Propellor specification

[DoubleEnder]

The bank manager is beginning to get upset, we are buying lots of gear! Next purchase is a new engine, a Kubota based 3cyl 20hp diesel. Lovely: small, light smooth, warrantied. I can hardly wait, but I'm puzzled over how to specify the prop. We have a full keel and a keel-hung rudder with the prop revolving in an aperture cut out of the rudder's leading edge.
The old prop is 15" diameter, pitch unknown. I want to replace it with a similar diameter fixed 2 blade, that will nestle behind the deadwood when we are sailing. Questions are what pitch, and what gearbox ratio? I can have 2:1 or 2.6:1

Any thoughts?

Thank you

 

[Peterduck]

Firstly, the Kubota will give you joy. I start my engine to show off for visitors because it starts so quickly!
Measuring the pitch on an existing prop is a ticklish business. Calculating which prop you need is even more so. The best reference on the subject is Dave Gerr's Propeller Handbook. It can get very deep [it's a very deep and arcane subject] for the neophyte, so I would recommend the section on propellers in Kinney's "Elements of Yacht Design". It has a number of nomograms to use to get very close to the specification that you need. It also has a quick way and a more accurate but longwinded way to do the necessary calculations.
Marine screws become more efficient the slower they rotate, so get a 2.6:1 reduction in preference to a 2:1. The prop aperture will determine the maximum diameter, leaving an inch clearance between blades and deadwood.
Peter.

 

[dickh]

Go to the propellor suppliers who will work out the correct size for you once you give them all the boat information.

 

[SwedishLass]

I went through the mill last year replacing a broken 2 bladed feathering with a three bladed. I have the little beta 482 13hp based on the Kubota unit with a 2:1 reduction. I settled for the 3 bladed 12x9 recommended by beta and have not been disappointed. Revs within 200 of max, speed within 1/2 knot of max hull speed when bottom clean.
(The black arts and advisors gave me numbers 13x10 13x9 13x8 12x10 12x9 in varying proportions)
On the two blade issue can you ensure it will lie behind the deadwood?
This link may prove useful it came up with all the right numbers for me.
http://www.alberg30.org/maintenance/MechanicalPropulsion/Propeller/propcalc.xls

Grahame, Fairey Atalanta A65 www.hillgf.freeserve.co.uk/joann