Can someone give a brief explanation of what happens when you put to light a line on the rod, say a 7wt line on a 9 wt rod. Also, what would happen if you put a 10 or 11 wt line on that 9 weight rod?

Well, quite a lot has been written on this so I won't begin to try to cover it all again. But basically what is going on is that the weight of the line loads the rod up. In other words it puts a bend in it that then unloads and transfers the energy which is seen in the loop that the line describes as it shoots forward (or backward, depending upon which end of the cast you are on).

Too light a line results in more difficulty getting the rod to load. One would have to really get some rod tip speed to get the line to form a loop. Too heavy a line and the rod would have a harder time accelerating the line and getting the energy into the line.

Now these terms "too heavy" and "too light" are somewhat relative and somewhat subjective in that I might find a certain line and rod combination pleasant to cast while the next angler might find it "too fast" or "too slow"

I strongly recommend studying the articles at this link for an exact description of what is going on with rod weights and line weights. Its not the easiest read in the world but it's well worth it to get a grasp of what the author is saying. The guy is a PhD research chemist but he's also an angler so I'll leave the word to him

http://www.common-cents.info/

The simple answer is you might put a 7wt line on a 9 wt rod if you want to consistently cast at least 70+ feet and you have the ability to pick up about 40' of line to start a cast.

With the 10 or 11 wt on a 9wt rod you are limiting your ultimate casting distance, but, it is possible to pick up only 5 or 10 feet of line and make a 30-50 foot cast within a matter of seconds.

One situation for long blind casting, the other for quick, short sightcasting.

In any case you're too new to worry about all this.

go big

OK, I'm going to throw my 2c in here. Rods are designed for a specific line weight. When you put a heavier line on your rod, say a 9wt line on an 8wt rod, you slow the action of the rod down considerably.If you add say a 10-12 wt line to an 8 wt rod you run the very real risk of breaking your rod. If you add a lighter line, say a 7wt to a 9 wt, as rrhyne 56 said, the rod will be more difficult to load. IMHO there is no improvement in performance so the practice should be avoided. Learn good casting techniques, practice,practice,practice, then practice some more. and you will be able make any cast you want consistently

I was asking because I'd thought I had too light a line on my rod. I was trying to work out which weight I should try next; 8 or 9. I'm using a 7 at the moment. I think i'm going buy a cortland clouser sea line, in 9wt.
My casting semed to be doing ok recently,of course, i always want more distance, and i was out the other night, messing about, but I was casting, and really driving the tip fast, and the rod started straining, but it was powering back up and springing the line out. I figured the rod wasn't loading up with the 7wt line. It loading up made it a lot easier and quicker it seemed to get the line out.
I figure if the 9wt makes the rod slow, i definately know it likes an 8. If i get an 8, and it works good, ill be wondering if a 9 would be better the whole time, lol.

My rod is 8/9 wt st croix leg-end (older rod, I bought it used). It feels like a medium action, if I translate the action in comparison to my other non-fly rods. I usually use an ultra fast or fast taper for my baitcasting, and fast for spinning gear. I do have some medium action IM6 all stars that feel very similar, but I very rarely use. It's kind of hard to tell, as the only other fly rod I've casted is a (relatively cheap in the world of fly rods) scientific anglers 6 wt I bought the Mrs (and she's never used!), and I was at a baitshop yesterday, and I was flexing 2 different Loomis rods, and another St. Croix, and they all pretty much felt just like the one I have!!! I'd expected the Loomis ones to be a lot stiffer. Perhaps it's because there's no weight on it, but I can give my non-fly rods a shake and tell the action straight away, with no weight on it. Kind of baffled me. I checked the other rods out, because of the casting situation the night before, and was considering that I may need a faster rod... Especially as that's what I use for all my other fishing, and may well suit me a lot better.
The advice on Al Gores internet says that beginners SHOULD use medium action, for timing purposes. I bought the 7wt line thats on it, because it was cheap and it was specifically a sea line, and it's made by Rio. If there had of been an 8wt, I would have got that, but non was available in my price range (we have a kid coming, so frivolous spending has taken a vacation).

That's a pretty rampant myth, but it's just that, a myth. Rods are not designed to carry an exact grain weight, if they were, you could only ever carry exactly 30' of line in the air, no more, no less, and, certain flies would then "break your rod."

cruzion; if your rod isn't working with the line, you may very well step up or down 1 or 2 line weights. Beyond that, at your beginner level, you will probably have diminishing returns. Going up a weight or two will have a subtle but important change on the feel of the rod, and, for most people, a very *minor* change in feel is all that is required to make things feel better. Of course, getting with a casting instructor always helps too.

I go one wieght higher and one weight lower and never have much problems.(ie) 8 wieght rod 7 wt - 9wt line. The only thing I recommend, when you change the line, do some practice casting (get use to the change) before hitting the water. I agree with LH Dave.... a casting instructor would be a great benefit if your just starting out.. or if you are like I was when I had the kids at home and money was short... just watch someone casting for a while.. brag on their casting abilities and they (a flyfisherman who just became an instant instructor) will generally stop what they are doing, tell you how easy it is and give you a few tips. Most of the time if they are casting decently they will have some good advise share.
Good luck and Tight lines

Man, I've been trolling these boards for 2 weeks to get to watch someone, lol.
I've seen someone fly fishing in corpus twice! Once he was on a surfboard, with a 3ft rod, and not really doing anything, the other time I was driving down JFK, and he was wading...so I didn't get to watch then, either!
An instructor is right out at the moment; the mrs just went and ordered a room full of baby furniture for 6 weeks time :|
We've got like 6 months before it's born. I have no idea why the impetous...but I ain't gonna argue, either :|
She needs to let me go out and find it a Sage 10wt....

Hey Long Haired Dave, it is not a myth! I'm a flyfishing guide in MT and I have actually seen it happen twice! Once with a 7wt rod(Scott) with a 9 wt line and a 4wt(orvis) with a 6wt line. Both rods broke above the ferrules. They did not break in two pieces but split the blank. Both breaks appeared to be identical. Now I can't say for sure if the rods were previously damaged or not but the location of the breaks and the fact that both rods were from different manufacturers and both were overlined make this seem like more than a coincidence. If you'll note, both rods were overlined by two line sizes. My thoughts are that you should use your rod the way it was designed. Sure it will work if you overline it but a screwdriver can work as a chisel and a kitchen knife can work as a screwdriver too. They're just not the most effective tools for the job. Cruzion, I'll be in your area from Dec 1-March 1. Email me at scenicvalleylodge@msn.com. I'd be happy to give you some pointers when I get there. I know what it's like trying to get geared up when a kids on the way. We just make do until they grow up and leave. Just think of all the new technology and gear that will be available in 18 years It sounds like you St Croix is an 8 wt.In my experience most rod makers that designate rods like yours ie 8/9 wt 5/6 wt put the preferred line weight first.

Llama I'm not going to beat this to death, but your idea that 2 line weights will kill a rod just doesn't make sense. If it did, I wouldn't be able to cast 30 feet of LC-13 on my 9 weight (roughly a 13 weight head). I have also cast that same 13 weight head on a 6 weight. Yeah, it's painful to cast on a 6 weight, but, it doesn't break, either. That's a 7 line weight overline.

Shooting heads are recommended to be uplined by 2 weights.

Any of the depth-charge style lines like 500 grain, 750 grain, etc, well exceed the rods they are intended to be used on. However these rods don't break from the lines, either.

Your average "good" caster will already be add 2 line weights of stress easily once they work out more than 30 feet of line-- since more than 30 feet of line weighs more than the designated grain weight. Some people like to say every 10 extra feet adds a line weight, but, that is just a vast over-generalization. Gets the point across, though.

I'm sorry you had 2 rods break, but it wasn't from being overlined.

Rods are made for a certain line weight. If you want to go one line weight up, say a 11wt line on a 10wt fast action rod, it has its advantages, but its disadvantages as well. The heavier line in my experience will throw harder into the wind, but you lose some distance on it. If you load the rod with the same weight line you will can technically throw it further but the winds will affect it more.

Just know that in all honesty, loading one weight heavier is all personal preference, try it out and see if it works for you, its the only way to really know.

As for the rod breaking 2wt overloaded, i believe it, must've been a scott or a sage or something...possibly one of them new winston boron composites;)

Welcome to TX. Just FYI, local etiquette prescribes that you don't bring back an old post just so you can stick your foot in your mouth.

Dave,
I won a TFO TICR-X 8 weight convertible with a two- handed conversion kit. Rick Pope recommended 11 weight line for this rod, so I purchsed Rio Outbound in the 11. I can't believe that Rick would make such a recommendation unless he felt the rod could handle the up-lining. So in my mind, what you state makes sense.
Regards,
Les