Diffuser ring experiment

Hi!
My first post here so lets se how it turns out...

A couple of days ago I made a little experiment with a Michigan Wheel aluminum PJ (or PI?) 83 prop. (think it is a 21" pitch)
It has been one of my best ie fastest props up to date but of course I thought that I could make it even better!
The fastest is still a Raker 22" with 45,1knots @ "I didn't dare to look" rpm.

Boat "Smiler 500"
Engine johnson 70hp -91
5" setback due to a CMC PT135 trim&tilt bracket
Propshaft 4,5" below bottom.
Prop have some cup added.

I went out and tested top speed with diffuser ring in place and ended up at 41,7 knots (GPS) @5600 RPM (Tiny tach) for a two ways run.

Back to the jetty, brought out the old hacksaw and removed the diffuser ring as I thought that it would reduce the drag due to "frontal area" of the lower unit torpedo.

Out again and this time I got these figures: 41,9 knots @6100.

So removing the diffuser ring yielded 500 more rpm but no speed, 6100 rpm is of course better for the engine but got me wondering what is going on
I did notice that the prop lacked some of the bite, ie ability to take trim.

Could some one please try to explain what the ring really does!?
Does it change the pressure distribution over the blades so that i now experience more ventilation or cavitation??

A Google search returns nothing but the old explanation that the ring prevents exhaust gases to back up over the blades on take-off.

please don't mind the spelling/ grammar, I hope I got my message through

 

No one Surely someone have at least an idea what the diffuser ring does.
Could the information be found in some book? If so, please point me in the right direction

Does this question perhaps belong in an other subforum?

If so, is there anyway it could be moved...

 

picture of "diffuser ring" ???

 

The idea behind the diffuser ring (I'll abbreviate it into DR) is just what you have heard about - to avoid the aeration of prop blades by the exhust gases, acting as a physical barrier. The shape commonly given probably reflects the necessity to find a compromise between a hole-shot situation, when a maximum thrust is necessary, and the top-speed situation, where minimum drag is required.

The diverging form of the DR tends to widen the prop wake, which ideally would prefer to shrink. Hence it creates a slight back pressure which increases a load on the prop disc, and hence the torque. Any stuff placed inside the prop wake creates a back pressure and increases the diameter of the wake. That's why you have registered a lower RPM at WOT with the DR. The term back pressure here is intended to mean a pressure measurable behind the prop disc.
A wider wake could also be useful at very low speeds so I guess part of advantage during the hole-shot comes from there too (apart the primary purpose of backflow-barrier for the exhaust gases).

Without the DR, there is no obstacle behind the prop. Hence the wake is narrower, the back pressure is lower and so is the disc loading. Which means higher RPMs at WOT.
In theory you should get a higher speed with no DR. Also because the engine, just like the prop, doesn't like back pressure in the exhaust lines - so you should have slightly more power available with no DR. That's the part which puzzles me, the rest appears to be pretty clear. Perhaps if you made your runs on a measured mile the results would have been more precise and more clearly in favour of the no-DR setup.

Cheers

 

Ok "Kvicken", I've been abroad for a few days, will get back to you with a "wider" explanation later, but those rings of varying profiles are there to reduce exhaust backflow into the low-pressure region along the hub outside perimeter. This gas flow will enter the suction side of the blade, spoiling its thrust.

......and ooops, Slavi was slightly ahead of me, so there you have additional info!

 

Diffuser ring...
Not my prop, borrowed/stole the pic...

 

Thanks!
I'm looking forward to that explanation

 

Thanks for your answer!
Would a measured mile be more accurate than GPS?
I thought that I was kind of careful when I did a two way test within 20min using GPS!
Even though I doesn't know the theory behind the DR the lack of top speed gain got me really puzzled too...

Unfortunately there will be no more testing before the winter, the boat is in storage since last weekend

 

I think your test valid and that the results speak volumes.

-Tom

 

It helps to stop the exhaust getting to the back of the blades in a turn

 

That's not a diffuser, looks like a nozzle.

Diffusers are by defintion parts of ducts or pipes across which the fluid pressure increases. For incompressible flows (water or low-speed gas flow, for example) it means that they have an increasing section-area.
Viceversa, nozzles (for incompressible flows) are convergent . My comments were referring to diffusers, not to the nozzles.

An example of a diffuser ring:

 

Looks the same to me.

"A rose by any other name is still a rose."

-Tom

 

The one in the first pic looks convergent, the one in the second pic is divergent. If so, they are completely different - both by form and by their fluid dynamic action.
The first one is a rose, the second one is a tulip.

 

I think one had a patent so the next one was the alternative

OMC did patent thru prop exhaust but I'm not sure who had the con-divergence first

 

Don't know what it IS but I do know both are called diffuser ring.

Made a quick sketch of a section of a prop where the upper geometri shows what my prop looked like, the lower one is what you (Rightly from a geometrical point of view) are calling a diffuser. I have been under the impression that they do the same job??

In what way, would this alter your explanation?

Thanks for all replies so far everybody!