Propellor rpm tied up vs under way.

[Icarus]

There is a difference between being stationary and moving through the water, obviously the being stationary the engine will labour hard because of resistance.
Assuming you have the correct prop diameter/pitch the engine should only reach about ⅔ of of top RPM while stationary
Feathering props aren't as efficient because of their blade shape isn't the same as a in a fixed prop profile..
IMO your prop is underpitched.
I had the same issue with the Autostream 3 blade prop 17" dia on my 40 ft ketch, 40 hp 2:1 reduction, max 2800 RPM...the prop was adjusted with a 20° angle which is equivalent to a 12" pitch, this setting was underpitched , I had to increase the pitch to about 13.5", and motoring at conformable 2200/ Max torque, the boat reaches about 90% of hull speed easily
I can push it along faster , however RPM increase sharply and little is gained.

[Icarus]

Never heard of certifying a prop pitch for engine warranty.

[Rucksta]

Instead of an arbitrary number like 90% or 75% of max rpm try starting with the premise
that an efficient installation has max static rpm at or slightly above max torque.

Factors other than prop selection may affect efficiency e.g.
prop aperture, shading of blades by hull or appendages, environmental conditions.

A static test, stern into the seawall at low tide produces a lower rpm than bow in at high tide.

Engines with max torque close to max rpm are harder to prop tune as the sweet spot is very narrow.
Measurment and test conditions become more critical.

[GILow]

Originally Posted by Icarus

Never heard of certifying a prop pitch for engine warranty.

Absolutely essential, and normal. How can a warranty apply if the engine has an incorrect prop fitted?

[Wotname]

Originally Posted by Rothblum

Have you had this actual experience? From a theoretical standpoint, I think the opposite would happen. There will be less loading of the prop and shaft if the boat is not permitted to move forward. The torque is proportional to the thrust, roughly, and at zero boat speed and max rpm, the flow over the prop is likely to be fully separated or cavitating. The lift on each prop blade will be close to zero, or much, much less than the lift generated by fully attached flow, so the torque will also be greatly reduced. I have learned that what I think should happen is often not borne out, so if you have had practical experience, I can't argue with that.

My actual experience supports the concept that the engine will be more loaded when boat is tied to the dock. My 2GM20 would make max rpm (3600) at WOT when underway in calm conditions with a clean bottom and prop. Tied to the dock, the best rpm at WOT was around 3300/3400 rpm.

My understanding of the principle is a correctly sized (diameter / pitch / rpm) prop will be producing maximum lift (thrust) when the water speed matches hull speed. The angle of attack (AOA) is a function of the prop blade pitch and water speed. Decrease the water speed to zero, the AOA increases and the prop blades stall. The drag of a stalled blade dramatically increases which is what overloads the engine. The water flow around a stalled blade becomes turbulent and this leads to the cavitation.

But I could be wrong...

[Lee Jerry]

A typical propeller curve:


Curves from a family of Wageningen B-Series props:


Prop curves do not account for cavitation.

[skipperpete]

Originally Posted by GILow

Absolutely essential, and normal. How can a warranty apply if the engine has an incorrect prop fitted?

And its one of the items on the 3 pages of the Yanmar commissioning and sea trial checklist for small engines, page one is the prestart inspection, page 2 is startup, idling and adjustments and page 3 is underway observations and tests. Either egt or high idle vs full load can be used to verify correct propeller matching.

[GILow]

Well, after all that the certifier came and went.

Happy enough with the install and gave me some good advice on rubber hoses vs others.

He did run the engine up in the pen but I’m not actually sure if he checked the revs at that point.