Lately I've talked to a few guys who put a 90" C203 off of a 180K on a 185 on floats. Has anyone seen this done and was it done legally? I've heard this from a a few unrelated sources and they swear by it so it makes me curious.

I know tip speed would be an issue. I've never run the numbers but I know my 86" Mac is plenty loud and the common knowledge is that tip speed may kill some efficiency.

Somewhat relevant to what your asking:

180K came with an 0-470-U, 8.6:1 compression rated @ 2400RPM, the prop is a 90" C204 seaplane prop. I would love to hang this on my -U/TS, however due to noise tests, cannot get it field approved above the 2400RPM that it is rated for, which defeats the purpose. Ive dug fairly far down this rabbit hole for 180's, it always ends at the noise issue. That said, I hear it pulls pretty good above 2400RPM [emoji6]

Very interested to know if anyone has gotten this prop approved at higher RPM. It is absurd to me that it cannot be approved, but an 88" Blackmack is OK haha!

Side note, seems like the 180K POH actually says 2450RPM for TO, been awhile...If there was ever a C203 approved on the -U, that is news to me.

I wanted to put the O470-U and 90" in our 180J. That combo on the K model is a realy good performer, esp if you then wind it up to 2600 RPM.. Couldn't find anyway to get it done. Did the TX O520 with 8:5-1 and MT 3 blade instead. Goes pretty well. I couldn't see how the
90" would work on a 185 at 2850?

Skalywag wrote: the prop is a 90" C204 seaplane prop. \

I think you're right, too many numbers rattling around in my head.

I think you guys are also correct concerning noise. I'm sure that would be a problem if someone went the field approval route. Quite honestly what interests me is cruise. If I'm running around without the pod on, I'm usually turning 2200 RPM. I am guessing that long blade swinging slow like that might pull the bird through the air at a respectable clip.

AEROPOD wrote:...I think you guys are also correct concerning noise. I'm sure that would be a problem if someone went the field approval route. ....

Besides noise issues, propeller approval has a lot to do with harmonics.
Different Continental engines have different counterbalancing dampers.
Incompatibility with those dampers is the reason why a C203 Mac isnt approved on a Ponked 470.
Might be that different length blades change those harmonics also.
So while an 82 or 88" 204 (dunno if they even make one) might be OK,
maybe a 90 incher wouldn't be.
That prop business is as much of a black art as a science IMHO.
Only the wizards know how it all works.