Purpose of "Diffuser" ring..back of prop

[Scaaty]

I have many (older) outboards. The Johnson's have diffuser rings that are more or less opposite of what Merc does. Johnny's capture water more or les, but Mercs splay it outward. Reason I ask is I just got off a Ebay a SS Prop from a 70's 115 Merc for the 135hp Flatbotom I'm putting together Its a cupped 19p, and I also have a 25p Aluminum. On the Ebay 19p, someone has taken a brass diffuser ring and brazed it on the hub (hub inside has no outward splay). Have to admit it sure looks cool! Now it's on the motor now, and the boat will soon hit the water (total rebuild on the motor..need 10 hours breakin first), but as I care less if it Cav's a bit in reverse, just a wondering on the in's and out's of the reason for two designs. Of course I have props that run straight to the rear, 2 blades with a skinny hub, old Johnson's tapered.. interested in opinions. Here a pic, click to enlarge..back button to well, come back​

 

[Scaaty]

Re: Purpose of "Diffuser" ring..back of prop

Come on..SOMEBODY must know an answer!

 

[Dhadley]

Re: Purpose of "Diffuser" ring..back of prop

The theory is that it helps capture water and keeps the blades from venting at higher speed (on plane speed). OMC had the wide part of the ring facing forward on the original SST's and then turned it around on the SST II's. On some applications it will indeed make a difference if it's gone, some it won't matter if it's there or not.

There's also theory about the length of the barrel and how it affects holeshot vs. top end.

 

[Scaaty]

Re: Purpose of "Diffuser" ring..back of prop

OK, I can picture that in my mind...Thanks!

 

[Dhadley]

Re: Purpose of "Diffuser" ring..back of prop

Anytime!

It all has to do with where the vortec is as it passes over the gearcase at a given speed. An old hot rod trick is to add a small ridge to the gearcase right in front of the prop. (since over hub props like a "chopper" doesn't have a barrel)