longshanks
Petty Officer 2nd Class
- Joined
- Jun 12, 2006
- Messages
- 188
OK guys, I am looking for some advice on how to get RPMs significantly higher than they presently are. I have a 2006 Merc 50HP four stroke motor, which is supposed to run 5500-6000 RPMs. Motor was just re-built by Mercury, under warranty (previous owner), so I am essentially running a new power head. My boat is a 15' heavy aluminum skiff, with a 6 degree hull, with a small tunnel. The motor is mounted with the anti-vent plate just above the top of the tunnel. I have tried three props.
1) 11 pitch by 12" Pirahna composite. 5000 WoT, at around 25mph.
2) 9 pitch by 12.25" Pirahna composite. 4000 WoT, hardly on plane.
3) 11 pitch by 11.75" Mercury Vengeance stainless. 5200 WoT, at around 27mph. Best performer of this bunch, but still around 800 RPMs low. With this prop, the motor occasionally cavitates when operating in rough water. So, the motor is about as high as I would like to mount it, for operation in the conditions I use it.
The obvious solution to raise RPMs would be to go down in pitch. However, the opposite seems true based on the props I've tried. The only explanation I can come up with is that blade size/prop diameter is playing a large role. As Silvertip suggested, maybe this motor will work better with 10-11" diameter props?
My feeling is that the boat is somewhat underpowered, but I am happy with the motor, the performance, etc. as it stands now. I just need to find a smaller prop (either pitch, diameter, or both), that will allow this motor to spin at the proper speed. Any suggestions?
1) 11 pitch by 12" Pirahna composite. 5000 WoT, at around 25mph.
2) 9 pitch by 12.25" Pirahna composite. 4000 WoT, hardly on plane.
3) 11 pitch by 11.75" Mercury Vengeance stainless. 5200 WoT, at around 27mph. Best performer of this bunch, but still around 800 RPMs low. With this prop, the motor occasionally cavitates when operating in rough water. So, the motor is about as high as I would like to mount it, for operation in the conditions I use it.
The obvious solution to raise RPMs would be to go down in pitch. However, the opposite seems true based on the props I've tried. The only explanation I can come up with is that blade size/prop diameter is playing a large role. As Silvertip suggested, maybe this motor will work better with 10-11" diameter props?
My feeling is that the boat is somewhat underpowered, but I am happy with the motor, the performance, etc. as it stands now. I just need to find a smaller prop (either pitch, diameter, or both), that will allow this motor to spin at the proper speed. Any suggestions?