Volvo Duo Prop-Sizing

[kghowe]

Can you help me determine which props I should have on my boat. I've got to replace both pairs due to damage and corrosion.

First the good news, both props are A6's. However both the foward props have different VP part numbers. Port side fwd prop is 15mm dia larger than rear where as the stb fwd prop is roughly the same size as the rear one. Which is right? From memory my Targa had one large on the front and small at the rear (prop nut end). Part numbers are 854779 (Port) and 852251 on stb. Engines are AD41's. Incidently both engines seem fine and can pull 3500-3600 rev's.

Any ideas out there? How do you size a prop anyway? Surely its not down to suck it and see.

Kevin
SBS

 

[GC1]

Ring 0208 874 7059 and all your questions will be answered

 

[[2068]]

Latest part nos are:
A6 Front Prop: 854779
A6 Rear Prop: 854789
A6 set of both props: 854769

Prop size is worked out by estimation, then trial&error, by boat manufacturer such that engine can still reach rated rpms with the biggest (pitched) prop possible, then they usually fit one size smaller to cope with people, kit, barnacles etc.

Each change in duoprop size makes about 3knots difference. Of course, that doesn't mean that changing up one size will make your boat go 3kts faster, it only WOULD make your boat go 3kts faster if the engines somehow suddenly produced a bit more power to still get the rated rpms...

Hope I haven't confused too much.
dv.