Best fin for Drysuit diving?

[Jeff Toorish]

Every time I am on an airplane and watch that giant plane take off I think about "lift". That is what bio-fins do. The bio-fin provides lift using The Bernoulli principle. A leading edge and a trailing edge.

Could you explain this? To be honest, I don's see how Bernoulli's equation would apply (I'm not saying it doesn't, just that I'm not seeing it). I would think that even if it did it would only apply in one direction. For example, when one leg is moving in one direction, you theoretically attain lift on that side, but the other leg is moving in a different direction so it would have the opposite affect, causing an equivelant amount of "drag".

Also, how would that work with various kicks? For example I frog kick mostly --while my dive buddy scissor kicks mostly. So, our fins are in very different positions.

And if Bernoilli's Principal does apply to bio-fins --is that different from other fins?

Again, I'm not suggesting your statement isn't true, only that I don't completely understand it and this is interesting to me.

Jeff

 

[deco_martini]

Could you explain this? To be honest, I don's see how Bernoulli's equation would apply (I'm not saying it doesn't, just that I'm not seeing it). I would think that even if it did it would only apply in one direction. For example, when one leg is moving in one direction, you theoretically attain lift on that side, but the other leg is moving in a different direction so it would have the opposite affect, causing an equivelant amount of "drag".

Also, how would that work with various kicks? For example I frog kick mostly --while my dive buddy scissor kicks mostly. So, our fins are in very different positions.

And if Bernoilli's Principal does apply to bio-fins --is that different from other fins?

Again, I'm not suggesting your statement isn't true, only that I don't completely understand it and this is interesting to me.

Jeff

Let me try explaining this. Put on your fins out of the water. Kick them as hard as you can. Now observe: did I lift off the ground and fly?

If the answer to that question is "no", then the foil properties of the fin underwater are probably negligible to its overall classic performance characteristics.

 

[Jeff Toorish]

Let me try explaining this. Put on your fins out of the water. Kick them as hard as you can. Now observe: did I lift off the ground and fly?

If the answer to that question is "no", then the foil properties of the fin underwater are probably negligible to its overall classic performance characteristics.

Thanks --I'm a pilot, I understand Bernoulli's equation. What I don't understand is how it would apply to these fins specifically, which is why I asked the question.

Jeff

 

[themagni]

I like my Force Fin SD-1 Military Fins up here.

I've got Force Fins as well, no issues with drysuit / integrated boot. Mine are the high-vis yellow, and many a buddy has commented on how easy they are to spot.

If you do get them, I would strongly recommend the bungy strap. It makes the don/doff stupidly easy.

Apparently a FF will stay on your foot if the strap breaks. I haven't experienced this myself, but a former co-worker had it happen to him.

 

[themagni]

Let me try explaining this. Put on your fins out of the water. Kick them as hard as you can. Now observe: did I lift off the ground and fly?

I would buy those fins.

 

[meesier42]

if you understand lift then you already understand that most fins do not establish the laminar flow over the blade to generate lift, further the angle of attack (nearly perpendicular) does not generate flow over the lifting body to generate the differential pressure across the body and create lift. The only way to establish "lift" in the water with fins is to flex them significantly such that they actually have laminar flow traveling down the blade while kicking and generating the pressure. In this case your leg is creating the pressure and the angle of attack allows the pressure to be converted to flow and propel you forward. So really, its Bernoulli backward, pressure creating flow vice flow creating pressure as in an airplane wing.

A split fin increases the angle of attack of the blade by flexing from the outer ribs towards the center, unfortunately the angle of attack is perpendicular to the direction of travel and no "lift" can be generated. They can perform fantastic water channeling thus allowing them to actually be pretty good fins.

I know of only 2 "fins" that actually generate lift. The Aqueon (although its been recently re-invented as the PowerSwim) and ForceFins. The Aqueon used mechanical linkages to establish the correct angle of attack while using very short rigid blades. The ForceFin establishes the high angle of attack by allowing significant flex and its short blade is the longest possible while maintaining a laminar flow in water.

Heel straps- no you don't need to use them at all with ForceFins, for boat diving I don't bother with them, shore diving, I normally do as the surf could rip them off right when I need them, but I don't have to worry about falling overboard when I am on the beach.

Back to the OP- to help you, please answer the following questions so that I can help identify the best fins for you
1) What is your swimming and scuba background? ie how long, comfort in water, how frequently, etc..
2) what body type are you and what type of kick do you prefer. ie. tight flutter, wide slow flutter, frog, all?
3) how much gear do you intend to carry and in what environments, certain fins are better for certain purposes.
4) do you have any other requirements, ie you always trim feet heavy/light? or have bad knees/hips/ankles

 

[Bob Evans]

Deco, you're a ..lot of fun... time to light your cigar under a balloon and lift up up up and away.

 

[deco_martini]

Thanks --I'm a pilot, I understand Bernoulli's equation. What I don't understand is how it would apply to these fins specifically, which is why I asked the question.

Jeff

Forgive me if I sounded condescending, I just have grown weary about some of the claims I have seen associated with scuba equipment lately. I was making a point about the silly claims manufacturers can make that can sound just scientific enough to make you say "huh, is that right?" and not any sort of slight against you.

Scuba is so profoundly rooted in scientific principles that it is kind of annoying when we throw those out the window when it comes to our accessories. Imagine if a regulator maker approached you and asked you to test a new regulator that worked due to conservation of angular momentum.

I doubt the foil properties of a fin really come into play in a tangible way at low velocity. Maybe if we drug someone behind a boat and they kept their legs perfectly still, we could see if they rose in the water column due to the the foils on their feet.

 

[meesier42]

rising in the water would prove what exactly? any flat plate in a comparatively high density fluid (water to air) can create a large lift vector at low velocity, but in order to be useful would have to be in line with the divers body and thus propel the diver forward, not upward. Of course the key to attaining this lift vector is the angle of attack and flow across the planeform. This is exactly why every single scientific study done to date has shown "fins where successive trailing edge segments that progressed at 90 to the horizontal produce the most thrust during the power phase" as the significant angle of attack sets up the lift vector to propel the diver forward. The terms in the lift equation are density, speed, plane form area, and a term called CL. CL incorporates angle of attack and shape of the foil as well as many other factors.

Point being, yes, the foil properties can provide significant advantages at the low velocity. and yes, there are fin designs on the market that use this fact, as I have already stated. Hopefully, I have shed some light on this topic to JeffToorish.

To answer the OP, I see you are posting in a couple of threads with the same question worded differently. But you have yet to answer my questions to you so that I, and the remainder of the intelligent people on here can make a recommendation to you and actually back up are recommendations with something other than "it worked for me"

 

[deco_martini]

rising in the water would prove what exactly? any flat plate in a comparatively high density fluid (water to air) can create a large lift vector at low velocity, but in order to be useful would have to be in line with the divers body and thus propel the diver forward, not upward. Of course the key to attaining this lift vector is the angle of attack and flow across the planeform. This is exactly why every single scientific study done to date has shown "fins where successive trailing edge segments that progressed at 90 to the horizontal produce the most thrust during the power phase" as the significant angle of attack sets up the lift vector to propel the diver forward. The terms in the lift equation are density, speed, plane form area, and a term called CL. CL incorporates angle of attack and shape of the foil as well as many other factors.

Point being, yes, the foil properties can provide significant advantages at the low velocity. and yes, there are fin designs on the market that use this fact, as I have already stated. Hopefully, I have shed some light on this topic to JeffToorish.

To answer the OP, I see you are posting in a couple of threads with the same question worded differently. But you have yet to answer my questions to you so that I, and the remainder of the intelligent people on here can make a recommendation to you and actually back up are recommendations with something other than "it worked for me"

I think we both agree that the wings on a plane however are a very different case than a pair of fins. Dragging someone behind a boat would give the same sort of foil proof of concept that the manufacturer's claim of "works like a plane" would be equivalent to.

A plane has wings but also has engines. The engines propel and the shape of the wings doesn't change much. In the case of the fins mentioned, if they were to work the same way a plane does (and as the person claims), the diver would have to put rockets on his back and let his fins merely act as pure foils.