Sherman 17-pdr (Firefly) - conversion restrictions

Discussion in 'Weapons, Technology & Equipment' started by Gary Kennedy, Sep 18, 2020.

  1. Gary Kennedy

    Gary Kennedy Member

    A thread on another forum asked about the restrictions faced in which Sherman tank types could be converted to 17-pdr (Firefly) standard.

    I was reminded of the the old Sherman Firefly site belonging to Mark Hayward, who went on to publish a book on the vehicle. The site is long gone but the waybackmachine still brings up a few saved pages including the Homepage linked (hopefully) below;

    Sherman Firefly

    One of the things I recalled was his mention of there being no reason given as to why Firefly conversions were restricted to the Sherman V in the first instance, then latterly the Sherman I. The relevant passge is below in blue, and the lines in the bold are the ones that stuck in my memory.

    "PRO file; WO 165/137 contains “Policy and Progress Report, Appendix AA” dated June 1945. It states; “Project: Sherman IC/VC…These tanks are the Sherman I (M4) and Sherman V (M4A4) converted in the UK to mount the 17 Pdr gun and .3 Browning coaxially”. In this report there is no mention of other Shermans marks being converted. Hybrids can be accounted for as it was just a late M4 with a cast hull front. Operationally it made no difference to the unit, so it was probably not recorded separately. Similar statements exist in the equivalent 1943 (December) and 1944 reports (June and December) without mention of other versions. Production Fireflies on other Sherman variants have yet to be conclusively proven by photographs or documents other than as test vehicles or post war museum mock-ups. Weekly reports of tank holdings in 1945 (WO 219/3350, 3353) list only IC’s and VC’s. WO 165/136 contains an August 1944 letter from R M Weeks. It states “Not all the 75mm Shermans are capable of mounting the 17 pr gun.” Frustratingly it does not say which ones! But it is obvious that for some reason, some 75mm Shermans were unsuitable candidates for the 17 Pdr gun. As M4, M4 Hybrid and M4A4 were. The obvious ones this statement must apply to for what ever reasons are; M4A1, M4A2, M4A3 (Stop press new information on a few US M4A3 conversions has just come to light) and M4A6 for which no indisputable proof of C conversion exists."

    There was a cogent, well thought out presentation on the other forum that argued the restriction was due to the need to produce a 'conversion kit' that could be applied to any tank deemed suitable. Key points highlighted are the need to extend the turret bustle (to allow clearance in the turret for recoil and to house the No.19 set), and the changes required for ammunition stowage, which in turn lead to the deletion of the bow machine gun. Due to the difference in the bustle on the Sherman II and III (and there being no Sherman IV in British service to contend with) the argument was that these two types were eliminated from consideration, leaving the I and the V, with the initial emphasis being on the Sherman V.

    In emails I swapped a very long time ago with Mike Taylor, he noted correspondence in WO205/151 that said;

    “It will be noted that during Nov / Dec (1944) all regts should receive two Shermans 17pr per troop, and be maintained on this basis till end Mar. After this date it is most unlikely, owing to lack of suitable Shermans for conversion to 17pr, that any further numbers of these will become available, and therefore we may expect during Apr / May (1945) that Shermans 17pr in this theatre will be wasting, and with no hope of replacement.”

    This change in issue from one per Tp to two is underlined in the few remaining 21AG AFV returns available for Jan-May 1945, and there is a definite downward trend in the number of 17-pdrs stated, accompanied by a steady increase in the number of Ic as opposed to Vc.

    I'm a self-confessed non-tankie and have no basis to dismiss the argument. It sounds convincing but it doesn't seem to have any supporting references or sources. I was intrigued though to know whether, in the years since the Hayward book was published, anything else has come to light on why some Sherman types were excluded from conversion to 17-pdr standard. Was it all about the bustle?

    Dave55 likes this.
  2. Listy

    Listy Well-Known Member

    I recall this book:

    goes into extensive detail on what sorts of Sherman's were needed to be converted. It was down to a certain combination of turret, gun mount and traverse gear if memory serves.
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  3. Ewen Scott

    Ewen Scott Well-Known Member

    There was a new Firefly book published at the beginning of 2020, but I’ve not heard anything to suggest that it adds anything new to Mark Hayward’s research. If it does I’m all ears. The Osprey book adds nothing to Mark Hayward’s book.

    As for a difference in turret bustles being the reason, I’m not at all convinced. The various Sherman turrets can be viewed over at the Sherman Minutia site.
    Sherman 75mm turrets

    The relevant ones are mostly in the top row and apply equally to all the main versions of the Sherman. I’d be interested in seeing that discussion if you could post the link.

    There were however certain features of the Sherman turret that had to be present to make it suitable for conversion:
    Wide gun mantlet, as it required modified to take the 17pdr.
    Hydraulic turret traverse gear from a single manufacturer, Oilgear.

    Most of the 75mm Shermans received by Britain in 1944/45 were remanufactured vehicles including 1600+ M4A4 and 500+ M4A2, which were the two most numerous versions received under Lend lease. They were also the two that the US Army didn’t want after comparative testing. But what made the M4A2 unsuitable remains a mystery. Maybe Britain got to specify parts going into the remanufacturing process, at least on the M4A4.

    There were only just under 2000 Sherman I Composite tanks produced between Aug 1943 and Jan 1944. They seem to feature strongly amongst the Firefly conversions in 1944. It has just dawned on me that most were built by Chrysler, who also built all the M4A4 Sherman V and did the remanufacturing of the the M4A4. Maybe that says something about the parts that went into them. When you look at other types of kit you quite often find that Lend lease to Britain came mostly from particular companies / factories. So maybe there is a connection but that is pure speculation.

    Of course by May 1945 production of the Comet tank was well underway, with the prospect of it replacing Shermans in more than the 11th Armoured Division.

    You can easily lose hours over at the Sherman Minutia site if you want to delve more deeply into Sherman history.
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  4. Gary Kennedy

    Gary Kennedy Member

  5. Ewen Scott

    Ewen Scott Well-Known Member

    Thanks for the link.

    Having read it I’m even less inclined to believe the turret bustle had any influence on the selection of the base vehicle for conversion. Over on Sherman Minutia the reason given for the introduction of the so called high bustle turret was the introduction of the large hatch hull, where the turret bustle required “lifted” to clear the hatches when the turret was turned towards the rear. Hybrid/Composites received both low and high bustle turrets, but the split is unknown. But only about 50 of 1976 were thought to be small hatch.

    When it comes to the Firefly, Mark Hayward, p53, notes that Firefly Ic and Ic Hybrids had both high and low bustle turrets. So the bustle location doesn’t seem to make a difference.

    There were differences in the ammunition stowage between the Vc and the Ic/Hybrid. So the hull shape may offer an explanation exclusion of the M4A1. along with the fact that few were delivered and many were early 1942/43 deliveries. But that doesn’t explain the M4A2.
  6. TTH

    TTH Senior Member

    I will say one thing. In the 30s, 40s, and early 50s Chrysler was known in this country for making rather unexciting and unstylish cars which were nonetheless very solid and reliable. After their unhappy experiences with some less-than-reliable British-built tanks I would expect that the RAC would have wanted only the most dependable tanks from the American arsenal. It would be interesting to compare serviceability rates between Chrysler-built Shermans and those from Ford, GM, and other manufacturers.
  7. Trux

    Trux 21 AG

    I remembered looking at this matter some years ago but could not find my material yesterday.

    The question is perhaps the wrong way round. There were certain requirements as mentioned above.

    To be suitable for conversion the tank had to have:
    A petrol engine.
    A wide M34A1 mantlet.
    Hydraulic turret traverse.
    and of course be available. Only new and unissued vehicles were converted.

    Which tanks had petrol engines is easy but for the rest...!

    TTH likes this.
  8. Ewen Scott

    Ewen Scott Well-Known Member

    I think that is an impossible task. Not all the plants built all the Sherman models. And most models changed a lot over time, virtually only the engine being the common factor in each model over time.

    Looking at Chrysler in particular, they ran the US Govt owned (built in 1940/41) Detroit Tank Arsenal until 1982 when they sold the whole Defense Division to General Dynamics. If they had been producing a sub-standard product, the WW2 USGovt would have stepped in and changed the management. They proved that with other companies e.g. Brewster.

    When Sherman production was rationalised towards the end of 1943, from 11 manufacturers to 3, Detroit Tank Arsenal was one of the three.

    In 1942/43 the plant was the sole producer of the M4A4 with the Chrysler Multibank Engine, 5 petrol engines strapped together driving a single output shaft to the gearbox. It was complicated and the US Army grew to dislike it for its unreliability, using it almost solely in the USA for training.Yet it became Britain’s Sherman of choice with, eventually, about 7200 of the 7499 production run being received. You don’t see complaints about reliability. It was found to be a good engine if properly maintained.

    In the field I don’t think any user sought to distinguish between for example an M4A1 built in one of the 4 factories involved. They were just M4A1.
  9. Ewen Scott

    Ewen Scott Well-Known Member

    Depends what you mean by new and unissued. Those M4A4 Sherman V being received in 1944/45 from the US were largely remanufactured tanks, from the some 1600 vehicles used by the US Army in the States for training in 1942/43. The had been returned to the factory and rebuilt and brought up to the latest standard where necessary. They were a prime source of base vehicles.

    ISTR reading somewhere that by late 1944 there had to be a search amongst the tank parks and units to find suitable vehicles to convert, suggesting some at least were used.
  10. Trux

    Trux 21 AG

    I have found my original source from many years ago. It is of course the David Fletcher Osprey book referred to by Listy in Post 2. Well worth reading.

  11. Gary Kennedy

    Gary Kennedy Member

    Thanks very much to all for the information. One question does occur re the conversions, as to whether these were done centrally, or as was suggested in the thread that caused me to ask here, whether there was some 'kit' that allowed REME Workshops (Tank Troops or Armoured Brigade) to undertake the work? I suppose I would lean to them being done higher up the food chain than Workshop level.

  12. Trux

    Trux 21 AG


    A lot of work was involved. Fitting a new gun sounds simple but there was a long list of other modifications, a major one being to install storage for the larger ammunition rounds.

  13. Chris C

    Chris C Canadian

    Including removing the hull gunner position and installing ammunition stowage in that space.

    By contrast, the change for the M10 was apparently simpler and large numbers of M10s in Italy were converted in-theatre to M10Cs.

    I have Peter Brown's book and I can check if there is anything on this issue here, but it is primarily a photo book.
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  14. idler

    idler GeneralList

    As I understand it, the Firefly conversions (I'm of the opinion Firefly and Mayfly meant the conversion kits rather than the converted M4s and M10s) were carried out by Royal Ordnance Factories. ROF Hayes has been linked to this programme.
    Chris C likes this.
  15. Ewen Scott

    Ewen Scott Well-Known Member

    When you look at the internal photos of the M10c and compare it with the Firefly it is easy to see how much simpler the conversion actually was.

    There is more space around the gun itself and no roof, so the breech can be maneuvered in from above, into a mounting that was designed from the outset to take different weapons, including the 17pdr via fitment of a couple of lugs to the gun itself. Some modification of the mantlet was required via welding in a special casting to reduce the diameter of the hole for the gun. The turret itself was hand turned so no powered turret mechanism to worry about. Although the ammo was reduced from 54 to 50 rounds they were all in the same place before and after conversion - six ready use on the rear wall of the turret and 4x11 round racks in the vehicle sponsons above the tracks. The only difference was the length of the rounds which forced some stowage in between the two racks on each side to be moved elsewhere. No turret basket to get in the way or to be worked around. The whole turret was able to be rebalanced easily by the addition of a counterweight to the barrel just behind the muzzle brake. No rearrangement of the front end of the fighting compartment as the radio operator position remained in place.

    1,128 M10 were supplied to Britain under Lend Lease in 1944 and another 520 in 1945. Of these 790 were converted to M10c between May and Dec 1944 with another 227 following until April 1945. These are reported to have been converted at the Royal Ordnance Factories in Woolwich, Nottingham, Ellesmere Port & Radcliffe. There is some evidence, that I've not seen, that there were an unquantified number of additional conversions in tank troop workshops. I doubt however that that number would be "large", given that 62% of all the vehicles delivered were converted in the UK and that is before taking account of M10s lost before conversion, and that many M10s were still in service in May 1945 with units in Italy.

    In comparison, a Sherman conversion involved a lot of heavy engineering to the steelwork. Sealing up the bow gun mount, cutting a second hatch in the turret roof in most vehicles and cutting a hole in the rear turret bustle to give access to the new armoured radio box fitted to the rear of the turret, as well as modifications to the turret mantlet to accomodate the bigger gun. At least 6 factories were involved in Firefly conversions - ROF in Woolwich, Nottingham (Chilwell), Leeds, Radcliffe in Manchester, Hayes and Cardiff (note many of the same factories as were doing M10c conversions).The internal modifications to the ammo storage were designed to ensure that as many of the 78 large 17pdr rounds (Mk Vc) or 75 rounds (Mk Ic & Hybrid) were stored below the level of the sponsons to minimise the chances of Fireflies brewing up when hit.

    Going back to the Sherman versions selected for conversion and there being no wartime M4A2 conversions, it is curious to note that the M10 was based on the M4A2 engine and running gear and was converted with the 17pdr. So if there is a technical problem with the M4A2 it looks to me like it lay above the tracks.

    The more I look at the problem though, I think it comes down simply to availibility. When the Firefly programme began the most numerous version in the UK had to be the MkV given how it formed the main version in NWE in mid 1944. It then switches to the Mk I. In terms of 1944 deliveries the M4/M4A1 saw the largest numbers delivered (mostly the former). Relatively few (501) of the Sherman III (M4A2) were delivered in 1944 and many of the earlier deliveries had gone to the Med. So once you pick the two most numerous versions, which had differences anyway, why complicate things with a third where there is difficulty getting more stock.

    At the same time Sherman DD production switched from the Mk V to the Mk III in 1944, with eventual British production being 400 to 293, all in 1944. Many of the early DD tanks were recycled and stripped of their modifications.

    The Sherman V was also in demand in 1944 as the basis of the Crab Flail Tank due to its engine power. Although the Sherman III had the same hp it never seems to have been considered for that conversion programme. Again why? The most logical explanation seems to be availibility.
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  16. ceolredmonger

    ceolredmonger Member

    My feeling is that practicality had a lot to do with what got converted once mass conversion was approved at dedicated workshop level. Sending batches of appropriate vehicles immediately on arrival for conversion was administratively easier than trawling around depots and workshops for vehicles which would require shipping and bringing back to spec. Hence the types being relatively late models for that type of glacis (the later 47° glacis wasn't suitable). Hence lots of M4 hybrids/big hatch have 17pdrs.
    I came across this researching M10 17pdr - most were later turret types (vertical rear edge on side) as this was the type arriving. Counter-intuitively later conversions were on earlier vehicles - (angled rear edge on turret side) which had been withdrawn and refurbished.
  17. ceolredmonger

    ceolredmonger Member

    Information on the Firefly/Mayfly conversion kits for Sherman and M10 is sparse, I am dubious they were used. Any hard evidence for Italian 'In Theatre' conversions would be useful. It would make sense to have a workshop in N Africa as L-L vehicles were shipped direct from the USA before issue.
    Division / Brigade workshops were involved in upgrades, as they would be for any vehicle. An ERY Sherman VC gunner told me that they lent on the fitters hard to get the add-on side armour as it was designed for the original 75mm ammunition stowage. Only when all the 75s were done did they grudgingly get some
  18. idler

    idler GeneralList

    Just to clarify the 'kits', I'm thinking in terms of components delivered to the ROFs carrying out the conversions, not a bundle of bits issued to lower workshops.
  19. Ewen Scott

    Ewen Scott Well-Known Member

    The only “second generation” (47 degree blacks) Shermans received by the British in any quantity were 1330 M4A1(76mm) Sherman IIa and 593 M4(105mm) Sherman Ib and they were only accepted reluctantly from about May 1944. The Ib was not suitable as in production form it had no powered traverse mechanism fitted and a different turret mantlet. The IIa had a completely different turret which would have necessitated a complete Firefly redesign, even if the modified M4A1 hull was deemed suitable.

    Re the final point about the M10 conversions, the preferred late models for conversion had the duck billed counterweight attached to the rear of the turret.
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  20. Ewen Scott

    Ewen Scott Well-Known Member

    The reason for prioritising the add on armour on the 75mm Shermans is simple and logical. They were at greater risk.

    The reason the Sherman got a bad reputation for brewing up was, not due to their being largely petrol fuelled, but that their was a lot of ammunition stored in the hull sponsons above the tracks and in the turret. The add on armour was designed to protect those sponson racks. When the Fireflies were converted all bar about 5 ready use rounds were carried below sponson level. The same thing happened in the second generation production Shermans, with the added protection of wet storage, which didn’t get the add on armour.

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