Hi Brian, It depends on the boat but on my 34+ (36 LOA and 30.9 LWL) and My setup is Yanmar 3GM30F / Kanzaki KM3P / 2.61 My theoretical basic hull speed is a little over 7.00 knots
When I bought the boat in came with a fixed 3 bladed prop RH 15D X 11P (Probably a Michigan Wheel) installed and 2 bladed 16 X 12 Martec (Eliptec Mark III) in a box. Both of the props are pitched to max out at the engine's continuous rating output of 3400 RPM. With a clean hull the 3 bladed prop makes 6.3 - 6.5 knots at (at the Yanmar recommended 80%) cruising RPM of 2850-2900 RPM and 7.0 knots at WOT / 3400 rpm. With the 16D X 12P Martec Folder it makes 5.8 - 6.2 at 2,850 - 2900 and 6.8 - 7.0 maxed-out. I do have a very clean hull with a recent burnished bottom. If you look at the 3GM30F Service manual performance graph max torque / best fuel economy is between 2,800-3,000 which explains the recommendation. Over-ptiched situation: The Martec prop was in bad need of a rebuild when I bought the boat so I contacted Martec and the guy strongly recommended I'd get new 16D X 14P blades along with the hub rebuild as he felt my existing blades were under-pitched. He was wrong. I did buy the 16D X 14P blades and it made the drivetrain vibrate badly at pretty much all throttle settings and actually slow down (Cavitating) at any throttle setting above 2,700 RPM. When I called back I was given every excuse in the book like bad alignment, bad mounts, slipping clutch, etc, etc. Since the Martec guy would not cooperate, I eventually decided I to just have my old 16 X 12 blades machined to the correct size locally / give it a try and all the issues disappeared. It still vibrates more than the fixed prop but it's reasonably smooth and does add .5 - 1.0 knot to my sailing speed. I did not have too much black smoke, my main "over-pitching" cues were slow down above 2,700 rpms and vibration. Also, I did have a lot of pitting on the prop, the bladed area was covered with small pea size oxidation bumps. You might want to do some testing on a calm and flat day to see if the boats does stop accelerating or slow down before getting to 3,400 RPM. I have also read that Martec is not the only culprit in the selling of over-pitched props, apparently many of the reps (Regardless of brand) tend to do that to make it look like their props are more efficient.. Good luck with it, PM me if you want more details. That vibration stuff is a bitc$$. Regards -Francois Rivard 1990 34+ "Take Five" Lake Lanier, GA
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