Gear Ratio VS Prop Pitch

Status
Not open for further replies.

Eric.G

Cadet
Joined
Jul 22, 2011
Messages
19
Quick question thats been on my mind lately.
What would be the need for different gear ratios in a boat when you could just prop up or down to meet the boats needs??

Thanks
Eric
 

Maclin

Admiral
Joined
May 27, 2007
Messages
6,761
Re: Gear Ratio VS Prop Pitch

Not enough range in prop pitch to cover the entire range of adjustments needed. Like saying all trucks can have the same differential ratio no matter the engine size or size of the truck and just adjust the effective ratio with tire height. Prop pitch is a fine-tuning element after the ratio gets you in the ball park.
 

Don S

Honorary Moderator Emeritus
Joined
Aug 31, 2004
Messages
62,321
Re: Gear Ratio VS Prop Pitch

You are missing one piece of information. Props have a range of usefullness (aka efficiency) on typical recreational boats. That is between 17 and 23" of pitch. Above that, you loose the ability to get on plane quickly, but can slowly gain top speed. Below 17 and you will have a tow boat but no top speed. Your engine also has a wide open throttle speed that is top rpm for the engine, but not lugging (overloading) it.
To keep the props in that 17 to 23 range is done by gear ratio in respect to the torque of the engine and the hp.
As an example, with Mercruiser Alpha drives. A SBC V8 will use a 1.5:1 gear ratio, while the 3.0L 4 cylinder will take a 1.98:1 gear ratio. But both may use the same exact 21" pitch prop.
 

Eric.G

Cadet
Joined
Jul 22, 2011
Messages
19
Re: Gear Ratio VS Prop Pitch

Ahhh - didnt know that about prop efficiency, as I've seen pitches from 10 up into the 30'' range. So I couldnt figure out why you wouldnt just prop way down for a 3.0L and find where the WOT was right and leave all the ratio's at, say 1.5 or so.
That explains it... Thanks Don!
 
Status
Not open for further replies.
Top