Hi, Is this thread still being monitored by the group? I have some more information on the DH Hydromatic propeller Type 45/4 used on Mk Vc Spitfires by the RAAF in the Pacific during WWII. One aspect of their design was that they only had 3 barrel bolts in the hub between each blade ( ie a total of 9 barrel bolts for the whole assembly on this 3 bladed propeller) rather than 4 bolts between each blade which was the more common layout for the larger hydromatic propellers (ala Hamilton Standard 23E50 on a C-47). Attached is a close up of a DH45/4 hyromatic prop installation on a No.79 Squadron RAAF Spitifre. The blades were of aluminium construction too, not laminated timber as per the Rotol blades.
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How are they different?

How are they different? …. To try to answer powerandpassion‘s question from the 10th April the main differences can be seen in the attached scans from two Hamilton Standard Manuals, these being Service Manual No. 110D for the 3D40 counterweight or bracket type propeller and Service Manual No. 140C for the 23D40 hydromatic propeller. As far as the two split hub halves are concerned the lower section in these images is the part closest to the engine which slides over the splined shaft of the engine first. It has a fairly similar appearance for both. The forward facing hub half is quite different however. The 3D40 counterweight prop basically incorporates a large hexagonal opening through which the three arms of the counterweight brackets can pass, whereas the 23D40 hydromatic prop forward facing half has a threaded round opening onto which the hydromatic dome is screwed. (see diawan’s entry from the 4th April 2015 with his attached picture of the trailered Merlin which is fitted with a hydromatic prop). Another notable difference is that the prop hub halves on the 3D40 are held together with 6 hub bolts whereas there are 9 hub bolts used on the 23D40 prop hub. The larger hydromatic prop hubs such as on the 23E50 and equivalent (see above Merlin prop image again) use 12 hub bolts. Hope this helps to show some differences.[ATTACH=CONFIG]237142[/ATTACH][ATTACH=CONFIG]237143[/ATTACH][ATTACH=CONFIG]237143[/ATTACH]