Carmichael low drag bulb

I've read old, old threads on this site before.

It appears there are some very knowledgeable people partaking.

I need to know the lowest drag form for a 15 knot mini-sub.

I mean mini. 22 inch diameter. Shoulder width.

Think of it like a manned torpedo. Actually, it a sports car,

no, make that a jet fighter style, sub. I'm looking for performance,

manoeuvrability, safety, efficiency, shallow water operation (30 feet max),

pressure hull. Has anyone established what hull form offers the least

resistant, good handling performance?

Thanking you in advance, Tom.

 

I believe the USN did what you seek post WWII with the USS Albacore. This hull resulted from extensive wind tunnel testing and performed to expectation in prototype and eventually operational subs. I believe over 40kts with diesel power.

I have tested this shape at scale level and at human power level. It is good. Basically an ellipsoid with a fineness ratio of 8. The aft end forms into a parabola for a single drive shaft. You need to keep control surface to a minimum.

If you intend to operate near surface most of the time then smaller diameter and larger fineness ratio will give the best result for the same displacement. Wave drag becomes noticeable within 3 diameters of the surface. At the surface the wave drag is substantial unless much much finer.

You will read about laminar flow hulls that are very slippery but I believe they are intended for speeds slower than you indicate.

One having a diameter of 22" will displace around 0.75 tonne so not that small. Drag say 600N at 15kts. Require about 5kW without allowing for prop losses.

Rick W.

 

When I was looking for low drag hull designs I did a lot of research on installed power and performance on subs. The objective was to determine best possible Cdv and I could use this for my own calculations.

It became frustrating to get actual speed data on subs. I was just re-reading some of the notes I found on USS Albacore (AGSS 569) and here is a classic:
<Phase V required modification for a final series of tests of a classified system designed to increase speed beyond what had been attained earlier. The nature and the results of this test remain classified at the time of this nomination. Ship design expert Norman Friedman gives Albacore's top speed when built as 26 knots and 33 knots when equipped with the silver-zinc batteries. The top speed of Albacore or any other submarine is stated by the Navy as "in excess of 20 knots." One former Albacore officer, forbidden to state any specific speed, said "Albacore has gone more in excess of twenty knots than any other submarine." [8]>

It would have been a wonderful experience to be in a position to make this statement. Simply knowing you are the best but forbidden to say by how much - maybe there is someone out there who actually knows by how much!

Rick W.

 

Rick, thank you for your reply.

It fills my heart to hear (read) someone who knows what they're talking about. I've had a frustrating, lonely few months researching this project.

I've seen a lot of what you posted, but appreciate the context you bring.

I do plan to minimize my control surfaces in an effort to minimize drag.
In fact, my original plan was only two elevators mounted behind a RICE
(Kurtz style) nozzle. These would operate independently to double as ailerons. In order to turn, I would first roll 90 degrees, using opposite elevator input, then using "up" elevator input, turn. I think it would be a thrill to drive.

To achieve these manoeuvring characteristics, it is my intension to superimpose the CG over the CB resulting in zero inherent (intrinsic)
stability and a highly responsive vessel.

I should also note here that my design is axsymmetric. It has no conning
tower. No sail. Very unconventional I know, but what a reduction in drag!
My new term for freeboard is: trailer. Other than in emergency, the only way in or out, would be on the trailer.

Fineness Ratio: I'm most interested in your comments here. I have
determined, certainly not conclusively and only from the vague info I've found, that 5-6 is optimal. Now Albacore showed 8. I am very flexible here. I just want what is going to give me the least resistance in a straight line AND while manoeuvring. I don't intend to operate near the surface, too
many hazards, (boats, logs, flotsom, jetsam, tideline debrit, etc.). It is also my impression that the shape I'm after has very poor performance characteristics at and near the surface.

It's very interesting to me your comment on laminar flow hulls not being practical at my speed objective, 15 knots.

5kw, so about 7Hp net or about 10 gross considering ~80% electric motor efficiency and about the same for a nozzled pair of contra-rotating props.
That's a lot of juice isn't it! ~8kw.

Finally, I'm making some headway here. Yahhhhhhhh

Swimmingly yours, Tom.

 

Tom
The quoted fineness ratio is for a given displacement constraint. If you have a minimum diameter constraint then shorter might be better.

You will find for airships the optimum fineness is about 5 but the optimum for water is finer. Probably related to the lower kinematic viscosity of water compared with air.

I have doubts about the laminar flow hull actually achieving that condition in a high speed maneuvering hull.

I have no idea what experience you have with these sort of boats but they can be nasty. I think something that you enter/exit from a trailer could be dangerous.

You find dollar for dollar that the A123 Lithium batteries are similar price to VRLA on a power basis. Much more expensive from an energy perspective. So for something that will run 15 minutes you might consider A123 lithium batteries as there will be a huge weight saving. It is a matter of how small can you squeeze into. You may need lead batteries for ballast.

There are many safety issues that need to be considered in such a craft. The thought of being trapped under water would make me a very reluctant submariner. Even free diving is far from my most relaxed activity.

Rick W.

 

I've seen papers on minimum drag axisymmetric shapes, but I've not seen any that also considered the handling qualities. I suspect you can tame the handling qualities by proper fin sizes, but what the tradeoffs would be between wetted area in the fin vs wetted area in the hull, I don't know.

You'd probably want to start here: Parsons, J. S.: The Optimum Shaping of Axisymmetric Bodies for Minimum Drag in Incompressible Flow. Ph.D. Thesis, Purdue University, 1972.

Take a look at Design of Fuselage Shapes for Natural Laminar Flow and Drag Reduction and Shape Optimization of Airship Bodies

Also, PARSONS, J.S., GOODSON, R.E., and GOLDSCHMIED, F.R., "Shaping of axisymmetric bodies for minimum drag in incompressible flow," Journal of Hydronautics 1974, 0022-1716, vol.8 no.3 (100-107).

I presume your sub is powered (or is it an underwater glider?). If powered, you may be able to have one or more suction slots around it. A Goldschmied body may be the way to go. These are the axisymmetric analogs of Griffith airfoils.

At the slot, you can get essentially a step increase in pressure. This allows you to have a favorable pressure gradient up to the slot, and then a favorable pressure gradient all the way to the tail. Depending on the Reynolds number, you could have laminar flow over the entire surface. See Aerodynamic Hull Design for HASPA LTA Optimization (attached).

And GOLDSCHMIED, FABIO R., "Fuselage self-propulsion by static-pressure thrust - Wind-tunnel verification", AIAA-1987-2935, AHS, and ASEE, Aircraft Design, Systems and Operations Meeting, Saint Louis, MO, Sept 14-16, 1987. 21 p.

A big problem is, you can get any answer you want, depending on how much flow separation you assume is present at the tail. See "Comments on 'Flow Separation Over Axisymmetric Afterbody Models'", attached.

 

Thank you Rick W. and Tom S. for your posts. Greatly appreciated.

Tom S, I will do some research into those publications.

Rick, my experience in, and on, the water is in underwater construction,
SCUBA (PADI) Instructor, Triathlon (swimming), Canadian Certified
tow-boat captain, sailing, windsurfing, Canadian Coast Guard Search and Rescue. I have an expired pilots license (the flying connection), and
a technologists deploma in Mechanical Engineering.

I'm also the father of a two year old, husband, have a healthy fear
of dying, and I'm not claustrophobic (as you may have already gathered).

I am considering epoxy or polyester resin with fiber, vacuum bagged,
as a construction material. I have considered steel but doubt it's feasible.
What really does scare me, is imploding and dying though.

I feel comfortable with my safety measures and certainly appreciate the
concern you have expressed.

Rick, I already have 72 lead/acid, sealed batteries. Each weighing ~2kg
or ~ 4 1/2 pounds. I'm not sure where you got the 15 minutes from.
I do want a 15 knot top speed, however. My displacement comes out a bit lower than your estimate at ~950 pounds. That's measured by a scale model I turned on the lathe and then shoved into a graduated cylinder.

Tom S, your final comments on laminar flow and slots is intriguing,
okay, exciting even. Wow!

Am I able to post a JPG drawing on here? If so, how?

I've got more questions but I thought I'd ask the toughest ones first.
Prop size? Variable pitch? High, flexible pitch? Number of blades?
Contra-rotating? RICE nozzle?
I understand it is a cubic relationship between velocity and battery,
or motor, power. And that cavitation is often a problem getting up to speed (15 knots). Here's an idea: 5 or 6 high pitch blades in a low clearance
RICE nozzle with direct (shaft) drive to an infinitely variable speed, high
torque, electric motor. Infinitely variable pitch would be wonderful,
but so would a million dollars. I've heard about automatically
variable, flex blades made of a plastic composite... Sounds almost
magical!

Tom
Victoria BC

 

For posting images you click over the paper clip icon on the reply screen and you get a pop-up window. See attached screen dump of part of this page.

Unless you have concerns about hitting objects I would just go for an open prop up around 20" diameter. Counter rotating would be good but you could control prop torque using other methods. Typical design is attached. This is a lightly loaded prop. You get an idea of the possible efficiency.

Counter rotating adds complexity in the drive. I would look to ballast/placement of batteries to keep it oriented. I know what you said earlier but this shape does not need to roll to turn. I expect the low CofG will provide the correct lean anyhow - there are lots of papers on sub dynamics. You could spin the prop faster to reduce torque but the efficiency comes down. You can still get over 80% at 2000rpm. I will leave you to calculate the torque and hence the required righting moment without any control surface input.

I can vouch for the ease of driving these hulls. I got the attached contraption up to almost 7knots and it suffered a lot of wave drag because the main hull was no more than 1ft below the surface. The outriggers also caused a lot more drag than intended. I fitted small foils on them for the second test. I did glide testing and tow tests on scale models as well. I could not get drag as low as Albacore but I was working near surface and did not realise till later how much this was costing me. You will see the hulls on SWATH craft tend to be much finer.

I would not operate the thing near deep water until I was very confident about every aspect of operation. At 8m/s you will go deep fast so you need a soft and shallow place to land.

I am more like 700kg on displacement for 0.55m diameter at 8 fineness??? Can you check your numbers on the model.

There is another thread with Mermaid building a radio controlled torpedo that could provide you with some useful data:
http://boatdesign.net/forums/showthread.php?t=23339&page=2
You might encourage Mermaid to get it done and provide some performance data.

Rick W.

 

If you intend to operate the sub with you in an air chamber then getting it heavy enough will be a challenge. Probably means a steel hull. You just cannot afford to build it light. Even your 144kg of batteries will not help a lot. You may need foils to get the required down force.

There is a lot of work done on human powered subs and they are water filled so pressure is equalised. Also means there is no unfilled space that has to be ballasted. I think current speeds are around 6kts so 15kts under power is not a particular challenge.

The 15 minutes running time was what I felt would be economic for a lithium battery that did not cost the earth. You are going to lose a lot in battery efficiency if you pull harder than 1C from VRLA. Ratings for VRLA are given at 10 hours. Hence you need about 10kWh of batteries at minimum - about 200kg I expect. These would give about 1 hour at full speed. Much further if you decrease speed. It is indeed a cube relationship between speed and power.

Rick W

 

You can get carried away with the need for the perfect shape. In the end I think you can get very close to optimum with easy to build developable shape with hard chines. I also think you will want to shape it around the available components within reason. You cannot afford much spare space if you want some sort of air filled cockpit. A 22" diameter tube (at the widest) is tiny inside. Need to mock it up and see what it is like. You will also need some way of "seeing". I know cameras have been tried in recumbent racers but riders cannot orientate. I believe you want a boat that will find the surface of its own accord if power fails.

Surface finish is over rated. I am almost certain you will not achieve laminar flow over much of the hull so a perfect finish is really window dressing.

If you made something that was welded up you could fair it out with heavy weight bog.

Rick W

 

Okay, I'll bite. What's "bog", as in "heavy weight bog"? A metal filler perhaps?
If I welded it up, it would have to be made out of 100's of pieces in order to get a teardrop shape.
I'll never get the spherical, cross sectional symmetry I would from a lathe turned, foam blank.

But, what I'm hearing here is, that laminar flow may not be achievable anyway on a 15 knot, highly manoeuvrable vessel.
I may as well take a cylinder, put an elliptical bow on and a parabolic tail and go from there. No?

Remember, I want a sub that will be super performance oriented. Rolls, inverted flight, 90 degree banked turns, breaching.
Think of this as a sub you wear. Am I a pacific white-sided dolphin, or a dahl's porpoise, trapped in a mans body? Perhaps.
I want to have fun in the top 30 feet of the ocean without killing myself.

Forward looking sonar, or collision avoidance sonar, may be a necessity here.
Something with a reasonable sweep rate suited to 15 knots.

I considered the camera/screen approach, but that would defeat half the purpose.

Forward looking eyes (horizontal from centre line) and down through 130 degrees or so, with 180 degree side visibility would be fine. If I want to see more to the side or up, I just roll with no affect on trajectory.

No fins, they produce drag. Neural buoyancy a must. I have room for more 4.5 pound batteries but am starting to run out of buoyancy.

I still need 100 pounds or so for ballast/trim tanks. I've already use up 75 pounds in my emerg. drop weight.

I've done the mock-up. 22" at my shoulders is fine. My hands will come down and forward to an array
of controls. I plan foot pedals for my independent elevator control and a couple of hand levels for independent
rudder controls. Independent, as in top half and bottom half. I'll try and get this drawing out.

What I'm after here is crisp roll response using four, independent, control surfaces and that's it.
No fins, no tilting props, no movable ballast. Simple and effective. A lot of driver control.

Also, when rolled 90 degrees, the rudders can act as elevators and the elevators as rudders.

Very unconventional, I know. If you think I'm a nut case, you may be right. Please, challenge my ideas.

It has to be pressurized, as much as I'd like to go ambient, my ears just wouldn't/couldn't take the rapid
changes. Nor could the cabin venting and repressurizing without getting all fancy, hi-tech, expensive, and if it ever fails, once, ouch!

I don't expect I'll run full throttle all the time. If it's low drag, I wont have to. Only to keep up with the seals!

But I wont be outrunning any orca's when they can reach 30 knots! (I spent three years as a whale watching guide/driver).

I picked 15 knots because I thought that's all the thrust I could produce and with calculated drag, that's the number that came out.
I'd welcome 20!

I want a nozzle for stability, like a big fancy rudder, but instead of creating drag, it may cancel out with improved prop efficiency.

Counter-rotating: two fold efficiency improvement. Improved prop efficiency, and no incurred drag from fin, aileron
or independent elevator compensation. I want to roll. Think of the centripetal-neutral feel of riding a motorbike in a corner.

You got what I said about the CofG and CofB being superimposed, right?

I read Mermaid the other day. That's what inspired me to blogg-on.

Okay, now lets see if I can get that drawing up.

Tom, Victoria BC

 

Tom
A quick response to hit some points.

The numbers I provided are for the elliptical hull shape. If you can create a laminar flow hull then you might do a lot better. BUT, and it is a big one, I think once you start to turn you will lose laminar flow.

The liminar flow hulls that I have seen photos of are for autonomous vehicles that move slowly and in a steady trajectory.

So assuming normal boundary layer turbulence there is no great advantage in a perfectly faired shape or perfectly smooth surface.

There are two ways to make it from steel. The cylinder with nose and tail cone or longitudinal leaves with hard chines. The minimum would be 4 leaves. You might go to 6 to get something like an hexagonal section. Point is you need to sort out what will go in it and where. This might actually have some role to play in the shape.

I do not know of any highly maneuverable underwater animals or boats that do not have some form of forward control surfaces. I am not sure about the Albacore. I think you need to find a good example of what you are doing or build a model before settling on the final design.

If you swing a large diameter prop with high aspect blades then there is no point in putting it in a nozzle. The nozzle just adds extra drag that will more than offset the tiny gain in efficiency. They offer an advantage of you are diameter constrained for some reason. Most of the steering force comes from control surfaces not vectored thrust. The latter only has value at slow speed. Adding the complexity of counter rotating props will likely lose efficiency rather than gain. The drag on foils to counter the torque from the prop will be miniscule I expect. If you ballast downward then there is no cost other than a bit of roll that can be counted with rudder trim. If counter rotating props were highly efficient you would see them on aircraft. Just go to a high efficiency prop in the first place. The loading you have is small. The reason you see them on things like the duoprop is that they are heavily loaded props that are low efficiency to start with.

I still see you will have difficulty getting enough weight into it. I would like to see more detail on the buoyancy model you tested. There is a big difference between your figure and mine.

For a surface boat you aim to keep weight down and I use glass microbubbles for making fairing paste or bog. I think you will want to fair this thing with something more dense like metal filler. Buoyancy will be your enemy.

The power requirement and performance can be determined to reasonable accuracy so the merit of various features can be worked out. I have seen a lot of effort wasted on detail of little merit while some of the main factors are forgotten about. A bit like having tiny ducted counter rotating prop when a large single prop could be fitted and achieve higher efficiency at a fraction of the cost.

I got your point about aligning CoG and CoB but I think you run the risk of being disoriented. Having some righting moment should not alter performance much and my first impression is that this would be a key safety feature.

Rick W

 

Tom
Attached shows what I consider would be the simplest shape to build from steel plate. It is 4.5mlong and 0.5m square. The drag at a depth of 2m doing 8m/s is 700N. The displacement is 680kg.

A rounded nose would help flow during turns.

If you wanted shoulder room you could make it 6 panels and make it wider than it is high. A matter of what you think you can squeeze into.

If it was 1/4" plate the hull would be 311kg so you are well on your way to the required displacement. It is a whole different story if you are going to operate it flooded. I am still not clear on this aspect.

Rick W

 

Tom,
I have a question about the ergonomics. You speak of wanting to roll 90 degrees to effect turns (or just for fun?), and that will mean that you'll be laying full-weight on your side during such a maneuver. It's clear that you'll be laying stretched straight out in the sub, so you won't be able to use knees or elbows to brace yourself. What have you got planned for body support in a steep roll? I'm thinking that if you roll sideways, you'd be laying on your arm and perhaps not be able to operate the controls with that hand. Your legs will flop over as well, so might make it problematic to operate your foot controls.

You could rig a network of webbing to support you in any position, but that sounds really scary if you think of emergency egress.

 

Great comments! Thank you Rick and "clmanges".

1 atmosphere air in the cabin, Rick.

We have different design techniques you and I, and that's okay.
I would first like to define my hull form and then design the filling.
My overall design objectives are efficiency, handling/performance
and safety. I'm pondering your dissorientation concerns.
Our buoyancy numbers do differ substantially, you're right.
I haven't really got to the details yet because I need to finalize
my hull shape first. I realize this may be a back and forth process,
but it's still where I've chosen to start. Hard chines are out for me.
No offence, and I may end up with them, but not for now.

I embrace your large prop ideas and would gladly dump all that other
stuff.

Clmanges, rigid bracing and g-force will keep me positioned.
I only anticipate the more radical manueovring at speed,
otherwise, the more conventional turning approach would be
implimented. I don't plan on ever exiting the vehicle in the water.
It will be possible if all other emergency measures fail.
I would lose the vessel to sinking if I were to exit due to flooding.
A scuba regulator and mask will be very close at hand.
Blowing the forward (and perhaps only) ballast tank.
Shallow water, close to the beach operation. As Rick has pointed out,
this will be a slow evolution to deeper , riskier operation.

Gentlemen, all of these points you raise are excellent pointsand I appreciate them more than you know, but what I need is to finalize my hull form before I can even begin to answer to them in detail.

I'll try again later (when my two year old isn't pestering me) to
get it up (so to speak...).

Tom.