Carmichael low drag bulb

A. You have not shown how you intend to get in and out of it.

B. I do not understand how you will set up the breathing system. How do you discharge waste gas. How do you separate the CO2.

I have already given my other views on the following.

1. Build it with positive buoyancy and forget the lead shot stuff. Could use small wings/vanes to generate the extra down force and improve control.

2. Remove the cowling on the prop unless you are certain you will hit something. It just adds drag.

3. Keep high density components low so at rest the boat is oriented. Will also likely provide enough righting moment to avoid counter rotating props.

4. Do a precise calculation on the buoyancy and all the component weights.

Another point after looking at the drawings. Bring the rudders further forward so the controls are not interfering with flow around the prop. They can just be fins coming out of the aft body. Vectored thrust is only of value at very low speed.

Rick W

 

You can get a lot of design cues from things like this:
http://www.youtube.com/watch?v=xL8UCPPAWEQ
You can bet they have explored all the avenues in search of efficiency.,

Most of what I have pointed out about props and controls can be seen here.

I know these are flooded vessels. On the other hand one of the things to realise is that the pilot has to generate the power with limbs working in water.

I believe the best speeds are about 6 knots. Not a lot better than an olympic swimmer at about 4 knots. I have noticed some of the swimmers are doing very deep dives on turns now to reduce wave drag. One guy in 200m backstroke was almost hitting the bottom of the pool on turns.

The HPS events are highly controlled activity because the dangers are significant.

I think you are overly concerned about the shape. It simply does not make that much difference once you get the fineness in the ballpark.

I have posted this link to a fellow who has been actively involved in the HPSs to see if he cares to contribute.

Rick W

 

Hi Rick, thanks for your response. I appreciate your continued efforts.

A. No, I haven't shown in and out procedure. Mostly because I don't feel it's relevant here. I need to finalize my hull form before I can get into that. But, to try and answer your question: on the trailer. Probably the nose cone will pop off in a symmetrical fashion. I suspect the tail assembly will be removable as well, or, I may make the entire inside assembly removable through the nose cone opening depending on where the widest point is on the hull. That depends of hull design.

B. Again, not important at this point, but, to try and answer your question: perhaps through a mouth piece from the 80 cft scuba tank. In the ambient design it was great. Compressed air would have passed through the motor compartment, cooling it, and then be ducted to the viewing window for condensation reduction, breathed, and then made it's way aft to the tale cone for venting. With the 1 ATM design, it may have to be vented directly from my mouth to the outside via a through-hull. I don't know, but I'm open to suggestion.

With your other points, it appears you and I are at a stand-off because:

1. I want neutral buoyancy so as not to waste thrust on staying down. Wings and vanes double the drag of the vessel for each protruberance as I understand it. Lead shot drop wieght is a saftely feature I'm not willing to compromise on. Besides, as you've pointed out, I may need added weight to overcome excess buoyancy.

2. I've been encouraged to have some sort of rudder by a fluid dynamics engineer. Are you familiar with the R300 submarine recently built in Texas?
His concern is instability without some kind of rudder. I feel the RICE nozzle will provide that stability while improving prop efficiency, giving a point of purchase to mount control surfaces on while minimizing drag by keeping those sufaces downstream of the props, in turbulent flow. If these surfaces are in front of the prop(s), they will decrease prop efficiency by disturbing the clean flow of water into them, no? My intension is to make those surfaces flexible so they curve, not pivot, to provide control and decrease drag. They would be fixed along the longitudinal axis of the vessel and curve the most at their outboard end. Also, by keeping these control surfaces so far aft, they have a greater moment arm for accentuated affect. No, I am not certain I will hit something.

3. I want zero intrinsic stability for superior manueovrability/handling. This would, as you've pointed out, make counter-rotating props very attractive. My understanding is CR can improve prop efficiency markedly.

4. Doing a precise buoyancy calculation would involve knowing what size and shape my hull is. I can't do that with out knowing the shape, then I can work out the size and know what my buoyancy figure will be. Then I can finalize what's going in and where.

It frustrates me that we think so differently Rick, and it must frustrate you too. For that, I appologize. I could be completely wrong with my whole idea/concept here, but I am going with it because I think it could work, dispite being so unconventional and, apperantly, unpopular.

I understand if your done with this and appreciate your most sincere efforts thus far.

Do you know of anyone else that may be interested and could help me out with my "outside the box" ideas?

All the best, unconventional, Submarine Tom.

 

Rick, I didn't see your second post until I had already replied to your first.

I too noticed those deep turn-around Olympic swimming dives, interesting.

I beg to differ on the importance of the hull form. Of course, in my case it appears obsessive because I have chosen this as my starting point and can't move forward until it's finalized. I believe in straight line speed you are right, it may not be so important, but, as soon as I start turning at speed, things are going to happen. I don't want to be sluffing speed off in turns, snap rolling (whatever that is), spinning around in circles, suddenly diving. All these possibilities have been expressed to me as concerns by those in the know??? I am really struggling here and this is starting not to be fun. All I have learned thus far about hull design is that further study is necessary and that it is incredibly complicated. You know, I am a marginally above average IQ, guy that has some pretty good street sense for 47 years of age. I've built lots of unconventional things in my life of which some have performed better than expected and some have seen me fall flat on my face. Someone out there has tried what I want to do before, probably lots of people. Where are they. Once I pick a shape, I'm committed to it. Lots of time, money and effort will go into this and I'd be tickled pink if it works. Wish me luck.

 

Tom
You need to look very closely at the video of the human powered subs. There are a lot of useful cues that you can take. Notice not many have counter rotating props or control surfaces behind the prop. You can bet that by the time you set up control levers on your rudders you will have much greater drag than that of simple blades ahead of the the prop with through-hull shafts and seals.

Your understanding of foils and props is limited and incorrect.

Sure if you stick a gun on the deck of a sub it will contribute significantly to drag but not a foil. A sail that is as wide as the hull will cause more than a little drag as well. All these things can be determined precisely.

The lift to drag on a moderate wing could be 20:1 so if you made the boat with 100kg of positive buoyancy it would cost 50N in drag to submerge it. Only 0.5kW at 20kts.

Compare the maneuverability of a jet fighter to a blimp. Large control surface equate to high forces that can change direction quickly. Have a look at some of the video clips of dolphins, sharks and wales. See how effectively they use forward control surfaces.

If you want something that is highly maneuverable make the body as small as you can and use large control surfaces.

I have attached the performance data for a NACA 3510-34 foil with an aspect of 4. You can see even with this moderately slim planform you can get L/D of 20.

Have a look at the forward control surfaces on this underwater flyer:
http://www.youtube.com/watch?v=fGf_LGX0594
Roughly around aspect of 4 and planform like a jet fighter wing.

Like I say, way too much emphasis on the fuselage. You need to understand the propulsion and control surfaces. These are the things that will give you the performance you seek.

If you want blimp maneuverability then stick with what you have. If you want to soar and turn with ease then look at what does that well in water or air and replicate as much as you can.

Rick W

 

I have a saying that I think is appropriate here - "polishing the turd". If you have a concept that is rubbish then it does not matter how much effort you put into it it is still rubbish. That is not to say what you have is rubbish. Clearly you have given clear thought to many aspects.

However design is an iterative process. You start with a concept and bring along all aspects together. Getting ever finer detail on every aspect. You cannot do one part in isolation of the others. It can be frustrating because sometimes you have to let go of something that you feel important. This is part of the learning process.

As an aside, my first foray into pedal boats was with a flapping foil. I got this to be a very efficient method of propulsion but I was speed limited because my boat started to bounce at about 70rpm. I kept at it for about 2 years until a guy asked why didn't I use a propeller. I pointed out that it was the way dolphins move and they are fast in the water. He simply said that if they could replicate a rotating shaft they would have propellers. Anyhow I started to make propellers and determined he was right. Not all of what I had learnt was nonsense because a prop is a rotating foil so I was able to apply what I had already learnt about foils to props. I now make very efficient props.

Some of your understanding is flawed. Or you have overemphasised certain features.

I think the quickest way to develop is to get your hands dirty. Do it in an easily affordable scale. Find out what is important and what is not.

Absolutely no point in getting the minimum drag hull if it does not respond to controls. Control surfaces give control not the shape of the fuselage.

Foils in water will generate huge forces for their size and do not contribute much to drag when in line. All can be determined in a matter of minutes to within a few percent.

Quite honestly I think the hull without forward control surfaces will be a tub of lard compared to what it could be with a couple of wings.

Rick W.

 

With all that said, I still don't have a hull form.

I've seen these videos. I've been researching this for five months now, and I'm just going around in circles. This is water, not air. A fineness ratio of four IS a blimp. Your obviously not understanding what I'm saying. My fault, not yours. An athlete can produce about 400 watts for up to about ten minutes. My calculations show I need about 8 000 watts (80% efficiencies means about 10 000 watts gross) to go 15 knots. That's a difference of an order of 20 from a HPV. Quite a bit more torque wouldn't you say? Especially from an electric motor.

It appears you can't help me Rick, and I'm not finding your comments terribly motivating at this point, although I really do appreciate all your efforts. I will ask you one last time, do you know of anyone that can?

 

You're not going to be able to go right to the answer in a straight line. You have to design in several spirals. The first lap around doesn't have to be all that accurate - its main purpose is to clarify your thinking and develop requirements.

I think you're a bit hung up on the hull form at this stage. There simply isn't a single best shape - only the most suitable shape for your particular requirements. Pick a hull shape that is sort of like what you envision. One way to do this is to adapt a two-dimensional section by taking its coordinates to the 2/3 power. Or start with an existing high-performance shape like this one.

Scale it to the approximate dimensions to hold the pilot, and calculate the volume. From your 15 kt speed and the cube root of the volume, you can calculate the volume Reynolds number [Re_vol = (volume)^1/3 * speed / knematic viscosity]. Take a look at the Lutz & Wagner paper on drag reduction of airship bodies and see what Reynolds number range you fall into. That will tell you roughly what kind of shape would be best, and also how much laminar flow might be possible. If you look at Figure 8 in their paper, you can see it's better to over-estimate the design Reynolds number than it is to under-estimate it.

You won't have the computer programs to design an optimal shape, but you can look at where the maximum thickness ought to be and adapt a suitable airfoil section using the 2/3 power relationship. Chances are there will be more drag in the details of things like your prop shroud & struts, etc., than there will be between similar hull shapes, as long as the flow is fully attached.

Now you really do need to get concerned about packing everything inside. Designing the systems for your sub will take a lot more time and effort than will the hull itself. You'll need to see if you can fit everything into the volume you assumed. If not, you'll have to resize the hull and maybe even iterate on the shape. For example, the maximum thickness may be too far back, forcing you to go with a larger than desired frontal area to get adequate thickness where you need it. The shape of the nose for laminar flow may be incompatible with the need to have a hemispherical transparency there. The weights and ballast capacity have to be consistent.

Propulsion efficiency will be important, so you'll need to properly size the prop and design the shroud. From the power available and your estimates of drag of the hull, you can start to see if you'll meet your target speed. If not, something's got to give.

You'll want to develop a spreadsheet that has the key geometric parameters, weights, and performance calculations. If you can develop relationships for the weights and performance based on the geometry, you can use Excel's Solver to find optimum combinations.

If there are some design parameters that are crucial to achieving your objectives, then you may need to devise some tests to reduce the uncertainty in those parameters.

For example, say the drag coefficient of the hull is critical to hitting your speed bogey. You may want to build some subscale models and test them to determine the drag. Carmichael used a comparatively inexpensive method of testing by lowering buoyant subscale models to the bottom of a deep, still lake. When the model was released from the ballast weight, he timed how long it took to come to the surface. From that, he was able to calculate the terminal velocity and knowing the buoyancy, he had the drag at that velocity. That's a technique that doesn't require tow tank or wind tunnel facilities. By making repeated runs with different amounts of buoyancy, you can cover a range of speeds or Reynolds numbers.

Or you may need to measure propulsive efficiency. You might be able to adapt an electric trolling motor by mounting it in a model of your tail unit and measuring the force it produces and the electrical power it consumes.

So don't worry about optimizing at this stage. At this point, you don't even know what the really important aspects are. First you need to get to a feasible design. From there, you can move to a better design, and a still better design, and ...

 

NO. Power is torque times angular velocity. Those big HPS props are running around 100rpm. I suggested you run your prop at 1000rpm maybe 2000rpm if you are prepared to lose a little in efficiency. At 2000rpm, 8000W requires the SAME torque as 400W and 100rpm. You don't see the HPSs with single prop spinning uncontrollably. Just minor adjustment on the control surfaces to offset the small torque. Maybe even ballasted to give sufficient righting moment to overcome torque. If counter rotating props were far superior as you seem to think then all those HPSs would have them. They are the exception rather than the norm.

As I said you need to do the calculations and bring it all along together. You are making assumptions and design decisions based on insufficient understanding of the engineering.

No one can help you if you are not open to understanding and doing the sums. None of it is magic. It can all be calculated with great precision or adequate precision with little effort.

Saying things like this is water not air shows a lack of understanding. The behavior of bodies in air or water is similar (remember the Albacore hull was developed in a wind tunnel). There are adjustment made for the density and viscosity but otherswise exactly the same principles apply. If you want a hull to behave like a jet fighter in water it will look like a jet fighter but the control surfaces will be a bit smaller in proportion. If you want it to be ponderous and slow to respond as is a blimp it will look like a blimp.

If you do build a model or actual sub you will come to learn about the detail of what I am trying to point out. I have not built a sub so I do not have all the answers but I do understand the physics and engineering detail that you need to work through.

I also notice that virtually all those HPSs have the same fineness and shape I originally proposed. Keep in mind that they are designed to go in a straight line with little deviation other than to maintain course. If you want to maneuver at high rates then your control surfaces will need to be bigger.

Rick W

 

Submarine Tom,
For what it's worth, you've got my sympathy. As Tom Speer just said,

I had the same experience very recently with my own project; I asked questions which I thought could be answered straightforwardly, but they couldn't. It didn't take long for me to feel like I was being given the run-around, but that is not the case. Rick, and tspeer, and others here have immense experience and knowledge about this stuff, and put out tremendous effort to help people here. Believe this: if a simple answer were available, they'd much sooner just hand it over than to write thousands of words, taking time away from their own projects.

You've obviously got a personal (perhaps emotional) investment in a particular notion of your project; you're attached to it. Buddhism says that attachment leads to suffering (I have no religion, but they all have some useful wisdom). Let go of that notion and start over. These guys can --and will -- still help you, but it's really hard to suffer and accomplish anything at the same time.

And -- (sly grin) even though I have no religion, if you want, I'll give you a suggestion for a 'detachment ritual.'

Your concept is delightful, and I'd like to see it succeed.

Curtis

 

This is exactly the kind of thing we're talking about. Clearly, this statement cannot be true. A tiny fin added to the hull will not double the drag. The amount of drag will be proportional to the area of the fin, so it's probably not going to double the drag until the wetted area of the fin starts to approach the wetted area of the hull.

Since you want to maneuver, there will have to be side force developed by something. If it's not a wing or fin, then it will be the hull. The drag due to lift is inversely proportional to the square of the span of the lifting surface/object. Since the hull is narrow, the drag due to lift of the hull alone will be high. Adding wings will dramatically reduce the drag due to lift when maneuvering. And the hull can only produce a small amount of lift before flow separation sets in, also raising the drag and quite possibly not allowing you to maneuver sharply at all.

Is that important? It depends on how much maneuvering you do vs straight line cruising, and how much wing you have to add to get the turning radius you need. Design is all about trade-offs - how much can you give in one area to get what you want in another area? You won't be able to do that without making some quantitative estimates.

This is where engineering requirements come in. You've really only give us three: single occupant, 15 kt, human power (~1/3 hp). Qualitatively, you've said you need to maneuver, but you haven't expressed that in quantitative engineering terms, yet. How sharply does the craft have to turn (turn rate or turn radius) at what speed? Does it have to be directionally stable, or what is the maximum time-to-double you consider acceptable for the pilot to stabilize manually? These aren't easy questions to answer, but they are critical to coming up with an acceptable design.

Even for just the requirements you've stated, you can start to flow the requirements down to various subsystems. For example, thrust horsepower is thrust times velocity. Thrust equals drag in steady state operation. There will be various losses between your power source and the thrust horsepower delivered to the water. So, say the drive chain is 90% efficient and the prop is 85% efficient. That means you can deliver maybe a quarter horsepower to the water. Convert horsepower to suitable units, divide that by the speed, and you get the drag. If I've done my sums right, that's 5.4 lb/24 N at 15 kt! Divide that by the dynamic pressure, and you have the drag area. Divide that by the skin friction coefficient you think you can achieve, and you have the maximum wetted area. From that you can estimate the maximum volume and see if you can begin to get into the craft.

If not, you have to go back to the beginning and up the horsepower or improve the efficiency of the drivetrain, or improve the efficiency of the prop, or reduce the skin friction. Once you have a set of values that all hang together, you have the requirements for the subsystems. You'll know how much power is delivered to the prop and what its efficiency has to be at what speed, which is what you need to design the prop. You'll know what kind of loss you stand in the drivetrain, and the prop design will tell you how much torque the drive has to withstand. And so on. Then you flow those requirements down into more detailed requirements for each part.

I know that isn't sexy, but it's what engineering is all about. You'll be able to come back to the forum and ask, "I need to design a prop that does X under Y conditions. How do I do that?" You'll be able to look at a shape in a technical paper and know if it's suitable or not, based on the design Reynolds number and the drag coefficient. Until you've done your homework, nobody can help you very much.

Not necessarily. Surfaces in the prop wash also have higher drag than they would have in the outside the flow. It may be better to mount them on the outside of the shroud where they get clean flow and also don't shed their wakes onto the prop.

You may well get enough stability from the shroud, but that will depend on how big the shroud is. It's going to depend on your hull form. That's another thing you have to estimate from sources like text books on submarine or aircraft fuselage design. In general, the hull by itself will be quite unstable.

This is not the most efficient way to design a control surface. You will be loading the outboard end excessively, and the drag due to the lift produced by the deflection will be high because the effective span will be short. Its better to load the surface more evenly along its span, unless it's rolling moment you're trying to produce. Technically, the minimum drag will be obtained when the normal-wash of the flow is uniform along the span of the surface.

All-moving or hinged surfaces are not much more drag than a flexible surface would be. I'd definitely accept the profile drag of a hinged flap over the induced drag of a continuously twisted surface like you're proposing.

Neutral stability and good maneuverability do not necessarily go hand-in-hand. For good handling, you will probably want to have a certain amount of positive stability so the craft will track straight, not burden the pilot excessively with having to control it, and not pick up drag by wandering around. Performance in the maneuver is also a separate issue from stability.

Exactly. So you need a candidate shape to start, then do the calculations, revise the shape, and do the calculations again. Don't think that you will be able to do the calculations once. You're going to do them dozens of times.

 

Curtis,

You make me laugh my friend.

Thank you.

As for the rest of you, you have offended me. You have created a monster.

I am no monster.

You know I appreciated your attempts to help me, this I have made abundantly clear.

Speaking of clarity. Would you please go back to my first post, on page one, as I have just done, and, carefully, reread it. Twice, as I have just done. Indulge me. Please. Then, have a beer, and rethink how you can best answer my plea.

Thank you, Tom.

P.S. Interestingly, the unsubscribe instructions for this forum are
impossible to figure out.

 

Tom,
Glad I could get a laugh out of you, but don't give up yet. No one has created any monsters here, and I don't think anyone has offended you. Look at it from their viewpoint; it must get frustrating at times, dealing with noobs like me (and you) who don't know what questions to ask because we're on the steepest part of the learning curve. And we get frustrated when we can't get simple answers to what we think are simple questions.

I just went back and read your first post again, and, having read all the rest of this thread, it looks to me as if the answer you seek is conditional, and you haven't stated all the conditions! Not your fault, because you probably didn't know there were so many (I sure didn't!). So, don't quit now; you're getting a free education, and I doubt if you'll find a better source anywhere. These guys impress me.

Take a break for a little bit, and then study all you can find. This isn't easy; it takes concentration and work, but it's worth it.

Curtis