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S-Class Torpedo Solution From A Novel I Am Reading

I'm reading a book call Crash Dive by Craig DiLouieed. It's ok so far, just getting into it. It starts out in a S-Boat in October '42 in Australia.

The author describes a way to fire torpedoes thusly.

Speed of ship + 3 = lead angle. So looking at the attached image from the book, a destroyer going 26 knots would have a 29 degree lead angle.

Shoot at 331 and it will hit at 0 degrees. Oh, 1,500 yards or less.

From the book:

Quote:

"Rusty said, “Here’s how we do it out here in the real world on a broken-down S-boat. We set up a zero gyro angle shot; we have to shoot our fish straight ahead. We estimate the target’s speed, add three to come up with a lead angle, and shoot at that point from between a thousand and 1,500 yards. Sometimes with visual contact, but often going only by sound bearings.”

Is this how they did it without a stadimeter? The picture shows how the fish went all screwy and messed up the attack.

I would think that there would have to be some accounting for the target's course/torpedo track angle.

Some calculations I had worked up at one point show a lead angle of 29 degrees for a 26 knot target at a track angle of 90 degrees is about right, if the sub were firing a high-speed Mark 14. For a Mark 10 that an S-boat would be using, the angle would be too small. My math shows about a 36 degree lead.

Set up a right triangle. The right angle is your track angle. The hypotenuse is your sighting line. Side A (target travel) is length 26 (speed of target). Side B (torpedo travel) is length 36 (speed of a Mark 10). A/B = tangent of the lead angle.
A/B = 26/36 = 0.72222...
Inverse tangent of 0.72222... is 35.83765 degrees.

:. the 29 degree lead angle shot will pass ahead of the target.

(Remember when you asked "When will I ever use trigonometry after school?" :))

Doesn't seem reasonable at all. Let's take two targets, one traveling 5 knots and the other traveling 15 knots. Now, since lead angle is proportional to speed, we know the lead angle for the 15 knot boat should be three times as large as the 5 knot boat.

However the formula says 5+3=8º for the slow boat and 15+3=18º for the fast one. 8x3=24, not 18, so I'm calling bull on that one. And of course it takes no account of enemy course relative to your own. That alone makes the rule invalid.

Here is another weird part:

Quote:

Reynolds punched the firing button. “Firing one.”
Frankie shuddered as the torpedo ejected from its tube, a ton of metal and explosives suddenly exiting the boat.
Reynolds counted eight seconds on his stopwatch and pressed the plunger for the second tube. “Firing two!”
Another eight seconds: “Firing three!”
Then: “Firing four! Secure all tubes.”
Four torpedoes in a longitudinal spread. If all went according to plan, the first torpedo would hit the cruiser close to the bow. The ship’s momentum would carry it forward, allowing the other fish to nail her both amidships and near the stern. As the cruiser was nearly six football fields in length, the odds looked good.
The torpedoes streamed in a single line toward the cruiser. At this range, nearly two minutes would pass before the contact-exploders struck the hull and detonated.

Eight seconds between shots!! That is quite the spread, then again, the cruiser was six football fields in length, but it was a Furutaka-class cruiser which is only about 600 feet in length, about 2 football fields. Maybe time to put the book down.

I was reading a book last night, War in the Boats, by William Ruhe, who served on S-37 during the war. He specifically mentioned the "speed plus three" rule as being used.

Quote:

Tex read the bearing from the top of the scope, looked at the Is-Was, and announced to the captain, "I'm using ten knots for target speed. We'll use thirteen degrees for the lead-angle for our zero-gyro-angle fish." Tex had craftily applied the old thumb rule of "speed plus three" to get a workable lead-angle."

The target was sunk.

Quote:

When I asked the captain he range and speed of the targets he said that he'd estimated the range as about 2,500 yards to the big target and it's speed as no more than nine knots. He had used the speed-plus-three thumb rule and corrected the 37's course to lead the target by twelve degrees so as to have straight bow shots for both torpedoes. the targets, he felt, were broadside-to when the torpedoes were fired and the they would produce a spread across the big ship of about fifty yards.

Two hits are reported.

Quote:

Originally Posted by james_nix (Post 2498461)

Eight seconds between shots!!

Ruhe also reports this timing between torpedoes.