boat speed and max rpm.

[joecathy]

We purchased a 2005 Beneteau 323 with with swing keel last year. It has a 20 hp Yanmar 2 cylinder. The boat was outfitted with a Volvo 3 blade folding prop. Top speed at 3000 rpm is 5.8 kts. 2 questions:
* Is this speed the same as with the stock 2 blade prop.
* What should be the top cruising RPM for this engine?
joe

 

[Philip_Oceanis 361]

5.8 knots at 3000rpm??????
No, no definately something is wrong here. Most probably you have the yanmar 2GM20 engine and its maximum rpm must be near 3600. If you can achieve only 3000rpm then either you have the wrong size propeller (or something wrong with it; does it feel right? any unusual vibrations? is it assemblied OK? what is the size of the propeller anyway?) or your propeller (or hull) is EXTREMELY dirty.
Cruising rpm is probably something like (75-80)%x3600 = 2700-2800rpm but I will leave the experts to answer this.

PS: Don't panic yet and start thinking that the 2-blade prop is better. The volvo folder is supposed to be a very good propeller and believe me even a slightly dirty propeller makes a lot of difference in speed (not sure how much max rpm can be affected though).