shrouded prop?

[AzDon]

In another recent thread, prop rake was being discussed pertaining to efficiency of less rake versus a prop with more rake containing and focusing the power rearward instead of throwing water off laterally.....
I'm wondering if anybody has ever considered or experimented with the idea of a prop having a shroud ring around it so it cant throw water off laterally(?)
There may be something collossally stupid about this idea, but I can't see what it would be!.... Anybody know?

[shipchips]

That is called a "cort nozzle" I have driven 240' vessels that had these nozzles and the thrust is very directional. Point the drive and that is where the vessel goes. Tugs use them and some ships. The only problem our company has had with them is the extra thrust comes at a cost, it is very hard on all of the running gear, shaft seals, bearings, gears.

[coltmodel70]

when i lived up in PA. , my boss's son had a alum flat fishin' boat. the older outboard had a cast alum ring around it. i didn't think much of it till he hit a stump in the river and broke it. it was like taking away 50% of its hp. and handling. this was only a 8-10? hp outboard

[coltmodel70]

wish i could think what the motor was now, but the prop shroud came factory on it, was machined for the ring to fit the lower unit

[RiverRacer]

It's a common thing on ships and low speed work boats, but I don't see it on high speed stuff, that shroud I think would cause handling problems!

[gfinishline]

"The less material I have in the water the better I like it". Back in the 1980's Jay Ryan (Menkens Az) was looking for a way to correct the prop failures in ski racing. I worked with him and gave him input on how different boats liked/disliked the prop modifications being tested. This is how the "3 blade SS 'full radius cup' props" came into being. Testing how far forward the 90 degree cup went (up the blade tips) , tuned the prop blast behind the boat. Some 'fast guys' reject this theory, but many others use it to great advantage. The 90 degree cup, actually bends the first few boundry layers of water compressed on the prop blades. It actually narrows down the cone of prop blast and concentrates the power in a focused smaller area. That is why those who like those props felt 'more control', and "smoother driving' overall. I have used the 'full radius cup' on drag 2 blades with equal gains. "I want the rudder to be the drag on the mass, not the prop".

[AzDon]

Maybe I wasn't clear..... what I'm picturing is not a seperate shroud fastened to the boat...... I'm visualizing a prop that has an integral ring around it instead of blade tips. The prop repair guys would probably hate the idea though because something like this would probably be tougher to damage!

[SnoC653]

Looking at aircraft props, why hasn't anyone made a q-tip boat prop or the swept back outside edge? aero dynamics and fluid dynamics share a lot of similarities.

[gfinishline]

I 'got you' on the first call. There have been props with "rings" around them, all low speed stuff. The 'ring area' between the blades is so much of a drag that fast boats would have control problems at skiing speeds. I still want as little surface area dragging thru the water as possable.

[RiverRacer]

"Everything" in the water is a drag even the water pickup!..

[gfinishline]

Sure, but the water pickup isn't 12" below the hull bottom. The leaverage created at distances below the riding surface really cause major changes in drag and control. Big fat drivers are drag too, but I ain't gettin' out for nobody!

[RiverRacer]

Without a doubt, but anything under the boat can change handling, look at those big ole brass type screen pickups, they are a deterent at speed!..

[Terrible toy]

Aircraft and helicopters have use ducted fans for some time. The shroud reduces thrust loss from the tip vortices of the fan. The weight of the shroud and mounting structure negates a lot of the advantage however. In water the density of water and drag of the shroud would far outweigh any blade vortex reduction.

[SnoC653]

Aircraft and helicopters have use ducted fans for some time. The shroud reduces thrust loss from the tip vortices of the fan. The weight of the shroud and mounting structure negates a lot of the advantage however. In water the density of water and drag of the shroud would far outweigh any blade vortex reduction.

But they only use the ducted fan for the tail rotor. The ducted fan never would survive in a main rotor application as the ducting would create way more drag that ecconomicly feasable to overcome. Swept tip blades are more commonly used.

[RiverRacer]

Water and air do not flow the same!...

[gfinishline]

"300 to 1 + -"

[SnoC653]

Water and air do not flow the same!...

I agree completely. Air has far more compresability than water does which alters the effects. But, flow is flow and the priciples of aerodynamics vs hydrodynamics are very compareable. The biggest difference between aircraft dynamics and boat dynamics is that boats have both hydro and areo dynamics to deal with. That is why they can't just generate a computer model and tell us how to set things up. It takes some good old fashioned trial and error sometimes. And that is a big part of why most of us come here. The knowlege you, george and some of the others impart on us, is worth more than all the priciples of dynamics combined. We get to find out what works and what doesn't from experience.

[shooter2]

I once spent an afternoon with Jamie Auld, he is one of the best at understanding boats. The man is brilliant and not just about hydros, but inboards as well. When Jamie speaks people listen.

Brian

[SnoC653]

I once spent an afternoon with Jamie Auld, he is one of the best at understanding boats. The man is brilliant and not just about hydros, but inboards as well. When Jamie speaks people listen.

Brian

Can we get him to join the site too? Soon we'll hold the key to all the great boating minds. Ok, so I'm reaching a little. But, you can't deny we have a great deal of that knowlege here already.