prop size

[SteveGorst]

I have been offered a 15 x 9 folding prop and can't find any documentation to confirm whether it will be ok with my boat. Does anyone out there know how to size props?

waterlength 6.6m
weight 2500kg
10 horsepower md5c
120s saildrive
target speed 5.7-6 knots

Do you think 15x9 would be ok?

 

[pvb]

Ask Volvo Penta...

Your best bet would be to ask Volvo Penta. Their parts catalogue recommendation is a 14x8 2-blade or 14x9 3-blade folding prop for an MD5C with S-drive, so the 15x9 might be a bit over-specced.

 

[sailorman]

try this

http://www.castlemarine.co.uk/pitch.htm

 

[SteveGorst]

Thanks for the link

To use the calculator I need to know the max rpm of the engine and the ratio of the gearbox. These are probably in the manual but they are on the boat. Has anyone any ideas? From my own research on the web they appear to be 3000 and 2.2 but the calculator tells me this is doesn't give me an ideal prop rpm for the boat.

 

[brianrunyard]

I've just put the figures:
10hp@3000 rpm;
2.2:1 reduction;
6.6 m LWL;
2500kg

and got 13"x11"
6.1 knots
No error messages

 

[SteveGorst]

funnily enough I don't either now....

There does seem to be a big discrepancy between volvo pentas 14x8 and the calculators 13 x 11 though. Would 15 x 9 be somewhere in between?

 

[LeonF]

Beware of oversizing. I moved froma two bladed 12x 10 to a three bladed 13 x 12. Was a bit green about prop size when i ordered it, know better now. I am about to have it re-pitched to 9 as it will not let the engine reach recommeneded max revs. As the boat got progressively more fouled thru the season the max revs dropped even more.