Dick O'Kane lead angle question

[Rockin Robbins]

I received a PM from taukarrie that was so good the I think it needs its own thread:

your first O'Kane method tutrial that is. first off, very well done. a lot of my most recent ciritcal questions were answered by watching it just once. one question though...

when estimating your lead angle you, as a rule of thumb, go for 15 degrees. but you also said that it doesnt really matter what that number is. my question is why would you choose 15 degrees and why does that not matter?

it seems to me that the lead angle estimate is crucial for this method and to regard it so lightly confuses me.

thanks for your time.

OK, I'm going to do this without reference to my video, so I may be correcting my own error here. Here is where language becomes very important and I believe that it's important that we be careful to use the same terminology as the military.

Why? Because knowing that lives are at stake, they take great pains in using carefully defined terms that can't be mistaken for each other. They are extremely disciplined in their use of terminology. They don't tolerate sloppiness too well. In this case the sloppiness may be mine, so let me set things straight.

Taukarrie, I think the problem comes with the definition of "lead angle." The lead angle is the difference between the shoot bearing or aiming bearing, along which you sight your periscope at the target, and the bearing the torpedo travels to impact the target you're looking at. Of course, if the TDC just sent the torpedo up the aiming bearing, by the time the torpedo got there the target would be gone! But you've told the TDC that the target is on a course at right angles to your own, traveling at 8 knots. From that the TDC does its analog calculation mumbo-jumbo and says, well, I have to send the torpedo 10º (or whatever lead angle) ahead of the aiming bearing.

You see, Dick O'Kane came about because of my stupid mistake. I was using Gutted's great chart, which he had just adapted for fleet boats. It looks like this:


OK, so you pick the target speed out of the first column, travel across the row to the column reflecting the torpedo speed and there's your lead angle. We'll take a target traveling left to right at 8 knots. For a fast torpedo your lead angle is 9º. You subtract 9 from 360, where you want the boom to occur, and sight your periscope up the 351 bearing. So your lead angle is 9º and your aiming bearing is 351º. When the target crosses the wire, you push the button, the torpedo zips up the zero bearing and kaboom!

So I did it wrong when I first tried it. I looked across the 8 knot line and picked the wrong column. That gave me the 14½º lead angle for the slow torpedo. I aimed up the 345½º bearing and pushed the button. The torpedo shot up the zero bearing just as it was supposed to but there was no boom. The target got there after the torpedo did. I was a bit miffed.

Then I had an idea. If I could screw that up, so could everyone else. And that chart only had columns for fast and slow Mark 14s. How about Mark 18s and Cuties? Heck, that's two more columns to make twice as many mistakes! The chart is great but we are not. How can we fix that?

It just so happens we have a built-in chart on board that automatically knows what torpedo and what torpedo speed we have selected. It NEVER makes a mistake. It's called the TDC. When we're using the chart, we tell the TDC where we want the torpedo to go and WE pick the lead angle. Let's do it backwards!

Instead, let the stupid people pick the aiming bearing only! The TDC can calculate the lead angle for us and send the torpedo up any bearing it needs to to make a boom. In the Dick O'Kane bearing we pick an arbitrary aiming bearing with a goal of getting somewhere close to a boom at the zero bearing. That's where the rules of thumb come from: under 15 knots, pick 350 or 10 as the aiming bearing, knowing that the lead angle is going to be somewhere around 10º. If you have a faster target than that pick 20º and aim up the 340 or 20 bearings, depending on which direction the target is coming from.

Now this aiming bearing means nothing at all to the solution! Let me explain. Our target is coming left to right at 8 knots. I'm going to choose my 10º offset and aim up the 350º bearing. The TDC is set for 8 knots, AoB 90º-the 10º correction for an aiming bearing of 350º. But that 10º isn't the lead angle! It is the correction for the aiming bearing. We're letting the TDC set the lead angle. And if you check Gutted's chart you can see that the TDC does its magic and calculates the lead angle as 9º. So it adds 9º to the aiming bearing of 350º and sends the torpedo up the 359º bearing. BOOM!

So aiming bearing: where you point your scope. THAT's what we arbitrarily pick by rule of thumb in the Dick O'Kane method!
torpedo track bearing: the path of your torpedo. That is calculated by the TDC much more precisely that we can with a chart!
lead angle: the angle between those two bearings

Clear as mud?