Hondo6

While the chain tension when stationary will be about the same (neglecting buoyancy effects), when in motion other factors won't be.

Liquids and gases/gas mixtures (like air) are both fluids. In any fluid, a moving object experiences form drag. In air, it’s called aerodynamic form drag; in water, it’s called hydrodynamic form drag. While form drag in gases/gas mixtures and liquids are governed by the same basic equation (D = ˝ρCAv^2), in virtually all cases there will be a large difference in scale.

All else being equal (shape, velocity, etc . . . ) form drag varies linearly with the density (“ρ” in the equation above) of the fluid through which an object is passing. It also increases with the square of velocity, so drag becomes far larger proportionally as speed increases. (For those interested, the other terms are “D”: form drag; “A”: frontal area; “v”: velocity; and “C”: coefficient of drag, which varies with the shape of the object concerned.)

Water is roughly 800 times denser than air. If the RD is underwater and exposed to the point it receives relatively unimpeded fluid flow past it, the increased form drag might be enough to be significant. My guess is that the derailleur body would be pushed back more than when in air at the same speed - since it will experience about 800 times more drag force pushing it back than in air at the same speed. Would that extra force move it backwards enough to cause problems? Dunno; that would depend on the strength of the RD's springs. But it's a helluva large increase. In theory, it could.

As noted above, going single speed would eliminate the issue.

But if a single-speed drivetrain won’t do and multi-speed gearing using an RD exposed to flow is necessary, it might be worth fabricating a faring and placing it in front of the RD to shield it somewhat and thus reduce the effect of form drag on the RD. It might even reduce form drag overall slightly (or might not, depending on it's area and shape). Making test runs with and without the faring would show definitively whether or not it was needed and/or helpful.

References: https://physics.info/drag/ (for discussion of form drag); Wikipedia (for comparative density of air and water)

djb

Hondo, makes for an interesting armchair topic doesnt it? I would have thought the really slow underwater speed wouldnt have an effect, but who knows. Will certainly be something for this group of students to encounter and perhaps have to work around. If going single speed is easy to figure out what gear would be better (guess they could do tests in a tank or something) or just resize a chain for whatever cog works better with the human engine and the reality of moving this thing through the water.
The poster Kooby is the one to ask his or her real life experiences.

still kinda fun just thinking of the ins and the outs isnt it?

Hondo6

Yes, analysis of stuff like this can be interesting and fun.

I have no idea at what speed wind resistance begins to affect a RD's geometry due to wind resistance on the RD body and cage. But I got curious, and played around a bit with a spreadsheet. The calculations were bit surprising.

In the equations for form drag, assuming that the coefficient of drag remains the same for the two fluids everything cancels out if you divide drag in water by drag in air except for density and speed. And the ratio of densities is a constant: 800. Plus, the units for velocity also cancel out. So a quick calculation can let you determine what speeds in water and air have roughly equivalent drag.

Turns out that 1 MPH/kph/whatever in water has the equivalent drag as between 29 and 29 MPH/kph/whatever. That speed probably isn't an issue. But remember, the velocity is squared in the equation.

A velocity of 2 in water calculates out as having drag equivalent to a velocity of between 58 and 59 in air. For 3, the numbers get silly: a velocity of 3 in water has the same drag as a velocity of almost 85(!) in air. I stopped there.

As I said: dunno at what air speed a RD begins to experience enough drag to affect its geometry (and thus shifting) by rotating the cage, and I also have zero idea how fast a human-powered sub can go. But the relative difference in drag is certainly enough to make one wonder if the increased drag in water might have an effect on an exposed derailleur-type drivetrain.

djb

My totally unknowledgeable opinion is that the slow speed underwater will not be an issue to a Rd, but a shield in front of it would certainly deal with the walking speed ish this craft will be going at.

The one underwater ghost shifting example here could very well have been a crappy drivetrain or misadjusted shifter, bent hanger, who knows.

It would be fun if the parent asking the question comes back with a progress report, but thats to be seen.

kooby

In our sub, we tested the drivetrain with the RD on land and didn't have problems, but if i recall correctly we didn't test it with resistance on the propeller, so that may have been a problem. Also its possible the RD wasn't in an ideal position and there was less chain wrap on the cassette. I think in our case it may have just been a perfect storm: viscous environment, old chain+cassette, lots of resistance, poor geometry, etc.

kooby

Well... I went back and found some pictures from our sub...

Definitely the angle the RD is mounted at is not correct, also i think the chain might be too short?

Darth Lefty 

This is a really cool project.

I did a lot of events like this through my school career though not a submarine specifically. A really efficient RC airplane, various rocket events, a wind tunnel model, a “joule mobile,” a Rube Goldberg contest.

Make pilot safety #1 priority (HPV subs have an original sin) and thorough testing very next. Performance third. Finish the thing weeks in advance and get it running well. If you show up with a craft you just finished you will spend the whole event testing and never get on the board.

djb

neat photo.
*It makes me recall this from Formula One, anyone else follow F1 and the "fourth pedal" that a photographer finally snapped a photo of when one of the Maclarens broke down on track and he stuck his camera with flash into the cockpit and snapped the shot that brought their neat braking system to light?



https://www.mclaren.com/racing/insid...pedal-3153421/



We can see that the chain was not routed through both rd pulleys, so theres that already thats a factor (or not) but the fact that the rd is mounted onto a solid block of aluminum that doesnt allow for proper positioning could have been the problem also.
When I mentioned the derailleur hanger, I hadn't visualized how the drivetrain setup would be like, but I wonder if a hanger type setup would allow more adjustment, ie slight bending of a steel piece, to get the rd right.
Mind you, I'm sure you guys did the alignment measurements of this block that the rd was attached to, but who knows. Also, I don't know what factor running the chain only through one jockey wheel was, but if possible, keeping the drivetrain in the same working configuration as on a bike would seem like the least problematic method, as its a known factor in how it works.

After all this interesting talk, I suspect the parent who asked this question came to a "bicycle forum" just to get an answer about the bearings and lube, and perhaps got put off by the speculating by all of us, and / or the one grumpy guy here.

to the parent, if you see this, suggest to your son and team to really look at the basics of a working bike in regards to rd placement, chain alignment distances in regard to crankset and cassette, and try to mimic this setup as best as possible while dealing with the space constraints and gear transfer stuff going on to turn a propshaft.
If they go single speed, then just have some prelengthened chains for different cogs until they figure out what gear is more appropriate. A bike store could even give them some old chains to use that will serve the purpose well enough.

Beezer

Thanks so much for replying! We have no idea what to expect. Any other ibsights into what typically goes wrong? Was control an issue?

kooby

Yeah, control absolutely was a problem for us. Our control fins were way too small, they would have worked well at higher speeds, but we never managed to get the sub up to those speeds. I think they were about 10x10 inches for each fin? Our quick-fix was to cut up some plastic boxes and use duct tape to attach them to the fins to increase their size. I think a good fin size should be 12x24 inches.
Also, having a good cart to transport the sub from the waiting area to the basin will save you a lot of energy and frustration.

I will probably be going to this year's edition of ISR, so I hope to see your son's team there!

Beezer

Awesome! That bit of advice is gold. Because we are our own small team and I am not an engineer we have had to go at this based on gut feel and some admittedly shaky calculations. Knowing what was successful is a real help!

Beezer

Thx this is great. This is a wet sub, but do you think there is any advantage to trying to run the bearings dry by using a box snd lip seals? Might be too complicated but its an idea

Beezer

event is over 4 or 5 days I believe ​​​​​​​

Beezer

omg thanks so much. You folks are gold! Nothing beats real world experience! Thx!

Beezer

I know this is a bike forum, so I will not go further with any sub questions, but here is one of the episodes where we chop up his old mountain bike for the drivetrain. If interested in the non bike-related progress you can check out the youtube channel. Thx for all the help!

FBinNY 

Not for your application. Time is on your side here. Whatever water damage occurs will be too slow to matter.

FBinNY 

Thanks, but I was more focused on the time length of individual heats & the air consumption considerations.

Ironfish653

Probably not worth the headache. Trying to run a 'dry' bearing, means sooner or later, you'll be lubricating those bearings with....water. You could, I suppose; but it'd mean constructing an entire new bearing housing, since you can't just stick a seal on the outside of the existing hub / crank spindle.

I'd plan for the bearings to get 'wet' Depending on what kind of bearing it is, simply filling the housing with grease would be sufficient; there's no 'space' for the water to go if it gets past the seals, but I doubt you'll be seeing the kind of water pressure or shaft speed that UHD-310 is seeing up in post #23
For a 'sealed cartridge' bearing, hosing it down with White Lightning 'Epic' or 'Wet' should be enough to keep the bearings happy for the duration of a trial heat.

Remember, get all the water out / off of it as soon as it's out of the pool for the day. WD-40 is good for getting water off of surfaces and out of cracks, but it also tends to 'thin out' other grease and lube, so be ready to re-lube after everything is dried off.

Good Luck!

djb

unless the bottom bracket and bearing is an old style ball bearing type, which is "easier" to really load up the ball bearings with a ton of grease, a "sealed cartridge" bearing really can't be re-lubed, re putting grease where the actual bearing is.

its probable that these folks wont have any experience opening up a bottom bracket (bb) nor have the tools, so to the parent--if at all possible, ask around if someone you know has the tools and experience, and they could open up the bike before it is chopped up and cram it full of marine grease.
I'm sure you'll all be having lots of other things to think about as the project progresses, and if lucky, a well packed bottom bracket bearing will be fine enough as is for the duration of the project if it is packed well with grease.

to the parent, look up the types of bottom bracket bearings, so when you get a donor bike to chop up, you'll have an idea of what to do to grease it if the team ends up doing it yourselves. Its a fairly simple job, it just comes down to having the right tools to do it, depending on the bb type.

this is an old style loose ball bearing type bearing, its still common on low end bikes, which might be what you end up as a donor bike.

and a sealed bottom bracket type

hopefully you can find a helpful bike person with the tools who is happy to help out.