Day or night strategic bombing?

[stona]

The RAF conducted extensive testing of no allowance shooting with upward slanting cannon apart from the recent Defiant practice so the Luftwaffe practice should have come as no surprise.
View attachment 366171 View attachment 366172

Numerous British specifications of the inter war period required various systems for 'no allowance' shooting.
It was the increase in armament, which led to the armament being positioned away from the fuselage and in or on the wings that led to the abandonment of the requirement. This went hand in hand with a huge increase in the speed of fighters too.
Schragemusik was not a surprise to the British, they understood perfectly how it worked. It just took some time for the evidence that the Luftwaffe was using such a system to be gathered and confirmed.
Cheers
Steve

Edit. Thinking about the fixed angle of a Shragemusik installation, it would not always have been exactly a 'no allowance' system, dependent upon the speed of the fighter. We can imagine that, to an observer in the fighter, the stream of projectiles might appear to curve backwards or forwards. In a perfect 'no allowance' solution they would appear to take a straight line to the target. Given the short range at which the system was used this is hardly relevant, but it is a valid point and might help in an understanding of exactly what 'no allowance' means.

 

[pbehn]

For information

Asperger syndrome - Wikipedia

 

[stona]

Re Pathfinder Force

I though some might like to see what Donald Bennett wrote about Harris' visit, in October 1942, to the recently established PFF at Wyton. By the way, there goes another Harris myth, that he never got 'out and about' to see the men under his command
Bennett wrote of the visit that it was

"an inspiration and a stimulant of the greatest value...the personnel of the PFF are all most impressed by the interest you are showing in their work and in their results. It is a tremendous incentive to them."

This is fairly typical of Harris. He opposed the formation of an elite force within Bomber Command, but once it was a 'fait accompli' he supported it and backed it. He was even solicitous of Bennett's personal safety. In August 1943 when Bennett asked for an exemption from the rule which forbade AOCs from flying on operations so that he could act as Master Bomber on the Peenemunde raid, he was told in no uncertain terms that he was far too valuable to be allowed to go.

"You must give up the idea of going on operations for an indefinite period. We depend very largely on your personal knowledge for the exploitation of Pathfinder methods during the next critical months...and you know too much. If you are taken prisoner the Germans have their own means and methods of extracting information involuntarily, and I have no intention of allowing to operate these methods on key personnel."

There was no answer to that, though Bennett did try unsuccessfully to be allowed to fly a Berlin raid the same year.

Cheers

Steve

 

[stona]

I would simply cite Harris's .....opposition to guidance from his superior Freeman and his opposition to providing aircraft for Coastal Command.

A little more on this. When Harris took over as Deputy Chief of the Air Staff, before taking over at Bomber Command, he was involved in naval matters. He re-established his working relationship with Tom Phillips, now Vice Chief of Naval Staff, in November 1940. It was to him he offered his thoughts on convoy defence against U-boats and long range aircraft. He also urged the adoption of the catapult fighter. Soon afterwards he was advising his old friend Portal along similar lines and also suggesting ways of protecting coastal shipping.
The question of how far the bombers of Coastal Command should be allowed to attack objectives not directly connected to naval warfare was raised, as was the far more fundamental question of who should own the Command. In November there was serious discussion about transferring Coastal Command to the Admiralty, prompted by Beaverbrook, but the Admiralty had for some time been seeking to re-establish control over what it called 'shore based aviation' meaning Coastal Command. In December the Defence Committee's decision was that operational control, but NOT command would rest with the Navy. It was Harris and Phillips who formed a small sub-committee to work out how this might be done. The remarkable cooperation which was to exist between Coastal Command, still part of the RAF, and the Navy is testament to their work.
Harris had no doubts about the critical importance of the U-boat war, however much he disagreed with the Navy of the roles of air power in fighting it. He accepted that the essential minimum of reconnaissance and striking effort should be put into the convoy areas to keep the submarines down and make them 'less resolute' in their attacks. He foresaw the advantages that ASV radar would bring and, as he told Professor Blackett (personal scientific adviser to C-in-C Coastal Command) that the B-24 would be the best aircraft for the job. He was correct on both counts. However, he made it clear to both Portal and Phillips that the correct use of the bomber force was to attack the German and French ports and factories where what he called 'the kernel' of the problem was to be found, not to waste thousands of hours of flying over wide ocean spaces.

Everything that Harris did as DCAS was to build up Britain's strategic bomber force to carry the war to Germany. He, Churchill, Portal and other senior figures in Bomber Command were reaching the conclusion in December 1940 that Bomber Command would need to target industrial areas as a whole, in a general bombing campaign, to inflict serious damage on Germany.
In December 1940 Harris protested to Freeman about the 2:1 ratio in planned fighter to bomber production planned and, worse in his eyes, the 8:1 ratio planned for purchases from the USA.
Next he had a go at Portal, asking why two fighter forces, one for day, one for night, were required now that the Luftwaffe had switched to night bombing. He also detailed various 'diversions' affecting Bomber Command and taking away many of its crews. This was just about anything that didn't involve bombing Germany, minelaying, the Mediterranean war, Coastal Command duties, the war against the beams, even the increasing demands of Dalton's SOE !
He would write

"One very sure way of losing the war is to reach a condition of stalemate whereby the whole populace becomes fed up with going on with it. If we continue to postpone the development of our bomber potential by such means...we may well find ourselves on the road to this particular kind of ruin."

It worked, in January 1941 the Prime Minister authorised the expansion of Bomber Command.

That's enough for now. These things are never simple, never black and white. Harris argued for the bomber force because he believed it gave a good chance of winning the war, not from some selfish or petty reasons. As I said some time ago, a lot of nonsense is written about him, arguably Britain's most controversial war time commander.

I read an article in the Independent on one of the anniversaries of the Dresden raid that showed such a breathtaking ignorance of the issues involved, even why the raid took place, that I sometimes fear for the memory of not just Harris, but the men who served under him.

Cheers

Steve

 

[pbehn]

That's enough for now. These things are never simple, never black and white. Harris argued for the bomber force because he believed it gave a good chance of winning the war, not from some selfish or petty reasons. As I said some time ago, a lot of nonsense is written about him, arguably Britain's most controversial war time commander.

Great post Steve, however Harris was made a controversial figure in a process that started even before the war ended. My uncle was in Bomber Command in the early years of the war, invalided out with health problems caused by lack of heated suits. He joined up to bomb Hitlers black heart out and that is what he did. I live close to the ex RAF airfields of Middleton St George and Croft, talking to the elderly in the past who actually lived through it I have not met anyone who didnt have 100% support. I was born in 1959 and have seen the anti Harris rhetoric rise through my lifetime.

Churchill said this July 14 1941
We ask no favours of the enemy. We seek from them no compunction. On the contrary, if tonight the people of London were asked to cast their vote whether a convention should be entered into to stop the bombing of all cities, the overwhelming majority would cry, "No, we will mete out to the Germans the measure, and more than the measure, that they have meted out to us." The people of London with one voice would say to Hitler: "You have committed every crime under the sun. Where you have been the least resisted there you have been the most brutal. It was you who began the indiscriminate bombing. We remember Warsaw in the very first few days of the war. We remember Rotterdam. We have been newly reminded of your habits by the hideous massacre of Belgrade. We know too well the bestial assault you are making upon the Russian people, to whom our hearts go out in their valiant struggle. We will have no truce or parley with you, or the grisly gang who work your wicked will. You do your worst and we will do our best." Perhaps it may be our turn soon; perhaps it may be our turn now.
National Churchill Museum | Winston Churchill's Do Your Worst; We'll Do Our Best Speech


If Russia had fallen there is absolutely no doubt that Hitler would have flattened and starved the UK if he could, Harris was doing his job, I have a dim view of those who change the scope of that job to suit the sensibilities of those born much later.

 

[stona]

Great post Steve, however Harris was made a controversial figure in a process that started even before the war ended. My uncle was in Bomber Command in the early years of the war, invalided out with health problems caused by lack of heated suits.

That lack of equipment was certainly NOT Harris' fault either, as your uncle would probably have known.
In 1939 Harris was writing letters to Ludlow Hewitt and any body else he thought could help about all sorts of equipment for both aircraft and aircrew he felt were needed. This varied from windscreen wipers, cockpit heating and armour plating, to bombs and bomb doors, all of which were needed or should be improved. At this early stage he described the Hampden as 'typical Handley Page junk' and his relationship with that company would go downhill from there

Following the Heligoland Bight fiasco he was describing the Hampden's rear gun mount as

"a rickety, ill designed, badly made piece of work which would not pass muster as a component of a bit of agricultural machinery."

In November 1939 he was pressing Tizard over why the RAF still had no self sealing tanks

"Presumably Farnborough are still trying to make our self sealing tanks sing 'God Save the King', and meanwhile our people will die for lack of them, while our enemies live."

This month he also pressed Ludlow Hewitt about flying clothing which he considered inadequate and incapable of keeping crews properly warm. He also tackled Farnborough over body armour for air gunners. He had several run ins with Farnborough, which, he thought, invariably dragged its feet, had no concept of the urgency that the war had brought, and was effectively allowing RAF crews to die needlessly.

"Half the essential equipment which we so urgently require is not at our disposal merely for the reasons that most of the authorities, and Farnborough in particular, cannot get it into their heads that half a loaf is better than no bread, that they will invariably make the best enemy of the good, and the hopeless delays are always occasioned while they try to put final and usually unessential touches to some already reasonable serviceable article."

This down to earth attitude certainly rubbed some people up the wrong way, but it was the men (and women) of the RAF whose best interests he was fighting for. In later years many of those for whom he fought do seem, somehow, to have understood this.

Cheers

Steve

 

[pbehn]

That lack of equipment was certainly NOT Harris' fault either, as your uncle would probably have known.

For those who were doing the job things seemed different to the view from the top. When he joined up my uncle had seen very few aeroplanes and hadn't seen a monoplane. A Hampden to him in 1939 was the ultimate in high tech, it is only when you see better "tech" that you know what is what. He was off flying and in the observer corps before he learned that heating a suit was possible.

 

[tomo pauk]

Excellent stuff, many thanks

 

[Shortround6]

Well, one does wonder about some of Harris's biases or prejudices.

At this early stage he described the Hampden as 'typical Handley Page junk' and his relationship with that company would go downhill from there

Following the Heligoland Bight fiasco he was describing the Hampden's rear gun mount as

"a rickety, ill designed, badly made piece of work which would not pass muster as a component of a bit of agricultural machinery."

From about 1930 to 1939 Handley page had made TWO aircraft that made it into RAF service aside from the Hampden.
The Heyford and the Harrow. Both built in rather small quantities and in peace time so some sort of acceptance inspection should have caught build quality problems.
I certainly don't know about the quality of the build of those two aircraft but the designs share as much responsibility with the AIr Ministry as they do the Handley Page design staff as do the Hampden and Halifax.
I also would think but could be wrong, that bits of kit like gun mounts, would have to be approved by the air ministry regardless of who made them.

From WIki so could very well be false:
" He also had a low opinion of the Navy; he commented that there were three things which should never be allowed on a well-run yacht "a wheelbarrow, an umbrella and a naval officer"

Given this opinion of the RN (if true) the idea of letting them use land based bombers for anti-sub use has a slightly different shadow on it.

 

[stona]

Well, one does wonder about some of Harris's biases or prejudices.
From WIki so could very well be false:
" He also had a low opinion of the Navy; he commented that there were three things which should never be allowed on a well-run yacht "a wheelbarrow, an umbrella and a naval officer"

Given this opinion of the RN (if true) the idea of letting them use land based bombers for anti-sub use has a slightly different shadow on it.

He may well have said something a lot like that. It's not exactly original, but was in common usage among fishermen and merchant sailors long before Harris' time. It usually lists those three items as the most useless things to have on board, but there are other versions.

"Four things shalt thou not see aboard a yacht for its comfort - a cow, a wheelbarrow, an umbrella and a naval officer"

"Nothing is less useful on a Schooner/goulette than a ladder, a gardening hose, or a Navy Officer"

The origins are lost in the mists of time.

I'm sure he had as low an opinion of some officers both of his own service and the other two.
The facts are that he had a very good working relationship with Phillips and the Naval Staff in general. He did have a very different view from that of the Navy about the roles to be played by air power in the fight against the U-boats, but he always acknowledged that it was a fight that had to be fought. He considered it a better use of his aircraft to attack them where he knew they could be found, in the factories, dockyards and bases.

Cheers

Steve

 

[Zipper730]

That lack of equipment was certainly NOT Harris' fault either, as your uncle would probably have known.
In 1939 Harris was writing letters to Ludlow Hewitt and any body else he thought could help about all sorts of equipment for both aircraft and aircrew he felt were needed. This varied from windscreen wipers, cockpit heating and armour plating, to bombs and bomb doors, all of which were needed or should be improved. At this early stage he described the Hampden as 'typical Handley Page junk' and his relationship with that company would go downhill from there

Following the Heligoland Bight fiasco he was describing the Hampden's rear gun mount as

"a rickety, ill designed, badly made piece of work which would not pass muster as a component of a bit of agricultural machinery."

In November 1939 he was pressing Tizard over why the RAF still had no self sealing tanks

"Presumably Farnborough are still trying to make our self sealing tanks sing 'God Save the King', and meanwhile our people will die for lack of them, while our enemies live."

This month he also pressed Ludlow Hewitt about flying clothing which he considered inadequate and incapable of keeping crews properly warm. He also tackled Farnborough over body armour for air gunners. He had several run ins with Farnborough, which, he thought, invariably dragged its feet, had no concept of the urgency that the war had brought, and was effectively allowing RAF crews to die needlessly.

"Half the essential equipment which we so urgently require is not at our disposal merely for the reasons that most of the authorities, and Farnborough in particular, cannot get it into their heads that half a loaf is better than no bread, that they will invariably make the best enemy of the good, and the hopeless delays are always occasioned while they try to put final and usually unessential touches to some already reasonable serviceable article."

He was blunt and gruff, but he made many good points: I would not have told the contractor his work was mostly junk as I would want to cultivate a relationship that's remotely decent (people who like you tend to cooperate more), though I would tell them if I thought it was unsatisfactory.

The idea of providing heating for crews, body armor were common sense ideas, as were self sealing tanks. I personally also agree that I'd rather have a product that does something adequately well than one that does nothing. I'm not sure if we even realized that in the USAF.

 

[stona]

To be fair his remarks about the Hampden were for his fellow officers and the Air Ministry, not Handley Page himself or the company.
Harris certainly had an intense dislike for Handley Page himself and consequently the company. Harris would later suggest that it would be a mistake to cease production of the Hampden, which he described as "a simplified construction type", in favour of the "over complicated, under-defended Halifax". In his campaign to stop Halifax production and increase Lancaster production he could be disingenuous, once claiming that it carried only half the Lancaster's bomb load, which is something of an exaggeration. He described Handley Page as "not an aircraft manufacturer, just a financier, with all that implies and more". He wrote a formal letter to the Air Ministry about the Halifax III, which was achieving "no ponderable improvement over its predecessors and whose continued short range was adversely affecting the employment of the Lancaster when the two types were operating together". I could give many more examples. Once again, he was only trying to get the best tool for the job in hand, and that was the Lancaster. He was presented with any number of perfectly reasonable and valid reasons why Halifax production could not be converted to Lancaster production without a substantial loss in overall heavy bomber production, but to no avail.
Nobody would describe Montgomery as an easy man either. Many successful commanders share some of these traits, Dowding was another, though Harris had doubted that he could work with Dowding when he was told that he was to become his SASO at Fighter Command. Dowding had been Air Member for Research and Development when Harris was in Plans and the two had never seen eye to eye. In the end Park went to Fighter Command and Harris to the Middle East, so any friction was averted.
On 7th September 1940 Harris wrote Dowding a nice letter saying
"All my fellows are full of admiration for the magnificent efforts of the fighters, and are almost getting to the stage where they are beginning to hope that you will leave a few for us."
There was mutual respect and Harris stood by Dowding following his removal after the BoB. Indeed it was Harris who would later call him
"the only commander who won one of the decisive battles of history and got sacked for his pains."

Cheers

Steve

 

[Graeme]

For information

Asperger syndrome - Wikipedia

My youngest son has Aspergers - which makes life interesting.

 

[Shortround6]

To be fair his remarks about the Hampden were for his fellow officers and the Air Ministry, not Handley Page himself or the company.
Harris certainly had an intense dislike for Handley Page himself and consequently the company. Harris would later suggest that it would be a mistake to cease production of the Hampden, which he described as "a simplified construction type", in favour of the "over complicated, under-defended Halifax". In his campaign to stop Halifax production and increase Lancaster production he could be disingenuous, once claiming that it carried only half the Lancaster's bomb load, which is something of an exaggeration. He described Handley Page as "not an aircraft manufacturer, just a financier, with all that implies and more". He wrote a formal letter to the Air Ministry about the Halifax III, which was achieving "no ponderable improvement over its predecessors and whose continued short range was adversely affecting the employment of the Lancaster when the two types were operating together". I could give many more examples. Once again, he was only trying to get the best tool for the job in hand, and that was the Lancaster. He was presented with any number of perfectly reasonable and valid reasons why Halifax production could not be converted to Lancaster production without a substantial loss in overall heavy bomber production, but to no avail.

So Basically Harris would lie when it suited him?
"...under-defended Halifax"
Now the initial Halifax had no top turret and in fact had an almost identical armament to the Wellington (also under defended?), however by the time the Lancaster reached squadron service the Halifax was being fitted with a similar if not identical top turret and since the belly turret on the Lancaster was essentially useless the MK II Halifax had an identical defensive armament in guns and layout to the Lancaster. The Halifax later got a 4 gun top turret and reduced the nose armament to one gun. But the nose turret was usually one of the first things to go on both planes when special purpose conversions were done. Very late Lancasters did get different armament but that was well after the Halifax vs Lancaster controversy.

"half the Lancaster's bomb load, which is something of an exaggeration."
Well this one certainly depends on version and desired range. Normal max bomb load for the Halifax was 13,000lbs (at almost absurdly short ranges) compared to 14,000lbs for the Lancaster at a useful range. At somewhat longer ranges we have.
Halifax II.................5250lbs.............1660 mile range..................1500lbs........2100mile range
Halifax III................7,000lbs............1985 mile range..................3000lbs........2510mile range
Lancaster 1&III.....10.000lbs............2250 mile range..................7000lbs........2680 mile range

Now the need for ranges of 2000 miles for most missions is doubtful (under 700 miles from Bristol to Berlin so 2000 miles allows for some large doglegs) and we are back to the incendiary vs HE bomb load problem. Once you shift to incendiaries the load becomes more volume limited than weight limited.
I would also note that the Halifax II figures given above are for Merlin XX engines as originally introduced while the Lancaster figures are for later model Merlins with 200-300 more HP depending on altitude.

Halifax III, which was achieving "no ponderable improvement over its predecessors and whose continued short range was adversely affecting the employment of the Lancaster when the two types were operating together"

Well, this is somewhat shown to be, shall we say, in error by the figures above (taken from the RAF data sheets) and I would note the Halifax MK III had cruising speeds 10-18mph faster than the MK II and within 1-2mph of the Lancaster.
Cruising speeds being both most economical and max lean mixture cruise.

Max take-off weight for the Halifax II was 60,000lbs, for the MK III was 65,000lbs and for the Lancaster 68,000lbs which helps explain the Lancaster's superiority.

Please note that I am NOT claiming the Halifax as as good as the Lancaster, it wasn't. It just wasn't anywhere near as bad as Harris claimed and if fitted with the big vertical stabilizers earlier operational losses might have been lowered sooner than waiting for the MK III.

Edit: I would also note that some pilot's notes for the Lancaster give different take-off weights. Early Lancasters may have been limited to 63,000lbs max take-off weight before later modifications. Subtracting around 5000lbs from the fuel/bomb-load brings the early Lancaster much closer to the Halifax.

 

[swampyankee]

The very action of forcing the bomber to fly higher also reduced their accuracy, making the sort of precision bombing US doctrine had advocated, impossible to achieve.
The Americans were well aware of the problem. Attached is a page from an 8th AF Bombing Accuracy analysis which shows both the decrease in accuracy with altitude and the trend to ever increasing bombing altitudes that was forced on the bombers in an effort to reduce losses.
View attachment 366013


Cheers

Steve

It's interesting the B-24 seems to show better worse accuracy. One wonders if it's a real difference or an artifact of poor test methods.

(Error corrected: better changed to worse; I thank Wuzak for pointing it out)

 

[wuzak]

It's interesting the B-24 seems to show better accuracy. One wonders if it's a real difference or an artifact of poor test methods.

It's actually worse - except above 24,000ft where the B-24 gets more bombs inside a 1,000ft circle, but less inside a 500ft circle.

At all the other altitudes the B-24 has less bombs inside both a 500ft and 1,000ft circle.

 

[stona]

The problem Harris had with the Halifax, apart from its operational limitations when compared to the Lancaster, was that it killed more of his men than the Lancaster did. He thought that they were being needlessly lost, the solution being more Lancasters. Very often it was a concern for the men under his command that drove his outbursts. He was rather uncomplimentary about the Stirling too.
He was a man frustrated by the red tape of the RAF and Air Ministry, particularly once the war had started.

"One gets the impression that the automatic reaction to every request is a negative...I have long ago adopted in my own Headquarters the principle that in any matters of everyday routine no-one has the power to say 'no' to a unit. If they think it ought to be said they have to come to me. Yet all our urgent operational requirements seem to go meandering through a maze of offices and, no matter how urgent, to be subjected to endless scrutiny, delay, obstruction, idle chatter and superfluous minuting by whole legions of departmental subordinates, some of whom quite obviously haven't the vaguest idea what it is all about."

And while they did this, his men were dying.

I have never been able to establish where his utter contempt for Handley Page originated. I thought that it might be in the first war, but if it was I've yet to find where.

Cheers

Steve

 

[stona]

I may now no a reason for Harris' low opinion of the Navy. When playing rugby for his school's Ist XV (Allhallows Grammar School) he suffered a 156 - 0 defeat to the Royal Naval College Dartmouth

Re:Montgomery. Harris first met him in 1928 when he was sent by Trenchard and Salmond to the Army Staff College at Camberley, effectively to argue the RAF's case for air power on what was an influential course. The then Lieutenant Colonel Montgomery was on the directing staff, and this is what Harris made of him.

"...a very good soldier who will make a damned good general. Incidentally he is the first soldier I have come across who has a proper grasp of the vital role of a tactical air force in land battles."

When in 1929 Harris moved up to the 'Senior Division', he was the only RAF representative, but now the Navy was also represented by Commander John Leach. It seems Leach was not held responsible for earlier results on the rugby field and the two became, and remained, good friends. Leach (and Phillips) were both killed aboard HMS Prince of Wales in December 1941.

Cheers

Steve

 

[Shortround6]

The problem Harris had with the Halifax, apart from its operational limitations when compared to the Lancaster, was that it killed more of his men than the Lancaster did. He thought that they were being needlessly lost, the solution being more Lancasters. Very often it was a concern for the men under his command that drove his outbursts. He was rather uncomplimentary about the Stirling too.

For a person who didn't have an unreasoning hatred of a aircraft company (or it's owner) the first rational step would be find out why there was a difference in losses, then see if the difference/s could be fixed. Step 3 would be to see if the fix/es could be implemented quickly or not and only in the last case, (fix/es could not be implemented or not without undue delay) should the extreme measure of switching production be implemented. Harris did his men no favors by trying jumping to step 4 right away.
There were at least 3 changes that reduced the loss rate of Halifax's unrelated to the engine switch.
1. was the change to big fins which eliminated the rudder lock over problem. This may have been major.
2. was the slightly extended wing tips. This may have been minor and more to due with higher operating altitudes?
3. Early Halifax's had a rather strange segregated electric system. 3 engines had generators (1000watt) each of which ran a different part of the aircraft. depending on which engine (or generator) was lost different parts of the aircraft (and different turrets) lost power. Later Halifax's got three 1500watt generators, a bit more battery power and the ability to run all parts of the aircraft from 1 or 2 generators. How big a part of of aircraft losses this was I don't know. Both late model Merlin powered and Hercules powered Halifax's got the new electrical system.
other changes that might have affected late war losses were some Merlin Halifaxes got Melrin 22 engines could be run at 7lb boost lean compared to 4lbs lean for the Melrin XX. A bit more power in engine out situations. And the provision in 1944 of a nitrogen purge system for the fuel tanks to reduce the fire risk.

I will also repeat that Harris (and others) didn't seem to grasp that the goal of anti-sub patrols wasn't to sink to U-boats but to prevent the U-boats from sinking ships/shipping. A bit like Barrage Balloons didn't knock down very many enemy bombers but forced them to bomb from higher altitudes with less accuracy or pick other targets.
A "spotted" sub, even if not attacked could be plotted and instructions given to convoys to route around known u-boat locations. Subs could be forces to submerge (even without dropping bombs) and movement and spotting restricted. U-boats could only patrol for a certain number of days and the more time spent under water the less productive the patrol would be (a smaller area swept by the u-boat=less chance of finding a convoy or shipping). Attacking U-boats at sea would also be safer for the attacking planes as they only had to worry about the AA guns on the U-boat vs the AA guns defending the port/shipyard locations.

 

[Elmas]

Not to say that sinking a sub at sea meant 50 trained seamen killed.

As if a bombing attack to a pen or to a shipyard could destroy 50 skilled workers together with one half-finished sub.

But probably, when a better idea was formed about the real damage caused to the shipyards

U-boat Archive - British Interrogation Reports

things changed.