Dinghy prop 4 blade super weirdness

[nfbr]

Originally Posted by Squanderbucks

The OP is starting off with a wrong first assumption. Two props with the same pitch and diameter but different numbers of blades are not equal. If they were why switch.
In general the fewer the blades the faster the boat but it’s load carrying ability is reduced. So a heavy boat needs more blades to increase the push. But since the greater push from more blades requires reducing the pitch to allow the engine to reach its fully loaded operating RPM’s.
For a n inflatable etc it should be propped so it reaches top RPM when carrying its normal MAX load. Running a boat at full load and not propped to allow full RPM willl shorten the longevity of the engine.
For most boats like my powerboat it should be propped to allow the boat to reach max RPM. Plus 100-150 RPM at full load. This allows for decrease in performance when bottom gets a little less than pristine new paint.
Even if you don’t run your boat at top RPM the proper Prop is determined as above. Else you will be overloading at all rpm’s along the curve.
Going from three blades to four requires either reducing the pitch or diameter or a little of both.

Correct. Else it will lug and not hit full RPM.
This is hitting rev limiter. Its cavitating or slipping. Like when a prop gets sargassum or weed on it. So the reverse behaviour of what you suggest.

[Dooglas]

Originally Posted by nfbr

seems to ...... hit rev limiter when over 50% throttle.

A 4 blade prop of the same diameter and pitch as a 3 blade prop is certainly not the same prop. I'm with Rumrace - too much prop.

[pcmm]

Hitting the rev limiter at less throttle doesn’t make sense. Your new prop INCREASES the load on the engine, unless you’ve spun the hub? In which case the prop would be slipping on the hub allowing your engine to over rev…

[nfbr]

Originally Posted by pcmm

Hitting the rev limiter at less throttle doesn’t make sense. Your new prop INCREASES the load on the engine, unless you’ve spun the hub? In which case the prop would be slipping on the hub allowing your engine to over rev…

Yep - why I posted. Got me scrathing my head.
Gopro time I think.

[pcmm]

Originally Posted by nfbr

Yep - why I posted. Got me scrathing my head.
Gopro time I think.

You can check by turning the prop by hand (please remove the spark plugs first!!!!!!!!). Put the outboard in gear and turn the prop. If you can turn the prop without forcing the engine to turnover, it’s spun on the shaft.

[nfbr]

Originally Posted by pcmm

You can check by turning the prop by hand (please remove the spark plugs first!!!!!!!!). Put the outboard in gear and turn the prop. If you can turn the prop without forcing the engine to turnover, it’s spun on the shaft.

That was first test. It doesn't turn. Plugs in, kill switch out. Suzuki EFI's are high compression. Pretty sure it's either cavitation or ventilation.
I'm wondering if the extra torque is sucking air down at transition speeds and ventilating the prop. Depth is optimal, factory plate submerged etc, but maybe....

[Grattaway]

I would definitely put my money on cavitation
Fins might assist solve the problem or 3 blades with different pitch to achieve your objective