Propellor shrouds? - PeachParts Mercedes-Benz Forum

t walgamuth · #1 05-20-2011, 09:54 PM

The other day at lunch somebody was talking about Manatees and getting hit by propellors and such and I said I was astonished that it is still legal to buy open propellors considering how dangerous they are.

Someone said that shrouding a prop would kill the power and I said if designed properly it would make them more efficient.

So the fellow who disagreed with me sent me some quotations today and I googled it and found some stuff and it seems that it all depends on many things.

So do we have any experts on Propellors who can talk about shrouding them for safety and or for changed performance capabilities?

And what about those jet drive options? I know wave runners have those so they must be fairly efficient.

Emmerich · #2 05-20-2011, 10:15 PM

Jet drives are very inefficient. On a real boat (as opposed to jet ski) they require a lot of HP. Thats why you see monster V8's. And they suck at slow speed, no control and forget about reverse, not sure they even go in reverse. The diameter of the impeller is tiny and must be really spun up to get going. Imagine putting a drill bit into a shake and trying to mix it as compared to a blender with large blades.

layback40 · #3 05-20-2011, 10:47 PM

To minimize drag, you need laminar flow of the air. Shrouds then can be used to improve the airflow past the prop.
The air speeds involved do not allow this. It will be in the turbulent region & so any obstruction by a shroud will cause significant drag.
Any good fluid mechanics text will explain this.

Skippy · #4 05-20-2011, 11:05 PM

Quote:

Originally Posted by layback40

To minimize drag, you need laminar flow of the air. Shrouds then can be used to improve the airflow past the prop.
The air speeds involved do not allow this. It will be in the turbulent region & so any obstruction by a shroud will cause significant drag.
Any good fluid mechanics text will explain this.

I didn't know manatees flew

layback40 · #5 05-20-2011, 11:10 PM

Quote:

Originally Posted by Skippy

I didn't know manatees flew

Same principles applies to a prop in water. ~ Fluid mechanics applies to both liquids & gasses.

t walgamuth · #6 05-21-2011, 07:35 AM

The Navy developed a setup for the props on their landing boats for the seals that seems to have worked very well and made it very safe for their men.

For pleasure craft the safety aspect seems pretty significant. Especailly a ski boat where you have people getting in and out.

I have a beautiful Daughter of my first cousin (what is that cousin once removed?) who has enormous scars on her theighs from her grandfather backing over her by accident. She nearly bled to death. My uncle was deeply affected by this accident and I don't think he ever used that boat much after that.

Apparently they can increase fuel effeciency too under some conditions and can increase directional control when maneuvering.

A drawback to shrouding an outboard prop is the possibility of hitting the bottom. Making it strong enough to resist such impacts will naturally increase drag.

layback40 · #7 05-21-2011, 09:15 AM

Quote:

Originally Posted by t walgamuth

The Navy developed a setup for the props on their landing boats for the seals that seems to have worked very well and made it very safe for their men.

For pleasure craft the safety aspect seems pretty significant. Especailly a ski boat where you have people getting in and out.

I have a beautiful Daughter of my first cousin (what is that cousin once removed?) who has enormous scars on her theighs from her grandfather backing over her by accident. She nearly bled to death. My uncle was deeply affected by this accident and I don't think he ever used that boat much after that.

Apparently they can increase fuel effeciency too under some conditions and can increase directional control when maneuvering.

A drawback to shrouding an outboard prop is the possibility of hitting the bottom. Making it strong enough to resist such impacts will naturally increase drag.

I think the fuel efficiency bit is like all these add ons to improve mileage of a car.
There is a skeg on an outboard that hangs down below the prop. A piece of pipe around the prop would not need to hit the bottom as the skeg does that.

Emmerich · #8 05-21-2011, 11:57 AM

Back to the original post, manatees are dumb as bricks, they hear a sound, pop up and catch a prop. I swam with them in Florida and they all have scars. Shrouding props would be like putting cushions on the outside of cars to protect people too stupid to get out of the way. Not very practical. In manatee zones you just drive slower.

tbomachines · #9 05-21-2011, 12:30 PM

kerry · #10 05-21-2011, 12:54 PM

Don't tugboats often have shrouded props?

Chas H · #11 05-21-2011, 01:02 PM

Quote:

Originally Posted by kerry

Don't tugboats often have shrouded props?

I think the shroud might be a wire cage to keep ropes and lines from getting tangled up. Some lobster boats have them to keep the trap float lines out of the propeller.

Hatterasguy · #12 05-21-2011, 01:14 PM

Shrouds increase drag and decrees efficiency, I know some company does make them for smaller boats though I have seen a couple on outboards. I think they are more resilient in shallow water/heavy weeds.

Its not really much of an issue though.

kerry · #13 05-21-2011, 01:38 PM

http://www.salmargemi.com/engine_outfitting.html

Shrouded props

t walgamuth · #14 05-21-2011, 02:57 PM

In a tug they provide more torque and in a swiviling application like that they provide more steering response.

Chas H · #15 05-21-2011, 03:31 PM

I've seen a cruise ship with the same sort of propellers. There are 4 on the bottom, driven by electricity, and they can swivel around for docking manuvers.