I have a set of Sharrows on my twin 300 yamahas. Definitely not snake oil, though agree they are expensive. Spent over $500K USD on my boat so what's another $10K, really. Didn't buy them for the increased fuel economy, though that was a bonus. I was having issues staying on plane at lower RPMs, tried multiple different props (3 vs 4 blades, mutiple pitches, longer tunnels, etc). Nothing got me where I wanted to be so ordered a set of Sharrows and was beyond happy with the improvement. Better grip was needed for 3-5' following seas, better tracking in the slop (reduced bow steer), and much quieter cruising. Better grip in reverse is also real, docking is moderately better and a bonus.
As for prop repair, I purchased the Sharrow Care support for $700 USD per year to get full replacements for any damage incurred. Sharrow is a small company but sells a solid product and has excellent customer service. I wasn't sold by all the online paid endorsements. Friend of mine have a set on their water taxi and convinced me they're real.
Here's why I bought them: better hole shot, lower planing and cruising speed, and quieter cruising at much lower RPMs. Improved safety and comfort. This graph -- based on data collected in both directions and averaged -- best demonstrates why I bought them. The hole is gone, replaced with a straight line.
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If they ever introduce Sharrows for the 25hp motors, I'll buy one for my kicker. Better maneuverability while getting broadside to a fish is worth it to me.
Barry