stainless intake grate

[Rivernut]

I'm having a friend weld me a stainless intake rock grate for my American Turbine SD-309. I have broken mine a few times, some without hitting anything, just harmonics I think. We are planning on heating it after it is made in a furnace to relieve stress that may be in the stock ones. Also sharpening the leading edges. Would putting in a sideways bar in the middle reduce flow too much? It is hooked to a GM 350, 4400 RPM's @ 53 MPH. Any advice about making a new / better one?

[UBFJ #454]

River -
Your asking for trouble using stainless. The metal is far too brittle.

[malcolm]

I've got a home made stainless grate on my Jac WJ. It's pretty bulky and I imagine it's slowing me down, but it's never broken.

[Rivernut]

Brittle? Stainless is stronger I thought.

[v-drive]

You're right Rivernut. The only way it's brittle is when it is a forging. Snaps quick.
:cool: V-drive

[roco racing]

dont use stainless and dont powdercoat it, just spray paint it, frequently check for cracks.

[Jetmugg]

"Stainless" steels have a huge range of mechanical, physical, and corrosion properties. What is important is the grade of steel used. 300 series stainless steels are extremely ductile and corrosion resistant, but to not have particularly high tensile strength. These are the most common stainless steels. 400 series stainless steels are capable of being hardened and tempered to extremely high strength levels, with a corresponding drop in ductility.
There are literally hundreds of different grades of "stainless" steels suited for particular applications. You just need to make an informed decision regarding material choices.
Steve M. - metallurgical engineer.

[DEL51]

My friend made a stainless loader for me and it worked so well that he ended up making 3 more for some other boats.I had it for 7 years and it still looks great.I could ask him what grade of material he used, if you're interested.I did have him sharpen the edges but not kife edged.Its more like an airplane wing.
[ December 20, 2002, 07:53 PM: Message edited by: DEL51 ]

[jweeks123]

Del, the shape of airplane wings changes quite a bit with the speed. how did you decide which wing shape to use?
jw

[DEL51]

The shape was not something we thought long and hard about.The leading edge was more rounded than sharpened and unlike a true airplane wing,the sides were identical.Imagine this shape > with a tip that is slightly rounded.I thought this would prevent chipping or cracking if I struck a rock. I wonder if making one side flat and one rounded would make any difference and what side should be rounded.Someone may have already thought this out.Maybe froggystyle, he is always investigating stuff.

[Rivernut]

My jet boat is a bass boat and is used a lot in shallow rivers with gravel bars and rocks. I hit stuff every once in a while, so the grate must be able to stand up to rocks being crammed in between the slots. The one I have is being repaired and duplicated. It had cracks in all five of the bars in the water path. Seemed like it cracked far too easily for what little trouble I've gotten into with this boat. The center piece broke on its own once or twice from harmonic vibrations, I had hit nothing.

[gstark]

Thanks Jetmugg for providing facts as opposed to opinion. 300 series CRES material has been the workhorse material of choice for a couple of decades in high temp aerospace pneumatics and cryogenic systems. Now with engine bleed temps exceeding 1000F and some over 1300F, even the best Inco materials aren't cutting the mustard. The ductility and ease of fabrication of the 300's combined with reasonable strength make for a good combination.
If anyone of the folks out there have had the opportunity to spend time in a vibration room, a real appreciation for dynamic behavior results.
Just picture a 36" dia loud speaker capable of moving plus/minus 1.25 inches at 20 hz, able to drive to 20,000 hz and capable of generating 25,000 pounds of force.
My guess is that many of the failures that have occured are due to the dynamic response of the grate structure. Unsupported or unstiffened edges and particularily cantilevered beams resonate at fundamental frequencies and most likely these are getting excited as well as their harmonics. The resonance creates displacements at these frequencies which can only be seen with a strobe - or felt by hand (not that you can actually do this on a boat). These displacements create physical strain in the welds (commonly called stress, which by the way exists only due to mathematical relationships, much the same way as torque and HP).
Fatigue cycles can build rapidly with the eventual failure of the structure. Any metallurgist should be able to take a 1 second look at the failure and make a judgement if the failure was due to fatigue or static overload.
We did development work on the new C17 APU exhaust duct and the exit grid/turning vanes were dancing all over the place, failing. We sequentially stiffened the structure to drive the frequencies up and beyond the excitation range and hey have been working like a charm.
I've never tested in water, that would be a little hard to do - so I don't know what effect the kinematic/dynamic viscosities and density would have on response. I'd need to think about this a little.
Enough rambling for now.

[LVjetboy]

Steve, I'm thinking of making a v-blocker out of SS...seems the v will strengthen the mount point. What do you think? If SS will work, what kind should I use?
jer

[gstark]

LV - you can use any of the austenitic 300 series, 304, 316, 321, 347. These are the most common. There are L-grades to some of these that change sensitivity to stress corrosion cracking, if my memory serves me.
Any of these materials are easy to blank, shear, cut and weld using TIG/MIG or stick arc.
If you need a source for material, let me know.

[Rivernut]

I got my new iron gate done (not the stainless yet). We took out one of the members so there are 6 instead of 7, and used 3/16" instead of the 1/8". The difference in flex is very noticable. The openings seem very close to the originals, so there should be no greater risk to the impeller. I would have posted a picture of it but I don't have space on my site to add short-term pics.

[LVjetboy]

Thanks for the offer gstark! I'm be emailing you soon.
jer

[Jetmugg]

Jer:
Here are some comparisons, and some of my thoughts on material selection. Any of the 300 series SS will be relatively easy to fabricate, and will offer more than adequate corrosion protection in fresh water. The tensile strength of this series of SS is in the neighborhood of 80,000 psi. This is pretty similar to plain carbon mild steel in terms of strength.
If you want to move to something more sophisticated (which I know you like), a precipitation hardening or duplex grade of stainless can offer over twice the tensile strength, and still have excellent impact properties. Examples of such dual phase materials would be 15-5 PH or 17-4 PH (precipitation hardening) or one of the true duplex (austenite and ferrite) containing stainless grades such as Ferralium (trade name) or alloy 2205. These are metallurgically superior materials, with strength up to 200,000 psi depending on heat treatment. The down side of these materials are the cost of the required heat treatments. This is serious stuff (no backyard barbecue stress relief). This will require a solution anneal up around 2100F, water quench, probably a cryogenic treatment to ensure complete transformation, and then an aging treatment. All of this needs to be done after the welding is complete.
For my money, I'd recommend that you consider fabbing the grate out of a higher strength low-alloy steel (i.e. 4130, 300M, HY-TUFF) which can offer strength up around 200,000 psi, and be much easier to fabricate and heat treat. Use the money you'd save and have the grate ceramic coated for corrosion protection.
For something really high-end, find a place to get your hands on some AerMet 100 (aircraft grade landing gear material) which has strength up over 300,000 psi. Then you could build the grate really thin but very strong. If it's good enough for military landing gear, it should be good enough for a jetboat.
My 2 cents (and then some)
SteveM.

[jweeks123]

Thanks for your input, gstark and jetmugg. technical info provided by professional engineers is sorely lacky on these boards. you guys have raised the bar here.
a question - what are your thoughts about building something out of materials that are way beyond what's necessary? in other words, if guys are building loaders from mild steel and they work, why would you use 200ksi or 300ksi material. I wonder if anyone has analyzed the loads on a loader?
jw

[gstark]

As with good design practice, one should mate the best choice of material with the best configuration. Examination of the failure modes is a definite necessity here, based on reading of other threads. Are failure due to fatigue or static overload ? If fatigue is the failure mode, then a material offering excellent S-N (stress vs. no of cycles) characteristics is desired. If static strength is exceeded, then a material with increased Fty (yield strength) is desired.
Inco 625 is roughly 2x the tensile yield of mild steel, with superior fatigue characteristics. It is easy to form, fab and weld. Inco 718 in the STA condition offers 140-180 KSI tensile yield, but is ultimate critical. Its fatigue life characteristics are similar to Inco 625 although the two curves do cross. A short 3 hr age at 1400F followed by air cooling is used commonly to attain 140KSI Fty with excellent % elongation.
Just about every fighter, commercial, or regional aircraft today is flying with hi-pressure, hi- temp Inco heat exchangers and pneumatic systems.
Jetmugg - correct me if I am wrong, but the PH alloys (17-4 and 15-7) tend to loose ductility/% elongation with increase in strength. This would be opposite what one would want in a structure, as this in my opinion.
The bottom line is what is driving the failure. If dynamics are the root cause, then virtually any design fab'd from any material will fail eventually. So using a 300 KSI material doesn't solve the problem in and of itself. Designing to minimize the structural response or moving the natural frequencies of the structure away from excitation frequencies it is the whole answer. Combine this with material selection and you should have no problems.
If this topic builds enough momentum I will see about making my vib room available for testing. This is a real eye-opener, because 90% of all failures we experience in testing occur in the vib room, not in LCF or endurance testing.
Analyzing the loads on a loader is likely a very difficult thing to do. Too many assumptions on boundary conditions are required. It would probably be easier, and dollars ahead to instrument with triax strain gages and measure actual strain.

[LVjetboy]

OMG, you guys are way outta my league! But I love the ideas floatin' here. Thanks JetMugg and GStark. I think if jet drives got a fraction of the engineering and analysis put into aircraft, we'd go a lot faster. Good ol' trial and error (experiment) has it's place, but has it's limits too. And once the pro retires, then where are we? Not recorded or written, well just start all over again and invent the wheel. I've seen a huge void in technical jet boat material.
Jweeks123, I seriously doubt anyone's done a stress analysis on a loader...more like, if it breaks, well then it wasn't strong enough??
Build another boat?
Something else touched on briefly, maybe just as critical as type of metal: loader design and weld quality. Finger grates may be more susceptible to vibration fatigue than race loaders, open center race loaders more susceptible than closed, etc. Ramp length may influence both loading and natural frequency. Where supports are located, the thickness of metal and quality of weld joint all make it hard to predict failure...or generalize in blanket statements about all loaders. And there are quite a few designs out there.
Real world, the fact that steel rusts and paint won’t stay on doesn’t help either. Rust causes pitting and stress risers. Given enough rust damage, cracking and failure may follow. Even with a good design made from quality material.
And from what I’ve seen, I’m not impressed at all with quality. Most welds look like something I’d do in high school metal shop. Base plates warped and pieces uneven. Squared corners and globs of weld junk everywhere. Ya they work ok, but water flow can’t be happy with this situation. Jet drive known as inefficient? Wonder why. And for something we stick in our intakes, in front of some fine tuned high-dollar machinery spinning at extreme rpm, just seems there’s a market for better quality alternatives.
jer

[Rivernut]

Thanks for all of the input...some hardcore science going on here. Are the manufacturers listening? American Turbine, Berkley? Your grates suck in more way than one! Mine looked like high school shop class with crooked bars and porous welds. The new and improved "low tech redneck grate" was made with thicker cold-rolled generic stuff from the local refab shop. One less bar on the improved design. The welds are shorter becuase a lot of the cracks were originating from them. The bars are straight and evenly spaced, not twistd like the original. The leading and trailing edges are rounded, not sharpened. And the hole will line up when drilled. A few of the bolts were stripped from the get go and were helicoiled. We are holding off on the military version until further testing and tournament victories to fund this project. You guys that cut and mold steel into toys are gods. Below are the before and after pics. Note the cracks on the original black grate. http://gallery.alloutdoorschat.com/Rivernut/jetgrates.jpg

[DEL51]

Good Job rivernut. I hope everything works out.The type of loader I had made was for a different application.You can easily see the improvement on your photos.