing prop is painted so I can't see the numbers even if they were there.  My prop now works, but it has some dings and had a small 1/4 inch piece that was bent and I was able to form it back to normal.  I just want to make sure I get the right one.

[5 Dollar Fishing Game]

It has 3 blades now and the clearance it has above the tallest point of the top most blade is about a quarter of an inch.

[5 Dollar Fishing Game]

So I did my research further and learned how to measure the diameter.  The distance from the center of the hub is 7 inches.  Times that by 2 and I have a 14 inch diameter prop.  The different websites I looked at show 2 pitch options 9 and 11.  I cannot tell which I have now and do not know which pitch to get?

[5 Dollar Fishing Game]

Found this on a website:

 

"Pitch is defined as "the distance a propeller would move in one revolution if it were moving through a soft solid, like a screw through wood." For example, a 21-pitch propeller would move forward 21 inches in one revolution.

Think of a propeller as you would a car's axle ratio. The lower the ratio, the more pulling power from a standstill. The same is true with a prop. The lower the pitch, the better your hole-shot. However, this comes at a price: top speed. The lower pitch makes the engine reach maximum rpm at slower speeds.

Conversely, a higher pitch will deliver greater top speeds, but slower acceleration. Be aware that lower-horsepower engines can bog down if fitted with a propeller with too high a pitch and diameter, and that can wear heavily on internal engine parts.

If you're changing pitch on a recreational boat, remember that each inch of pitch is worth about 200 rpm. Lowering the pitch will increase rpm and vice versa. For example, going from a 23 pitch to a 21 pitch will increase engine rpm by about 400 revolutions."

 

 

With this in mind, I have a hydrofoil stabilizer and my hole shots are insanely better since I installed it.  So should I go with the 11 pitch and gain some on top end?  The 9 pitch wouldn't reach a higher top end but the hole shot would be better.  Again, the hydrofoil stabilizer would more than make up for that.

[Way2slow]

if its a OMC/BRP prop, take the nut and washer off and the numbers should be around that inner edge.  Without knowing the current pitch and diameter, and the rpm the engine turns with it, it would almost impossible to know the size you need to replace it with.

[5 Dollar Fishing Game]

Would the numbers be stamped on the washer? Or stamped on the prop itself were the exhaust comes out?

[5 Dollar Fishing Game]

Found it stamped on the prop.

13.75 x 15

[5 Dollar Fishing Game]

Isn't the pitch kinda high???? Now I really don't know what pitch to get LOL

[slonezp]

Changing pitch will change rpm's. The motor has a maximum rpm it can run safely. Without knowing what rpm you are running at WOT and what rpm's the motor is rated for, you can't make an educated decision on what prop is needed. Props can be repaired and tuned. That might be a better option

[Way2slow]

Now you need to know what RPM the motor was turning at WOT. Without that, its still just a guess. You want it propped to turn within a couple hundred RPM of the manufactures max rated rpm with a normal load. Even if it goes over a few, that's better than being three or four hundred low.

As for being high, no, that's about a normal pitch for that motor on the size boat they normally run on. 15"-17" pitch are pretty much the most common size unless you are running an extremely heavy boat or extremely light boat. If it's very under powered, it might have needed a 13" pitch but again, need the engine rpm to know anything for sure.

Two critical gauges that I won't run without, water pressure and tach. Everything else are just nice to have.

[WIGuide]

I love reading the progression of these posts  

[5 Dollar Fishing Game]

I have learned so much about props and pitches in the last two days thank you guys !!!! :-)