Diving Performance - Beyond Drag (article Series And Discussion)

[ronscuba]

fjpatrum, I was not trying to insult you when I mentioned your qty of dives. It is just surprising your opinions are so strong given the limited amount of dive experience you have. People generally form opinions based on their experiences.

You've repeatedly mentioned the large majority of scuba divers have no interest in speed. With only 50 dives, you have not dove with many divers. Why do people buy new fins ? Scubadiving Magazine fin speed tests articles were very popular. They were repeated for years and had an impact on the popularity of split fins and full foot fins.

When you read the fin section on scubaboard don't you see people mentioning power, performance or speed in the fin threads ?

On a dive boat, I hear people talking about how much they like their new fins, how they are faster than their old fins. I hear this type of conversation frequently because many divers like to discuss their gear.

I am not saying my opinion is automatically correct because I have more dives than you, but my experience with the divers I meet on dive boats is there are plenty of people interested in more speed. I like to dive in different locations. Been to many different tropical locations. The conversation is common.

 

[Monofin]

fjpatrum,

Welcome back. I hope you will stick around as I do think you have useful input here. I would like to write an extended reply but not tonight or tomorrow.

I to am an engineer also and would like to have an engineering and safety discussion. I expect that is the useful information you say needs to be given to relate to the typical scuba diver.

This is a big deal to me.

 

[Monofin]

Well before you go, I need you to explain what the data is that applies to the audience. If you don't care about swimming speed, what am I supposed to show that is relevant to you?

A comparison of speed and another of endurance of a "Standard Scuba" kit versus your fins, tank modifications, and snorkel when the water is warmer.

 

[REVAN]

You keep insisting there’s a “20% efficiency gain” but you don’t back it up with any actual data of that.

Well, I have to admit, this is confusing me a bit. You are right that I didn't back that up with any data, but that's because I didn't make the claim here to warrant backing it up with any actual data. Maybe the reason you are so strongly opposed to my claims, is that you are confused as to what they are. What am I missing?

For clarity:

The only claims that I actually made in the report about my kit were that "I was able to cruise efficiently at sustainable speeds of about 2 knots.", that I could sprint at a speed of at least 2.5 knots at something less than max effort, but which caused excessive regulator free-flow, and that I thought my max effort would probably yield something a little less than 3 knots if I were to adjust the regulator to be able to handle the higher speeds. For the first 2 claims, I had video evidence to back it up. I provided the pool length and you can get the times and kick counts from the videos and do the calculations yourself if you want. In general, I tried to keep my claims to observable evidence, and not distract the subject with a bunch of conjecture.

I guess people will have to either take me at my word that my effort was cruise level when I was doing 2 knots in the pool, or watch the videos and make a judgement call that they think I was lying and that it was not a sustainable cruise level effort. One reason I didn't cut the dolphin video is so that people can see the performance over a long period of time to assess workload and respiration rates.

I wanted people to watch the videos and make their own conclusions. If you are already able to achieve 2 to 2.5 knots, or faster with your scuba kit, that's great. You are doing as good or better than me and I'd like to see how you assembled your kit to accomplish this level of performance. If you can only cruise at 0.8, 1.0, or 1.2 knots, but would like better efficiency to be able to cruise faster, I am inviting you to look at what I have done for inspiration on how you might achieve better performance for yourself. That's primarily what the articles were presently about; that, and the call to do even better in the future.

 

[REVAN]

@fjpatrum : It occurred to me that you may be talking about the drag of the tank being 20% better than a standard scuba tank. I seem to vaguely recall having a discussion with you on tank drag several months ago on a separate thread regarding buoyancy issues. In this report, I did not do a detailed breakdown of where the performance came from, but maybe that is what you were expecting to see. I just built the best kit I could imagine that used a standard back mounted scuba tank and bi-fins to see how good I can get it. If you want an estimate for what the drag differences are for adding fairings to a scuba tank, I'll direct your attention to the following from a mechanical engineering textbook:

The standard scuba tank will be a little worse than 'B', as it has the valve, regulator and hoses in front of it. 'C' is very representative of my tank with the fairings in place.

If we ignore the valve and regulator issues, this reference indicates the fairings are likely to be lowering the tank drag to about 0.15/1.1 = 14% of the standard scuba tank, a drag reduction of 86% for the tank component of the kit.

It is plausible that I may have at some other time made a conservative claim that I lowered my tank drag to 20%. Maybe you inverted this to think I meant 20% better instead of 80% better. Could this be what you are thinking of?​

Now there is more to a scuba kit than just the tank. They also tend to have a lot of hoses. Note figure 'D', the small unstreamlined cylinder. That is the size cylinder that has the same drag as 'C'. This is why we want to take those hoses and arrange them in a way to keep them close to the body and in our boundary layer to shade them from the freestream flow. Hoses can make a lot of drag by themselves.

Since you are an engineer, I would think you would already be familiar with this example, as it is fairly common engineering knowledge. I guess I was not expecting resistance on this issue.

FYI: I got the above image from: An Introduction to Mechanical Engineering:

 

[John C. Ratliff]

As I have stated, I have been looking at diver efficiency for quite some time. Here are three photos I took of my system, using a Canon T70 camera in an Ikelite housing. The T70 is a film camera I was using in 2001 to get these photos, as it had a neat back on it which allowed me to program several shots at 30 second intervals. So these are from the film era, and I found them when I was looking through my photos, cleaning up my office, last week.

By the way, I was using my UDS-1 scuba unit, produced by U.S. Divers Company, and it is a fairly streamlined unit. I can no longer use it as the tanks are of a weird opening, huge by today's standards, but there are no eddy current test fixtures for it, and it is the suspect aluminum cylinder which requires eddy current testing. So it is now, unfortunately, simply sitting in my garage. I'm also wearing my Para-Sea BC, which is my own design that' I have been using since the 1980s. It produces quite a bit of drag, but not so much as a regular BC since it "hugs" my body nicely.

If you look at the photos, I have three different swimming configurations.

--The first is the way divers usually swim, using a flutter kick with arms down. You can see the drag produced by examining the cross-sectional area that the diver presents to the water.

--The middle photo shows that simply by extending my hands in front of me, I greatly improve the cross-sectional area for streamlining.

--The third photos shows me using the Hammerhead Unit, and the dolphin kick. You can see further improvement in the cross-sectional area presented to the water, which subsequent lessening of the drag component that REVAN talks about above.

I still have a lot of "garbage" on me, which is the Para-Sea BC, and there is a significant drag component by the material from the BC. But this gives some additional information to you to see how to look at these improvements in streamlining.

SeaRat

PS--I just found the video I shot last summer of the Hammerhead Unit in use in the Clackamas River. I'll see what I can do to put a video together.

 

[caseybird]

Revan,
Looking at your diagrams from your 10:39 post:
Turn a steel tank upside down, lead with the rounded end, build a tail fairing that enclosed the valve and reg and you've done half the job at no extra cost.
Thoughts?

 

[caseybird]

Also, looking over the same diagram, a nylon tank strap and the huge clamp is a disaster as far as drag is concerned. Times 2 in a lot of setups.
Perhaps a single stainless steel band wth a latch placed out of the flow? Has to be better.
SS

 

[REVAN]

Revan,
Looking at your diagrams from your 10:39 post:
Turn a steel tank upside down, lead with the rounded end, build a tail fairing that enclosed the valve and reg and you've done half the job at no extra cost.
Thoughts?

That can work also. As they say, there's more than one way to skin a cat. Steel tanks (at least my HP tanks) are not fully rounded on the bottom. They still have a bit of a corner to them so there may be a little penalty for that. If I remember right, LP steel tanks are hemispherical, but they are also a lot larger tank. I'm sure the front fairing with the contoured interface for my head is better streamlined and far more comfortable. It is also the easy part to make of the two (that is, if they were being manufactured). The tail cone is the harder part to make being both larger and having more parts to it.

I also wanted my kit to be able to work with standard aluminum 80s for when I travel. I live in the desert. The only diving I can do without traveling is in my pool. So for me, making something that travels well is critical to the equipment's overall functionality, and I don't want to travel with tanks.

 

[ronscuba]

A comparison of speed and another of endurance of a "Standard Scuba" kit versus your fins, tank modifications, and snorkel when the water is warmer.

Scubadiving Magazine fin speed tests had some decent differences between fastest and slowest fins. The 2007 article is still online. 12 average divers, speeds and time averaged. Fastest fin averaged 2.4 mph, slowest 1.7 mph. In the slalom course, fastest fin 55 seconds, slowest 77 seconds.

Would be interesting to quantify the effects of streamlining BCD, hoses and tank. Another 1 mph and 15 second drop sounds reasonable.