[PUP] :PPM-propeller efficiency

[John Marshall]

Do trawler manufacturers select the prop for perfect match at full RPM?

I don't know a lot about this, but given trawlers are rarely run wide  
open, I'd think they would be focused on getting the best engine-prop  
efficiency at S/L of 1.2 or so?

Or does that have a huge downside in terms of reserve power or whatever?

John Marshall
N55-20 Serendipity

On Nov 23, 2008, at 8:40 AM, Bob Frenier wrote:

> I hope we can get back to the very lively and interesting topic of the
> mythical perfect passage maker. It sure perked up this list when it  
> started.
>
>
>
> That brings me to the efficiency of the propeller. Whenever I look at
> engine manufacturers' charts, there appears to be an extremely low  
> level of
> propeller efficiency at the engine's lowest rpms in a propeller that  
> has
> been selected to be perfectly pitched and sized at maximum engine  
> rpms. And
> since we have discussed at length the fairly extreme difference in  
> fuel burn
> at low rpms vs. high rpms, aren't we faced with a dilemma? In other  
> words,
> just at the rpm which brings out the best efficiency in the engine,  
> the
> efficiency of the propeller stinks. Leaving aside for a moment whether
> buying a controllable pitch propeller ("CPP") will save enough fuel  
> to pay
> for itself, won't a CPP increase propeller efficiency dramatically  
> at those
> low rpms where the engine sips fuel? And won't the CPP put a perfect  
> load on
> the engine at any rpm, so running slow doesn't gum things up?