The Dorsal Fin
It was inevitable. In my empathic research in the realm of dolphins I had gratefully made use of the monofin. My empty hands demanded and produced the 'WaterWing' and for quite a while I thought this was as close as I could get. Of course the thought of a dorsal fin had been tickling my spine for a longer time, but more as a loose end, then as a serious option. Also the books unanimously ascribed it the function as course-keeper, something that only became interesting in paces that went way above mine.
Until I came across a theory, I don't remember where, that only took shape after lengthy contemplation. I could not help noticing, that in simple submersion, I experienced a slight stickiness from the surface. As if the liquidity of water was slowed down. But so many things happen at that moment: breathing requires exact timing, the dive-angle needs to be bladed in, the monofin needs a light overstrike so it can be slapped back in the water (and not just above) and then soon compensating is in order.
However this is not a welcome delay. Especially when I am moving 'in traveling mode', surfacing only for air, this takes me out of my rhythm.
According to this theory the dorsal fin directs the turbulence that originates from submergence to the surface and thereby speeds up the process.
The first question that came to me was why water at the surface responds in a different way then deeper, 'full' water. My grasp was that the afflux of water to a receding volume is swifter in 'full' water, because at the surface it is limited by air.
Then how about the diversion of the turbulence? The water flows in from all sides, but before it 'collides' the turbulences are kept separated and are ordered laminary by the dorsal fin.
But because theory is no match for practice I decided to try out the dorsal fin myself. As a backpack I used the mounting of my four litre tank and for wood I took African padouk, because its specific weight is just a little less than that of water. For dimensions I based myself on the blueprints of Dudok van Heel(a Dutch dolphinist pioneer), but I made the fin about 20% higher to compensate for body length and speed. I preserved it with two layers of epoxy.
The first try-out was in the swimming pool. After I worked my way through the various humoristic remarks, my very first findings were positive, albeit not spectacular. But I did notice a slight increase in speed when leaving the surface. There was, however, also a counterforce. The mounting was hollow and made of plastic. Moreover it was opened up with lots of holes and slits that indeed filled up after submerging, but thereby causing 'drag'. It became clear to me that there was something to be gained here.
I flattened the bottom of the fin and screwed it on a sheet of glasfiber that originated from an old monofin. This I took to the pool and it felt better, but members of my divingclub drew my attention to the straight sheet sticking out where my shoulders bend forward, so the bottom side was catching plenty of water. Nonetheless I have dragged a club mate, who spontaneously clung on to my dorsal fin, half way through the swimming pool.
Back home I rounded the bottom of the fin after the shape of my back. I launched my neoprened self into my waterhole, the 'Veenmeer'. The funny thing is that at first I paid all my attention to specific effect. And it was there, but not in such measure that it was decisive for progression. Only when I went down along the weed curtains on the other side of the lake, I became one with my fins again. It felt as if I was under viscosis: I was gliding so lightly, so easy, so effortlessly through the water, as if it had given up its resistance. This was no longer the water I knew, this was a more fleeting fluid.
Mr. Grey drew up the paradox named after him in 1936. He calculated the theoretical top speed of whales, based on their water resistance and physical energy. Then he found, that in reality they go 3 to 4 times faster. I have always thought that whales make use of properties of water that we do not know of.
Now I can articulate this aspect: they transform the water from an a-select mass into a laminary jet-phase, which they 'lift' through using differences in pressure.
Right now I'd like to dream up a way to speed up time as I can hardly wait for a trunks try-out in the Veenmeer.
And thus the dorsal fin has become an integral part of my underwater personality.
This text has been published before in Dutch in the diving magazine 'Onderwatersport'.
Jan Ploeg, Norg, Holland, March 26th 2003