Dinghy prop 4 blade super weirdness

[nfbr]

20hp Suzuki on OC tender 350

Swapped from 3 blade to 4 blade. Same brand solasz
Same pitch and dia

Now it won’t plane, seems to cavitate and hit rev limiter when over 50% throttle.

Engine position not changed.

I’m very confused. The 11” and 8” pitch 3 blades are fine
The 8” 4 blade won’t plane.

Any ideas?

[Rumrace]

You should drop diameter
You are over propped. You’ve increased your blade are to increase torque but lost the pitch. Keep the pitch loose diameter.
Check max RPM of engine and never allow pitch or diameter exceed it.

[rslifkin]

Originally Posted by Rumrace

You should drop diameter
You are over propped. You’ve increased your blade are to increase torque but lost the pitch. Keep the pitch loose diameter.
Check max RPM of engine and never allow pitch or diameter exceed it.

If he's hitting the rev limiter it's probably not over propped.

[nfbr]

I should add
With driver only I can get on plane if gentle
With 5 ppl, it just starts and spins at 6-7knts, bow up
Overrev light flashing

[kapnd]

So what’s the question here?
Now that you’ve proven that you bought the wrong size propeller, it’s time to go back to one that you know works.

[nfbr]

Originally Posted by kapnd

So what’s the question here?
Now that you’ve proven that you bought the wrong size propeller, it’s time to go back to one that you know works.

Wrong size? It's the same size. Just 3 vs 4 blades.

[rbk]

Sounds like it’s cavitation causing less resistance and limiting out. Lower pitch might solve it, rarely see 4 blades on small outboards, they tend to act more like a blender rather than a prop.

[wholybee]

Originally Posted by nfbr

Wrong size? It's the same size. Just 3 vs 4 blades.

Changing from 3 to 4 blades is a change in size. Was there a problem you were trying to solve with the change?

[kapnd]

Read Dave Gerrs book, “propeller handbook” if you’re interested in knowing why it doesn’t work.

[OldManMirage]

There is a ton of informative sites out there but the general rule is when going from 3 blade to 4 blade you drop 1" - or more - in pitch. Where did you get your new prop ? Many times dealers will be able to help figure out what you need.

[rslifkin]

I wonder... Does the prop have a rubber hub? If so, it could be bad and spinning on the hub at higher power settings.

[nfbr]

Originally Posted by wholybee

Changing from 3 to 4 blades is a change in size. Was there a problem you were trying to solve with the change?

Kids growing into older teens means 5 ppl and gear in dinghy. Aiming for better planing transition. 10 to 8” pitch helped a lot.
Next step was 4 blade.

[vpbarkley]

You might consider a Piranha propeller. They have interchangeable blades in different pitches you can try. Performance is close to that of a stainless steel prop.

https://piranha.com/

[pcmm]

Originally Posted by nfbr

20hp Suzuki on OC tender 350

Swapped from 3 blade to 4 blade. Same brand solasz
Same pitch and dia

Now it won’t plane, seems to cavitate and hit rev limiter when over 50% throttle.

Engine position not changed.

I’m very confused. The 11” and 8” pitch 3 blades are fine
The 8” 4 blade won’t plane.

Any ideas?

Your results aren’t really surprising! Adding an extra blade isn’t just a case of “more is better”. You’ve added 33% more blade area but you haven’t compensated by either dropping diameter or decreasing pitch a bit. Go back to the place you bought the prop and have them actually do the calculations

[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.