what don't I understand about the way Beersmith appears not to calculate the boil ibu correctly when adding a WP?

But then again, I swear that the dress is white with gold trim. ;O)

Hmmm. Alright. I hear what you're saying and I'm not arguing the logic of it. Empirically, it seems like the rise in IBUs should continue to follow the same curve as long as the wort is hot, correct?

If that empirical assumption is correct, then it should show up in real world examples, right? In other words, if I measure my IBUs in a lab, they will either confirm or challenge BeerSmith's model.

BeerSmith doesn't create its own formulas. It integrates accepted formulas and the most widely used is Tinseth. In the Tinseth model, about half of your utilization comes in the first 15 minutes, but that utilization is cut by about half for the next 15 minutes. So, as a single hop contribution, it would seem that you're correct in assuming the calculation is short by about 5 IBUs.

So, how does that play out according to real world lab measurements?

I brew commercially. The rig that provided the following data is 15 bbl steam fired. The boil time is 90 minutes, the minimum WP is 20 min, the stand time is 30 min and the chill is 40 min. So, a kettle hop charge is potentially amplified by an additional 90 minutes.

In these two examples, I extended the calculated time to the total contact time in hot wort. The wort was never below 190F.

Example 1: Dortmund Lager, OG 1.045, Recipe IBU 22. There's a charge at 20 min and at WP. Extended contact time is 26 IBU for an 18% increase. Lab result was 21.7 IBU.

Example 2: Red Ale, OG 1.065, Recipe IBU 36. There's about 17 IBU in the bittering charge. The rest is 10 min and WP. Extended contact time is 48 IBU (the bittering only rises by 2) for a 33% rise. Lab results were 35 to 37.

What this says to me is that BeerSmith gets this pretty correct. Before the addition of WP calcs, I had the hop utilization at 110%. It's at 100% with the WP calculation in place. So, my IBU model is the same as yours.

I know that with WP hops the initial bitterness is more intense, but as fermentation goes on it diminishes significantly. If I had the same wort balance with a 60 minute addition, I know the resulting beer would be over-bittered. I attribute some of this to hop tannins that precipitate and perhaps that the resulting isomers aren't as "sturdy" as they might be if fully boiled.

In the next few months, I'll be moving up to the largest system I've ever brewed on and that may change my recipe design. At the same time, I'll get to review some new research on the subject of >50 IBU hop effects. Hopefully, there will be surprises for me from both!