Sterndrive gear ratio versus pitch of propellor?

[jerama88]

Hi, Can some one help. Can I change a same type sterndrive from 1.61 to 2.16. This will obviously change top end/ rpm/ tourqe. If I keep the same diameter of prop but increase pitch by say 5 inches am I getting back what I lost or are there other factors to think about?

 

[Frank Acampora]

Re: Sterndrive gear ratio versus pitch of propellor?

On my outboards I usually go from 1.7 to 2.0 and increase pitch by 2 inches. This yields the same theoretical travel through the water but because the prop is turning a slower RPM, there is less parasitic drag. More horsepower is available to drive the boat and I usually gain from 3 to 8 MPH. I don't know if this would be true for 4 cycle I/O engine/outdrives.

 

[Bondo]

Re: Sterndrive gear ratio versus pitch of propellor?

Hi, Can some one help. Can I change a same type sterndrive from 1.61 to 2.16. This will obviously change top end/ rpm/ tourqe. If I keep the same diameter of prop but increase pitch by say 5 inches am I getting back what I lost or are there other factors to think about?

Ayuh,.... What sterndrive,..?? What pitch is the Right pitch prop on the 1.61:1 yer runnin' now,..??

The window of the available pitches can be a limitin' factor,...

My guess is, yer gonna need 6" maybe 8" of more pitch, to hold the motor's rpms down,...

'n What motor,..??

 

[Maclin]

Re: Sterndrive gear ratio versus pitch of propellor?

Jerama, you might mention the engine you have (VP Diesel), to get the best advice here as well.

 

[Maclin]

Re: Sterndrive gear ratio versus pitch of propellor?

Technical on the Volvo AQAD40B, you only have 3600 RPM's to work with...:

 

[jerama88]

Re: Sterndrive gear ratio versus pitch of propellor?

Thanks heaps. I have a Volvo Penta AQAD 40B with a 280 volvo out drive. 1.61 ratio has a 16 inch diameter x 16 inch pitch and it sits perfect at around 3500 WOT.
I did some calculations and Im picking 21 inch pitch would do it? Not sure thou...that size prop is available or I could go 23 inch pitch...

 

[jerama88]

Re: Sterndrive gear ratio versus pitch of propellor?

On my outboards I usually go from 1.7 to 2.0 and increase pitch by 2 inches. This yields the same theoretical travel through the water but because the prop is turning a slower RPM, there is less parasitic drag. More horsepower is available to drive the boat and I usually gain from 3 to 8 MPH. I don't know if this would be true for 4 cycle I/O engine/outdrives.

Sounds what I wanted to hear really. Thank you

 

[jerama88]

Re: Sterndrive gear ratio versus pitch of propellor?

Ayuh,.... What sterndrive,..?? What pitch is the Right pitch prop on the 1.61:1 yer runnin' now,..??

The window of the available pitches can be a limitin' factor,...

My guess is, yer gonna need 6" maybe 8" of more pitch, to hold the motor's rpms down,...

'n What motor,..??

As above thanks for any more info

 

[jestor68]

Re: Sterndrive gear ratio versus pitch of propellor?

Since that gear ratio is matched to the diesel engine max rpm, why would you want to change gear ratio?

If you change to the ratio you mentioned and go up to a 19 pitch, the theoretical speed drops by 2 mph.

Looking at it another way, adjusting prop diameter/pitch might be a better approach than messing with the gear ratio.

Since a inch of diameter is roughly equal to two inches of pitch, switching to a 15 X 17 prop should allow the motor to attain about the same rpm, while raising the theoretical speed by several mph.

I would therefore recommend playing with prop size rather than changing gear ratios. The 1.61 gear ratio favors less diameter and more pitch.

I guess the guys at Volvo know what they are doing when they match gear ratios to engine types.

 

[jerama88]

Re: Sterndrive gear ratio versus pitch of propellor?

Since that gear ratio is matched to the diesel engine max rpm, why would you want to change gear ratio?

If you change to the ratio you mentioned and go up to a 19 pitch, the theoretical speed drops by 2 mph.

I guess the guys at Volvo know what they are doing when they match gear ratios to engine types.

Thanks heaps...19 pitch sounds great..how do I get that equation or table..cant find much on it
. The only reason I am doing it is because I have a complete drive (2.15) in the shed ready to go and the old one(1.61 ratio) is not in good condition.

 

[jestor68]

Re: Sterndrive gear ratio versus pitch of propellor?

Thanks heaps...19 pitch sounds great..how do I get that equation or table..cant find much on it
. The only reason I am doing it is because I have a complete drive (2.15) in the shed ready to go and the old one(1.61 ratio) is not in good condition.

Change the "good" drive to the 1.61 ratio if possible. It might require going all the way to a 21 pitch(same diameter) to equal the theoretical speed of the 1.61 ratio, provided the engine can still attain 3500 rpm.

You can use the(BAM) prop slip calculator to get an idea of what happens with different gear ratios versus pitch at a given engine rpm. Just use the blanks for ratio, pitch, and rpm; then it shows theoretical speed(not accounting for slip). Just google "prop slip calculator".

Since these are just estimates, it will require some experimenting. So it would be nice if you had a dealer willing to let you try a few different props.