physics and sink tips - Spey Pages
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post #1 of 25 (permalink) Old 02-19-2019, 03:42 PM Thread Starter
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physics and sink tips

I was a humanities and fine arts major so before i looked it up i couldn't even spell physics. But here is my dilemma. rio versitips 7wt S3, S6, weigh 95gr. so i guess density is the determining factor for sink rate. My problem is they don't make a S8 in a 7wt. Now the 8 wt. tips weigh 109gr. My question is can i cut back an 8wt S8 tip to make it a 95gr. tip and still get the S8 sink rate. These are 15ft tips so if my math is correct I would have to cut off 23" to get it back to 95gr. Or will 14gr make that much difference and just leave the 8wt tip at 15 ft. Thanks Rob
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post #2 of 25 (permalink) Old 02-19-2019, 03:45 PM
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I'd leave it at 15 ft and go cast it. If you don't like the turnover, tag a bit off. I don't think you'll notice the grains, but maybe the length.
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post #3 of 25 (permalink) Old 02-19-2019, 04:35 PM
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The sink RATE, FWIW, is a property of the stuff the tips are made of, and the radius of the tip, not the lenght. So you are good there.

But the total depth is proportional to the sink rate AND the length, so if you cut the s8 back to 13’ like you are suggesting then the DEPTH it will sink goes down like 8 x (13/15) ~ 7, so the same depth as a 15’ S7 tip in the same situation. Maybe that is not as useful or worth the extra expense.

The physics of the turnover at the connection between the line and tip, an important feature matching tip to a line, depends as much on the gr/f (weight per unit lenght) of the tip as the total weight, so you really can’t change that by shortening the tip.

But SLSS is right, should be fine at the slightly higher weight of the 8wt tip. You may even like it better. If you are having turnover issues my guess is you are more likely to have them with the other 15’ tips as well, so consider cutting them all back to 13’ or something. But I doubt you will even notice - that is such a small difference in comparison to what many people do all the time, like switch freely between say t6, t8 and t11 on the same head. It depends a little on the head - on a skagit head it will make no difference. A little more on a scandi head, but while the difference is a little bit above then manufacturing variation itself, it’s not too far off of even that for those two tip weights.

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post #4 of 25 (permalink) Old 02-19-2019, 04:41 PM Thread Starter
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Thanks guys, I'll leave as is and see how it turns over. If there is a problem I'll start cutting back 6" at a time and test it. But the length may be more important that that 14gr. of weight. Rob

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post #5 of 25 (permalink) Old 02-19-2019, 05:17 PM
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Sink Tips - research

For more information than you probably need or want, visit FlyFishingResearch. These guys have done some rather serious study of the dynamics of sink tips in flowing water, and provide some pretty easy to follow explanations of the physics.

Fly Fishing Research

Jim

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post #6 of 25 (permalink) Old 02-21-2019, 09:17 PM
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Its not about total grain of the tip that is the limiting factor. Its the grain per foot matching tip to the end of the head that makes up for a balanced line system. Will 8wt tip work on a head that has the tip that of a 7wt sink tip, absolutely but its not "ideal". I wish they made s8 in 7wt as well. I think core thickness is the limiting factor for them.
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post #7 of 25 (permalink) Old 02-21-2019, 09:19 PM
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Just to add...

You are probably better of cutting up some t8 than bucherig a $30 s8 tip.
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post #8 of 25 (permalink) Old 02-22-2019, 12:49 AM
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Originally Posted by domantas View Post
Just to add...

You are probably better of cutting up some t8 than bucherig a $30 s8 tip.
Yeah, an even longer section of t8 if you can cast it will do in a pinch. T8 is pretty damn close to the 7 wt s6 stuff!

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post #9 of 25 (permalink) Old 02-22-2019, 04:03 AM
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Following SLSS: Don't obsess over the grains in the versitips. You can usually go up or down a size or two without an issue. Worry about length more. Definitely try before you cut.

Also keep in mind these are density compensated tips (unless I misremember). Not like level tips (T-8 etc.). If you cut from the front you are cutting the fastest sinking section. If you want to maintain sink rate and drop weight/length cut from the back.
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post #10 of 25 (permalink) Old 02-22-2019, 11:48 AM
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How to Design Fly Lines

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Originally Posted by domantas View Post
Just to add...

You are probably better of cutting up some t8 than bucherig a $30 s8 tip.
When you start cutting & cobbling together fly lines/heads & sink tips, you are in fact designing a fly line system. This booklet will set you back $20, but it lays it all out, including formulas & tables. Screw up one $30 sink tip & you'll wish you had first bought this little book.
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post #11 of 25 (permalink) Old 02-22-2019, 12:03 PM
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When you start cutting & cobbling together fly lines/heads & sink tips, you are in fact designing a fly line system. This booklet will set you back $20, but it lays it all out, including formulas & tables. Screw up one $30 sink tip & you'll wish you had first bought this little book.
What booklet? Thank you.
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post #12 of 25 (permalink) Old 02-22-2019, 12:51 PM
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What booklet? Thank you.
Al Buhr's book on fly line design. Poppy has it at the Red Shed.
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post #13 of 25 (permalink) Old 02-22-2019, 01:09 PM
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Following SLSS: Don't obsess over the grains in the versitips. You can usually go up or down a size or two without an issue. Worry about length more. Definitely try before you cut.

Also keep in mind these are density compensated tips (unless I misremember). Not like level tips (T-8 etc.). If you cut from the front you are cutting the fastest sinking section. If you want to maintain sink rate and drop weight/length cut from the back.
He called them “versitips” but I’m pretty sure these are the same as Rio replacement tips - I only have one scandi versitip line, but the tips that came with the line are Rio replacement tips. He can correct me if I’m wrong, but I’m not aware of a numbering system for the weight of tips for any other Rio product.

So if we are talking Rio replacement tips (7 and 8 weight are about 7.5 and 8.5 grains per foot respectively, etc.) then I alway tell people to think of them a level tips. Yes they have a couple of feet at the front that tapers a bit that Rio itself says is to help a bit with turnover. It is pretty minor, not like a versileader at all for example, and you have to look very closely even to see it. So I always tell people to just regard them as level tips, since for practical purposes they are. Rest assured a level tip will hang in a straight line in a constant current. There is a mathematical treatment of this problem in the fly fishing research page that was linked above, as well as a rigorous derivation of why a line of constant density sinks faster the larger the diameter that troutless is referring to above. The Rio replacement tips are compensated in a different way, namely so that all the tips in a given weight set will have the same grains per foot - as discussed above this number is a very important one for matching tips to lines.

Troutless brought up a very interesting point but I have no idea if the short fraction that is slightly tapered in the front is also density compensated in the usual sense, for the decreasing sink rate with radius. I suspect not but I could be wrong. At any rate the taper is very slight and only in the very front. I feel like you would have to be a pretty sensitive caster to detect the effect of that taper on the end. At any rate when I cut them back it is always from the front, thus removing the taper completely, and nail knot a piece of 30# maxima chameleon (relatively stiff) to the front end and put the new loop in that. I’ve don’t this with maybe 40 tips in the last few years, usually cutting 15 footers back to 12-13 feet. They all cast beautifully, but I’m guessing the ability to fine tune the gr/ft to match the line you get with the replacement tips is where the biggest payoff in turnover comes from, not the minor taper on those tips in the very front.

The reason why t8 is very close to the Rio 7wt s6 replacement tips is that those are 7.5 gr/ft - t8 is 8 gr/ft and its sink rate is about 6 ft/second. Both are for practical purposes basically level, and will sink in a straight line.

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post #14 of 25 (permalink) Old 02-22-2019, 01:15 PM
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Also keep in mind these are density compensated tips (unless I misremember). Not like level tips (T-8 etc.). If you cut from the front you are cutting the fastest sinking section. If you want to maintain sink rate and drop weight/length cut from the back.
Unfortunately density compensated tips don't work quite as well as they could (in theory). The skinny forward tip end is lighter (grains/foot) and doesn't sink any faster than the rear - certainly not in moving water. However, their taper is noticeable for casting and line turnover. I suggest to trim based more on those properties. Keep the front taper (chop from the rear) if you want finesse in the front, or keep the fat rear (chop off the skinny front) if you want powerful turnover for bigger bugs
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post #15 of 25 (permalink) Old 02-22-2019, 01:24 PM
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Al Buhr's book on fly line design. Poppy has it at the Red Shed.
This is a supper cool little booklet and if it is the type of thing that interest you is a one of a kind source and well worth the money. Here is some basic info.
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