There's nothing wrong with that idea at all Epi however I think you will still have to use a bit of top-up water. Let's consider a 1.050 brew where you want a "Volume into Fermentor" of 19 L (5gal). Let's also assume a 90 minute boil and a pot diameter of 34 cm (13.4 in) and that you use a hop sock to minimise trub. Here's what your numbers will look like in the draft BIABAcus if you did a full-volume mash...
Here's how I'd handle it...
Firstly forget your traditional mashing ratios. Concentrate on putting as much water as you can into your mash. Efficiency is mainly affected by how much water contacts the grain, not when. So, in this scenario, I would sparge with 5 L (1.85 G)and top up during the boil with 2 L (1.3 G) as soon as is possible. Now your volumes will look like this.... If you're wondering why the numbers between VIK and EOBV-A don't add up, remember that you will be adding those 2 L in during the boil.
Remember that there are several different types of efficiency measurements. Here's what the BIABacus predicts for this brew... See how EIK and EOBE are identical and are much higher than EIF? The most important efficiency measurements to focus on are EIK and EOBE as these figures are undistorted by kettle trub. EIK and EOBE tell you if you have a mashing problem. A low EIF however could mean you have a mashing problem, a trub management problem or both.
I'm assuming that when you mentiuoned 50%, you meant EIF. The only situation where a 50% EIF would be normal is in the case of an extremely
high gravity brew so there defintiely is a problem.
I think if you go with the above figures, assuming I have made the right assumptions on the gravity brew you will be doing and your evaporatin rate, your figures will improve dramatically.
Let me know if you have any questions on the above. As usual, I have probably written too much in one hit*.
* I actually have more to say (believe it or not) on how to go about sparging easily with the equipment you have. You'll be able to avoid putting a spigot on your kettle.
As you already know, your kettle is too small to do this. While sparging in the manner you describe will solve the 'Mash Volume' exceeding your kettle size it does not solve the fact that the "Volume into Kettle" is too high to do safely without boiling over.
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