[Public-List] Revisiting propellers...

[Gordon Laco via Public-List]

Hello friends,

 

So, I’ve got my replacement prop from the very generous Kelsey at Campbell Props.  

 

I can’t wait to try it out once SURPRISE is back in the water…  to remind all of the backstory… 

 

We had a factory installed two bladed 13” diameter prop with a 7” pitch.  We were basically quite happy with it, but I wanted to experiment.  We bought a Campbell ‘Sailor’ three blade prop 11” diameter and 5” pitch at their recommendation.  We wanted the engine to be at a higher RPM at our 5.2knots cruising speed under power.  In practice last summer, we experienced complete success with regard to higher RPM… both charging and cooling were dramatically improved.  However, we were disappointed at loss of top end speed and apparent higher fuel consumption.  (we had hoped that although the engine was spinning faster, it shouldn’t have been working as hard as in lugging…)

 

The replacement, all shiny and new on my desk as I write this, is 12” diameter (one inch greater) the same 5” pitch, and the blades are slightly cupped.

 

My objective is to keep the RPM, but have less slip.   Slip is, I know we all know, the difference between how far the prop advances through the water per revolution, and how far it would advance if it were cutting through a solid, per revolution, as in if it were a screw going into wood.  Theoretically a 5” pitch means it would advance 5” per turn… but in a fluid like water, and pushing a big heavy boat, it slips some.  The increased diameter means a little more blade area… and the slight cupping increases the hydrodynamic efficiency of that area.

 

Well. There it is.  Have to try it out now!

 

Gord #426 Surprise