shrouded propeller v.s nonshrouded opeller

[kach22i]

http://smalloutboards.com/pumpjet.htm

Pump-Jet lower units are designed provide a safe, strong performing, and reliable alternative to standard props. These lower units will fit Johnson/Evinrude 20-35hp outboards that have thru-hub exhaust and the gear shift connection contained within the water intake screen ( OMC outboards made from the early 1980's-today. Use 1992 as a cross reference). The gears and housing are the same as a standard lower unit with the exception of the rotor device and casing. As you can see from the photos below water is redirected and funneled from the rotor blade into a narrowing opening towards the back of the casing. AFA Marine has boat tested these lower unit devices and we were very impressed with the performance. We noticed improved acceleration and turning ability with this device. If you are looking to convert your Johnson/Evinrude 20-35hp outboard motor to a Pump Jet system this product is for you. All that is needed is to remove your existing lower unit and replace it with this one. The gears, shift rod connection, shaft, water tube, and bolt configuration are the same.

In the 1990's, the U.S. Military recognized the need to develop an outboard motor which could be used safely night and day for training exercises with men and equipment in the water. Attempts to adapt cages and guards to open propellers resulted in unacceptable performance losses and decreased engine reliability. The Pump Jet provides added performance, safety, and reliability to outboard motors.

[chartman]

You mention the 1990's U.S. Military efforts. We have a copy of the U.S. Marine Corps "Safety Evaluation of the Pump Jet Propulsion Assembly for the Military Amphibious Reconnaisance System" 30 Sept 1992 report on our Propeller Guard Information Center site at:
http://rbbi.com/pgic/propellergtest/...t/pj95usmc.pdf

This document spent many years online in a USCG docket, as part of ACT's 1995 response to a USCG request for public comments, but was removed earlier this year when the USCG moved to a new docket system and dropped some of their older materials.

If anybody is interested, we also have a lengthy document from ACT that was part of their submission. It includes some performance charts and other information.

chartman

[Guest625101138]

chartman
I am interested in the performance data. It is clearly well designed. It is not a backyard experiment.

My expectation is that the top speed will be reduced but like the brief comments earlier you would expect better acceleration. It might provide overall performance benefit in heavily loaded applications as are being discussed on other threads here.

My view is that it could become a standard design if it offers a performance and fuel economy advantage. So there are benefits all round including the main benefit of safety. It might be extremely useful for ski boats where acceleration is important and risk of prop strike reasonably high.

[kach22i]

Originally Posted by chartman

You mention the 1990's U.S. Military efforts.

Nice little document there.

Do you have anything with more photos or diagrams?

I'm an architect, not a engineer and like pretty pictures.

[Village_Idiot]

Biggest disadvantage I see is the lack of propellor interchangability. Different boat designs/layouts can be fine-tuned for performance by trying different props with varying pitch, diameter, blade number, cupping, rake, skew, etc. - I don't see a way to do that with this setup.

[chartman]

I temporarily posted the full 1995 letter from ACT (less the Marine Corps test report I linked to separately earlier). Their letter discusses the drive and provides a propjet vs. prop speed comparison curve for a Zodiac F470 at various weight loads. It is about a 4 megabyte pdf file
http://www.rbbi.com/pgic/propellergt...et/actpj95.pdf

I will leave that file up about a week and then take it down (dont want to leave a file that big on our site). If you think you might want to see it in the future, save a copy to your computer.

I will permanently leave up a copy of the performance curve from that letter. It is at
http://www.rbbi.com/pgic/propellergt...ctpj95perf.pdf
Please note that data is very old and they have probably improved since then.

My understanding is pumpjets are generally available in small sizes (typically less than 50 HP). The availability of larger units has been caught by the "which comes first" problem. Tooling is relatively expensive and manufacturers do not want to commit to making larger models till demand is proven, but demand cant be developed and proven without larger units to sell.

A few larger units have been handbuilt which is very expensive. I seem to recall a 140 HP unit being built by ACT. I will see if I can get someone over there to join in the conversation.

chartman

[Guest625101138]

chartman
Thank you for the comparison curve. More or less as you would expect under light load and very heavy load however the ducting does a good job in the medium load state. Better than I expected.

[Holiday]

I have another case for using a prop guard, and if effieciency could be improved or the loss minimized it would be great. I have a Chrysler 26 with a 10 hp auxilary outboard. The problem with the outboard version of this boat is that the kick up rudder can strike the prop. Can anyone provide guidance in finding the proper tip clearance and Ai/Ae ratios, or perhaps comments on alternative designs such as a simple short lenther ring aroung the prop tips?

[Guest625101138]

I expect the tip clearance to be as small as the tolerances will allow. It gets down to the method of manufacture and any flexibility in the supports of the duct.

The area ratio is a function of the velocity ratio. If you have the existing diameter of the prop and the top speed the boat can achieve with the 10HP you can determine the velocity ratio. Typically it is around 1.2 for outboard props but it could be higher if you have a little prop on a slow moving boat.

[chartman]

Several companies make ring guards for small outboards. These are generally NOT designed ducts to increase performance, they are just basic ring guards for small outboards. You might just buy one of them, or get some ideas from their designs and try to improve upon them for your specific application.

Mac's Troll Prop Saver
http://www.propsavers.com

Outboard Propeller Guard
from Bite-Me-Too
http://pacificcoast.net/~bitemetoo

Propeller Guard
from Tanner Manufacturing
http://www.tannermfg.com/tannermfg.htm

PropGuard (New Zealand)
http://www.propguard.co.nz

In today's marketplace, companies are reluctant to devote the engineering and manufacturing costs associated with putting contours on the interior of ring guards this small (10HP). Another factor mentioned by Rick earlier, is trying to securely mount a ring very close to the prop blade tips in a manner that will keep the ring centered on the propeller on outboards this small, especially when the ring itself may not be real stiff. That is why you see the basic ring guards listed above in the marketplace.

If efficiency is a major limitation to you, you can probably recover horsepower more economically by cleaning the bottom of your boat, making sure it is correctly propped, throwing out the stuff you no longer use (lighten the load), repairing dings in the prop, running a stainless steel prop, reducing the amount of time you run wide open, etc. than by paying additional for a contoured guard over a conventional ring guard for an outboard this small.

If you elect to build your own, you might get some ideas from the zillion guard pictures resulting from a Google Image search at:
http://images.google.com/images?hl=e...h+Images&gbv=2

Chartman

[ancient kayaker]

chartman
I am interested in the performance data. It is clearly well designed. It is not a backyard experiment.

My expectation is that the top speed will be reduced but like the brief comments earlier you would expect better acceleration. It might provide overall performance benefit in heavily loaded applications as are being discussed on other threads here.

My view is that it could become a standard design if it offers a performance and fuel economy advantage. So there are benefits all round including the main benefit of safety. It might be extremely useful for ski boats where acceleration is important and risk of prop strike reasonably high.

With recent concerns about availability and cost of fuel being fresh in our memory, the fuel economy advantage should be attractive in the marketplace. Speed is not the main issue in all power boats, for example our local lakes have large numbers of pontoon boats that aren't very quick. I have recent memories of the amazing amount of gas one of these can consume. Perhaps they might achieve similar speed with a smaller, more economical motor if a shrouded prop can increase thrust.

[Peter123]

FYI,

http://www.statesman.com/news/local/...le-527456.html

Jurors find boat manufacturer partly liable
Company ordered to pay $3.8 million in damages, medical expenses
A federal jury Monday found a boat manufacturer partially liable for a 2005 incident on Lake Austin in which a teen's leg was severed by a propeller. Jurors ordered the company to pay $3.8 million in medical expenses and damages.

After deliberating for about seven hours, jurors found that the Brunswick Corp. shared more than half of the blame for the accident that severely injured Jacob Brochtrup, who was then 18.

Brunswick officials said in a statement after the verdict that they "stand behind our products, which are used safely and properly by boaters around the world."

Jurors found that Brochtrup also was responsible, as was the driver of the boat.

"I think they made a well thought-out, informed decision, and I think it was the right decision," said Brochtrup, now 22. "Based on the evidence that was presented, I think the case was proven pretty well."

Brochtrup sued Sea Ray Boats Inc. and Mercury Marine — Brunswick is their parent company — in 2007, saying they were liable for his injury.

He had been celebrating the July Fourth weekend wakeboarding with three friends at Emma Long Metropolitan Park when the accident happened. Brochtrup had just finished his turn on the wakeboard when a tow rope popped off the back of the white Sea Ray ski boat.

Brochtrup jumped out of the boat to grab the line. Unaware that Brochtrup was in the water behind him, 18-year-old driver Patrick Houston put his family's boat in reverse.

The propeller caught the top of Brochtrup's right leg and twisted it around, chopping deep into flesh, muscle and bone.

The suit said that the wound to Brochtrup's leg was so large that he had lost most of his blood and that it caused his heart to stop. He had been in cardiac arrest for at least 45 minutes, and a STAR Flight helicopter delivered him to the emergency room clinically dead.

Some doctors called him a "one-in-a-million survivor."

According to the suit, the manufacturer of the boat and motor did not have safety devices, including guards or covers, to prevent Brochtrup from becoming entangled or stuck.

"While we at Brunswick remain sympathetic to the plaintiff for this unfortunate accident, we are nevertheless disappointed with today's verdict," Brunswick officials said in the statement Monday. "We will evaluate our options in this matter going forward, including a possible appeal."

Austin attorney Robby Alden, who represented Brochtrup, said the decision marks the first successful case against the boating industry by a person injured by a motor. A U.S. Supreme Court decision in 2002 allowed such cases to go forward.

Boat makers prevailed in two similar suits nationally that involved older-model boats, he said. And jurors in two previous trials of Brochtrup's case deadlocked, resulting in mistrials.

During the latest trial, which began last week, Alden said he sought to show jurors that manufacturers could make boats and motors safer by installing guards on propellers and placing a shield over the back. The concept for a device was created years ago, he said, but the industry has resisted adopting it.

Before the verdict, Brunswick attorney Woody Norwood of New Orleans would only say: "We are very sorry about his injury. It was a very unfortunate accident."

According to Monday's decision, Brunswick was 66 percent responsible for the accident, and Brochtrup and the boat's driver each were 17 percent liable. The driver wasn't part of the suit and will pay no damages.

Most of the damages were for Brochtrup's past and future medical expenses. However, he also received $100,000 for his disfigurement and $264,000 for physical pain.

"I think the amounts for the award were fair," Alden said. "I'm happy about it. Hopefully, they will start making a change to protect people."

Brochtrup attended the trial and was in court for the verdict. Since his accident, he said, he has learned to live with one leg, but he hopes to receive a prosthesis soon. In recent months, he has been studying to become an audio engineer, and he plans to work in the recording industry.

"It's not what I would have wanted, but I'm just trying to enjoy life," he said.

[portacruise]

Sorry, maybe this has been mentioned before, but what about just attaching the ring directly to the propeller tips so the ring rotates with the prop itself? I have heard kort nozzles have been built this way and also have several desk fans that have the blades constructed in such a fashion. Should be safer than a regular blade tip and you don't have concern with debris or friction between the prop ends and separate ring at close tolerance. Don't know about efficiency, though.

Porta

Originally Posted by Rick Willoughby

I expect the tip clearance to be as small as the tolerances will allow. It gets down to the method of manufacture and any flexibility in the supports of the duct.

The area ratio is a function of the velocity ratio. If you have the existing diameter of the prop and the top speed the boat can achieve with the 10HP you can determine the velocity ratio. Typically it is around 1.2 for outboard props but it could be higher if you have a little prop on a slow moving boat.

Rick W

[baeckmo]

Originally Posted by portacruise

Sorry, maybe this has been mentioned before, but what about just attaching the ring directly to the propeller tips so the ring rotates with the prop itself? Porta

Forget it, the relative velocity along the tip is so high, that the friction losses will be far worse than with the fixed nozzle!

[chartman]

Originally Posted by portacruise

Sorry, maybe this has been mentioned before, but what about just attaching the ring directly to the propeller tips so the ring rotates with the prop itself?
Porta

RingProp (and several of their predecessors) tried to commercialize that concept (ringed props) out of Australia and the U.K over about a 25 year period and the company failed in 2006. We provide a history of that development on our site, but you will probably find the following link more relevant to your interests.

The Australian army did some testing on RingProps and provide a photo at:
http://www.defence.gov.au/news/armyn...69/letters.htm

As long as we on propeller safety developments, have you seen the Environmental Safety Propeller from Australia? It looks like a normal prop, has slightly blunted edges, and delivers the performance of a normal prop. We have video of people putting their hands and feet in them while rotating it at:
http://rbbi.com/pgic/ptech/safetypro...ypropeller.htm

Chartman / gary