Looking for a unique DIY project

[Akimbo]

How interested are you in wreck diving? You mentioned DPVs. What about a wet submersible that is designed more for carrying larger gas supplies, tools, artifacts, and supporting decompression stops in safety and comfort than speed and distance? More freight elevator than racer.

It could give you experience in free-form lofted designs, weight management, hydrodynamics, power efficiency, and human factors — all relevant to aerospace engineering. There is nothing like a tight budget to spur innovation.

 

[bshey]

"There is nothing like a tight budget to spur innovation"

Sounds ridiculous, but its completely true.

That idea is kind of what i was talking about during my first post. This sort of thing:

I was wondering how feasible a gas engine would be. I am looking at small 3-5 hp 2 stroke water cooled rc boat engines. Cooling wouldn't be an issue if i hooked up a small pump to run water through the cooling jacket. I would have a small scuba cylinder with a first and second stage regulator hooked directly up to the air intake. The engine would get exactly how much air it needs and none when its off. Electric start, and i have thought up a way to expel exhaust:

I would have the exhaust hooked up to an IV type bag with a metal contact, so that when the bag inflates a certain amount, contact is made, and a small pump turns on to pump the gas out the bag and out a one way valve (into the water). I might even use a weak magnet to help contact remain for a while, so that the pump isnt turning on and off like a machine gun... I could even hook up a catalytic converter in line. OR if i really wanted to be environmentally friendly (and keep buoyancy rock solid the whole time) i could have it pump the gas into another low pressure (bigger) cylinder for storage, and be able to vent it at the surface.

I would then have a centripetal clutch in line with the engine, and after that a 10:1 gear reduction unit. I might then use a magnetic drive coupling to took up to the prop (trolling motor prop).

Here are my issues:

-I cannot find a suitable gear reduction unit that can handle an input of 13,000 rpm @ 3.5 hp

-I am unsure of how i would mount the prop on the outside of the vehicle, and what bearings would work in salt water. If i went with a conventional sealed drive shaft, i cannot find one.

-I dont know how i could make the shell out of fiberglass able to withstand pressures down to a reasonable depth, and i don't know how i would make the mold. Carve the shape out of foam block, and make a mold out of that??? Also, that would be an expensive mold. At least $500.

-I dont know how i could control throttle


Any ideas of how to fix these issues? If they would be fixed, would this even be possible?

 

[Akimbo]

…Any ideas of how to fix these issues? If they would be fixed, would this even be possible?

What issues, did you post something after that image with the model? :shocked2:

Seriously… Gasoline internal combustion engines are way outside my area but smaller jet skis come to mind. Maybe you can learn something from talking to those mechanics?

 

[bshey]

thats odd. the whole post showed up for me. i will retry.

I was wondering how feasible a gas engine would be. I am looking at small 3-5 hp 2 stroke water cooled rc boat engines. Cooling wouldn't be an issue if i hooked up a small pump to run water through the cooling jacket. I would have a small scuba cylinder with a first and second stage regulator hooked directly up to the air intake. The engine would get exactly how much air it needs and none when its off. Electric start, and i have thought up a way to expel exhaust:

I would have the exhaust hooked up to an IV type bag with a metal contact, so that when the bag inflates a certain amount, contact is made, and a small pump turns on to pump the gas out the bag and out a one way valve (into the water). I might even use a weak magnet to help contact remain for a while, so that the pump isnt turning on and off like a machine gun... I could even hook up a catalytic converter in line. OR if i really wanted to be environmentally friendly (and keep buoyancy rock solid the whole time) i could have it pump the gas into another low pressure (bigger) cylinder for storage, and be able to vent it at the surface.

I would then have a centripetal clutch in line with the engine, and after that a 10:1 gear reduction unit. I might then use a magnetic drive coupling to took up to the prop (trolling motor prop).

Here are my issues:

-I cannot find a suitable gear reduction unit that can handle an input of 13,000 rpm @ 3.5 hp

-I am unsure of how i would mount the prop on the outside of the vehicle, and what bearings would work in salt water. If i went with a conventional sealed drive shaft, i cannot find one.

-I dont know how i could make the shell out of fiberglass able to withstand pressures down to a reasonable depth, and i don't know how i would make the mold. Carve the shape out of foam block, and make a mold out of that??? Also, that would be an expensive mold. At least $500.

-I dont know how i could control throttle

 

[fjpatrum]

"There is nothing like a tight budget to spur innovation"

Sounds ridiculous, but its completely true.

That idea is kind of what i was talking about during my first post. This sort of thing:View attachment 139737

I was wondering how feasible a gas engine would be. I am looking at small 3-5 hp 2 stroke water cooled rc boat engines. Cooling wouldn't be an issue if i hooked up a small pump to run water through the cooling jacket. I would have a small scuba cylinder with a first and second stage regulator hooked directly up to the air intake. The engine would get exactly how much air it needs and none when its off. Electric start, and i have thought up a way to expel exhaust:

I would have the exhaust hooked up to an IV type bag with a metal contact, so that when the bag inflates a certain amount, contact is made, and a small pump turns on to pump the gas out the bag and out a one way valve (into the water). I might even use a weak magnet to help contact remain for a while, so that the pump isnt turning on and off like a machine gun... I could even hook up a catalytic converter in line. OR if i really wanted to be environmentally friendly (and keep buoyancy rock solid the whole time) i could have it pump the gas into another low pressure (bigger) cylinder for storage, and be able to vent it at the surface.

I would then have a centripetal clutch in line with the engine, and after that a 10:1 gear reduction unit. I might then use a magnetic drive coupling to took up to the prop (trolling motor prop).

Here are my issues:

-I cannot find a suitable gear reduction unit that can handle an input of 13,000 rpm @ 3.5 hp

-I am unsure of how i would mount the prop on the outside of the vehicle, and what bearings would work in salt water. If i went with a conventional sealed drive shaft, i cannot find one.

-I dont know how i could make the shell out of fiberglass able to withstand pressures down to a reasonable depth, and i don't know how i would make the mold. Carve the shape out of foam block, and make a mold out of that??? Also, that would be an expensive mold. At least $500.

-I dont know how i could control throttle


Any ideas of how to fix these issues? If they would be fixed, would this even be possible?

My thoughts on a gas engine... don't bother. I suspect you'd need a lot more air flow than a "small cylinder" (define that please) would provide and you'd need a larger tank than would be feasible unless you built it as part of the body/frame of the vehicle. Not to mention the whole issue of sealing the engine and keeping it dry. I suspect RC boat engines wouldn't be suitable for long term submersion, either, though needing air might make that a non-issue since you'd have to seal the chamber the engine was in to give it air. As for venting the exhaust safely, again, I think you'll find the exhaust difficult to contain in a reasonable sized chamber though obviously at depth it wouldn't need to be as large.

I think, overall, weight, complexity, and NRE requirements might just make it simpler/faster/better to use electric motors with good battery power supplies to do it. I think you'll find your budget stretches further this way too, since the big cost factor would be getting good batteries which can be sourced from lots of battery manufacturers. Then you can focus on the vehicle design and build, rather than the propulsion design, per se.

As for a fiberglass body, look at the way wooden kayaks are built (strip boats as well as stitch & glue boats). You could easily build a frame and then skin it with fiberglass and a thin layer of wood (maybe 2-4mm thick) that would still be reasonably light and sturdy enough for the pressures down to recreational depths (I think.)

 

[Diver0001]

My thoughts on a gas engine... don't bother. I suspect you'd need a lot more air flow than a "small cylinder" (define that please) would provide and you'd need a larger tank than would be feasible unless you built it as part of the body/frame of the vehicle. Not to mention the whole issue of sealing the engine and keeping it dry.

I liked his idea but I was thinking that it wouldn't be easy to keep water out of the engine.

For the exhaust he could attach something akin to a 2nd stage as well. I think his initial design was too complicated and it won't work. A regulator type exhaust chamber would keep it water free enough, and whatever water does get in would probably turn to steam from the heat.... and he could also vent it straight into the water provided the exhaust valve was mounted deeper than the exhaust outlet from the engine. The problem then becomes thinking in 3 dimensions..... what happens if you go upside down with it?

I don't know how much air it would use but his idea of using a regulator to deliver air on demand certainly wasn't bad and I think it's worth doing some calculations to see if it's feasible.

The big question becomes this: how water tight can the inlet connection be? Because even a small amount of water in the engine will turn it off. This will need to be very robust and I'm not at all convinced that it would work without a lot of modifications. Also lawn mower engines have moving parts that will be in direct contact with the water, which isn't going to make it work better....

What might work better is to make a dry chamber out of a water tight box. The box would house the entire engine so even the moving parts don't contact the water. The box could be kept at ambient pressure with a scuba regulator, which would avoid all of the problems with heat melting the plastic bits and the engine could be supplied with any air it needed by the regulator. Exhaust would still have to go via an intermediate chamber into the water. We once helped a guy design a dry box for a hobby submarine what worked exactly like this, although his sub used an electric motor.

R..

 

[bshey]

Thanks for the great feedback. I was calculating the air consumption, and came across some bad news. The engine size i was planing on using would consume about 10 cubic feet of air per min at full throttle. Thats 8 mins of run time with a regular 80. Also, at $8 a fill, thats a dollar a min. definitely NOT worth it.

What i would have done was sealed the engine in a box, and used a magnetic coupling.


I am thinking that i may want to do the same style of scooter though, but with 2 trolling motors, electric scooter speed controllers and golf cart batteries. Kind of like a "freighter" as Akimbo said. Maybe a little faster though.

Are there any other motor/prop solutions though? 2 name brand Trolling motors at 55 lbs each are around $1000. Thats a TON. How much thrust would one need to move the diver and craft at a decent pace?

 

[Diver0001]

I think you can calculate it to a certain extent. Mass isn't a huge problem under water but buoyancy and drag will be (I don't remember the number but IIRC drag is on the order of 800 times higher in water than in air). Inertia will keep the train rolling if your kit generates enough force to overcome drag once you get it started, even if getting it up to speed is done with minimal acceleration. The issue will be maneuverability and user perception. A typical diver will think he's "moving quickly" if he's moving at more than 2km/h and he/she will probably need to get to that speed within a few seconds in order to have a perception of significant acceleration.

R..

 

[bshey]

Ok so i found some great 55lbs trolling motors for about $370.

For the hull: Could i build a frame our of wood (maybe with some metal reinforcements) and then just fiberglass over the inside and out. Leaving the wood completely sealed? Could i even use polyurethane foam in some places (curvy bits because its exceptionally sand-able)? Would water have any chance of getting in if i really covered everything? I was thinking that maybe a flooded hull with sealed off electronics might be beneficial. I know it would add alot of mass, but I wouldn't need counterweights, so it wouldn't weigh a ton out of water.

I will see if i can scan some drawings in soon to see what you guys think.

 

[Rich Keller]

Yeah, I know. That's why I suggested the inertial guidance. Get oriented via GPS on the surface then gyroscopes and a computer underwater. I'm not sure this is actually doable---measuring distance traveled on a diver seems problematic---but maybe the OP can figure something out.

Your wireless trasmitter idea could work. I believe there are already systems that transmit an accoustic signal from a boat or other fixed point that can be detected by a hand held receiver as a simple return beacon.

Deploying multiple transmitters would allow the receiver to triangulate position in 3D. I just read The Azorian Project, a book about the recovory of a portion of a Soviet submarine & they used such a system in the salvage.

Multipal transmitters would work just like the old Loran C system worked before there were GPS satellites, but as you said it has already been done. The only thing new that could be developed from that is working off a single point instead of multiple transmitters. The problem with an acoustic signal from a boat is that thermal-clines can distort or block the signal and the boat is never a fixed point.