1.5 or 1.65 ratio- need education

[PA-Checkmate]

What's the difference of each ratio assuming running the same stock I/O power in hulls of same length? When I was checkmate shopping noticed both would run the same top speed.

 

[karzrus]

It's as much about which Propeller you want to run as anything.

Since most brands/styles of propellers have a limited range of pitches and some pitches are more easily found then others, having the right gear ratio will put you comfortably into the middle of the pitch range for the type of propellers that work best on your hull.

 

[Jimway]

Here you go. A big block motor, or small block, or 'y' block, or half block, for that matter, turning 4800 rpm will be spinning the prop at 3200 rpm at 1.5 gear ratio. Go to 1.65 and you will be spinning that same prop at 2900 (and some change) rpm. Just to foment things a little, in theory, the faster a prop is turning, the more power is going to be needed to turn it. A slower turning prop with more pitch will be more efficient than a faster one with less pitch even though in theory, they will calculate at the same speed. Not too far down this path, we are going to come to the 'No replacement for displacement' part. Either that or a transmission in between the power plant and outdrive, or a variable pitch prop, not to forget about some kind of supercharging also.

 

[RNGRDVE]

You'll run a larger prop with the 1.65 when comparing similar setups. My old 253 had a 1.65 and I ran a 28 itch bravo one. Jason's 253 with 1.5 and similar power ran a 26 pitch bravo one.

 

[Jimway]

Which conveniently brings us to the 200 rpm change per inch of prop pitch... On the educational side, higher reduction in the upper case means more stress on the drive shaft lower gears and prop shaft and gear floor.

 

[Gimme Fuel]

Equates to the 1.65 wanting about 2" more pitch than the 1.5 as Dave mentioned above.

 

[corey331]

I was talking to Jason and Harold Kunkel about this a couple of weeks ago when we were discussing prop possibilities on my boat. They both said, "If you have an I/O Checkmate, you pretty much always want a 1.5". I can't explain it scientifically like others have, but according to them, A LOT of testing has gone into them making that statement.

 

[PA-Checkmate]

Thanks for the feedback. Recently purchased a Convincor 270 496 MAG-HO... and just wondering about the ratios which mine has 1.5.

 

[Craig]

Here was a situation I personally saw:

I had both a Convincor 270 with 496 HO and 1.65 ratio and a 270 with 496 HO and a 1.5 ratio. I ran a 28 Bravo on the 1.65 and a 26 Bravo on the 1.5. The 28 Bravo was about 1-2 mph faster. Both props were stock out of the box. The 28 Bravo ran a tad higher RPMs. I don't remember the exact #'s. I bought the props from Teague Marine and one of the techs there told me that the 28's are a little different animal and if your set up can turn one it should be faster. (BTW: Teague use to be a great place to buy Merc props, fair priced and no sales tax. Not sure how they are now.)

Anyway, I ran a 27 Mirage Plus on the boat with the 1.65 and a 25 Mirage plus on the 1.5. With the Mirages, they ran same speed, same rpms.

I agree though, that once you get into the faster set-ups with higher HP and higher pitch props you would want the 1.5 and possibly even 1.35 ratio.

-Craig

 

[ameliano111]

When you're looking at gear ratios, it completely changes how the engine's power is delivered to the propeller. A lower numerical ratio like 1.5 means the prop turns more times per engine revolution, which is usually great for top-end speed if you have the horsepower to swing it, while a 1.65 ratio gives you more mechanical advantage, better hole shot, and helps the engine stay in its powerband if you're carrying a heavier load or running less displacement.

Figuring out that sweet spot requires a lot of real-world testing and a practical, diagnostic approach rather than just guessing based on a spreadsheet. You have to look at how the RPMs respond under load and analyze why certain setups cause the engine to lug while others let it breathe.

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