Offshoreonly.com - 1.5 or 1.65 ratio- need education

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PA.WOODCHUCK

10-24-2018 01:09 PM

1.5 or 1.65 ratio- need education

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.

Baja Rooster

10-24-2018 01:44 PM

I am no expert, but do have a Baja with 1.65 and have looked into it.

The difference is good for about 2” pitch. I run a 25” where similar boats run a 23”.

Better top end as youre swinging a bigger prop with more bite. Fountain equipped some of their boats with 1.65” for this reason.

Harder on the drive because all things being equal youll swing a bigger prop.

Spares are harder to come by but often cheaper for the lower demand.

Other than that the difference is pretty insignificant.

Tinkerer

10-24-2018 07:42 PM

With the same HP you will swing a bigger prop on the 1.5 ratio. I have been told there is about 600 RPM difference between the two ratios.
Usually you need an engine with lots of torque to spin the higher ratio.
I had a 509 ( 650 HP ) in my Daytona with 1.5 ratio -2 shorty drive and then went to a -3 shorty 1.64 ratio and couldn't spin a prop that would bring my RPM back up the 600 RPM.
The 509 didn't have enough torque to spin the 1.64 ratio.
I built a 598 ( 800+ HP ) and have NO problem spinning the correct prop.

SABER28

10-24-2018 11:13 PM

wait? aren't both replies saying the opposite? i'm confused. mine are 1.32 ratio, whats the reason for them?

Griff

10-25-2018 12:24 AM

Quote:

Originally Posted by Tinkerer (Post 4655938)

I think you are thinking backwards. A 1.65 will spin a bigger prop than a 1.5. About 2-3 inches more of pitch
I think you went from 1.50 and tried a 1.36.

LostinBoston

10-25-2018 11:43 AM

Quote:

Originally Posted by PA.WOODCHUCK (Post 4655876)

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.

It depends on the boat, prop, and speed. A higher reduction ratio means that for every 1 engine rpm, the prop is turning that much less. Set up a prop calc spreadsheet to determine equal theoretical speed based on rpm x pitch. Props have their own efficiency as well. Spinning a higher pitch prop slower will reduce rotational drag and increase torque at the prop, but may not be able to reach full potential of the hull due to pitch limitations. It will also add stress to your gears. A lower pitch prop spinning at a higher rpm may be more efficient.

Griff

10-25-2018 01:04 PM

Quote:

Originally Posted by PA.WOODCHUCK (Post 4655876)

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.

If propped correctly and with the same prop slip%, they should run the same speeds.
The 1.65 was often used for higher altitude applications.
The engine will not make as much as hp at higher altitudes, so in order to give more prop options, they went with 1.65 ratios.

F-2 Speedy

10-25-2018 02:18 PM

IF you're on the limiter with a 1.50 spinning a 34p and want to go faster you go to a 1.36 and break the XR.......lol

Tinkerer

10-25-2018 07:50 PM

My bad- long day at work.
Spinning a prop faster takes more torque even if it is a smaller pitch.

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