Our dear Britton was kind enough to loan me his Indigo propeller so that I could obtain the comparison data. This isn?t Practical Sailor, but I tried to keep the data as even-handed as possible. The same boat carrying the same load, performing opposing runs on the same stretch of river in nearly identical weather. The bottom was neither clean nor fouled, a simple mild coat of slime, which I think to be fairly typical for cruising boats. Ignition timing and carburetor jet settings were not re-tuned for the Indigo series of test runs.
Here we go:
I?ve included the charge data because this is much of my motivation for investigating the propeller swap. The low rpm allowed by the 2blade propeller was limiting the alternator?s charging ability, which is of particular concern to me because only about 10% of my motoring is done at speeds faster than ?river cruise?.
Upon review of the data, Tom Stevens of Indigo offered comment:
I found no appreciable difference in reversing/stopping power between the two propellors. (He was right about the ability to achieve higher RPM at WOT, by the way*)I have always had concern myself about the hulls like Triton and Alberg 30 mainly from an astern performance standpoint. With that 4" wide deadwood, a lot of the astern column of water being pushed by the prop slams into the deadwood, negating your astern efforts. In the ahead direction, I have got to believe that the vibration is reduced reduced with my prop also. Unless your A4 is rather tired, you really should be able to get more RPM at WOT. With my Tartan 34 tied up in her slip, I can get 2000 RPM WOT. My belief is that a static dock trial on most any hull should give about the same results.
Propwalk is another hot topic. In forward gear, I found almost zero propwalk with the Indigo propeller. I left the helm unattended for a full two minutes with no remarkable change in course. By contrast, the two-blade propeller generated enough propwalk that I had to secure the helm for any unattended period of more than ten seconds or so.
The Indigo does generate some propwalk in reverse gear, but it?s still an improvement.
In reverse at roughly 1100 rpm, with the helm centered, the 2blade prop caused a swing of 30 degrees in the 30? distance required to clear my dock. The Indigo prop caused only 10 degrees of swing in the same distance.
In light of these results, I think it?s fair to say that the performance of the Indigo propeller is in fact hindered by the small keel aperture, as these improvement numbers don?t seem to measure up to the improvements found with other applications in which the prop sees clear water. This is no surprise, really. There is improvement, however, and it might just be enough to forgive the additional drag under sail.
*After the propswap testing, I converted to electronic ignition and retuned the engine. I now can achieve 2400 rpm in a static dock test.
