Shrouded propellers on trolling motors

[1idahoguy]

It?s been a few years since I was in college, studying fluid dynamics and all that. But I?m wondering...why aren?t more (any?) electric trolling motors manufactured with shrouded propellers?

It?s a well-known phenomenon that shrouded propellers have greater thrust efficiency vs. non-shrouded at low speeds ? exactly the kind of conditions that electric trolling motors operate. Obviously, building a motor with a shroud is more costly. And perhaps the added cost isn?t outweighed by the greater efficiency. But I believe many of us would at least consider a shrouded design when making a purchase.

Anyone here with some background in marine technology/manufacturing have any comments?

 

[gm280]

Not any expert on fluid dynamics but if I'm not mistaken, in order for there to be any increase in thrust output, the prop has to be extremely close to the shroud to gain anything. That being said, imagine, if you will, a trolling motor hitting anything in the water. There goes that thrust advantage. And IF the manufactures could build a strong enough shroud to avoid such damage, then the increase of water resistance and weight would negate the thrust increase. That would be my opinion as to why they don't build them like that. When you see folks with broken props and bent shafts, those are very hard hits...

 

[steelespike]

I think trolling motors are often in or near weedy waters.I think a naked prop is more likely to get through weeds
without having to stop to clear the prop.

 

[1idahoguy]

Those are very valid points.

I wonder if a prop had a shroud (or ring) build such that it was attached to and rotated with the blades...if that kind of design would also net the same thrust efficiencies as a fixed shroud. Hmmm...

 

[gm280]

Ha, with 3D printer available these day, buy one and print one out and see how it works...

 

[Texasmark]

I think trolling motors are often in or near weedy waters.I think a naked prop is more likely to get through weeds
without having to stop to clear the prop.

Absolutely. I once had a Minn Kota with a 13" blade thinking about thrust and "moment of inertia" on the large diameter and all that good stuff when I selected it. In reality if you want thrust, get a motor with a big motor. The little 4 bladed Motor guide prop that should have the prop with the least calculated thrust actually is the most practical if you bought your boat to fish where Bass live. In open water and continuous high motor speeds, 13" is great. Don't need a shroud in open water.

Mark