posted 01-07-2011 09:44 AM ET (US)            

Loss of speed. Increased drag and fuel. There are some avail for some engines. The only statement that I heard was from a RIB service company in Houston. Hi said that the guard might keep you from shoving your foot into the side of a prop. Without guard, the shape of the prop might tend to push your foot outward. The guard would; however, form a tube and any appendage that got into it would be quickly lost. Sort of down the disposal.

posted 01-07-2011 08:26 PM ET (US)            

Drag was negligible at 4 mph. Great invention. There were lots of dings on the prop guard; none on the prop.

http://i31.photobucket.com/albums/c391/boatdryver/Stbd-Qtr-web.jpg

Most Tugs and other commercial vessels use huge prop guards in on the Inland Passage to Alaska because of trees in the water so they can run at night. they accept the drag-induced extra fuel consumption in return for being able to run around the clock. Of course they aren't trying to go 25 knots

JimL

posted 01-11-2011 03:24 PM ET (US)            

One reader pointed out that guards may increase injury if a limb was caught inside the guard. Perhaps, but a look at the physics of the situation might suggest otherwise.

A planing boat at 24 mph and 3600 RPM, with a nominal reduction gear of 2:1, swinging a 15 inch diameter propeller, has propeller blade tip speeds of about 80 mph. Damage from impact is related to the square of the speed, so the damage potential from the blades is about 11 times that from the hull or gearcase itself.

At idle speed of 5 mph at 1000 RPM, the blades are going 22 mph and have about 15 times the damage potential as the gearcase itself. At idle speeds, humans can withstand collisions with the gearcase but not with the blades.

In Florida huge tracts of inland water are restricted in speed for manatee protection. Manatees are somewhat more rugged than people and can probably withstand gearcase collisions on plane, but cannot do well with prop blade strikes. Perhaps protected propeller boats can in the future obtain some relief from the speed restrictions upon a demonstrated safety increase for these marine mammals.

The subject of drag with propeller guards has been discussed earlier. Guards can provide an "endplate" effect that actually increases the efficiency of the propeller. The guard itself is more draggy. The net effect probably depends on the design of the guard, the speed of the boat, and other factors.

Interesting is the idea, first articulated here, that guards may be linked to a control that allows limited vectoring of thrust. For instance, a guard may be designed to produce stern lift by vectoring discharged water downward. This would obviate the need for ventilation plate wings and maybe for trim tabs on some boats.

posted 01-13-2011 09:49 AM ET (US)            

But returning to guards, Jimh says the manufacturers would have done it if it was worth doing. Jimh, if you believe manufacturers have a primary interest in either the environment or even safety to manatees or swimmers, I have underwater land to sell you. Unfortunately or fortunately, in this free enterprise system, corporations have little option but to maximize profits. Unless the government intervenes with regulation, saving the manatees is just not profitable.

I wonder why there is a rush to ad hominem argument whenever a subject is discussed that elicits knee jerk disagreement. I won't go into it, but pcrussell50, my fluid dynamics credentials are pretty solid. I have taught fluid mechanics, aerodynamics, and computational fluid dynamics at the graduate level in a major university for years. Not that it matters, because I haven't asked anyone to believe anything that cannot be demonstrated easily.

The "endplate" effect is real, and it increases the efficiency of a wing (or a prop blade). Ducted fans have been used, off and on, in airplanes and they work. Sometimes the tradeoff is worth it, sometimes not. Putting a shroud (guard) on an existing propeller design may not be optimal; probably a prop designed to run with a shroud would do a lot better. No one has commented on the possibility of thrust vectoring with a guard, something I find very interesting and maybe worth some work.

I could find no USCG tests condemning guards; there is quite a bit of information on the internet though, and it is easily googled. It suggests this is a field ripe for technical development with a real need. Obviously, someone is concerned enough about propeller safety to reduce legal speeds to idle over huge expanses of inland water in Florida.

Anyway, people believe what they want to believe. But sometimes reality is much more interesting.

posted 01-13-2011 01:44 PM ET (US)            

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I wonder why there is a rush to ad hominem argument whenever a subject is discussed that elicits knee jerk disagreement.

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It's not knee jerk, my friend. I promise you. I've been not only a student of, but an operator of, high speed fans, both ducted, and unducted, for my entire professional career. Fuel prices being what they are, these topics are of very great interest to my employer, who has many engineers in it's employee, including some on loan from Boeing, GE, a Pratt and Whitney, and Rolls Royce. Oh, and my argument was not ad hominem. It was purely fact-based. My sarcasm at your application of the equation for kinetic energy was an ad hominem attack. I admit it. But it is important to separate which of our statements are ad hominem attacks from the statements that are true, lest the baby be thrown out with the bath water.

quote:

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I won't go into it, but pcrussell50, my fluid dynamics credentials are pretty solid. I have taught fluid mechanics, aerodynamics, and computational fluid dynamics at the graduate level in a major university for years.[QUOTE]

This is good news. Now my attack feels a little less as hominem, to me anyway.

[QUOTE]Not that it matters, because I haven't asked anyone to believe anything that cannot be demonstrated easily.

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I grant you on technicality, that you have stopped short of demanding federal regulations that boats be equipped with these things.

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The "endplate" effect is real, and it increases the efficiency of a wing (or a prop blade). Ducted fans have been used, off and on, in airplanes and they work. Sometimes the tradeoff is worth it, sometimes not.

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No questions about endplate effect from me. I'm fully versed on it. If I were operating a heavy pontoon boat, that would go no faster than 15 mph, (about 6.7m/s), at 5500 engine rpm, I would use a Dura-Jet. But that form of boating of course, has no place in this forum. You must also know that as you get much over 8 meters per second in water, endplate effect drops rapidly to a second-order state as drag and structural loading become the dominant factors.

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Putting a shroud (guard) on an existing propeller design may not be optimal

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May not be optimal?! Coop, you are the master of understatement, my friend! That made me chuckle.

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probably a prop designed to run with a shroud would do a lot better.

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Agreed. Such a system would do much better. In fact, there already is such a system. The Marines use it on one of their "Black", military series Evinrudes, for tactical beaching ops. It is much better than anything else out there, with an engineered housing, a stator, and a proper impeller properly fitted to the housing and designed to work with the stator. And it still sucks. It sucks so very badly it's hard to describe. It's devastatingly inefficient at recreational boater speeds. In fact, I'm not even sure it's _possible_ to reach recreational boater speeds with it. At least DuraJet has the intellectual honesty to publish their test data... where they did not even reach 20 mph... at 5500 engine rpm!? Now why do you suppose they did not publish any data in the speed regime that say, Boston Whaler boaters operate? At least I can tell you that the endplate effect was working well up to about 18 mph though. Because their graph showed something along the lines of a 5% or greater increase in efficiency over a prop... at 10 mph... and if I were a pontoon boater, I would look at getting a DuraJet, really. I would.

I launched an initial salvo. You fired back. Now let's talk turkey. Here's where we can hopefully tap into your expertise. You must know that when it comes to turbomachinery, the devil is in the details... very small details. Clearances not at the level of thousandths, but of ten-thousandths, make or break these things. This is one of the reasons (along with high temperature superalloys), that gas turbine engines cost not 10 times more, but 100 times more, per horsepower, than recips... and they STILL have greater brake specific fuel consumption. Now, you're an engineer. One of the things that differentiates and engineer from an pure theorist is, he is charged with making things that are of practical utility. That means finding a way to balance cost versus performance. Nobody is going to pay say, $150,000 for a 40hp outboard even if it is optimized by CFD and painstaking aerospace experimentation, and contains exotic alloys. It just won't fly, (pun intended). If it were up to you, as it would be if government forced this upon us, how much should we be expected to pay for motors that drive our Boston Whalers at their current speeds, a few mph either side of say, 40mph? Double? Triple? More? Or do you think it can be done for not even double current costs? If so, how? Maybe through public funding?

This is not like a seatbelt or mandatory helmet law... not in any way.