Diffuser ring experiment

[Submarine Tom]

Ah yes, I see very clearly now the difference. My powers of observation are slipping.

Kvicken, what on earth motivated you to cut it off in the first place? You know, manufactures don't go looking for ways to spend extra money to put things on that aren't necessary...

-Tom

[daiquiri]

Well, dear Kvicken, thank you for bringing out this question about diffuser rings, it has allowed me to learn something new and to get rid of some brainless hearsay ideas.

Lets start with demolishing preconcieved ideas and false facts, written by myself:

Originally Posted by daiquiri

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.

Dead wrong!

The idea behind the diffuser ring is to reduce the backpressure acting on the exhaust tube of the engine, particularily in the low-speed regimes.
This is the original patent of the device: http://www.google.com/patents?id=mNB...page&q&f=false

Since the pressure behind the prop disc at low speeds is bigger than the pressure of the exhaust gases flowing through the exhaust duct, when the two flows (water and gas) meet together at the trailing edge of the hub (with no DR) the flowlines of the water stream bend inwards and creates an obstruction (or back pressure) for the gas flow. It is a bad working condition for the engine, where it cannot give the full power.
The purpose of the ring is to both augment the pressure of the exhaust gases (hence the name - diffuser, as noted in my first post) and to create a backpressure for the waterflow coming from the prop. The net effect of the DR is an increased diameter of the propeller's wake and an increased cross-area for the passage of the exhaust gases behind the hub.

This part of my first post remains true:
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.

So it's the double and opposite action you have there, when DR is mounted:
1) the back pressure of the water flow increases, thus decreasing the propeller efficiency
2) the back pressure of the exhaust gas flow decreases, thus increasing the efficiency of the engine at zero or low speeds, and possibly (but imho not necessarily) also at high speeds.

The overall propulsive efficiency will depend on the net result between the two opposing actions.

The real gain happens during the hole shot, when the increase of engine power output due to reduced back pressure gives a better acceleration.

At high speed, the prop has less efficiency, and the engine - well, who can tell. In your case, the results of your experiment indicate the possibility that (with that particular DR) the prop has lost a bit of efficiency while the engine has gained some, giving a zero net gain in terms of speed.

With no DR, the opposite happens - still with zero net gain.

Standard disclaimer: it is all imho. But after having read the patent I have much stronger reasons to believe it is closer to the truth than what can be read around in the internet.
Thanks again for starting this constructive thread and for letting me learn something new.

Cheers

[daiquiri]

Originally Posted by Kvicken

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?

They both do the same thing, but one of the two is doing it better imho.
The one in the upper part of the drawing does the job by just deflecting outwards the waterflow, but it doesn't help the gas flow to expand correctly. It is too draggy.
The one in the lower part is both aerodynamically (exhaust gas flow) and hydrodynamically (prop water flow) more efficient and less draggy.
Cheers

[powerabout]

and most props do not have either anymore

[Kvicken]

Originally Posted by Submarine Tom

Ah yes, I see very clearly now the difference. My powers of observation are slipping.

Kvicken, what on earth motivated you to cut it off in the first place? You know, manufactures don't go looking for ways to spend extra money to put things on that aren't necessary...

-Tom

That was an easy Q to answer!

First: Every consumer product is a compromise and something manufactured to work in as many applications as possible. (I know that because I work with product development) This prop was made in the seventies or (earlier?) probably aimed for sport boats i.e. not really what I have. My boat is a pretty light weight small offshore race boat. Most likely they wanted good take off and midrange rpm performance. I on the other hand could live without them since I've got other good cruising props on hand and my primary interest is top speed.

Second: I'm curious, and since this modification could quite easily be reversed it seemed to be a good idea. Daiquiris post (kind of) supports the theory

[Kvicken]

Originally Posted by powerabout

and most props do not have either anymore

And this post kind of brings forward the next interesting Q!

What have the manufacturers done to the props other than making the hub / barrel longer?

I've seen props lately with quite short hubs as well, from what I've read they perform well but maybe that is because people run them really high!?

[powerabout]

well its either Mum and Pop doing whatever with a boat or a very small part of the market trying to go as fast as they can and are happy to put up with poor performance at slow speed
Top of the line performace props do not have thru hub, over the hub is fastest

[Kvicken]

Originally Posted by daiquiri

They both do the same thing, but one of the two is doing it better imho.
The one in the upper part of the drawing does the job by just deflecting outwards the waterflow, but it doesn't help the gas flow to expand correctly. It is too draggy.
The one in the lower part is both aerodynamically (exhaust gas flow) and hydrodynamically (prop water flow) more efficient and less draggy.
Cheers

Maybe I should convince my brother to turn me a new diffuser ring like the lower part in my sketch then
On the other hand there will no testing until spring