Diffusers on Propeller Hubs

[Jilly - 5]

I recently went from a small hub (40 hp Johnson) to a large hub (re-built 1983 75 hp Johnson). On the 75 hp, the prop has a ring aft of the blades whose purpose appears to "flare out" passing water. On other props (Hustler, for example), there is a flaring of the prop housing itself, rather than a ring like the OMC. I have to assume this ring on the OMC and/or the flaring on the Hustler has some purpose. But I also have to assume that they are a drag on foward motion. Both these assumptions then lead me to believe that the ring/flaring is a compromise to accomplish some larger purpose.<br /><br />Question: What does the ring/flaring accomplish?<br />Question: Can I remove the ring on my OMC prop?<br />Question: If I remove the ring, what will happen? <br /><br />You should know that I am not a freak for jack-rabbit starts or extreme hi-end speed. My 75 pushes a 17' skiff for crusing & fishing.

 

[fast_c_class]

Re: Diffusers on Propeller Hubs

The Diffuser ring prevents the exhaust gases from backing up on the blades which produces cavitation during take-off.<br /><br />you could but why bother, if its on there its there for a reason, taking it off wont do anything as far as gaining mph.<br /><br />removing the ring could cause excessive ventilation of the prop hurting your performance. (ive run without one though without any performance decreased or increases)<br /><br />a little history told to me from john janeky at DAH props: the ring was invented by an engineer DON A HENDERSON at mercury soon after the first 100hp tower of power inline 6's came out. these big engines were propelling boats to speeds never seen before by production engines. many peaple would get up to about 80mph and the engine would ventilate very bad and torque to the right. it was found that exhaust gasses were slowly working there way up to the point of the gearcase, once they completed the journy to the front of the case bam!!! a low pressure area caused by the bubbles cause the prop to vent and quickly pull to the right, a term now know as BLOWOUT. Don A. Henderson the engineer (DAH) invented the ring to cure this. MERCS VERSION WAS A FLARE, Don later moved over to omc unfortuanatly the design was patented by merc. DON THEN CREATED THE RING DESIGN FOR OMC.