# Building a submarine



## FC4 (Jan 17, 2013)

Hi all,
       I'm in a group of undergrad engineering students who've been given the task of designing and building a 345mm length submarine which runs on a CO2 canister. We have chosen an oscillating CO2 engine as the power-plant and it runs to a prop. We've modeled the initial designs on 3D modelling software but we are having issues on the maths side of things. How can we calculate the rpm of the crank and how can we select appropriate gearing so we get good acceleration but don't effectively do a burnout. Any equations or other help would be gratefully received.
Thanks,
           Alex


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## Tin Falcon (Jan 17, 2013)

> We have chosen an oscillating CO2 engine as the power-plant


IMHO not a good choice got to the Florida Association of Model engineers web site there are plans for various CO2 engines there. an oscillator will drain you tank . 


basic power formula (P x L x A x N)/33,000

P= pressure
L= Lengths of stroke
Area of piston
number revolutions per minute
33,000 converts foot lbs per minute to horse power 
dimensions are in feet. 



Please post an intro when you get a chance.
Tin


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## Entropy455 (Jan 17, 2013)

Dont forget to divide 33000 by (2 times pi)

If N is radians per minute, then the equation is good. However if N is in rpm, then 33000 should be approximately 5252.

Propeller design is pretty involved. As a starting point, I would recommend going to a boat store and looking at "small" electric trolling motors. Take note of the propeller dimensions (size, pitch, blade count, ect), Also note the DC horsepower rating of the motor, and the rpm in which the propeller is advertized to turn while in operation. This will give you a good idea of the power requirements of a small prop. If you size your CO2 engine to big, you'll cavitate and run out of fuel quickly. If you size your CO2 engine too small, you won't overcome drive-line friction.

How deep, how fast, and how far must the sub move? What size CO2 tank are you going to use? Will you be racing other subs?


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## lensman57 (Jan 17, 2013)

FC4 said:


> Hi all,
> I'm in a group of undergrad engineering students who've been given the task of designing and building a 345mm length submarine which runs on a CO2 canister. We have chosen an oscillating CO2 engine as the power-plant and it runs to a prop. We've modeled the initial designs on 3D modelling software but we are having issues on the maths side of things. How can we calculate the rpm of the crank and how can we select appropriate gearing so we get good acceleration but don't effectively do a burnout. Any equations or other help would be gratefully received.
> Thanks,
> Alex


 
Hi FC4,

I think the engine below is the ticket, you might have to tinker with the equeation from TIN to get your calculations right.

[ame]http://www.youtube.com/watch?v=Jc7LWZQ_ZXM[/ame]

Good luck,,

A.G


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## TorontoBuilder (Jan 17, 2013)

Entropy455 said:


> Dont forget to divide 33000 by (2 times pi)
> 
> If N is radians per minute, then the equation is good. However if N is in rpm, then 33000 should be approximately 5252.
> 
> ...



Entropy,

The OP said the design will be 345 mm or just under 14 inches... ergo the co2 tank will likely be small, as will the prop requirements. This will ultimately lead to the appropriate engine size.

Their best bet is to look at existing model props, especially those designed to provide good propulsion at low rpm. Raboesch model 174 4 blade prop would be best. 

They are designed to run at lower rpm which would conserve their co2 supply.


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## OrangeAlpine (Jan 17, 2013)

Way back when I was  a teenager, I built a small battery powered boat.  I had no access to a prop, so I built one.  It was built using a drill press, tin snips, file and soldering iron.  It was way too large, so I built another one, then modified it's pitch.  They are really easy to make.  Maybe not as refined and efficient as a commercial unit, but way better than a gearbox.

I think you will find that building an engine, then customizing the prop for the application will be the easiest out.  For sure it would be the quickest way to approximate the final prop, be it shop made or commercial.

While the wobbler may not be the most efficient choice, considering size of your project and the complexity of alternates, I think overall ,it would be a good choice.  That last thing you need is a powerplant that will take a year to complete.

Bill


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## TorontoBuilder (Jan 17, 2013)

Perhaps FC4 will tell us the metrics that will be used to evaluate their project?

Will it be endurance, max speed, etc. Has a CO2 tank capacity been prescribed for the project or is this open to the constraints of the overall sub size?

Engine, prop design and gearing if any should be based on the metrics to be used to evaluate the project... i.e. if it is endurance or speed, i'd select a good commercially available prop design for the task that has proven engineering and manufacturing. Then i'd find the best multi-cylinder oscillating engine design that I can find.. modify the design to provide suitable power output. 

I'd then find the max efficiency range of the motor, and try to run the motor in that range while under full load. Then I'd calculate any necessary gearing needed to bring the prop rpm into the correct range for max efficiency of the prop design. 

I'd regulate the CO2 to the engine using a pressure regulator to drive the engine at the rpm that provides the max efficiency of the engine design. I'd use a 42g disposable CO2 cartridge which has a thread to accept a standard or custom pressure regulator.


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## Journeyman (Jan 17, 2013)

Compressed CO2 might be difficult to use as when released through the motor there are going to be problems with bouyancy as the gas will expand into the hull. There are plenty of model submarines about but as far as i know they all use elecric motors for motive power.

Cheers
John


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## TorontoBuilder (Jan 17, 2013)

The sub can be designed for automatic buoyancy compensation sort of like a buoyancy vest for scuba diving. A through hull fitting attached to a pressure release valve can vent the CO2 exhaust in order to maintain a standard hull pressure which minimizes impact of expanding CO2 gas within the hull. 

Initial neutral buoyancy and subsequent sink and rise can be implemented using water ballast only.

Damn this thread makes the boat builder in me want to build a sub...


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## MuellerNick (Jan 17, 2013)

> there are going to be problems with bouyancy as the gas will expand into the hull.



As long as the gas doesn't blow up the hull or disposes water in the hull, nothing will happen.


Nick


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## TorontoBuilder (Jan 17, 2013)

Nick given the small design, and I'd expect the desire to keep the design as light as possible, breaching the hull integrity is a very real possibility...


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## MuellerNick (Jan 17, 2013)

> and I'd expect the desire to keep the design as light as possible,



Looking at U-boats, I don't have the impression that they are as light as possible. They are as heavy as possible. Just light enough so they swim.


Nick


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## TorontoBuilder (Jan 17, 2013)

Ah, but this isn't a real U-boat which has to resist tons of pressure due to deep depths, or implosions of depth charges... 

Secondly, they have the design constraints, either self imposed or by their prof that it is to be 345mm long, use CO2 power, and have an inefficient oscillating engine. Those constraints alone dictate a very light design if it is to function as a sub and not some aquarium fixture at the bottom of a tank.


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## Entropy455 (Jan 17, 2013)

I served 8 years in the US Navy on Submarines. (I&#8217;m the guy on the right).

My boat displaced about 20,000 tons submerged.

Most submarines require lead ballast. The lead serves two purposes &#8211; one, permitting the boat to submerge, and more importantly two, to keep the &#8220;top-side&#8221; of boat in the upright position. Trust me - you don&#8217;t want to be within a round cylindrical object during a high sea-state, unless you&#8217;ve got a lot of mass keeping the keel at 6 o'clock position.

Air has a density of about 0.0000431 pounds per cubic inch.
Steel has a density of about 0.284 pounds per cubic inch.
Lead has a density of about 0.410 pounds per cubic inch.
Water has a density of about 0.037 pounds per cubic inch.

The design goal for all submarines is neutral buoyancy (so you don&#8217;t pop up to the surface, or sink to the bottom, when you lose propulsion). Thus one design goal for this model submarine should also be to have a neutral buoyancy (an overall density approaching that of water). I.E. the sum of the masses of air, steel, brass, copper, bronze, lead, etc that makeup the submarine, divided by the volume of water displaced by the submarine, should equal about 0.037 pounds per cubic inch. This will ensure the ability to submerge the boat without having to use a lot of power to hold it under, or to keep it off of the bottom.


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## FC4 (Jan 17, 2013)

Thanks for the rapid responses. I should have included more specs in my question- my bad
16 gram cylinder of liquid CO2- energy source
The purpose of the craft is to beat all rivals in a 10 metre sprint in a shallow tank: economy only needs to be considered in the sense that we shouldn't run out of CO2 too early.
Alex


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## TorontoBuilder (Jan 17, 2013)

Great information Entropy...

It also helps make my point, since given the typical hull shape of a sub, basically a somewhat distorted prolate spheroid... with the bow being tending to be more bulbous than the stern which is more tapered ... I calculate the volume to be ~ 90 cubic inches for a 13.58" x 4" beam sub of the typical hull. 

I have done a very similar exercise many times in the past having designed model blimps and calculated the volume of helium and lifting capability of the envelop... even more critical than subs.

Given the density of water we have a displacement of ~3.33 lbs. To achieve neutral buoyancy the hull and entire contents should total 3.33 lbs.

So very light design and materials required... the weight of the engine, CO2, prop, shafting, regulator and CO2 cartridge will eat up a lot of that allowance already... and hopefully RC gear.


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## TorontoBuilder (Jan 17, 2013)

FC4,

Okay, well I dont think you will run out of fuel no matter how inefficient your engine....

16 grams liquid CO2 will yield ~488 cubic inches of gas at standard temp and pressure (30.5 cubic inches gas per gram). I say approximately because the filling of standard cartridges is somewhat inaccurate.

Given typical oscillating twin engine (.5 bore, .62 stroke running perhaps as high as 1000 rpm) should run for ~2 minutes on that charge. Plenty of time to cover the distance of 10 meters.

I have a spreadsheet which adjusts the run time based on engine bore, stroke and rpm if you want to explore the variables. I am not familiar enough with the theoretical maximum rpm for oscillating engines myself... but maybe someone else here can add that. I'd design for the largest bore and stroke that can fit within the hull and remain below the design displacement...

As an aside, given the design goal I'd use a a system that would guarantee I'd achieve the maximum theoretical hull velocity given the drag coefficient of your hull design - therefore I'd use a steam turbine directly coupled to a 45 mm 6 blade prop like the one here. I'd control the rpm via gas regulator and shoot for 9000 rpm and use utilize every last drop of fuel... with an oscillating engine you will not likely use all your fuel.


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## Tin Falcon (Jan 17, 2013)

> Don&#8217;t forget to divide 33000 by (2 times pi)
> 
> If N is radians per minute, then the equation is good. However if N is in rpm, then 33000 should be approximately 5252.



Nope do not worry about that not needed 

basic power formula (P x L x A x N)/33,000

P= pressure at the piston or enginge intake
L= Lengths of stroke in feet
Area of piston in ft^2
number of stokes per minute. (not rpm) similar but accounts for multiple strokes per revolution.
33,000 converts foot lbs per minute to horse power 
dimensions are in feet. 

If you use an engine designed for steam or air you need a regulator.  If you use a co2 engine design  you do not need a regulator. 

Tin


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## TorontoBuilder (Jan 17, 2013)

Tin,

How would you not require a regulator with CO2? Or rather why not?


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## Tin Falcon (Jan 17, 2013)

CO2 engines have a momentary open valve and are designed for the relatively high pressure of the co2. 

Oscillatory are nice and easy to make but the valve design is inherently inefficient. 
Tin


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## Entropy455 (Jan 17, 2013)

So the ultimate design goal is to rip though 10-meters of water as fast as possible.

Does the sub need to be fully submerged for the entire run? Are you disqualified if you bounce off the bottom, or broach the surface?

16 grams of CO2 is not a lot. One thing to remember is that a compressed CO2 engine is still a thermodynamic heat engine. There are things to can do to increase its efficiency.

The power source is the thermal expansion of compressed CO2. I say thermal expansion, because internal energy is converted into flow-work as the gas expands (see enthalpy). From the ideal gas law, if you expand CO2 without any external heat input, PV^gama is a content (polytrophic process) and the CO2 will be quite cold after it expands, with much lower pressures. You can obtain appreciably higher expansion rates (which means more crankshaft horsepower from your CO2 power source) if you introduce heat into the gas upstream of the engine. See the ideal gas law (PV=NRT). Both expansion pressure and expansion volume will be appreciably higher when heat is introduced into the expanding gas. Additional bonus: the heat capacity of CO2 increases significantly at elevated pressures  thus theres a real energy return to be had if you can heat the CO2 upstream of the engine early on.

Heres a bit of engineering:

16 grams of liquid CO2 at 77 degrees F, will have a pressure of 914 psig, and take up a volume of about 1.38 cubic inches. If you introduce 1.84 BTU of energy into the CO2 upstream of the engine during the initial enthalpy of vaporization (using the surrounding water as a heat source, or possibly some other self-contained heat source, perhaps DC electric) - the CO2 will flash into a gas while maintaining a pressure of 914 pisg, at a temperature of 77 degrees F, with a volume of 4.09 cubic inches. Without this introduction of heat, both the gas temperature and pressure would drop appreciably during initial vaporization.

1.84 BTU might not seem like a lot of energy, but over a ten second race, its equal to 0.25 extra horsepower  which is a boatload for a model submarine (pun indented).


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## kf2qd (Jan 17, 2013)

A uniflow would probably work better for CO2. A schrader valve in the cylinder head and exhaust ports at BDC for exhaust. It would be more conservative on CO2 usage. A conventional stem engine design would have many more places for CO2 to leak out and any leakage would cripple performance.

And you will have to deal with CO2 usage. As the pressure inside the sub changes the motor will slow. How much CO2 and how much space. (Hope that doesn't spoil the fun for your teacher...) Though if you vent any CO2 the total weight of the sub will change.


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## Tin Falcon (Jan 17, 2013)

If you go to the FAME site http://www.floridaame.org/ then click on the galley, then non combustion engines you will see several co2 engines. some of those photos are links to plans take a look at Dave kerzels modular engine a good explanation of how co2 engines work. It only makes sense to use an engine designed for CO2 if that is the required"fuel" or energy source. 
The v-2 is nice little engine and should have plenty of power for a small sub. or use the modular design for aa mutiple cylinder engine. 
Tin


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## Entropy455 (Jan 17, 2013)

Tin Falcon said:


> basic power formula (P x L x A x N)/33,000
> 
> P= pressure
> L= Lengths of stroke
> ...


I saw number revolutions per minute and incorrectly assumed you were talking about rotational power.

ft-lbs of torque is not the same as ft-lbs of energy. One is a force vector; the other is an energy magnitude.

 Hp = (P x L x A xN)/33,000

Pressure in lbs per square foot (P), times bore area insquare feet (A), times cylinder stroke in feet (L), times the number of cylinder power-strokes per minute (N), divided by 33000, is a perfectly valid equation for horsepower.

My bad.


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## OrangeAlpine (Jan 17, 2013)

How about a CO2 rocket powered sub?  

Consider:  No engine, which means no problems dealing with clearances as the engine cools, no lube problems, no prop to optimize, no prop shaft seal, no waste gases, just a tiny pressure regulator that can be adjusted to ensure the tank is empty at the end of the 10 meter course.

Bill


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## TorontoBuilder (Jan 17, 2013)

More nice numbers, and why I thought a pressure regulator would be required, since I don't know any oscillating engine designed for 900 psi. Please someone correct me if I am wrong and point me to the design. 

In fact that pressure is approaching the high end of the max pressure for annealed copper tubing... so you had better trust your supplier to send the right product on fittings and tubing. 

If it could handle the pressure, 900 psi in a two cylinder single acting engine with a 1/2" bore and 5/8" stroke turning at 800 rpm will generate 0.446 HP versus 0.049 hp @ 100 psi.

Given the OPs originally stated criteria (oscillating engine) I don't think there would be a need to consider adding heat because their system likely couldn't handle the pressures.

Now if this was free form design challenge then maximizing the psi via heated tank and vapour lines would be advantage. I'd run my CO2 line to the engine thru the hull and around the hull exterior like a keel mounted condenser in a steam launch would be simple in the space allowed.


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## TorontoBuilder (Jan 17, 2013)

OrangeAlpine said:


> How about a CO2 rocket powered sub?
> 
> Consider:  No engine, which means no problems dealing with clearances as the engine cools, no lube problems, no prop to optimize, no prop shaft seal, no waste gases, just a tiny pressure regulator that can be adjusted to ensure the tank is empty at the end of the 10 meter course.
> 
> Bill



Ha, I had the same thought, rocket nozzle at stern... but what about icing of the nozzle?  Especially if the tank is very close to the nozzle... doesn't CO2 expand into both vapour and some solids... and drop in temperature as expands?


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## TorontoBuilder (Jan 17, 2013)

Entropy455 said:


> Hp = (P x L x A xN)/33,000
> 
> Pressure in lbs per square foot (P), times bore area in square feet (A), times cylinder stroke in feet (L), times the number of cylinder power-strokes per minute (N), divided by 33000, is a perfectly valid equation for horsepower.



I know most model engine builder dont work in square feet, so an easy correction is to add conversion for using inch measurements....

Hp = (P x L x A x N) / (12 x 33,000)

Multiply the denominator 33,000 by 12

Don't forget to also add a factor for multiple cylinder engines or less apparent, double acting steam engine designs.

Hp = (P x L x A x N x C) / (12 x 33,000)

where C = number of cylinders


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## dman (Jan 17, 2013)

from what I understand of what entropy was saying the heat isn't to increase pressure. its to prevent liquid co2 and/or dry ice from entering the engine and increase total volume that moves through the engine allowing for either more rpm or a larger engine to produce more torque. thus needing a bigger prop or altered prop pitch. yet not run out of gas within the same distance.


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## TorontoBuilder (Jan 17, 2013)

dman said:


> from what I understand of what entropy was saying the heat isn't to increase pressure. its to prevent liquid co2 and/or dry ice from entering the engine and increase total volume that moves through the engine allowing for either more rpm or a larger engine to produce more torque. thus needing a bigger prop or altered prop pitch. yet not run out of gas within the same distance.



I don't think you are correct. Temperature and pressure are related... temperature decreases resulting in pressure losses. As CO2 is released at 900 psi it drops in pressure and temperature due to the expansion out of the tank. 

If you reheat the "fuel" lines prior to entering the engine you regain pressure lost due to the expansion and pressure drop leaving the tank...

That it may also melt any dry ice that forms is bonus. 

It has been many years since I took my refrigeration course, and we didn't work with CO2 but pressure laws are same.


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## OrangeAlpine (Jan 17, 2013)

TorontoBuilder said:


> Ha, I had the same thought, rocket nozzle at stern... but what about icing of the nozzle?  Especially if the tank is very close to the nozzle... doesn't CO2 expand into both vapour and some solids... and drop in temperature as expands?


I figure that the nozzle could be designed to be water heated, which is more than can be said about the engine.  

Bill


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## TorontoBuilder (Jan 17, 2013)

Perhaps we could have a challenge... everyone design and build a model sub, open engine class... but with a 45 gram disposable CO2 cartridge as the "fuel" supply. 45g cartridges are a standard size and have threaded connection. I haven't ever seen 16g cartridges.

Fastest recorded time to cover 10M distance in an indoor pool (water temp between 70 -76 F) without breaching the surface wins. Times verified by video recording posted to youtube...


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## Tin Falcon (Jan 17, 2013)

basic power formula (P x L x A x N)/33,000

P= pressure
L= Lengths of stroke
Area of piston
number revolutions per minute
33,000 converts foot lbs per minute to horse power 
dimensions are in feet. 

Sorry for the confusion I caused I was going by memory. I hve page 4 of the KN Harris engine book in front of me .

P Mean effective pressure at the piston. I assume PSI Have you everseen a gage read in pounds per square foot. 
L stoke in feet
A piston area in square INCHES 
N number of stokes per minute in a double acting engine. 
Tin


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## dman (Jan 17, 2013)

TorontoBuilder said:


> I don't think you are correct. Temperature and pressure are related... temperature decreases resulting in pressure losses. As CO2 is released at 900 psi it drops in pressure and temperature due to the expansion out of the tank.
> 
> If you reheat the "fuel" lines prior to entering the engine you regain pressure lost due to the expansion and pressure drop leaving the tank...
> 
> ...



I may have misunderstood the comment about the tubing being unable to handle the pressure. I thought the comment assumed the heat would increase pressure beyond the 914psi in the cartridge but now I see it could as easily mean a pressure drop could save it from failure.


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## TorontoBuilder (Jan 17, 2013)

Tin Falcon said:


> basic power formula (P x L x A x N)/33,000
> 
> P= pressure
> L= Lengths of stroke
> ...




Tin,

Does your reference say implicitly that the formula is for double acting engine?

I have same formula in regarding Greeley's half horse engine and the calculation requires factoring in the double action of the cylinder...

See here:


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## TorontoBuilder (Jan 17, 2013)

sorry Tin I should have trusted you...

I checked Harris and it does state in double acting cylinder...



So now I am confused what Greeley did with his calculation...


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## Swifty (Jan 17, 2013)

I tend to agree with "OrangeAlpine" regarding working with the pitch of the prop as well as the size of engine. You may want to make a temporary prop for a start, changing the pitch with a pair of pliers until you hit on the best combination of motor and pitch. When this is done a final prop can be made. You will only know what's best by running it in water, fully submerged as weight and resistance will all come into play.

Paul.


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## Tin Falcon (Jan 18, 2013)

TB : as far as double action vs single and multiple cylinder I would think stokes per minute should cover all engines except for multiple expansion then one would have to calculate each cylinder seperately and add the totals. 
I would think it would work for single action single cylinder as well.
As far as the trust thing I do try to post accurate and complete information. but I am human and I do sometime post in a hurry. So extra eyes to validate is appreciated. the reason I cited Harris is I wanted to quote a respected reference . The first attempts were somewhat from memory and not totally complete and accurate. sorry for the confusion guys. 
Tin


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## metal (Jan 18, 2013)

Nice challenge! 
Another angle: when the speed of the vessel is limited by the hull friction, reduce the friction by discharging the used propellant along the hull, as done with torpedoes.
Good luck & keep us informed!

Mike


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## akitene (Jan 18, 2013)

Hi FC4,

Your project sounds great. Here's a link to a French site dealing with model submarine propeller construction. The translation is not that good but the propeller is really nice.

Hope this helps!

Akitene


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## FC4 (Jan 19, 2013)

Hey again,
              good to see so many of you helping us out. In response to Entropy's post (#21) we assume the craft must be underwater throughout the run but in terms of steering the craft is allowed to bump its way along the sides. 
In response to the "rocket" fans out there: the project is marked considering the complexity of the design, especially the engine. 
Thanks for that prop. link akitene and others who've posted about props. A design limitation I forgot to share is that we have a day to actually build the thing so we must rapid prototype most parts and keep machining to a minimum to save time. Also we'll use a stock propeller.


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## Tin Falcon (Jan 19, 2013)

> A design limitation I forgot to share is that we have a day to actually build the thing so we must rapid prototype most parts and keep machining to a minimum to save time. Also we'll use a stock propeller.


An important detail you left out . That does add to the challenge. 
IMHO a preassigned and proven CO2 engine will help insure success. does the engine need to be made as well in that time frame. If you can use a commercial product look  the airhog toys. I have an old aircraft fuselage and engine they also made buggy's find one of those at a yard sale and you have a power plant design that could be rp manufactured. .


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## lensman57 (Jan 19, 2013)

FC4 said:


> Hey again,
> good to see so many of you helping us out. In response to Entropy's post (#21) we assume the craft must be underwater throughout the run but in terms of steering the craft is allowed to bump its way along the sides.
> In response to the "rocket" fans out there: the project is marked considering the complexity of the design, especially the engine.
> Thanks for that prop. link akitene and others who've posted about props. A design limitation I forgot to share is that we have a day to actually build the thing so we must rapid prototype most parts and keep machining to a minimum to save time. Also we'll use a stock propeller.


 
Hi FC4,

One day to complete is not very long to start building engines and propellers, if the design brief allows it why not just use a direct discharge of the gas through a varible nozzle or control valve if need be. If you go down this route, allow for the loss of the gas weight to control the buoyancy.

A.G


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## TorontoBuilder (Jan 19, 2013)

Tin Falcon said:


> TB : as far as double action vs single and multiple cylinder I would think stokes per minute should cover all engines except for multiple expansion then one would have to calculate each cylinder seperately and add the totals.
> Tin



Thanks Tin... you are absolutely correct.

After reviewing Greeley's formula I see that he did use number of strokes for the value "N". 

I apologize to everyone for causing some confusion here. I was thinking Greeley used rpm for N rather than strokes. Greeley simply expressed N using the root calculation  i.e. N = RPM x 2, with 2 being the factor to account for the double power stroke per engine revolution.

Tin was correct that N covers all engines except for multiple expansion engines. I blame Harris for my momentary confusion... because his statement "N = number of stokes per minute in a double acting engine" is actually misleading. It is number of strokes per minute in any engine! 

I was however on the right track, since I was stating that the formula required a factor for the # of cylinders and action of the engine... which is the case. 

So for anyone else like me who has less experience with the power formula, if you need a reminder of how N is actually calculated, the root calculation is engine RPM x # of cylinders x # of power strokes per revolution.

For the following engine types the calculations are as follows:

single acting single cylinder engine (N = rpm x 1);
double acting single cylinder (N = rpm x 2);
single acting multiple cylinder (N = rpm x # of cylinders): 
double acting multiple cylinder (N = rpm x 2 x # of cylinders).

My brain is so much happier when it understands all factors required for a calculation... now I can finally let this go and bookmark this page in case I ever need a reminder.


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## TorontoBuilder (Jan 19, 2013)

lensman57 said:


> Hi FC4,
> 
> One day to complete is not very long to start building engines and propellers, if the design brief allows it why not just use a direct discharge of the gas through a varible nozzle or control valve if need be. If you go down this route, allow for the loss of the gas weight to control the buoyancy.
> 
> A.G



I concur with A.G. given those times constraints I'd use a gas nozzle design... unless the rules permit you to purchase or build an engine prior to the competition. 

I am a huge fan of junk yard wars, and having tried similar types of projects I know for a fact how difficult it is to even complete a project that can meet all the goals of the challenge. KISS principles usually win out under such conditions.


Addendum...
But if you can pre-purchase components, I'd go turbine direct connected to one of the Raboesch 6 bladed brass props I mentioned, using their prop shafts. 

If the rules permitted pre-purchased components, I'd also try a stamp basic processor, a gyroscope input, rear rudder and forward bow planes and write a basic stabilization program... this will allow your sub to run straight and compensate for the discharge of gas by adjusting the bow planes.


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## sunworksco (Jan 20, 2013)

Before you go down the wrong path, you may want to register on the www.subcommittee.com submarine forum. Everything there is tried and true.


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## rpledm (Jan 21, 2013)

Nothing scales directly so this group needs a bit more info.
First: Is this to be a scale sub or does anything go?
Second: What are you using as a control? Micro-controller? R/C? Clockwork?
Third: How much help is your Prof allowing you to have?
A lot of other questions and suggestions will follow as a result of these questions and answers. You should also ask yourself, "Is this my Prof?"


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## mrehmus (Jan 21, 2013)

Exhaust the gas from the engine to outside the hull. Set it up correctly and you will get a bit of jet propulsion.

How are you going to control depth? Basic and primary question of every submarine sailor.

Remember, there are only two kinds of ships, Submarines and targets.


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## Lakc (Jan 21, 2013)

Turning liquid co2 to a useable gas for an engine is an extremely endothermic reaction. Using the most of the gas you can get at a high enough pressure looks to be the key. At large flow rates, your pressure will drop very quickly.


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## FC4 (Jan 22, 2013)

rpledm said:


> Nothing scales directly so this group needs a bit more info.
> First: Is this to be a scale sub or does anything go?
> Second: What are you using as a control? Micro-controller? R/C? Clockwork?
> Third: How much help is your Prof allowing you to have?
> A lot of other questions and suggestions will follow as a result of these questions and answers. You should also ask yourself, "Is this my Prof?"



1) The project is for a miniature sub: it won't be scaled up.

2) There are no electronic control systems in it seeing as we only have 8 hours to build the thing, so we are planning to design it to have neutral buoyancy and steering-wise we've got an impeller-style design like a torpedo so fins drive the thing in a straight line. At the moment we've got a one-way crank attached to the piston so the prop rotates the right way. Also our design is a SINGLE cylinder oscillating engine. Most commentators on this thread have mentioned a double cylinder system: is it imperative we have two?

3) Regarding help from you guys; I consider this research and we will or have actually design(ed) each part so our Prof. should be fine with it. 
Another point: we are allowed to buy parts but I think that is limited to, say, ball bearings and gears and tubing, rather than complete engine systems which we could then tune for our needs.

Hope that helps you help me,
                                       Alex


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## GerryB (Jan 22, 2013)

G.Day Alex,
I think your project for building a Submarine should be looked at from a different angle.
What you are expected to build is to put it blindly is a Torpedo.
A Torpedo is nothing more than an unmanned Submarine.
Designed to go a distance to the target,submerged to a certain depth,as fast as possible. 
During the first years of the IIWorld War,Torpedo's were powered by compressed gasses,later by electric batteries.
It was the  use of Gas that gave away the telltale approach of a Torpedo as it was ejected from Motor exhaust.
I have built two Submarines (Submersibles) Both over 6ft long,Radio Controled
and i can tell you that there is more to getting them to work correctly other than the propulsion system.
Hope this is of use,best of luck with your project.
GerryB


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## FC4 (Jan 23, 2013)

Yes our design is essentially that of a torpedo. The original design was self-levelling and had other eccentricities but given the short build period we have to keep it simple.


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## mrehmus (Jan 23, 2013)

...
During the first years of the IIWorld War,Torpedo's were powered by compressed gasses,later by electric batteries.
It was the use of Gas that gave away the telltale approach of a Torpedo as it was ejected from Motor exhaust.
...
GerryB[/QUOTE]

No compressed gas propulsion in U.S. torpedos. All the Mk 14's we fired in the 60's were steam powered using compressed air and methanol to heat water. 

The Mk 14 was the main torpedo in WWII and was introduced in 1941. We used Mk 14s in the 1960's on both nuclear-powered Fast Attack and Missile subs on which I served. 

The Mk 10, introduced in 1915, which the Mk 14 replaced, was also steam powered, generated by burning methanol. The Mk 10 was also used during WWII.

The Mk 18, which was battery-powered was introduced late in WWII but did not replace the Mk 14. It took the Mk 37 and then the Mk 45 to do that.

http://en.wikipedia.org/wiki/Mark_14_torpedo for more information.


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