Negative buoyant fins

[meesier42]

You are talking about 2 different things now. Buoyancy in water is one thing, and weight is something else. I can attach and inflate a 250 pound weight bag to an anchor and it will be neutrally buoyant in water, but try to put that on your feet and kick with it.... NOT!

Its not just the buoyancy of the fin that matters, it is its overall weight regardless of it being in or out of the water. You still need to move that weight in the water. The heavier the fin, the more force is needed to move the fin in the water, regardless of its buoyancy in water. It still has mass, and that tires a diver. An efficient fin that has less weight is optimal. Now, I understand if you want the heavier weighted fin for trim, but my advice is to adjust your tank position, and use a lighter fin.

Not me, I have been using ForceFins for years, and I fixed my trim 10 years ago, they are lighter than pretty much all others on the market and I understand how heavy fins are inefficient.

My point was to say that it would be helpful in these discussions of buoyancy to have buoyancy numbers vice the "my Jets are really heavy" dribble that people have stated. Even very heavy fins are not really all that negatively buoyant and I agree that fins will rarely correct and out of trim situation, yes natural rubber is negative, we all know that, but how negative?

I checked my ForceFin Pros, size XL, wiegh just under 4lbs for the pair, and are about 0.5lbs per pair negative. 0.5#'s isn't much trim balast.

I would like to see what the Jet Fin (along with many other fins) are. Its a simple test, use a handheld fish scale to weigh them while immersed in the water (fin and straps, preferably with and without spring straps), the submerged weight is the negative buoyancy. The water needs to (or should be) salt water, as salt water is more dense and the negative buoyancy will be reduced in saltwater as compared to the measurement in fresh (~3.5% less).

 

[pescador775]

Jim does raise an interesting point from Newton's laws of motion. In this argument, the higher the mass of the fin the harder to initiate motion and harder to stop and reverse motion. I can't argue with the basic physics but frankly, I don't believe that it makes a practical difference in effort or performance with regard to fins, especially when doing a short flutter as with BioFins. Forward thrust is mostly a product of blade motion and a heavy, flexible fin might have an advantage in some way, don't know for sure but it could positively affect storage of energy in the blade, at least when comparing the same blade materials. The effects of mass on kick effort could be "submerged" pretty thoroughly when compared to other fin factors. For years, I used fins with 1# lead weights bolted to the bottoms of the foot pockets. Yes, it is slightly harder to kick but I never had a problem with it. The idea is to prevent surface splash which is itself a source of energy loss.

 

[meesier42]

as I think about it, the fin doesn't move that far or that fast and is moving a water that is much heavier than the fin is in the first place, I am not sure how much a difference a heavy vs light fin would make given they are otherwise similar. This is something that I have never really considered before.
I think we should first try to find out just how negative (or positive) different fins are.

 

[vicdiver656]

Yep, Scubapro Jets couldn't be any more negatively buoyant!! The only hitch with them is you have to work extra hard to propel yourself, as they are a blade fin, as opposed to a split fin.

 

[vicdiver656]

Jets with springs seems like the best bet. thanks

Depending on the size of the boot, XSScuba also makes Turtle Fins, which are a close brother to Jet Fins. As far as I know, they sell different boot sizes, and might be less expensive than Jets

 

[pescador775]

XS Scuba sells the Power Fin, I think.