Traction DPV

[DeepBound]

You know how propellors are inherently inefficient, because half of their energy goes into moving water, rather than moving the boat or scooter?

Someone I know wants to build a homemade scooter based on the idea of a snowmobile. It would have a track and move along the bottom, over the sand, rock, or maybe even silt, and push against the bottom instead of against the water. He reasons that this should be able to go twice as far with the same battery.

Do you know if there's already something like this out there?

 

[medic001918]

I don't know of anything like that out there, but having contact with the bottom is something that I can't see working out without ruining visibility. And in the wrong hands, causing damage to delicate marine life. All in all, I'd say it's not such a good idea. Just my two cents.

Shane

 

[DeepBound]

It would be OK on sandy bottoms, or just to get to the site on a silty bottom. Then you could follow your track back to the entry.

 

[Sideband]

Sounds like a silt throwing, vis destroying, coral reef killing machine. Yes I have heard of such a thing before. They are called student divers.

 

[broncobowsher]

But as the track goes over the top it will be pushing water forward.
then the frictional loss of the track drive, gear boxes, idler wheels.

There are too many losses to be had in an effort to overcome that ineffiency. Now trying to get the 10 losses to come up less then the 1 potential gain, that would be the trick.

Did you know that a jet drive boat is more efficient then a prop? Only if set up right.
From my aircraft engine archives, large slow props are more efficient then small fast ones. An OX-15 rated at 90HP and a large prop turning 1800 RPM delivers the same performance as a newer ~120HP engine turning at 2600 RPM with a smaller prop (to keep the blade tips sub-sonic)

 

[WJL]

It would be OK on sandy bottoms, or just to get to the site on a silty bottom. Then you could follow your track back to the entry.

You can't be serious. If this were a good idea, why is it that no underwater vehicles use that propulsion method?

In case you maybe are serious, think about this proposed device for a minute. If you want it to work like a snowmobile, it has to have a firm grip on a relatively solid surface to work. So just to start with it has to have complete, total negative buoyancy. You will have to do more than just flood a big ballast tank with water - the water is neutral after all. It is going to have to be seriously heavy to press on the bottom hard enough to develop traction. It will need a great deal of energy to move its own weight around. That fact all by itself is going to make it very, very hard to deal with in the water, even if you figure out some sort of means to flood and vent water from the ballast tanks.

Now think about the surface you are travelling over. Unless you are diving over some hard smooth surface like a flooded concrete parking lot, the surface is going to be dirt particles of some sort, whether big particles like gravel or microscopic particles like silt. All such dirt particles are going to be in some degree of suspension - they're underwater after all. So they are going to be loose and floaty under the best possible circumstances. So your tractor belt is not going to be able to push on them very hard before they start to move around. As soon as they move, any amount at all, you are losing energy and efficiency, even if you can get moving at all. With a bottom composed of small particles, such as silt, the bottom is hardly even solid at all. Most lakes around here, you can shove your arm into the "bottom" all the way to the shoulder without much trouble. On such a surface all you are going to do with your tractor is dig a hole in the silt.

Of course we're ignoring the fact that there's no reason to suppose that there will always be a relatively smooth barrier-free bottom to drive over. And we're ignoring the problem and limitation that your whole dive will be right on the bottom whenever you're using this thing.

Even on the best possible hard bottom, your tractor is going to make a giant cloud of dirt and debris. The driver will be enveloped in it. Everyone nearby will lose all visibility. How can this possibly be acceptable?

 

[DeepBound]

The silt / sand / gravel / mud on the bottom is soft like you aid, which makes it similar to snow. Snowmobiles can even go over water for short distances without sinking if you're already travelling at a good speed. The whole point of the skis and the tread is so that it doesn't need a surface like pavement.

Of course, it would only be of use to get to the site, then you would park it and swim.

I share your concern about damaging coral, though. Where I live the bottom is covered with weeds or silt, so I think that's why this guy thinks its such a bright idea.

I'm not sure how negative it would have to be? I can push myself along the bottom very efficiently just by letting a bit of air out of my BC.

Anyway, I doubt he'll ever make the thing. Knowing him, he'll just drive his skidoo into the water and go until the engine ceases

 

[Nemrod]

Propellers are actually highly efficient. That is actually a professional statement based on real knowledge.

N

 

[JahJahwarrior]

Your buddy sounds like a friend of mine. My friend, though, wants to build a dive light. A canister dive light. I tell him I've read up on them alot and can give him all sorts of helpful info and pictures and diagrams....he doesn't want any of that, "all of them probably don't know what they are talking about and I can build a better one without help." This is coming from a guy whose only seen these things in stores.... so he got a light from a car, and made a steel pipe enclosure for it. Not sure what kind of front it has, but I know that alot of plastics have trouble, and most people use doubly thick glass. Also, don't know if he's thought about bouyancy of the lighthead, or how to hold it. I have no clue what kind of gland he used on the head, or the body. The cable was probably from an extension cord, (he's told me he's got it built, but I haven't seen it yet) and isn't rated for water exposure. The batteries, I don't know what kind they are. He built the "canister" out of a drybox..... anyways, this guy went extremely far out of his way to avoid any of the advice from the hundreds of people on the internet offering it after building their own contraptions. I don't think his light will work very well


What you see here is that a tractor idea for a DPV is stupid. Snow doesn't tend to float around, like silt will. And really, the "bottom" on many places is easy to push thing sinto for several feet, in those places you're buddy will litereally be using this thing under ground in order to get traction. Not to mention the silting up. If I was diving somewhere and someone silted the place out with a contraption like that, I'd slice their octo hose (forces them to the surface, keeps them from returning, won't kill them.)! One place I dive is very very very low flow right now, and if you silt it up, it'll stay silted up for hours. I really would attack anyone who used something like that there! (very least, I'd tell them and their buddy to surface, I'd surface too, and I'd tell them politely that they are being extremely rude to myself and other divers and that they need to stop, orleave. In other places, like ocean diving, you have coral to deal with. If you are running it in weeds....sooner or later, you'll get them stuck in the wheels of the thing.

Your buddy will feel a greater sense of accomplishment if he built a jet drive DPV, or a "normal" feeling of accomplishment if he builds his own propeller driving DPV. And, propellers might not be 100% efficient, but a propeller driven DPV will get other divers asking "dude, that's cool! Can I try it out?" rather than them slicing your air hoses, kicking your mask off or pulling off your fins and hiding them.