Volvo Penta duo prop upgrade from B to C

[Kepin]

Hello sailors, just came across this forum, as I'm looking for a piece of advice from more knowledgeable guys here. My boat was equipped with B4 set when I got it, I dont know what was an original prop set, but last winter I got a set of C5s and installed them. The time weather was permitting I have the boat on the water and found out that the prop set is too heavy ( too coarse ) for my application. Its getting on the plane very hard and WOT is 3100 rpm top with speed around 30 miles/hr. With B4 it was going around 40 miles/hr with WOT 4000 rpm. Should I replace it with C3 to get faster on the plane and to have top-end speed like with B4? Thank you in advance.

 

[QBhoy]

Hi

what are the details of the boat and engine driving her ?

 

[Kepin]

Hello QBhoy, She is Wellcraft Prima 243 powered by Volvo Penta 5.7 GL with Holley carb-r and DPS outdrive duoprop B4 ( original ) C5 ( current ) , 4500 Lb

 

[rlw]

From memory there is a a the is a 2 step difference between C and B props. C3 would equate to a B5. C series were mainly for high torque diesel drives. Volvo used to have a brochure matching the prop size to boat speeds and drive ratio's. Check out the link below.

http://seastriper.com/Propeller_guide_2006.pdf

 

[BruceK]

From memory there is a a the is a 2 step difference between C and B props. C3 would equate to a B5. C series were mainly for high torque diesel drives. Volvo used to have a brochure matching the prop size to boat speeds and drive ratio's. Check out the link below.

http://seastriper.com/Propeller_guide_2006.pdf

I think a lot there had to do with your particular gear ratio w.r.t. chart. (IIRC you were on 300's) Certainly on my gear ratio the C3 were not making enough revolutions to come close to those estimated speeds. Top end was 20 knts with the motors seriously under loaded.

 

[QBhoy]

I think a lot there had to do with your particular gear ratio w.r.t. chart. (IIRC you were on 300's) Certainly on my gear ratio the C3 were not making enough revolutions to come close to those estimated speeds. Top end was 20 knts with the motors seriously under loaded.

Could be. Suppose the rpms are different between them

 

[BruceK]

From the chart I believe it's meant to be interpreted as such:

if your boat is designed to do X knts and you have this engine and gearbox ratio..... line up the correct prop accordingly. So looking at a KAD300 with a drive ratio of something like 1.5:1 and saying a C3 will get you e.g. 30 knots and then looking at another engine e.g. KAD42 with a ~1.78:1 gear ratio driving a B5 to get 30 knts does not mean equivalency. The C3 for me was hopelessly under-propped while the B5 was significantly over-propped. The C3 doesnt even register for the KAD42.