Snorkels for freediving

[Toeter]

Hi there,

I,m new to the sport of freediving, but up to now I have lots of fun.
It is very difficult to get information over here about freediving gear.(I life in the Netherlands)
So it is great to find so much info on this site.
I'm missing one thing, info about snorkels for freedivers.
Maybe it is here somewhere but I can't find it.
So would you please be so kind to tell me your opinions on this subject.

[rcohn]

Snorkels for freediving are a relatively personal choice. Virtually all freedivers use solid tube snorkels, the flexible snorkels are for scuba. Most freedivers prefer a simple tubes free from any purges that might fail and add drag. Any online store that features freediving gear will feature this type. Some of us (including me) like purges, they make clearing easier. In rough water a few divers like dry snorkels, the Aqualung Impulse is popular here.

My tastes may be a little odd, no one makes my ideal snorkel so I compromise and change snorkels depending on conditions. I like a narrow bore tube, I get enough air and it has less dead air space. I like a straight long tube, bending it around your head does not reduce drag and it measurably reduces the height. Finally I like a purge. No one makes this combination. I have a long narrow bore straight tube with no purge, good in calm water with thin exposure protection where I may sink deeply when breathing up. I also have a couple of wide bore purge snorkels for rougher days and heavier wetsuits.

Hope this helps a little.

Ralph

[WreckWriter]

Toeter once bubbled...
I'm missing one thing, info about snorkels for freedivers.
Maybe it is here somewhere but I can't find it.
So would you please be so kind to tell me your opinions on this subject.

What you want is a large bore, slightly curved tube (curves sorta around your head). If you want a purge use only a simple bottom purge. Absolutley no "dry snorkel" gimicks, they're toys. The Scubapro "Shotgun" is a good freediving snorkel.

Tom

[rcohn]

WreckWriter once bubbled...
....., slightly curved tube (curves sorta around your head). .....

Why would you want it to curve around your head?

Ralph

[Toeter]

Thank you for the answer(s).
I dicided to buy a Shotgun from Scubapro.
Search over.
:thumb:

[WreckWriter]

rcohn once bubbled...
Why would you want it to curve around your head?

Ralph

Limit drag in every way possible. Also, believe it or not, a snorkle standing up straight will make noise by sorta vibrating in the water as you dive. Of course the noise part applies primarily to hunters but in my experience that's what most freedivers are.

Tom

[rcohn]

Drag is proportional to frontal area, curving the snorkel around your head doesn't limit drag at all. You'd need to curve it behind your head to reduce drag. It limits the height of the snorkel above the water. If the curved height is ok and you straighten the tube you could cut over an inch off, this would reduce drag. The only advantage I can see to curving the tube (beyond looking cooler) is that the opening of the snorkel is set at the center of your head so you have a greater range to roll to the side before submerging it. I haven't noticed any additional vibrations between a straight tube relative to a curved tube and I don't see a reason why vibrations would be reduced..

One way to reduce drag and the dead air space at the same time is to use a narrower bore. In the popular wide bore snorkels, you rebreathe more of the stale air you just exhaled. I find I get plenty of air from a narrower bore. Back in the 70s all the snorkels I saw/used had narrow bores and I thought they worked fine, wide bores are a relatively recent innovation. My narrow bore Cressi snorkel has less dead air space that a standard snorkel with greater height. Too bad it doesn't come with a purge. Lately I've been using a Scuba Pro Twin Valve Shotgun 2 snorkel more frequently.

Ralph

[WreckWriter]

You're basing your statement on what YOU notice, not what is reality. You may not notice the vibration but it's there, the fish can hear it. Vibration is reduced primarily because a long straight object will vibrate more than a shorter curved object.

There's been a lot of changes since the 70s, mostly for the better.

Tom

[rcohn]

WreckWriter once bubbled...
Vibration is reduced primarily because a long straight object will vibrate more than a shorter curved object.

Interesting assertion, would you care to provide a reference to support it?

Ralph

[WreckWriter]

rcohn once bubbled...


Interesting assertion, would you care to provide a reference to support it?

Ralph

Sorry Ralph, don't have a reference to support it. All I have is my 30 years experience free diving, a book I wrote on the subject, and the shelf full of trophies I won in free diving competition.

I don't know the science, I only know what was passed to me by the folks that taught me and which I have found to be (or at least appear to be) true over the years. When I punch down on a fish and he spooks before I get within shooting range, I have to decide what I believe spooked him and change that thing. My mentor called it "fish sense".

I've tried lots of different gear, this what works for me and why I believe it works. Your milage may vary and no science available :)

Tom

[MikeFerrara]

We could all pitch in and have a FEA done. But..I suspect that we need to define "vibrate more". I think a curved structure will have different resonant frequencies (I think higher). Of course we would have to define the excitation spectrum and measure (or predict through FEA) the response spectrum in order to determin the transfer function. There may very well be transverse resonances that are not seen with excitation in only one direction. Therefore, It could be difficult to design a test that would simulate moving through the water. But...assuming the same material and demensions, will curves add stiffness? If so, I think the resonant frequency will be higher. Would you agree? I would expect the excitation (moving through water) to be a low frequency. Would you agree? If that is the case, I would expect a lower magnitude response in the snorkel When moving through water).

WreckWriter, didn't you check any of this out?

Actually drag is dependant on more than just frontal area. Shape (yes frontal area is part of this) will change drag. But we are not just concerned with the drag of the snorkel but the drag of the diver and the snorkel combined. Having the snorkel as close as possible to the head may very well be more hydrodynamic. In either case, I think it will alter flow in one way or the other, therefore changing drag.

rcohn, do you ever provide references? I think in this case it could be done.

However I don't think it would be worth the work because rcohn can use any kind if snorkel he wants, for any reason he wants. On the other hand, maybe not, since I don't have references to prove it.

[WreckWriter]

MikeFerrara once bubbled...
WreckWriter, didn't you check any of this out?

I admit it was silly of me but no, I didn't! all I know is that when you're punching down hard to 70+ you can feel that puppy vibrating on the side of your head. With a curved tube, you feel it less.

Maybe it was silly of me to change my snorkel based on such unscientific evidence but hey.... :)

Tom

[MikeFerrara]

Tom,
I think if I were stating your case I would consider using your experience for a reference.

[WreckWriter]

MikeFerrara once bubbled...
Tom,
I think if I were stating your case I would consider using your experience for a reference.

Didn't I try that? It's really all I have, never tried any study of it other than in-water.

Tom

[rcohn]

WreckWriter once bubbled...
I've tried lots of different gear, this what works for me and why I believe it works. Your milage may vary and no science available :)

So why with no science available did you claim


a long straight object will vibrate more than a shorter curved object. Did you think I'd be so impressed I'd just shut up and go away?

Ralph

[rcohn]

WreckWriter once bubbled...
I admit it was silly of me but no, I didn't! all I know is that when you're punching down hard to 70+ you can feel that puppy vibrating on the side of your head. With a curved tube, you feel it less.

Maybe it was silly of me to change my snorkel based on such unscientific evidence but hey.... :)

Tom

So what was the big deal? I dive with both straight and curved snorkels and don't notice a big difference in vibration and said so. First you seem to claim the vibrations are too small for me to notice
You may not notice the vibration but it's there, the fish can hear it. Then you switch to claiming that
you can feel that puppy vibrating on the side of your head You seem to be very upset that anyone might have a different opinion. You give your opinion, I give mine, and maybe someone else will join in. You can feel free to cite your book and list your trophies and let the reader decide for himself. Unless you want to provide some measurable proof, it's all just a matter of subjective opinion so far.

Ralph

[rcohn]

MikeFerrara once bubbled...
We could all pitch in and have a FEA done. But..I suspect that we need to define "vibrate more". I think a curved structure will have different resonant frequencies (I think higher). Of course we would have to define the excitation spectrum and measure (or predict through FEA) the response spectrum in order to determin the transfer function. There may very well be transverse resonances that are not seen with excitation in only one direction. Therefore, It could be difficult to design a test that would simulate moving through the water. But...assuming the same material and demensions, will curves add stiffness? If so, I think the resonant frequency will be higher. Would you agree? I would expect the excitation (moving through water) to be a low frequency. Would you agree? If that is the case, I would expect a lower magnitude response in the snorkel When moving through water)..

A curved structure mainly shifts the nodal points. For example the frequency of a tuning fork, bent 180 degrees, remains the same as the straight bar (for the same harmonic mode). The nodal points shift to the bends and the stem is set at the anti-node where it produces the greater motion for power transfer to a resonator of some sort to enhance the sound. My reference is Kinsler and Frey, I’ll give the full reference and correct spelling tomorrow when I have it in hand.

The curved snorkel adds a new torsional mode that does not exist in a straight tube producing additional vibrational modes. Have you taken that into consideration? It doesn’t really matter because I believe you are on the wrong track from the start again. I don’t believe that the simple vibration modes of a bar are what we are dealing with here. While snorkels are made out of a wide variety of plastic stiffnesses, if you rigidly support one out the water and strike it to excite vibrations, you will see they damp out in a fraction of a second. With the viscous damping of the water the vibrations will die out even more quickly. The frequencies we experience with a snorkel moving through the water are much much lower.

I believe that key factors in snorkel vibration are the elasticity of the connection between the snorkel and the mouth and the snorkel and the head/mask strap. The snorkel vibrations you feel are not bar mode vibrations as in a tuning fork. In all cases, except for possibly the very softest of bendable snorkels (and of course flexible scuba snorkels), it is a rigid bar and changes in stiffness are irrelevant. What matters are the hydrodynamic effects and fluctuations (such as vortex shedding) that cause the rigid bar of the snorkel to move, tensioning and releasing the elastic connections at the mouthpiece and the mask strap. While I don’t believe curving the tube increases stiffness, I also do not believe the stiffness generally matters as long as it exceeds a minimum level. Curving does change the hydrodynamics and depending on the magnitude this might have some impact on vibrations. For example, the moment arm relative to the mouthpiece and mask strap obviously is reduced, which might act to reduce vibrations. Then again hydrodynamics between the tube and the head also change and I have no idea what the effect on vibrations would be here.


MikeFerrara once bubbled...
Actually drag is dependant on more than just frontal area. Shape (yes frontal area is part of this) will change drag. But we are not just concerned with the drag of the snorkel but the drag of the diver and the snorkel combined. Having the snorkel as close as possible to the head may very well be more hydrodynamic. In either case, I think it will alter flow in one way or the other, therefore changing drag.


Drag of a cylinder = (Cd)(A)(rho)(U^2)(0.5),
Where:
Cd – coefficient of drag = 1.2 for a cylinder
A – projected area in the plane normal to the flow
rho – fluid density
U – velocity
From: Streeter and Wylie, Fluid Mechanics,McGraw-Hill, pg 281, 1975. (This equation will be found in virtually any fluids text.)

As you can see drag is directly proportional to projected area and proportional to velocity squared.

First, I of course assume we are talking about keeping the shape (cross section) of the tube unchanged and only changing the bend. This goes back to the old issue of only changing a single parameter at a time so that valid comparisons can be made. Unless you’re once again seeking to obscure the answer? If the shape remains constant the frontal area remains constant and the major factor in drag should also remain constant.

Now it’s less clear what happens due to the close proximity of the top of the head. When the tube sticks straight up in the flow, the drag equation directly applies (at least for laminar flow). If the snorkel was shielded by the head, drag might be reduced. When the snorkel is positioned close to the top of the head, but not in direct contact, fluid must flow between the snorkel and the head. While I don’t know what happens in this case, it is likely the forcing a viscous fluid to flow through the narrow area, increasing the velocity and disturbing the flow around the head, might actually increase the drag relative to the freestanding tube. I believe the effect will be relatively small either way


MikeFerrara once bubbled...
rcohn, do you ever provide references? I think in this case it could be done I of course assumed anyone eager to discuss drag effects would be familiar with the elementary equation for drag in laminar flow, but if not you should be now. Do we have enough references or do you need more? I used the term “moment arm”, do you need a reference? We could look for some more elementary sources but they are limited as these tend to be advanced (college level) topics.

I haven’t yet seen you support a single argument or supply a single reference. Oh, if you still want the FEA done there are a few experts here (not me) who would be quite happy to do it for you, I can get a job set up anytime. However, I guarantee you won’t be happy with the price.

Ralph

[rcohn]

MikeFerrara once bubbled...
We could all pitch in and have a FEA done. But..I suspect that we need to define "vibrate more". I think a curved structure will have different resonant frequencies (I think higher). Of course we would have to define the excitation spectrum and measure (or predict through FEA) the response spectrum in order to determin the transfer function. There may very well be transverse resonances that are not seen with excitation in only one direction. Therefore, It could be difficult to design a test that would simulate moving through the water. But...assuming the same material and demensions, will curves add stiffness? If so, I think the resonant frequency will be higher. Would you agree? I would expect the excitation (moving through water) to be a low frequency. Would you agree? If that is the case, I would expect a lower magnitude response in the snorkel When moving through water)..

A curved structure mainly shifts the nodal points. For example the frequency of a tuning fork, bent 180 degrees, remains the same as the straight bar (for the same harmonic mode). The nodal points shift to the bends and the stem is set at the anti-node where it produces the greater motion for power transfer to a resonator of some sort to enhance the sound. My reference is Kinsler and Frey, I’ll give the full reference and correct spelling tomorrow when I have it in hand.

The curved snorkel adds a new torsional mode that does not exist in a straight tube producing additional vibrational modes. Have you taken that into consideration? It doesn’t really matter because I believe you are on the wrong track from the start again. I don’t believe that the simple vibration modes of a bar are what we are dealing with here. While snorkels are made out of a wide variety of plastic stiffnesses, if you rigidly support one out the water and strike it to excite vibrations, you will see they damp out in a fraction of a second. With the viscous damping of the water the vibrations will die out even more quickly. The frequencies we experience with a snorkel moving through the water are much much lower.

I believe that key factors in snorkel vibration are the elasticity of the connection between the snorkel and the mouth and the snorkel and the head/mask strap. The snorkel vibrations you feel are not bar mode vibrations as in a tuning fork. In all cases, except for possibly the very softest of bendable snorkels (and of course flexible scuba snorkels), it is a rigid bar and changes in stiffness are irrelevant. What matters are the hydrodynamic effects and fluctuations (such as vortex shedding) that cause the rigid bar of the snorkel to move, tensioning and releasing the elastic connections at the mouthpiece and the mask strap. While I don’t believe curving the tube increases stiffness, I also do not believe the stiffness generally matters as long as it exceeds a minimum level. Curving does change the hydrodynamics and depending on the magnitude this might have some impact on vibrations. For example, the moment arm relative to the mouthpiece and mask strap obviously is reduced, which might act to reduce vibrations. Then again hydrodynamics between the tube and the head also change and I have no idea what the effect on vibrations would be here.


MikeFerrara once bubbled...
Actually drag is dependant on more than just frontal area. Shape (yes frontal area is part of this) will change drag. But we are not just concerned with the drag of the snorkel but the drag of the diver and the snorkel combined. Having the snorkel as close as possible to the head may very well be more hydrodynamic. In either case, I think it will alter flow in one way or the other, therefore changing drag.


Drag of a cylinder = (Cd)(A)(rho)(U^2)(0.5),
Where:
Cd – coefficient of drag = 1.2 for a cylinder
A – projected area in the plane normal to the flow
rho – fluid density
U – velocity
From: Streeter and Wylie, Fluid Mechanics,McGraw-Hill, pg 281, 1975. (This equation will be found in virtually any fluids text.)

As you can see drag is directly proportional to projected area and proportional to velocity squared.

First, I of course assume we are talking about keeping the shape (cross section) of the tube unchanged and only changing the bend. This goes back to the old issue of only changing a single parameter at a time so that valid comparisons can be made. Unless you’re once again seeking to obscure the answer? If the shape remains constant the frontal area remains constant and the major factor in drag should also remain constant.

Now it’s less clear what happens due to the close proximity of the top of the head. When the tube sticks straight up in the flow, the drag equation directly applies (at least for laminar flow). If the snorkel was shielded by the head, drag might be reduced. When the snorkel is positioned close to the top of the head, but not in direct contact, fluid must flow between the snorkel and the head. While I don’t know what happens in this case, it is likely the forcing a viscous fluid to flow through the narrow area, increasing the velocity and disturbing the flow around the head, might actually increase the drag relative to the freestanding tube. I believe the effect will be relatively small either way


MikeFerrara once bubbled...
rcohn, do you ever provide references? I think in this case it could be done I of course assumed anyone eager to discuss drag effects would be familiar with the elementary equation for drag in laminar flow, but if not you should be now. Do we have enough references or do you need more? I used the term “moment arm”, do you need a reference? We could look for some more elementary sources but they are limited, as these tend to be advanced (college level) topics.

I haven’t yet seen you support a single argument or supply a single reference. Oh, if you still want the FEA done there are a few experts here (not me) who would be quite happy to do it for you, I can get a job set up anytime. However, I guarantee you won’t be happy with the price.

Ralph

[WreckWriter]

you have too much time on your hands.

You want to use a narrow bore straight snorkel, go for it. I'm sure you can still find one at K-Mart or similar stores.

The original poster asked for opinions, you gave yours, I gave mine. Like you said, let the reader decide.

Tom

[The Zer]

With all other factors being equal:

It would seem to me that a narrower bore would provide the diver with more O2 (fresh air) than a wide bore snorkel, due to the amount of exhaled air (left in the snorkel) that will be 'rebreathed'.

Any thoughts?

Of course, not to narrow as to greatly increase the static pressure/resistance.

[rcohn]

Missing reference from previous post, Kinsler and Frey, Fundamentals of Acoustics, J. Wiley and Sons 1962 page 77.

The change in stiffness due to snorkel curvature really should not matter in diving, but here is information the changes in bending with a curved beam anyway, from Roark and Young, Formulas for Stress and Strain, 5th ed., McGraw Hill, 1975. Chapter 8 "Curved Beams" page 209.


When a curved beam is bent in the plane of initial curvature, plane sections remain plane, but because of the different lengths of fibers on the inner and outer sides of the beam, the distribution of stress and strain is not linear; the neutral axis therefore does not pass through the centroid of the section and Eqs. 1 and 2 of Art. 7.1 do not apply. The error involved in their use is slight as long as the radius of curvature is more than about ten times the depth of the beam, but it becomes large for sharp curvatures.

Ralph

[Toeter]

I think that in the small/big bore discussion the problem of rebreathing your air is not that important, I think.

The air I rebreath is only a small portion of air I take in.
When I am snorkeling I vent about 3 to 4 liters of air each time.
I have 3 snorkels 'in the house' and they have a air content of 150 to 175 ml (all big bore).
So each time a take a breath I am rebreathing 175 ml of air (max).
In this air there is still 16% O2, so enough for me to breath, I only need 4%.
I think the content of a small bore will be around a 100 ml(?), so the difference is not that big.

Does this make sence or am I barking up the wrong tree here ?
:confused:

My main concern is the resistance I feel when I'm breathing.
I tried a couple last week and for me was the shotgun the winner, so I choose that one.

Respect !

[Toeter]

Today I realized I was barking up the wrong tree.

The supplie of O2 isn't the problem, but the level of CO2......:banging:

On every exhale you produce 4% CO2, that's not good to keep on rebreathing this.

But I understand that if your snorkel has a air content around the 150 ml that this causes no problems.