Mark's got the right idea, but it's too late for me. I've already got twins. I just wish they had designed the boat with twin keels to protect the twin props! I have seriously considered adding prop-protecting keels forward of the props. Maybe someday when my bank account exceeds my needs.
As PG mentioned, we had a couple of encounters with submerged logs this winter that created a pronounced vibration on the port prop at cruise RPMs. Three years ago I had a 'significant encounter with a submerged object" that caused $8000 damage to my running gear. Ever since then, I have had a subtle vibration in the port running gear that the mechanics could not trace. This latest impact only made that vibration more pronounced.
We were having the bottom scrubbed by a diver on a Friday and decided to have the 20x18 props pulled at that time. Once they were on deck, I knew we made the right decision. The stbd prop had noticeable damage to its tips and leading edges.
I delivered them that day to the only Prop Scan Prop Shop in CA, which just happened to be in my own backyard. Steve Thomas of Thomas Marine Propeller promised to have them scanned, tuned and delivered to my cockpit on the following Tuesday so my diver could reinstall them before leaving town. By Tuesday morning, I had a computer analysis of the as-found and as-tuned results in my email. The pitch of these 1976 date-stamped props remained at the 17.7 degrees found on the stbd prop to retain the WOT 2800 RPM.
Once they were installed, I took the boat out for a trial run to see if there was a difference. I immediately found that ALL vibrations were gone. I proceeded to the San Joaquin River to record the speeds at all signficant RPMs to see if there was a cruise speed improvement.
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Interesting, never heard of the prop scan before, but it makes sense!! I have to admit, I got to the part in your post where you took it out on the river for a test run and I thought you were going to say you hit another log.
I Love a happy ending.
I get some odd vibrations that seem to come and go, I haven't had my props checked in years, so maybe I had better check around Seattle for a shop that does that kind of prop balancing.
Thanks Al
Larry B