Sink Rates, Brass, Tungsten and the great unknown.

The dangers of mixing alcohol, spreadsheets and too little mathematical knowledge.

My real interest in sink rates of flies was sparked by an introduction to Czech style nymphing and spawned from that “mono nymphing”, particularly where it relates to catching yellowfish in the rapid waters of the Vaal and Orange Rivers. (  Favourite haunts of us at Inkwazi Fly Fishing). Very early on it became quite apparent that as anglers most of us, or a least me, were grossly over estimating the rate at which our flies went down to the fish, in particular in moving water. Many also did and still do misinterpret the reasons for the sink rate and don’t adjust our gear and flies accordingly, our conclusions being based on incorrect logic.

Now I have been pondering these variables for some time and a recent newsletter from the guys at Fly Talk, (well worth reading and interesting stuff that they put out), had me thinking about it all over again.

You can subscribe to their newsletter on this link Fly Talk Subscribe

Combine a little bit of interest in the subject, a dose of insomnia and a few whiskies and you are well on your way to hours of interesting hypothesis..

Here was the story in brief from Phillip at Fly Talk:

The flies used in the "Test"

They measured the sink rates of four apparently identical wooly buggers with tungsten and brass beads in two bead sizes. The average sink rates were:

3mm Brass                         8 sec / metre

3mm Tungsten                  5 sec/ metre

4mm Brass                         7 sec/metre

4mm Tungsten                  4 sec/metre

The tentative hypothesis was that a mm increase in bead diameter resulted in a 1 sec /metre increase  in sink rate..

Now I just love that the guys are thinking about this and I really do admire them for putting the ideas out there, it is useful stuff, however there are some considerations, (some mentioned in passing by Phillip I have to add), which throw a spanner in the works and it got me thinking. So armed with ADSL and unlimited access to google, Wikipedia and a spreadsheet I crunched some numbers.

Why I hadn’t listened as school when they were discussing the volumes of spheres and the densities of various metals lord only knows, if only my teachers had explained that it could affect the efficacy of my fishing I may have paid a little more attention.

So given the calculation for the volume of spheres,

(I ignored the hole in the middle, it was complicated enough already), the relative weights, densities and gains per mm increase in diameter were calculated for both Tungsten and brass beads..(Brass isn’t a standard material, being made up of various combinations of metals but I used an average of density of 8.4 g/cm-3)

Here are the results:

	Diameter	Volume Sphere cm-3	Gain in V Variation	Mass	Gain mass	%Gain
Tungsten Beads	0.2	0.004190476		0.080876
	0.3	0.014142857	0.009952	0.272957	0.192081	238
	0.4	0.03352381	0.019381	0.64701	0.374052	137
	0.5	0.06547619	0.031952	1.26369	0.616681	95
Brass Beads	0.2	0.004190476		0.0352
	0.3	0.014142857	0.009952	0.1188	0.0836	238
	0.4	0.03352381	0.019381	0.2816	0.1628	137
	0.5	0.06547619	0.031952	0.55	0.2684	95

The table shows a couple of interesting things, (listen I am not a mathematician so they were interesting to me, although perhaps obvious to the better informed). The gain in volume and therefore mass of the beads per mm increase in diameter gradually reduces as the beads get larger. That is to say you get less gain for a 1mm increase because obviously the increase in volume as a proportion of the size of the previous bead becomes less. I suppose that is to be expected but it also highlights that adding larger and larger beads is going to give you less of an effect, something worth remembering for the “bigger is better” brigade who seem to imagine that you can solve all ills by using larger beads.

The mass difference between the tungsten and brass is significant, nearly double in the case of Tungsten, (a 50% increase is what they say in the catalogues but twice the mass is perhaps a better way of expressing it).

So a 4mm Tungsten bead weighs a bit over twice as much as a 3mm tungsten bead.. and yet the sink rate isn’t affected by more than 20%. That is to say according to Phillip’s findings the 4mm tungsten version sank at 1 sec per metre faster than the 3mm.. So why doesn’t it sink twice as fast?

Why the bigger bead version doesn’t sink twice as fast.

The first really big thing to consider, which wasn’t measured is how fast the bead would sink on its own, it wouldn’t make any difference between the 3mm and 4mm bead, it is a function of density, perhaps a little for the additional drag of the larger bead, (I am not a physicist either) but it wouldn’t make much difference if any.

The second, really really big thing to consider is that a woolly bugger is perhaps the worst style of pattern to use for the experiment, even in non scientific terms it is obvious to me that the sink rate is far more a function of the woolly bugger’s profile, and inherent drag than it is affected by the mass or size of the bead.

I am quite sure that given a slimmer fly, such as a buzzer or Czech nymph you would see a dramatically different picture emerging.

Thirdly I would hypothesize that the maximum sink rate would be of the bead alone and that any variation is therefore an inherent property of the fly’s natural drag, profile etc. i.e. no version of a fly is going to sink as rapidly as the sink rate of the bead alone, that being an inherent maximum if you wish.

Which then only goes to show that the design of your sinking flies is more important than the size of the bead or even the material of the bead that you use.

Sure Tungsten will overcome the drag more effectively than brass, sure a slimmer fly will sink faster and more importantly a slimmer fly will show a marked improvement in sink rate per increase in bead size, at least until the overall density of the fly approaches that of pure tungsten.

Add to that thickness of the leader and you are into a whole new ball game.

Another point:

It struck me after looking over this discussion that perhaps the really interesting point is that a fly with a 4mm brass bead of a mass of approximately .28 of a gram sinks at 7 sec per metre, whilst one with a 3mm Tungsten bead with a mass of .27 of a gram sinks at 5 sec per metre and yet theoretically they weigh almost exactly the same.

Surely this is even more demonstrable proof that it is the density and not the mass which really has an effect on the fly. ? Something that I have been trying to get across to lot of people in the past.. Two flies of near identical mass which if Phillip’s experiment can be judged as accurate, sink at completely different rates.

I have had considerable discussions, verging on argument, with various anglers about the use of massively heavy flies, for Czech nymphing in particular, and the truth is that you gain less and less as you increase the size of the bead and gain a whole lot more if you consider using slimmer flies with less drag and thinner nylon for the same reason.

This I am sure will attract the interest of the mathematicians and physicists out there, which is fine, do give me some feedback, as said my skills in such areas are limited and errors in the formulae or conclusions are more  than likely. Double so after a late night and some scotch, but the point I think is pretty clear, the fly and leader are far more important variables than the bead and to be effective one should consider them very carefully.

The bead effectively increases the density of the overall fly or rig, but if you are trying to change the properties of something so inherently unsinkable as a woolly bugger the variations are minimal.

Given that the bead is by far the most dense part of the system it stands to reason it is more a question of the other “stuff” slowing things down than it is a question of the bead “speeding it up”. Which is perhaps why one of my most effective sinking patterns on the streams, for normal nymphing is a tiny brassie fished on thin tippet, it doesn’t weigh much, but the overall density is probably as good as that massive tungsten bead woolly bugger and in the end that is what counts.

Food for thought.

First Find the Fish.

Cape Piscatorial Society Newsletter

Thursday, August 06, 2009

Lakensvlei Dam

Would you believe that despite the onslaught of cold fronts I have actually managed some fishing this past week. Thanks both to Paddy Coleman a client and friend who wanted to fish Lakensvlei, and to Ian Lourens who kindly allowed me to once again use his boat.

We headed out of Cape Town late on Monday evening and had a very pleasant stay overnight at the Ceres Inn before heading out just after breakfast to hit the water. We had considered the option of foregoing the pleasures of a cooked breakfast and hitting the water at first light, but really come the frigid dawn the idea lost it’s appeal and we only pushed the boat out at around nine.

There was a slight breeze blowing down the dam and slightly towards Bob’s house which seemed pretty pleasant for drift boating.

You may recall that my last trip up there suggested that the fish were feeding in deeper water on daphnia and perhaps the odd crab and given that for Paddy it was his first attempt at drift boating I thought we would aim for a long drift down the middle so as to “settle into” the process if you will.

Now the international teams, who have a great deal more expertise at drift boat fishing than any of us South Africans will tell you first find the fish, then worry about the depth and finally the fly. This piece of advice has always stuck with me and I take it to mean that in a boat the first thing that you want to do is cover water.

There was a time when I would never have fished out in the middle but it has worked for me before, and with word from the locals that they had also been taking fish in deeper water, and given that there can’t be much to eat out there in the middle other than daphnia I put on a select of flies including a hot orange Booby on the top dropper. Orange is a traditional attractor colour for daphnia feeders, but this first drift was really supposed to be a matter of getting into the swing of things.

We hadn’t drifted thirty yards when I hit the first fish, a superb silvery rainbow and within two casts I hooked up his brother. Then another two casts and I was into fish again, it seemed like we had really cracked the code but the last fish had badly tangled the leader in the net and by the time I had it sorted we had drifted out of the action.

Aiming to repeat the drift we rowed back and started again but the notoriously fickle wind at the dam had us blowing much faster and in a different direction this time and we missed the fish. No matter what we did we couldn’t repeat the drift over what I was sure was a serious concentration of fish and we remained fishless for several hours despite working hard at finding them again.

We picked up a gorgeous brownie up in the inlet arm but that seemed to be a once off event and we found no more fish. Eventually the wind abated and swung back again and we could drift more in the region where we had contacted fish in the morning. Paddy had a similar experience to mine on one drift and after hours of trying he took two fish in two casts. I picked up one more and that was about it for the day. Eight fish in total, all taken on sinking lines and every single one of them took the orange booby.

So it turned out that we had found the depth and the fly , we had even found the fish, it was just tricky to stay on them with the variable winds.

All in all an interesting  day , although we worked really hard at it, success coming in fits and starts depending almost entirely it would seem on whether we could get the boat over that pod of fish. We didn’t kill any, so I can’t confirm that they were on daphnia. Perhaps it is better that I don’t know, nicer to imagine that one is right than to risk finding out the opposite.

Wherever you are fishing next, I hope that you will “find the fish”..