Nozzle for outboard motor

Does anyone know if there is a nozzle(like the ones on commercial boats)for a small (25hp) outboard. It's a ring that fits closely around the propeller improving its performance.

 

There are shrouds available for outboard props, but they are designed more for boats operating around swimmers, like surf rescue inflatables. They reduce performance. I can't image any outboard nozzle improving performance unless it was getting extra thrust at very low speeds moving heavy boats.

 

Google Propeller Guards for Outboards and you will see many guards and some of the manufacturers claim better performance
A propeller working in a duct with tight tip clearances will improve performance but there would have to be some reduction in this performance
due to the extra drag created by pushing the ring through the water.

Perhaps some of the manufacturers that you will find will actually have real data that proves their claims

 

No way they improve performance in the vast majority of applications, otherwise they would be standard fittings.

 

http://www.propguard.net/test.html

Mr E, attached is a test result from Propguard, not sure how accurate that it is but the manufacturer makes a strong case.

 

Evidently the forum has many discussions about this topic

Go to the search box, type in shrouded propeller and many will show up
To get down to a more concise selection type in "shrouded propellers v.s. nonshrouded propellers" to bring up a 2010 thread

Additionally, if you Google "shrouded propellers for outboards" and look in the picture section, there are some actual nozzles ie large inlet and a smaller outlet attached to some outboards
and the respective links to the picture

Another source is again search for "propeller ducts for outboard motors"

 

I'm not convinced these shrouds maintain performance on small outboards say 4 to 25Hp, at least that has not been my experience. I note one of those tests is with 230Hp almost ten times the OP's motor size. No reason to doubt the test data but it is a bit out of range for a 25Hp, and probably hull weight. The only performance benefit may be in turning at slow speed with a heavy or draggy hull as far as I can see.

The issue is maybe more a safety and regulatory one IMHO. Here in the UK some places insist on them, but most don't. Tbh it comes down largely to driver training, skill, awareness and seamanship to avoid any incidents.

Thanks for the links and advice Barry, useful to the OP.

 

At low and modest speeds these do offer some advantages, but once you get up over say about 25 knots, the drag associated with the extra gear hanging is a detractor. At lower speeds, steering response is improved and bottom damage in shallow water is greatly minimized. There's three types, most are simply a prop guard, designed to protect swimmers and prevent lines from getting at the prop. These will offer nothing but prop protection. The second type is an actual duct (some are vented), with a tight fitting inside diameter. This is the style you want. The last type is "tourist trap" product, seen in every seaside marina. It looks like a duct, but the gap is way too big, so it's just a prop shroud that doesn't do much, except protect the prop.

 

There was these http://www.propellersafety.com/ringprop/

And also a "??? Handler" that was once available as a closely fitted shroud for yamaha 9.9-4/

Jeff.

 

They are used on tug boats and anchor handlers
Not used on rig supply boats or container ships
If thats not proof of what they can and cant do nothing is

 

They increase thrust at low speed. That is the reason to use them in trawlers and tugs. At higher speeds the increase drag gives less performance that a standard propeller.

 

"The Handler" was a proper, close fitting nozzle that actually came with a matched prop but was pricy at about $500 back when they were on the market which I'm sure is why they didn't sell . All these propellor guards available today are not nozzles. The nozzles are to increase thrust so you can have more thrust at the same rpm ot the same thrust at lower rpm with lower fuel consumption.