Cupping a prop ?

[SEA3PO]

I completed my new diesel installation and now need to install a propeller to match the hull/motor ...My tug is a 21 Seahorse but very much like a R-21..(same dimensions)...the motor I installed is a new Universal 3M-20b....18 actual horses even though they say 20hp...

Now what prop to use ? I have a 13rh16 and it is too much prop...so I am thinking maybe a 13rh12 would be closer to what I need....my Universal reaches hull speed at about 1300 rpm...I think it should be around 2000-2500 rpm. (the Kubota will reach 3500 without any load)

I have been told not to get a cupped propeller as those are for speed boats ...Is this true...Anyone using a cupped prop ? I understand it will lower my rpm by 200-300 ...true?

Sorry to ask so many questions but all the calculators give me different answers...and the prop guys have no idea of our type of boats and full displacement boats in the same weight range...

Thanks
Joel
Seahorse 21 (1980)

 

[SEA3PO]

I reached the 13rh12 number by thinking that by bringing the 13rh16 down would increase the rpm by 1200rpm (plus 300 for each 1 of pitch removed)...the best performance /economy of the Universal/Kubota is at 2500 so I would be very close to that number...(not counting any cupping)

Joel
21 Seahorse tug

 

[NorthernFocus]

Cupping is typically done to prevent/eliminate cavitation. Usually not a problem at low RPM but possible. And yes cupping does effectively increase pitch. Not clear from your OP if you are suggesting re-pitching your old wheel from 16 inch to 12? If so have you talked with a wheel shop yet? That's an awful big change and may not be possible pending material etc. Personally if I could get away without cupping the wheel I'd go that route. One less complication.

 

[SEA3PO]

I found a prop on Craig's list...cheaper than just a simple repitch...yes I was told I could only repitch 2 pitches...so the 13rh12 is a new propeller to me but a very old non-cupped screw...(no cup) with a 1" bore to match my shaft. I hope to get it in a week to 10 days...anxious to see how it does..

Thanks
Joel

 

[SEA3PO]

I was told to clean it with catsup to make it look new again... if that does not do it...I have a sandblaster...

Joel
Seahorse 21

 

[SEA3PO]

What is the standard propeller on a R-21....with the 20hp...I would think it would be close to the same I need.

Thanks
Joel

 

[Captain Nice]

For reference, factory prop on 30hp Volvo 21EC is 13 x 12.125, .060 cup

CN

 

[REO6205]

Call Helmut's in San Rafael, they might have answers for you. Finding anybody that understands displacement hulls in your part of the state is difficult.
If Helmut doesn't have answers for you they'll be able to direct you to somebody who does.

http://www.helmutsmarine.com

 

[TexasEye]

?What is the Problem you are trying to resolve?

Your boat is low RPM and Low Speed.

Cupping increases load on blade and reduces engine RPM and increases Manifold Pressure (Power) so I suspect cupping a small diesel low rpm engine is like going up hill, heavy in High Gear...not a good thing.

Consider this!

1. Ventilation  Cause: Air from surface or exhaust gases as a result of incorrect trim, high speed/steep turns  Effect: Prop slip due to reduced water friction on prop blades  Symptoms: Increased RPM, loss of speed, can lead to cavitation.

2. Cavitation  Cause: Lowered pressure on blades  Effect: Formation of water vapor (boiling) can occur, rupture on the blade.  Symptoms: Cavitation burn on blades

3. Reduction: 

Blade cupping – the cup holds the water on the prop blade and reduces slippage and cavitation. 

Progressively pitched prop – reduces ventilation.