Bertram 31 Propeller size

[utilamayor]

I recent acquired a 1964 Bertram 31 outfitted with 2 Detroit 453 diesels. I just replaced transmissions to Borg Warner Velvet Drive 1.91:1 reduction. I do not have the slightest idea as far as what size props should be used. Any help or suggestions you may render would be sincerely appreciated.

[plain_sailing]

It depends on:
1) the geometrical tolerances of stern construction
2) speed of rotation of shafts (rpm or rps)
3) Shaft Horsepower
4) required speed
5) tolerance on cavitation
6) shape of hull (planing, displacement, semi displacement ??)
7) wake fraction

Reduction ratio of gear u have chosen is tip top .
If engines RPM is about 2500 then two propellers of 24 inches diameter each one are fine.
Try to choose pitch ratio about 0.7-0.8. Expanded area ratio should be about 0.5-0.6.
Choose 3 blades instead of four.
With the above characteristics u ' ll get a proppeller efficiency of about 57 %