Calling all Los Angeles SSN-688 class fans

[Tom Dougherty]

Many thanks, Tracy! I too thought I had tried that link permutation!
Frankly, the instructions for revising the relocated link files on the website are...obscure at best.

[Vepr157]

The I3B and I3M are your classic skewed propellers (skewback is an ok term, but I think still slightly colloquial). The I3B is the original 688 propeller, and the I3M is quite similar. I suspect the difference is either that the I3M has improved blade tip geometry or it can mount the hub vortex diffuser.

I've been doing some digging here and there to try and come up with "definitive information" (I'm not holding my breath) and came up with interesting information in SSN-762 Columbus Command History for 1994 (on page 4) of the Command Histories for Columbus on the Naval History and Heritage Command site.

Essentially, Columbus had the I3M propeller and "dihydrals" (page 3, paragraph 3 "narrative") installed in the first half of 1994 and when she reached the Pacific afterwards , she performed extensive testing in September. I'm not sure if the baseline they established was for her or for 688i's in general, indicating a rough timeline of when the I3M became available.`

I suspect that the I3M was developed in the '70s. The official history of DTMB (Where the Fleet Begins) mentions that in the '70s DTMB developed a modified blade tip for the 688 propeller to improve tip cavitation, and that older propellers had their tips cut off and the new ones "bolted" on (must have been flush fasteners or something). Also the SKU of the I3M has a similar number to other 688 propeller developed in the '70s.

The 688I was intended to have a hybrid propeller from the start, as evidenced by congressional testimony (the Navy just said they would have new and quieter propellers, but the artists' impressions show a hybrid propeller). I would guess that most, if not all, 688Is had hybrid propellers when they entered service.

So I would guess that in the case of the Columbus, the newer hybrid propeller was removed and the older I3M installed for testing purposes. The early '90s was a time of intense propulsor development in preparation for the Seawolf and NSSN. The Philadelphia even tested a few varieties of pumpjets.

Jacob