Royal Australian Navy Discussions and Updates

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ngatimozart

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It will be interesting to see what way NZ goes replacing Canterbury, do they go an Galicia size/style ? Or something along the lines of Endurance Class (or a couple).

Although Choules was purchased by the RAN, Phase 4C was not scratched or cancelled, or course new DWP will shed light. If it is still on the cards a local build of 3, 2 RAN and 1 NZ, Galicia sized would go nicely indeed :)
As mentioned by 40DS, the MRV Canterbury maybe with us for a while. I do feel that it may be replaced with something along the lines of an LPD after NZDF have gained experience and institutional knowledge in amphib ops. This is a completely new capability set to NZDF having very little history in amphib ops apart from Gallipoli and a landing in the Solomons during WW2 (NZ Army & USN). So I see Canterbury like a "baby walker" in the context that it is what we are using to learn how to do the amphib ops. They will learn lessons from it and I believe one is that a dock is essential so that is why I think the next step will be an LPD. Personally, I think sooner rather than later and no more than 13,000 tonnes with a minimum of four NH90 sized helo spots on the flight deck, plus good hangarage for said helos and two or three AW109 / SH2G(I) sized helos.
Tried to find, would be interested in the logistic supply behind US ESG's ? but only found same info on front line units. Does anyone have any info on what supply/fuel ships follow the groups around ?

Which also bring up the forthcoming replacement for both the RAN and RNZN for Success/Endeavour replacements ? If such a force and agreement does become a reality between both countries, NZ will also require a ship of same or similar class/size to fill in when needed

Cheers
We think (hope :D ) that maybe the MPSC (Endeavour replacement) might be a BMT Aegir design of around the 18,000 - 20,000 tonne mark. Nothing has been said or leaked but the current NZG has taken a recent liking to tagging onto pommy buys so they might have tagged onto the end of the RN Aegir contract. As 40DS said announcement due soonish.
 

40 deg south

Well-Known Member
As mentioned by 40DS, the MRV Canterbury maybe with us for a while. I do feel that it may be replaced with something along the lines of an LPD after NZDF have gained experience and institutional knowledge in amphib ops. This is a completely new capability set to NZDF having very little history in amphib ops apart from Gallipoli and a landing in the Solomons during WW2 (NZ Army & USN). So I see Canterbury like a "baby walker" in the context that it is what we are using to learn how to do the amphib ops. They will learn lessons from it and I believe one is that a dock is essential so that is why I think the next step will be an LPD. Personally, I think sooner rather than later and no more than 13,000 tonnes with a minimum of four NH90 sized helo spots on the flight deck, plus good hangarage for said helos and two or three AW109 / SH2G(I) sized helos.

We think (hope :D ) that maybe the MPSC (Endeavour replacement) might be a BMT Aegir design of around the 18,000 - 20,000 tonne mark. Nothing has been said or leaked but the current NZG has taken a recent liking to tagging onto pommy buys so they might have tagged onto the end of the RN Aegir contract. As 40DS said announcement due soonish.
Agree that Canterbury has been a brand new capability for RNZN, and they are on a steep learning curve. Dubious about replacement with a superior vessel in the foreseeable future, nice though that would be.

http://www.defence.govt.nz/pdfs/reports-publications/defence-capability-plan-2011.pdf
Looking back at the Defence Capability Plan p33), it envisaged the new tanker entering service in mid-2019, with the Littoral Warfare ship being delivered in 2018. Interesting that the RFI's went out in the opposite order.

I don't know much about the RAN, but that infallible source (!) Wikipedia had this to say
The RAN currently has two ships used for afloat support/replenishment at sea; HMAS Sirius is a fleet oiler, with a limited dry stores capability, while HMAS Success is a general dry stores/fuel replenishment vessel. The navy has initiated a project that will ultimately see two new purpose built vessels enter service by 2020. Sirius was purchased second hand (double hulled to meet new international regulations) in 2005 as MT Delos and converted to replace HMAS Westralia in 2006. Then, as Sirius reaches the end of its service life around 2018, a new vessel will be purpose built. At around the same time (approximately 2015), a replacement for Success will be constructed.
.

If correct, the time frame for the tanker builds matches NZ closely. I still think a joint purchase is far from certain, unless it offers good cost benefits on both sides of the Tasman. Australia might see advantages in a Navantia design, to maintain commonality with the rest of their Spanish-designed fleet. This is irrelevant to NZ. Similarly, Australia may wish to build locally for political/industrial reasons while NZ will be looking for an offshore-built turn-key solution.

ASC - Presenting the Aegir 18A

This could still fit with ASC's Aegir proposal, splitting the build between Korea and Australia. But it only makes sense to NZ if Aust/NZ agree on a common design, and adding ASC into the BMT/DSME mix doesn't raise costs.
 

Volkodav

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I just read a very interesting paper on the thinking behind Japans DDHs and it all comes down to being able to provide (and sustain) the minimum of 8 ASW helicopters per flotilla required to identify, track and engage a high end SSN or SSK. It was eye opening and I will try and link the document once I find it again and can properly credit it.

Reading between the lines of Japans ASW capabilities it is clear that the RAN has lacked the ability to conduct any meaningful ASW operations since the retirement of HMAS Melbourne in 1982 and that the procurement of the Romeos will do little if anything to change this situation unless a way is found to concentrate them i.e. deploy eight or more on a LHD on a regular basis.

Following the Japanese example it can be seen that a helicopter carrier is actually the most efficient and effective way to conduct ASW operations and looking at similar UK and Euro projects from the 60s and latter it is quite clear that the Japanese are not alone in their thinking.

If Australia is serious about ASW then a couple of helicopter carriers may be the only way forward.
 

John Newman

The Bunker Group
Agree that Canterbury has been a brand new capability for RNZN, and they are on a steep learning curve. Dubious about replacement with a superior vessel in the foreseeable future, nice though that would be.

http://www.defence.govt.nz/pdfs/reports-publications/defence-capability-plan-2011.pdf
Looking back at the Defence Capability Plan p33), it envisaged the new tanker entering service in mid-2019, with the Littoral Warfare ship being delivered in 2018. Interesting that the RFI's went out in the opposite order.

I don't know much about the RAN, but that infallible source (!) Wikipedia had this to say
.

If correct, the time frame for the tanker builds matches NZ closely. I still think a joint purchase is far from certain, unless it offers good cost benefits on both sides of the Tasman. Australia might see advantages in a Navantia design, to maintain commonality with the rest of their Spanish-designed fleet. This is irrelevant to NZ. Similarly, Australia may wish to build locally for political/industrial reasons while NZ will be looking for an offshore-built turn-key solution.

ASC - Presenting the Aegir 18A

This could still fit with ASC's Aegir proposal, splitting the build between Korea and Australia. But it only makes sense to NZ if Aust/NZ agree on a common design, and adding ASC into the BMT/DSME mix doesn't raise costs.

I don't know that I'd be holding my breath that Australia and NZ would go down the path of a 'joint' purchase, and regardless of a joint purchase, even ending up with the same class of ships, but hey, who knows!

If the Aegir ships built in SK do turn out to be as cheap as the various figures published for the UK and Norwegian ships being purchased, that's probably the path I reckon NZ would follow.

You would have to imagine that the Cantabria design would be the frontrunner for the RAN after having spent almost all of last year here, the reported commonality of systems with the LHD's and AWD's and added to that the existing relationship with BAE and Navantia, and the need to save job's at Williamstown too.

Obviously the right type of ship needs to be selected for the RAN, but if both those design do meet the requirement, it may come down to who offers the best industry package.

I would imagine that if the BAE / Cantabria bid was selected that the ships may be built in a similar fashion to the LHD's, hulls build in Spain, superstructure built in Williamstown and integrated in Williamstown (probably not much block work for Adelaide or Newcastle with this option).

On the other hand if the ASC / Aegir selection was made, and the proposed 3 hulls were selected, it would see the first two built in SK (and that is where it might be interesting if NZ was involved in the project too) and the third ship completely built here with block work going to both Williamstown and Newcastle.

But since ASC release that PDF regarding their proposal, I do remember reading somewhere (might have been APDR magazine) that ASC had said that their offering would also include the 'commonality' with systems on both the AWD's and LHD's and I also think they said that instead of building 2 in SK and one here, it could be done the other way too.

I remember just prior to the 2013 election the then Government was talking about speeding up the process for the replacement of Success and Sirius, but things seem to have gone a bit quiet since then.

I suspect that we may not see anything from Government till the new 2015 DWP has been released, but then again, in the last little while the new Government has made announcements about the P-8A's, Triton and of course the F-35A's, so maybe we might see some defence related announcements in the soon to be announced Federal Budget.
 

King Wally

Active Member
I just read a very interesting paper on the thinking behind Japans DDHs and it all comes down to being able to provide (and sustain) the minimum of 8 ASW helicopters per flotilla required to identify, track and engage a high end SSN or SSK. It was eye opening and I will try and link the document once I find it again and can properly credit it.

Reading between the lines of Japans ASW capabilities it is clear that the RAN has lacked the ability to conduct any meaningful ASW operations since the retirement of HMAS Melbourne in 1982 and that the procurement of the Romeos will do little if anything to change this situation unless a way is found to concentrate them i.e. deploy eight or more on a LHD on a regular basis.

Following the Japanese example it can be seen that a helicopter carrier is actually the most efficient and effective way to conduct ASW operations and looking at similar UK and Euro projects from the 60s and latter it is quite clear that the Japanese are not alone in their thinking.

If Australia is serious about ASW then a couple of helicopter carriers may be the only way forward.
The RAN could easily flood a LHD with Seahawks and go for town. I sure would be nervous pushing that thing into hostile sub infested waters though. Somewhat feels like a massive target? I will say I have considered it before, I just never knew if it would be a good idea or not?
 

Volkodav

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Verified Defense Pro
The RAN could easily flood a LHD with Seahawks and go for town. I sure would be nervous pushing that thing into hostile sub infested waters though. Somewhat feels like a massive target? I will say I have considered it before, I just never knew if it would be a good idea or not?
The LHD full of ASW helos would be a nice juicy target, a LHD full of troops and equipment would be even juicier. A purpose built helo carrier would be the way to go, it could be smaller and cheaper designed to complement the other escorts.

Looking back to the 70s and 80s it really is too bad money couldn't have been found for a helicopter carrier or three just to get the Sea Kings and Wessex to sea to complement the surface escorts.
 

aussienscale

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I would have thought the Aegir 18 would be a little on the big side for the RNZN ? But probably about the right size for the RAN, along with the Cantabria Class, Berlin etc.

A possible advantage of a joint project between Aus and NZ is the Aegir design, there is a smaller option available which I believe would be suitable for NZ, here is an older PDF from BMT, but still relevant

Cheers

http://media.bmt.org/bmt_media/resources/33/AEGIRPresentedatWMTCMarch06.pdf
 

Volkodav

The Bunker Group
Verified Defense Pro
Could have got Principe de Asturias cheap, even allowing for the cost of a refit.
It would have been a good fit too considering the significant amount of commonality in systems with the FFG7. To be honest something cheaper and or smaller such as an ASW version of Ocean, a Chakri Naruebet, or even a rehash of the Vosper Thorneycroft Harrier Carrier would have done the job quite efficiently and effectively.

Going forward I can't help but wonder if a small through deck design with frigate level sensor and defensive armament outfit in place of some of the ANZAC replacements would be affordable. Acquire three hulls to complement the AWDs (possibly as FFG replacements) and cut the ANZAC replacement project to 6 hulls. Use systems removed from the ANZACs and maybe the FFGs where possible to keep costs down. A DDG, DDH (small helicopter carrier), FFG combination would be far more capable than a DDG, FFG force especially if something like Crows Nest, with CEC fitted, was embarked. MCM and CSAR helicopters would also be a worth while and affordable addition. The helo carrier, if designed like Hyuga, to be capable of striking down an unfolded helo for maintenance or repair would be a force multiplier for any group it was a part of. Fitted with a ski jump or small catapult and basic arrester gear, as well as is large size permitting additional command and control spaces,would make it ideal for operating MALE UCAVs.
 

40 deg south

Well-Known Member
I would have thought the Aegir 18 would be a little on the big side for the RNZN ? But probably about the right size for the RAN, along with the Cantabria Class, Berlin etc.

A possible advantage of a joint project between Aus and NZ is the Aegir design, there is a smaller option available which I believe would be suitable for NZ, here is an older PDF from BMT, but still relevant

Cheers

http://media.bmt.org/bmt_media/resources/33/AEGIRPresentedatWMTCMarch06.pdf
Aussie
I was expecting something smaller too, but Ngati posted the specs from the RFI last year. Let me see if I can dig them out.

I just went through and added up the weights they wanted plus chucked in 500 tonnes for dry stores.

Item Tonnes
Diesel -------------------------------------------------8,000
Kerosene ---------------------------------------------1,700
12 x 20ft TEU @ 20 tonne / TEU -----------------2,400
26 x 10m vehicles @ 26 tonne / vehicle --------2,600
Landing Craft 2 x 65 tonne --------------------------130
Dry storage ---------------------------------------------500
Total --------------------------------------------------15,330 tonnes

So they must be looking at a 22 - 25,000 tonne fully laden ship at least, so it's going to be up near the Karl Doorman class anyway.

Read more: http://www.defencetalk.com/forums/n...y-discussions-updates-4854-196/#ixzz31ABVCMHJ
If ministers/Treasury sign off on a vessel this size, it will be a hell of a lot larger than anything else in the fleet. Possibly similar to the BMT/DSME vessel that the Norwegians have chosen.

I think the fact that BMT/DSME will have previously built five vessels of a similar type (ok, the MARS ones are much bigger, but overall design is similar) will provide a level of reassurance to brass and bean-counters alike. The recent LockMart Canada ANZAC upgrade announcement specifically quoted NZ's procurement chief talking about 'de-risking' the process by using a contractor that had just done a dozen frigate upgrades for RCN.

Of course, we have an election in September, which could kill the whole thing dead.
 
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ausklr76

New Member
Scott Morrison on ABC radio this morning talking about the possibility of a "coast guard" (didn't actually say those words) by the joining of the boarder protection force and customs. If I remember correctly they were talking about it in the 2010 election campaign and now that they are in power seem to be forging ahead with the idea.
Seems like a winner to me.
 

Bonza

Super Moderator
Staff member
Getting back to the subject of the ANZAC replacement, I've been interested to read about the capabilities of the Japanese Akizuki-class destroyer. 5000 tons (over 6000 fully loaded), 32 VLS with a mix of ESSM and ASROC, a 5-inch gun and 8 anti-ship missiles, onboard torpedo tubes and a couple of CIWS, with facilities for embarking a single Seahawk-size helo. Apparently the class is meant to act as ASW screening for the larger Kongos and other surface combatants. The only thing I had questions about was the electronic systems, as from my understanding some of these are indigenously developed Japanese systems and I would imagine the RAN would prefer something American or local.

Anyone else have any thoughts on the class? There could be limitations or details I don't know about as I've only really read a bit of info online.
 

Volkodav

The Bunker Group
Verified Defense Pro
Getting back to the subject of the ANZAC replacement, I've been interested to read about the capabilities of the Japanese Akizuki-class destroyer. 5000 tons (over 6000 fully loaded), 32 VLS with a mix of ESSM and ASROC, a 5-inch gun and 8 anti-ship missiles, onboard torpedo tubes and a couple of CIWS, with facilities for embarking a single Seahawk-size helo. Apparently the class is meant to act as ASW screening for the larger Kongos and other surface combatants. The only thing I had questions about was the electronic systems, as from my understanding some of these are indigenously developed Japanese systems and I would imagine the RAN would prefer something American or local.

Anyone else have any thoughts on the class? There could be limitations or details I don't know about as I've only really read a bit of info online.
Interestingly I believe the Akizuki class is an integral part of Japans 8 Escort, 8 Helicopter concept with five DDs of this and other classes making up an ASW Escort Flotilla along with a pair of DDGs (one or more with AEGIS) and a DDH (Hyuga or Izumo). The flotillas being designed to counter high end SSNs as well as to defend against stand off missile attack. The missing piece is the recognised need for a squadron of 10 STOVL fighters to intercept the missile launch aircraft before launch and a supporting AEW aircraft.
 

Bonza

Super Moderator
Staff member
Interestingly I believe the Akizuki class is an integral part of Japans 8 Escort, 8 Helicopter concept with five DDs of this and other classes making up an ASW Escort Flotilla along with a pair of DDGs (one or more with AEGIS) and a DDH (Hyuga or Izumo). The flotillas being designed to counter high end SSNs as well as to defend against stand off missile attack. The missing piece is the recognised need for a squadron of 10 STOVL fighters to intercept the missile launch aircraft before launch and a supporting AEW aircraft.
That's all well and good for the Japanese but I was more interested in thoughts as to the design in an Australian context. We aren't getting STOVL fighters any time soon.

Would the design be considered modern enough for the ANZAC replacement? Is there much room left for growth in the design? Etc etc...
 

Volkodav

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Verified Defense Pro
That's all well and good for the Japanese but I was more interested in thoughts as to the design in an Australian context. We aren't getting STOVL fighters any time soon.

Would the design be considered modern enough for the ANZAC replacement? Is there much room left for growth in the design? Etc etc...
I would suggest on a preliminary look that they would be too expensive to build and operate (large crew, four large GTs vs GT / DE options) as well as being to specialised in their ASW flotilla centric role. Nice ships but designed to fit CONOPS we don't have. Type 26 or FREMM would likely be better options for a straight MOTS replacement otherwise we should look outside the square and cover off the ASW and Land Attack missions with a smaller number of imported high end platforms complementing the DDGs and then build (locally and continuously) larger numbers of indigenous or modified MOTS GP light frigates / corvettes / sloops / OPVs to cover off other missions currently filled by FFGs, ANZCS, PBs, MCMVs and survey vessels.

Common hull 2000-3000 tonnes and able to land a Chinook and hanger a Romeo.

Light frigate - GT & diesel, full ANZAC ASMD level outfit i.e. CEAFAR, VAMPIR, 9LV, Mk41, ESSM, Harpoon, Mk45 5" etc. Replaces ANZACs in UN deployments, supports DDGs

Corvette - GT & diesel, Harpoon?, CAAM or RAM, 3" or 57mm gun USN LCS Module compatibility. Replaces ANZACs in regional deployments, supports DDGs and LHDs

Sloop - Diesel, CAAM or RAM, 3" or 57mm gun USN LCS Module compatibility. Replaces PBs, MCMVs and survey vessels in regional an global deployments can support LHDs

OPV - Diesel, Typhoon. Replaces PBs in EEZ and regional missions.
 

hairyman

Active Member
I notice that on the Australian Army site there is some discussion about getting JHSV vessels to replace Balipapan's. Is there any thought of any use for the RAN for the JHSV?:dance2
 

cdxbow

Well-Known Member
RAN surface fleet 20 years on

Following on from Volkodav post I have illustrated what a balanced fleet might look like 20 years hence.

The 15 patrol boats, 12 OCV and the 8 patrol frigates come out of the SEA 1180 budget, which is nominally 3-5 billion. These 35 'low end' hulls come out roughly to 3.5 billion, with a bit in the bank for mission modules and upgrades should this be desired. There are some rough figures in the graphic.

The patrol boats are a Damen type patrol boat with flight deck. Robust hulls made of steel. Given their role, I can't conceive why you wouldn't have at least a flight deck capable of handling a small UAV, and preferably a helo. These are going to built mostly overseas to keep the price to 35 million each.

The OCV as illustrated are a 100m trimaran hull, mainly for use in the littorals and northern waters. 6 would nominally be used as OPV for border control, the other 6 would nominally be minesweepers using mission modules, to replace the Huons. They would be highly modular, configurable and capable of supporting stanflex modules. Large flight & cargo deck with extensive accommodation should enable them to keep up with future developments. I would look developing other modules to suit eg ASW. They would need to be built to a price, about 120 m each.

Next come the Patrol frigates, these would perform similar roles to the OCV but designed for heavy seas, such as the Southern ocean. The hulls would have a deep draft & provide good sea keeping in heavy seas. I have said about 200m per hull, which is not outrageous when you consider the Danes built 9 Holland class (3750t) for 1.3 billion. They would use the same modules as the OCV. That would give you a total of 20 hulls supporting what ever mission modules the RAN develops. You could consider making them ice rated for use in Antarctic waters, though this would obviously make them more expensive.

The future frigate I have illustrated as 6 type 26 derived hulls and given them a nominal 1 billion each price tag.

The last two classes are those in the pipeline already, the very expensive AWD & the 2 LHDs.
 

Bonza

Super Moderator
Staff member
I would suggest on a preliminary look that they would be too expensive to build and operate (large crew, four large GTs vs GT / DE options) as well as being to specialised in their ASW flotilla centric role. Nice ships but designed to fit CONOPS we don't have. Type 26 or FREMM would likely be better options for a straight MOTS replacement otherwise we should look outside the square and cover off the ASW and Land Attack missions with a smaller number of imported high end platforms complementing the DDGs and then build (locally and continuously) larger numbers of indigenous or modified MOTS GP light frigates / corvettes / sloops / OPVs to cover off other missions currently filled by FFGs, ANZCS, PBs, MCMVs and survey vessels.

Common hull 2000-3000 tonnes and able to land a Chinook and hanger a Romeo.

Light frigate - GT & diesel, full ANZAC ASMD level outfit i.e. CEAFAR, VAMPIR, 9LV, Mk41, ESSM, Harpoon, Mk45 5" etc. Replaces ANZACs in UN deployments, supports DDGs

Corvette - GT & diesel, Harpoon?, CAAM or RAM, 3" or 57mm gun USN LCS Module compatibility. Replaces ANZACs in regional deployments, supports DDGs and LHDs

Sloop - Diesel, CAAM or RAM, 3" or 57mm gun USN LCS Module compatibility. Replaces PBs, MCMVs and survey vessels in regional an global deployments can support LHDs

OPV - Diesel, Typhoon. Replaces PBs in EEZ and regional missions.
Thanks for your thoughts regarding the Akizuki. I can see the appeal in building so many vessels locally but I very much doubt it's going to happen, and I don't know that I'd like to see future frigate purchases deferred in favour of smaller patrol frigates lest the RAN run in to the same issues as with the ANZACs. But that is just my opinion, and I'm not as well informed as some.
 
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alexsa

Super Moderator
Staff member
Verified Defense Pro
Following on from Volkodav post I have illustrated what a balanced fleet might look like 20 years hence.

The 15 patrol boats, 12 OCV and the 8 patrol frigates come out of the SEA 1180 budget, which is nominally 3-5 billion. These 35 'low end' hulls come out roughly to 3.5 billion, with a bit in the bank for mission modules and upgrades should this be desired. There are some rough figures in the graphic.

The patrol boats are a Damen type patrol boat with flight deck. Robust hulls made of steel. Given their role, I can't conceive why you wouldn't have at least a flight deck capable of handling a small UAV, and preferably a helo. These are going to built mostly overseas to keep the price to 35 million each.

The OCV as illustrated are a 100m trimaran hull, mainly for use in the littorals and northern waters. 6 would nominally be used as OPV for border control, the other 6 would nominally be minesweepers using mission modules, to replace the Huons. They would be highly modular, configurable and capable of supporting stanflex modules. Large flight & cargo deck with extensive accommodation should enable them to keep up with future developments. I would look developing other modules to suit eg ASW. They would need to be built to a price, about 120 m each.

Next come the Patrol frigates, these would perform similar roles to the OCV but designed for heavy seas, such as the Southern ocean. The hulls would have a deep draft & provide good sea keeping in heavy seas. I have said about 200m per hull, which is not outrageous when you consider the Danes built 9 Holland class (3750t) for 1.3 billion. They would use the same modules as the OCV. That would give you a total of 20 hulls supporting what ever mission modules the RAN develops. You could consider making them ice rated for use in Antarctic waters, though this would obviously make them more expensive.

The future frigate I have illustrated as 6 type 26 derived hulls and given them a nominal 1 billion each price tag.

The last two classes are those in the pipeline already, the very expensive AWD & the 2 LHDs.
Why a aluminium trimaran as a OCV ........... seriously. You would be better off with a proper OPV that can patrol and more lower tier frigates than three classes of ship one of which is a light weigh hull that will have less longevity if pushed hard. To give you and idea of cost of alluvium high speed hulls just 8 CCPB (57m) armed with 50 ca MG cost 350 million in 2010/11 figures. Net present value will be higher..
 

cdxbow

Well-Known Member
Did I say aluminium????

I became convinced a while ago that fire and fatigue issues made them unsuitable for warship hulls, unless you were prepared to institute an expensive aircraft like maintenance regimen.

Also the alleged savings in 'through life costs' don't seem to stand up in the real world, indeed the only published figures I could find for maintenance of aluminium hulls showed they were more expensive to maintain than steel, though to be fair it was a bit of comparing apples to oranges.

It's also about 3 times the cost to make per ton of hull, though it makes up a bit because it's about half the weight. Then you add the fire proofing stuff which reduces the weight benefit by roughly a third.

So an all aluminium hull was clearly out.

What about a roughly 50% steel, 50% aluminium ship? See att.
The lower steel hull is very well braced by the 2 decks, making what should be a very rigid structure.
 
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