Two Blade Fixed Pitch Propeller

[Frank 157]

Our Heavy 38 ft Cruising yacht has a 3 bladed Max Prop  but it came with a spare 2 blade fixed pitch propeller of the same Dia and Pitch (18 x 17) The 2 blade unit has a comparatively generous blade area and I am wondering if it would be more efficient than the Max Prop . My reasoning is that the Max Prop does in comparison does not seem to have an aerofoiled chord profile and has little if any twist in the blade. I am no expert on this topic but I assume  compromises have to be made for feathering units. My experience with  2 blade folding propeller on a sail drive is they seem to have plenty of "bite" assuming the motor and prop are matched to the vessel of course. 

We can "hide" the 2 blade unit behind the fat keel so I don't think there will be any extra drag under sail. 

I understand that we would have to fit the spare and do sea trials to be sure of any difference, however any comments from propeller experts would be appreciated.

 

chrs

 

 

[LBD 126]

Consider if the blades and water flow pass obstructions like the dead wood, rudder or strut. When spinning if the upper and lower blades of a two blade propeller are both in time with (Close proximity) obstructions a "chatter" can develop in the drive line or rudder.

The grey or blue instances can be fine... a two bladed prop in a position such as the red  hull could be a problem.

 

I will say if you only motor a little this would not likely be a problem but if you find your self motoring for days....

 

[harrytom 642]

my orininal salthouse 26 came fitted with a 3 blade and a spare 2 blade,swapped them over and 2 not as efficient  but drag was less,now the 2 blade ,marked shaft so blade be verticle and locked in gear. Found out quickly not a good pratice better to leave in free mode. Went back to 3 blade. not a big difference sailing cruising but would hinder if racing

[Frank 157]

Thanks for the replies , good points made