Gear ratios - please confirm

[stevens]

Hi guys,<br /><br />At a local boating forum, there is some dispute as to how to interpret gear ratios for outdrives.<br /><br />Could you please confirm the following:<br /><br />A common gear ratio of 1:1,5 would indicate that the engine turns 1.5 turns for every turn of the prop, right? Likewise, a ratio of 1:2 would mean that the engine turns twice as fast as the prop?<br /><br />The common ratios for Mercruiser drives would be 1:1.5, 1:1.65 or 1:2, right? What would you choose for which type of engine?<br /><br />I'd appreciate your help on this.

 

[tommays]

Re: Gear ratios - please confirm

that is correct but i go 2:1, 1.5:1 ,ect<br /><br />because on different types of machines there are 1:2 and other overdrive ratios<br /><br /><br />tommays

 

[Don S]

Re: Gear ratios - please confirm

With Mercruiser, the 1.65:1 was only used on the inline 6 Chevy engines. The 1.5:1 is a standard ratio for the 5.0 and 5.7 Chevy V8's, some of the big blocks and diesels used 2.1:1.

 

[stevens]

Re: Gear ratios - please confirm

Thanks.<br /><br />So, to be more specific, if you combine a so-called "260 hp V8 Bluewater" to an Alpha drive that was previously used with a diesel engine (i.e. 2:1 ratio), this wouldn't be the right match? The result would be that the prop wouldn't get sufficient rpm's and the boat would be slow?<br /><br />Short of changing the drive, are there any other ways of fixing this? Is it possible to change just gear ratios, or would a different prop be able to compensate?

 

[vipzach]

Re: Gear ratios - please confirm

I different prop would be your easiest fix, if the drive ratio is wrong. I would be more expensive to replace the gears.

 

[Don S]

Re: Gear ratios - please confirm

if you combine a so-called "260 hp V8 Bluewater" to an Alpha drive that was previously used with a diesel engine (i.e. 2:1 ratio), this wouldn't be the right match?

No, it wouldn't be the right match. And the chances of getting a prop to cure the difference is unlikely.<br />Props have a range on them, too steep a pitch or too little pitch and they are not very efficient, even if you could get the proper rpm. The boat just wouldn't perform.<br />That is why they make all different kinds of ratio's. Otherwise there would just be infinate number of different props for different engines and boats.

 

[MrBill]

Re: Gear ratios - please confirm

3.7L 4cyl used 1:1.84 Alpha drive ratio

 

[vipzach]

Re: Gear ratios - please confirm

I should have been more clear with my reply Don!<br /><br />Replacing the prop is about the cheapest half-assed fix you could do. Installing the correct drive ratio is better and your boat will perform correctly.

 

[TilliamWe]

Re: Gear ratios - please confirm

vip & don are both right.<br />You might get a prop that works, but you'll end up with something goofy. Like a 30 pitch! <br />Seems to me, I/O manufacturers use gear ratios that allow the boats to run anywhere from a 17 pitch to a 23 pitch prop. Those have got to be the vast majority of prop sizes that you will ever see.

 

[stevens]

Re: Gear ratios - please confirm

Thanks for your help so far. So, how about changing just the gears to get the right ratio - is it possible?

 

[TilliamWe]

Re: Gear ratios - please confirm

Sure, it's possible, but gets pretty darned expensive!