You have deliberately or otherwise misapplied the graph. Here is the section you took it from:
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The key sentence is
: "an analysis of the effect of merchant vessel speed on the safety of independent ships". You have ignored the benefits of convoys which had a lower loss rate than independently routed ships even ones as fast as the Victories. The cut off speed was 15 knots with ships below that speed required to travel in convoy. Obviously they were areas of the oceans were conveys were not a regular feature but by the tine the Victory ships appeared in mid 1944 the convoy system was well established.
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Ships in convey were in fact safer than even fast independent ships. As a consequence fast troopships with speed of 15 knots or greater and oil tankers with minimum speed of 14.5 knots were conveyed in their own fast convoys. From the same page as the graph:
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Clearly stating the most successful measure was escorted convoys.
The 15 knot limit was established in by the British early in the war as compromise between the loss rates verses the much faster turnaround time of independently routed ships. It proved to be a good number but in no way does it mean that fast independently routed ships were invulnerable. Interestingly the Cabinet ordered a reduction in minimum speed for 15 to 13 knots with unfortunate results. After some moths the 15 knot limit was reinstated.
From
Roskill The War At Sea 1939 - 1945 Volume 1 : the Defensive:
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The real reason that no Victory ships were sunk by U Boats is simply opportunity . By the time Victory ship appeared the U boat war had changed dramatically due to the Normandy invasion. Atlantic shipping was no longer under serious threat. At the time of D Day there only 31 Victories had been delivered and they were on the west coast of the US. Only 104 of them were delivered in 1944 along with 112 Haskell Class Attack Transports. The vast majority of these Victories stayed in the Pacific although some did serve as troopships in the Atlantic. There was virtually no opportunity for a U-boat to actually encounter a Victory in the open ocean.
The radically different nature of the U-boat war after D-Day is discussed in a different chapter of the same document you reference
HyperWar: Antisubmarine Warfare in World War II [Chapter 7]
The entire document is well worth a read.
This sub-thread needs to fade away but I wanted to "go on the record" to show in a simple diagram the point I was trying to get across. (Please excuse the "chicken-scratch" handwriting of mine in the labels on the diagram, but I had hand surgery five days ago and still have bandages on my writing hand.)
This diagram has a target (T) that is a Victory ship traveling at 18kts. The submarine (S) can make 17kts at best. (Typical for a VIIC doing long runs.). The sub is running on the surface. This one-knot advantage has the following effect: If the sub spots the target (moving to the right) at a point on the perpendicular of the target's track, or further to the right on that track, the sub will never be able to approach the target to get into a firing position, unless the target passes very close to the sub. This is the simple geometry of the problem. Unless the target's track passes within a short distance to the sub's present position, it cannot get into a reasonable firing position. This is because the sub has to track and approach the target on a hypotenuse of the right triangle (the longest "side") formed by the target's track, the perpendicular line to the target's track, and the sub's direct track (hypotenuse). Since the sub's track will always be longer that the target's track, the sub will never be able to "catch up" to the point he can attack at a reasonable range and angle. If the target has an even larger advantage (e.g. warship running at 20kts) the hopelessness of the situation is apparent. (This is why reminiscent books of WWII submariners and ships logs, etc. are replete with situations where the sub "could not approach target and get into firing position.") Running down the hypotenuse means that the sub's velocity vector has a component in the direction of the target track as well as a "forward" component to get ahead of (or equal to) the target's position. The target's vector, on the other hand, is a straight line down his chosen track.
There are 3 scenarios in the diagram based on the perpendicular distance to the target when spotted: 12nm, 8nm, 4nm. (The horizon for the top of cargo ship from a sub's coning tower is about 12-13nm in daylight, by my calculation, although this varies greatly depending on sea state, mist, fog, etc.) The position of the target is the tic-mark on the target track with the time and travel distance along his own track. The semi-circle is a 3000-yard "firing envelope" for the target (about 1.5nm). I chose this number simply because the examples in the US submarine torpedo firing manual rarely list any distance beyond this. The little "dot" in the semi-circle is half this distance perpendicular to the target, or about 1500yds. This is where the sub captain will "aim" his vector or, rather, where I draw his vector aiming here. This would be his "ideal" firing position, if the sub can reach this. The sub's position at this time is the tic mark at the end of its vector line.
Note that the longer the perpendicular distance to the target's track the greater the sub will eventually lag behind the firing range semi-circle. Conversely, the shorter the perpendicular distance to the target's track when spotted the more likely the sub will get into a firing position within the semi-circle. In the 4nm case, the sub can just get to the edge of the semi-circle (3000yds from target). At shorter distances than 4nm it will have an even better chance to "shoot." (This is because the distance to the track is smaller and, thus, the hypotenuse's length is closer to the target track's length.)
The diagram was made with meticulous attention to scale accuracy. (Note the smudges where I was off by 1 or 2mm and had to re-draw.) Looking at the big picture, imagine a submarine in the center of a big circle whose radius encompasses the sub's ability to spot a target at any direction. The actual angle that the target is approaching is immaterial, as the diagram is circularly symmetric and you just have to draw a line from the sub perpendicular to the target track and rotate the diagram sheet and you have the equivalent problem. The above arguments mean this: If the target has a speed advantage over the submarine, then the right two quadrants (right semi-circle of the big circle) are off limits as to the possibility of approaching the target to get into a good firing position. The greater the target speed advantage the closer its track must be to the sub position in order to get even a chance at a torpedo shot. The German's understood this fact well and is why they worried that, eventually, the majority of convoys might be all 17-18 knot Victory ships which would significantly reduce their success rate at tracking and attacking a convoy due to simple geometry. In this case their subs would have to spot the target in the "left" half semi-circle of the maximum spotting whole circle. Due to the vastness of the ocean and the randomness of finding convoys, this could not always be guaranteed and would probably not be the case in about 50% of the time, due vagaries of navigation and the state of the sea. They would always be unable to get in front of the target (convoy) if first spotted in the right semi-circle. In some cases this would apply as well to targets spotted well into the left spotting semi-circle, if the speed of the targets are much more than 1 knot greater than the sub's maximum speed and their track is perpendicularly far enough away from the sub.
