Torpedos will likely remain the most lethal anti-ship weapons for their size, and navies like the USN and Western fleets do not place enough emphasis on defences from them (or at least appear not to on the surface). Torpedo attack has been rare in the postwar world, however, the most well known of the few the ARA General Belgrano by two 350kg torpedo warheads sinking the 45 year old treaty limited hull never designed with any torpedo resistance given the priority to guns and citadel armour relative to similar Japanese cruisers and the performance of torpedos in that era.
Defence in depth is the only practical solution to dealing with the threat of torpedo attack, a large torpedo tolerant hull being the last line of defence. The first line of defence would be aircraft (actually satellites if we still have any, but these can't attack the threat platforms), the larger hull being suited to carrying a larger number of them relative to the costs of phased arrays and SAM's and RAM/CIW systems and analogous sub-surface systems. This being an open forum, however, I don't think it is wise to get into any detail concerning the many dozens of factors and scenarios involved in ASW and torpedo defences.
Theory being nice but not always relevant in the real world, torpedos are vulnerable to counterattack from supercavitating weapons since the typical torpedo is not very fast or difficult to detect and localize. I doubt these supercavitating rockets will ever achieve much range, but as defensive weapons and limited underwater travel ASW weapons they have some potential even with basic competent guidance. Again, I could add further here but do not think it prudent. Nothing exists at present as far as I am aware beyond experimental types, but ten years or so from now might see them come into service as a successor or supplement to ASROC et.al.
Reliance upon or in the absence of anti-torpedo weapons, the tolerance of the hull to torpedos provides the extra layer of defence from them along with the limited developed countermeasures available. As conceived, the large hull vessel would factor resistance to torpedo attack into it including avoidance of using the keel as a primary strength member. These ships would not be built large to incorporate a bewildering array of weapons and systems, they would be large to make them damage tolerant as expressed by a large margin of the displacement allocated to that end. It would be possible to squeeze everything into a hull half the size, but for the cost of a hundred million or so in steel lost anyways to greater reliance on auxilliaries, why would you want to given its vulnerability to relative modest attack for a ship costing over a billion and have further problems during mid-life refits?
Although one torpedo hit would likely result in the mission kill of the vessel, a task force would not have one frigate. However, a task force built around one or two air capable amphibious ships would have to withdraw since all the eggs have been put into one or two baskets. The frigate damaged by one to as many as three or four torpedos would have to withdraw, but assuming it is still afloat, the projection force aboard would have some capacity to transfer and rejoin the force at a different point in the operation. Tirpitz survived a 4 ton charge under the keel, and although never fully corrected the ship was available for service some months later. Moreover, torpedos do not always function properly such that launching four to six weapons does not mean all of them will reach the target or even explode at an optimal position under the hull, but that still remains a potent level of attack that could sink all but the largest ships.
The principal problem I have with the typical frigate or destroyer is that they get packed with a full array of systems that are so expensive that they become the de facto major surface combatants. The balance between offense and defence has become skewed out of rational proportion and too reliant on active defences to survive any length of time that warrants the expenditure for what they deliver. There is little defence in depth.
The large vessel would not be an agile ship, but 5000-10000t ships are not either, and when in close to other vessels its expensive armament will often be of no value given minimum range requirements. However, a corvette sized vessel with good seakeeping would be better suited to these subsidiary roles, which owing to the presence of the large frigate, would not require a full suite of systems. The active defences would comprise the modern equivalant of a short range point defence/CIW armament (RAM/Rocket Phalanx) supplemented with various remote weapons like 5.56mm miniguns, 0.50 machineguns, 40mm grenade launchers, 25mm chain guns, the 35mm cannons in storage in Montreal if suitable (?), and similar types, the passive defences focussed around compact citadel pattern protection of the crew stations and the best available PPE. Supplemental aft deck packages would compromise minehunting equipment that has been bought for the MCDV's and similar forms of towed passive/active arrays and towfish as well as an army 155mm autogun motor carriage and deck mount PGM's designed with this application in mind for later transfer ashore after the beachhead is established.
A full fledged task force requiring the deployment of over half the fleet being the extreme or maximum scenario, the basic frigate-corvette package would provide the most common deployment formation. In the absence of an embarked projection force, the extra capacity would permit higher endurance on station and some improvement to habitability for long deployments, and as the exception but not the norm mid-deployment crew exchanges to double the length of distant ship deployments. Although it could be argued that it is not likely that the navy would ever engage in any significant scale of amphibious operation like a generic Falkland Islands scenario, building the fleet to that end would allow it to deploy self sufficient formations approaching that level as part of a larger coalition task force if only to ensure the Dieppe tragedy is not repeated. The creation of this fleet would take a couple of decades, but it would have a functional role as soon as the first few vessels entered service.
It would prove politically easier to get the funds for building one ship every two years in sub-groups of two to four (ie.keeping contract awards well below the $10B threshold), interspersed with contracts for corvettes and lighter armed patrol ships, arctic patrol corsair/tenders, additional auxilliaries (if only two JSS are built now), and coast guard icebreakers and patrol vessels (which might have some commonality with the naval size equivalents), et.al., as well as the submarines (which might be produced through a different program approach of a government owned agency given the specialized nature of submarine design).
I have no doubt you are well aware of the complexity of this subject matter and that there are a large number of factors not mentioned here, but I have actually thought the matter through in depth over the course of many years. Their would doubtlessly be specific problems/difficulties I have not foreseen (been on the receiving end of opinions of people who know nothing of what they speak of from any direct experience too many times), but little that subsequent detail rework can not resolve. Ship design has always been an exercise in compromises to competing demands. I have no personal biases regarding any of it or anything related at all to career, business, or political posturing.
Quick Estimates (Ice class simple rating by 1st year feet of ice with qualifications, most double bow form)
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12 CPF's, 3-4 AAW DDH >>> 10 SOE Frigates [Class 8+] $15B
2 AOR/JSS >>> 2 JSS, 2 JSS/SOE [Class ?/8+] 3
1 Sealift (?) >>> 2 Bulk Liquid/Container Sealift [Class 0+] 1
6-10 AOPV's >>> 6 AOPCT (system commonality to corvettes) [Class 5+] 2
4 Diesel Submarines >>> 10 Arctic/coastal/task force Submarines 6 (8 boats)
12 MCDV >>> 10-16 Corvettes/OPV's (No.s depend on CCG, AOPV, et.al. factors)[Class 1+] 3
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6-9 Icebreakers >>> 10 Icebreakers, slightly larger-enhanced/commonality to AOPCT [Class 5/8+] 3
80 Various Patrol >>> 16-20 OPV's/Corvettes (& smaller craft incl. joint NR small h/c) [Class 1+] 3
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Navy Only 30/20=$1.5B/year
Yes, it is a lot of money, little of it allocated since this is not any actual government plan, automation (which does not always produce the savings hyped ahead of time, as per my own experiences with CNC manufacturing) may help manning issues but they will remain a problem, but this is the general framework around my own views on attaining a three ocean fleet with a nominal capacity to project a basic expeditionary force. However, I think the real problem is the people running the show few of whom appear to have anything but their own immediate interests in mind, and it is not really a question of funding.
It also is not a question of just the navy, but the air force and the army working to integrate a substantial element of their forces into the fold. Their are workable near term and long term solutions, but if the air force's recent game with pulling a Chinook is any indicator, it will be yet another Dieppe all over again.
