Why not a better solution for hydrodynamic efficiency?
What I mean is, why are all bravo-style drives and props designed with such an ill-conceived termination? it just doesn't make a whole lot of sense from an efficiency standpoint.
Besides the obvious need for some boats to vent exhaust through the prop, boats with through-hull exhaust may benefit from cleaning up the aft end of the prop.....

Thoughts?

I submit these photos for comparisson:

In stark contrast, now I show you this:

I'll take it a step further, I ran some CFD code on a couple of models, and the results are below in the next post.

Plots:

Flow:

[hijack on] One of these days I want to build my own Velocity with either diesel or GM SBC...one awesome homebuilt [hijack off]

There's a decent thread on diffuser rings in here somewhere. From what I recall, a lot of the need for it is time to plane, or even the ability to plane at all. That ring actually makes a difference. I know the Hydro Q-IVs I have on right now don't have a ring.
I don't remember if people have cut the ring off say a Bravo 1 and shared the results. But I know adding/removing the ring is done occasionally.
YMMV

The big varible is this; How much of the drive and prop are out of the water? This varies from boat to boat.

In the extreme case all but the bottom of the gear case and less than half the prop is in the water. Surfacing situations are where it's very difficult to model. Every boat and set up changes the condition in which the prop is surfacing thus changing the model. The varibles are endless.

In submerged aplications there is a great amount of research done but surfacing situations are a whole different story.

It is difficult to compare air and water. The variable are endless with depths and hull configurations which affect the amount and angle with which the water attacks the high pressure side of the blades.

We do remove the diffuser on many through hull propelllers. Some through hull exhaust boats still pull air through the drive unit and cause havoc in planing. Diffuser lengths can actually help as a planing surface on various applications.


Look at surface piercing props, they don't have tubes and we have tested that process and that is why they dont have much or anything for the aft end. It takes the right combination to run them.


Yes, relieving drag of most any sort can improve efficiancy on boats. But, in some cases, leaving the drag on (diffuser) can remidy other problems therefore becoming the lesser of two evils.

Thanks guys. Agreed, most if not all fundamentals can be thrown out in regimes where there's interaction at the air/water boundary, doesn't make it impossible just a bit more tricky. Incompressible air flow physics (low Mach numbers) has many striking similarities with that of liquid fluids - only differences are variable inputs like density, viscosity, and Reynold's number, but the mathematics are the same. Quantitatively, my preliminary estimates yield a slight reduction in overall system drag of about 1.5-1.8%. Qualitatively, simply looking at the flow profile one can deduce that there's better laminar adherence of the prop thrust stream. In applications where drives are deep, I still must wonder if there would be any benefit. I'm considering having a buddy machine up a pair of tail cones and giving it a shot. What could it hurt, right? Once I get going, I have to satisfy my curiosity.

Food for thought:

Oh, and I wanted to share a really cool video I found, check it out.
It's strobe pulse time-delay video of prop cavitation.
It gets good at -1:30, -1:08 and -0:40
Neat.
[YOUTUBE]mQvbispmUF4[/YOUTUBE]