Propeller Calculations

[Nomad Willy]

rgano,
Did they offer the same slightly different ratios for the 2.57-1 gear?

 

[rslifkin]

rgano,
Did they offer the same slightly different ratios for the 2.57-1 gear?

Not for the regular velvet drives. Only a couple of the ratio choices rotate opposite and the 2.57 and 2.91 aren't close to any that do. To get counter rotation with my 2.57 ratio velvet drives, I have counter rotating engines (gassers).

 

[Jdoc]

You don’t want to give away diameter, with that you lose blade area , efficiency and impact the propellers ability to absorb horsepower and turn it into torque.



Forget about it.

Thrust

 

[FF]

No need to guess,, Skeens Elements has tables to compare the different prop choices.


Images for skeens elements of yacht design

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[Martin J]

Change in Pitch chart

I have attached a change in pitch chart that many of you will find useful. I've had it a few years and cannot recall which prop manufacturer I got it from.

 

[rslifkin]

I wouldn't give up diameter, but if you end up with more blade area than necessary, going to a design with narrower blades for less area should be more efficient than going down in diameter, as the higher aspect ratio blades should have lower tip losses.

 

[rgano]

rgano,
Did they offer the same slightly different ratios for the 2.57-1 gear?

I do not know the answer to that. I was a GB neophyte at the time (1986) just interested in knowing if I could trail a shaft while trolling; so they went and dug up this elderly engineer who had been involved in the design. We talked only about my particular model and gear ratio. That was back I the day when people answered phones and would actually discuss things with customers.

 

[rslifkin]

I just checked the ratio list for the 71C and 72C Velvet Drives. Looks like 1.91:1 is the only ratio that rotates opposite of the engines. All others rotate the same as the engine (1.0, 1.52, 2.10, 2.57, 2.91). So 1.91 / 2.10 ratios would be the counter rotating pair, although personally, I consider that a pretty bad design kludge.