Rocket Fin II's... the alteration.

[Mambo Dave]

Well, I'm really not sure where to put this as it falls into a "modify your own gear" category that doesn't exist.

Long story ... kept long:

Let's talk Rocket and Jet fins, shall we?

Some like Rockets, some like Jets. Jets get used more often due to the DIR acceptance, their reported efficiency and what-not. Rockets exist for those who love them.

Years ago I had the regular rubber version of Rocket fins and loved them. Apparently they dry-rotted, got thrown out, and (fast forward) by the time I got back into diving the Rocket Fin II's were in existence. Awesome.

See here, Rocket Fin II’s were reportedly designed by, and are (we are left to assume) currently used by, the U.S. military’s special forces (or some branch of them). This made them perhaps a better buy in my mind – a more up-to-date fin made up up-to-date materials.

Of course the whole argument that Jets are still better exists – but that isn’t the point at the moment. The point is that I spent moolah (greenbacks) on Rocket II’s.

Furthermore, after ten dives I came to find that Rocket II’s are a highly specialized fin that are quite probably designed by, and for, those who would need to fin underwater with very heavy loads. See here, with their rib reinforcements (that Jets do not have) and their new-fangled plastic-rubber compound, these fins are beyond stiff. In fact their main flexion point seemed to be my ankles. One of my dive buddies verified this in an underwater fin trade-off maneuver half-way through a shore dive. Man, I was loving those split fins the rest of that dive… but I digress.

A good amount of those dives were within an amount of time that, when paired with cycling, my legs should have been able to adjust for the added strength these fins required – if that’s what they required – to become efficient with a myriad of kicks. I don’t believe strength is what these fins ask for as much as they are really asking for a very substantial load. Save for the one dive where two dive buddies loaded my thigh-pocket with at least six pounds of fishing weights from under the pier, great loads really haven’t been a consideration of my dive plans.

But let’s consider the possible “Pros” of RF2’s over Jets before some third-world country starts demanding our women-folk wear neoprene hoodies while we all convert to “The Fin of No Flex”:
1.) A new plasticky-rubber compound that seems like it might last longer than humankind plans to inhabit this floating orb.
2.) A full foot-pocket, with a bottom that reaches the back of the foot (made of the same stiffer material, it seems to give support for the overall kick and the extended-bottom protects the foot (I’ll leave it to you to make up the vile underwater or out-of-water nemesis it protects the foot against)).
3.) The RF2’s have traction on the soles. Why is this so awesome for a Rec Diver? I dunno.
4.) Without using a side-by-side comparison, the RF2’s seem to have a bit bigger paddle area than that of the JF’s. (arguably a mute point if the RF2’s still don’t work as well)
5.) Could the RF2’s ribbing help direct water that equates to more efficient kicks?
6.) RF2's have cool holes in them to hang them off of one's tactical gear should one surface from a shore dive in a war zone.

Upon examining a new set of Jet fins – equipped with the molded-in ‘Made in USA’ label that, I was assured, was lying (perhaps the old mold was made in the USA?) – I found that they were quite flexible at the paddle-part (herein NOT referred to as the ‘PP&#8217, while in comparison my RF2’s were steel I-beams.

Now for something really stroketastic: I decided to pare down parts of my RF2’s to allow some flexibility.

My initial thoughts were to go for ALL of the ribbing that exists on the RF2’s that does not exist on the Jets. Well, either I chickened out or I felt like I’d reached an acceptable first test point, but I stopped after cutting only one side’s outer-most ribbings.

Are they ugly? Heck yeah.

Do they flex more? Yes. As much as Jets? No, I don’t believe so. Not yet.

Why did I trim down one side’s – the top side’s – outer ribs instead of the bottom's? I wish I had a thought-out answer to offer, as I really should have considered this, but as of the time I first started cutting I really had planned on cutting BOTH sides off before testing them out tomorrow. After a side-by-side flexion test post-one-fin's top-side cutoffs, I came to the conclusion that I did achieve some… alteration of the initial design’s performance – enough so as to make me happy. If I am unhappy I can always cut a little more off, and more after that, until I finally reach that point at which I shell out the very-reasonable cost of the local Jet fin reincarnates.

The current version of my cut-aways, that leave all ribbing on the bottoms, may actually maintain their efficiency during frog-kicks. I'd like to be able to claim I thought about this beforehand, but again...

Will I sink at an unbelievable rate into the great abyss, never to be seen again? Probably.
Until then, comments are most welcome. I have some not-so-great pictures I can post if anyone really needs to see the pending disaster.

Best,

Mambo ‘DIWr’ Dave

 

[CWMurf]

Pictures man...

 

[Jason Ooi]

attach some photo please

 

[Tom Winters]

After what you did to those fins, I feel sorry for the poor SOB who is going to have to drag you through the water.

 

[Mambo Dave]

LOL, Tom, that's the whole point. Much like 'spinning' while cycling offers more efficiency through less effort (and higher revolutions), at some point I believe that super-stiff fins' requirement of strength over numbers-of-kicks per minute is a losing proposition (efficiency-wise). After confirming that, over a given distance and time, a more flexible fin (with averagely-favored design) left my legs less exhausted, and me with more energy and ability, I took the plunge to alter what I had instead of buying something new. Had these same fins been designed out of the old black rubber compounds perhaps they would be flexible enough for me, but then I bet they would not have passed the military's new specs.

It would be interesting to get a hold of the order that called for the vented-fin design by whatever military branch called for it. I have read that they called for a vented-fin design, and the Rocket Fin II came in as the lowest bid, but did it specify anything else in the overall design? This premise takes the stance that the military (the Navy?) has tested the available scuba products, have come up with multiple ways to define characteristics and performances of the equipment, and has duly requested the best equipment characteristics for given situations. Does this happen all the time? Of course not - it's the military. But I'd be willing to bet they actually have tested, and differentiated between, fin efficiencies for swim-suit swimmers, scuba gear-laden divers and fully-loaded combat divers/swimmers.

I believe I can kick well enough to do multiple dives without a noticable decrease in my ability - just not with the design of the RF2's as they were. I probably should have waited to post this until after the test dives... oh well, c'est la vie.

I'll do something about pictures later tonight.

Best,

Sinks Like a Rock Dave

 

[Mambo Dave]

Success, at least as far as I felt it.

Both fins are basically the same as the first altered fin in the picture.

I think that what I’ve got, now, is pretty sweet compared to what I had.

Upon examining Jet knock-offs again, Jets have no ribs on the tops of them. Jets do have ribs on the bottoms (albeit less substantial and made of the much more flexible old black rubber compound). I dove with the altered RF2's twice - one shore dive and on boat dive. My plan is to dive with these one or two more times before I decide to alter them again.

The next alteration, as the fins are still stiffer than most (actually, all I've felt except VOIT UDT Duck Feet Fins, which may be more efficient for a non-loaded diver to use as they seem to have a less-wide paddle-area to move water with each kick... I need to look at them again - strange fins), will be to trim back the inside two ribs. This would be in an effort to maintain the directional water guidance of the ribs left on the top, while allowing the fin to flex a little more. The goal, now, is to have a slightly stiffer fin than the Jets while keeping as much ribbing as possible.

When considering this next cut, I'm thinking it would be best for the diver to decide if they want the option of some form of balanced efficiency for the flutter and frog kicks, or perhaps a more efficient frog kick over a flutter. I'm far from being an expert here, but I'm not too sure many people are experts at fin design... the people who keep coming up with new designs each year must not know what they're doing if, every year, their designs are still compared to the original Jet fins for efficiency... oops. back to the point.

And I'm looking for input here: It seems to me that if I were to cut the bottom-inside two ribs to gain flexibility, I would now have fins that would direct water flow for both sweeps of a flutter kick. However, the cross-sectional thickness would pretty much be the same, thereby causing a little more drag (than a thinner-profile fin - it would be about the same as the pre-cut RF2's) when pulling the fins sideways for a frog kick. Reducing the ribs across the top, only, in a Jet-fin style, would leave fins with a thinner cross section that, in turn, would be slightly easier (thus more efficient) to push out to the sides during the non-thrust portion of the frog kick.

At the moment I pretty much use the frog kick 85% of the time. At first this was out of necessity as the aforementioned flexion point being my ankles was not an exaggeration, and it turned out to be pretty painful in the long run. But that 15% I don’t use the frog is when I really want some speed.

 

[Mambo Dave]

vs.

 

[Green_Manelishi]

Rocket fins and Turtle fins are both shorter and wider than Jet fins. Do you know how the blade stiffness compares between Rockets and Turtles?

And as a brief aside, I have one Dacor Turbo fin in my cellar. I wish I still had the Turbo IIs that I used in the 1970s.

 

[Mambo Dave]

Off hand I'm not sure how the RF2's compare to the Turtles. My LDS, Fill Express, has Turtles, so next time I'm there I'll pick one up and play with it.

Rocket fins may well be shorter than Jets (in fact it seems that I recall my original set being that way), but that isn't the fin I was addressing in the thread. I focused on RF2's.

 

[Green_Manelishi]

Off hand I'm not sure how the RF2's compare to the Turtles. My LDS, Fill Express, has Turtles, so next time I'm there I'll pick one up and play with it.

Rocket fins may well be shorter than Jets (in fact it seems that I recall my original set being that way), but that isn't the fin I was addressing in the thread. I focused on RF2's.

Yes I know. However, quick google and examination of both seems to indicate the RF and RFII are the same size and the mods to be ease of use rather than composition.
But I could be wrong.