I've been a bowhunter for 50 years and have learned the a larger thin walled arrow shaft is stiffer than smaller diameters with thicker walls. That said I've been pouring over various 9' 10wt fly blanks and I see a trend in every manufacture so far. Although the tip sizes seem to remain constant, butt diameters are consistantly larger on 2 piece blanks than on 4 piece blanks. I thought this was strange until I realized the extra 2 ferrules should actually stiffen the blank - increasing the ERN, load, rating, or whatever the correct term is - and to keep it within the desired wt rating, the butt has to smaller to start with on 4 piece blanks to offset this affect. If correct, I assume there are other diameter adjustments throughout the length of 4 piece blanks. And the multi-piece rods are always a bit heavier than 2 piece in the same wt range and I understand this as well.

But if the end result between a 2 and 4 piece blanks is identical ERNs, what differences would there be in casting and most important fish fighting? Thanks.

ERN is simply power. You cannot assume that a company's 1 and 2 piece blank model is designed upon the same identical parameters, although they sometimes are. If you measured two, and the ERNs were different, it might be the ferrules but it might be something else.

If you were take a blank and cut and install ferrules, you'd hope to use a design and material that wouldn't stiffen that area - your ferrule should flex along with the rod. However, you'd certainly increase the weight, which would reduce the speed (frequency) of the rod and make it feel and act less stiff.

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Thanks Tom but that didn't quite answer my question, I'll try it this way. St Croix makes both a 2 and 4 piece SCIV 9' 10wt blank. Both take a size 4 tip, the 42F9010.2 has a .430" butt dia, the 42F9010.4 has a .410" butt dia - why the diifference in butt diameters for the same length and weight blanks?

You have no way of knowing if they are designed to the same parameters. Remember, there is no standard for what constitutes a 10-weight rod. Those two blanks may be widely different in terms of action and power.

On the other hand, if you were to measure both of them for ERN, and find them to be the same, then the smaller diameter of the 4-piece model may well be to accommodate the ferrule sizes required for a 4-piece model.

But until you know that the power of the two blanks is the same, you can't make that assumption. I have many blanks, from the same manufacturer, in say, 2-piece and 4-piece models, which are totally different blanks and the designers have told me that they weren't designed around the same parameter. In other words, the 4-piece model wasn't intended to be the same as the 2-piece but with more pieces - it was designed with a totally different action and power in mind.

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