Higher RPM + Lower Top End Speed

SwiftCloud

SwiftCloud

GreatGrady Captain
Joined
Mar 22, 2006
Messages
66
Reaction score
1
Points
6
Location
Marblehead MA
Model
Marlin
I am assuming that I need my prop "adjusted" or replaced due to a pitch issue, but curious if anyone can think of any other cause for this issue:

My maximum RPM used to be 5500 and my top end speed about 39 mph. In the last year or so, I can get up to 6000 RPM and my top speed is 34-36 MPH. I have to get up to several hundred extra RPM to get the same speed at lower levels too. This is true at the beginning of the season and at mid-season (so not bottom growth).

I've had my prop reconditioned a few times (minor rock strikes), but the change was noticeable in between reconditionings (i.e. the reconditioning didn't change the RPM / speed issue back to what it was the first time around).

Thanks in advance for any suggestions.
 
If you are sure that the prop isn't cavitating, your prop hub might be slipping. With the key off, you can quickly test this by dropping the motor into gear and applying pressure to the prop to see if it will spin easily. It isn't a foolproof test though - the hub needs to be fairly worn. A more reliable method that I've read is to use a grease pencil or Sharpie marker to draw a line across the inner and outer hubs. Run the motor at WOT under normal load, then check your mark to see if the lines are still matched up. If they aren't, have your prop hub replaced.
 
More info would help. Assuming that this condition wasn't always happening,I am not so sure that the prop is spun. If the prop were spun, it is more likely to spin under the greatest load. That is often when getting on plane and not at the top end. Hitting the throttle hard at rest will often cause a quicker than normal jump in revs but with a slower than normal increase in speed. So if the prop is not spun, what can account for the lower speed and higher revs? Cavitation as mentioned can be an issue and that can be caused by a lot of things, a damaged blade on the prob or turbulent water flow past the lower unit. That can be caused by something on the hull that disrupts water flow. Check the prop closely for cracks. The loss in effective pitch can be due to blade flex (or bent blades)
 
Agree with seasick and assuming you have ruled out a prop hub/flex issue, no change in the bottom condition and identical engine and trim tab positions. An simple increase in the roughness on the surface of the prop itself could decrease its efficiency. I would also guess a significant change in total vessel weight, loading configuration (center of gravity) could result in a change in power (RPM) required or propeller efficiency (how it hits the front side flow) for a given speed as well. Although unlikely, I wouldn't be too quick to rule out a gauge error for RPM (or speed especially if not GPS determined.)
 
Thank you for the thoughts.

I didn't know a prop could be a little spun and will check that out.

Speed has only been determined by GPS. No changes to cruising area, load, or bottom.
 
Props can lose some diameter during reconditioning depending on how damaged they are. You should probably replace the prop. If you reconditioned it twice it is probably a half inch to an inch smaller than it started out.
 
I believe the most simple explanation is that your prop has changed size or pitch or both during the reconditioning. Higher RPMs and lower speed mean that you're pushing less water. When I went from a 17 pitch to a 15 pitch my top RPMs went from 5500 to 6000.
 
lgusto said:
I believe the most simple explanation is that your prop has changed size or pitch or both during the reconditioning. Higher RPMs and lower speed mean that you're pushing less water. When I went from a 17 pitch to a 15 pitch my top RPMs went from 5500 to 6000.
Click to expand...
When I originally commented, I had missed the part about the reconditionings. In view of that I agree with your statement about the parameters of the prop probably changing. Diameter is easy to check, pitch is a little more difficult. Your experience in RPM change is consistent with the 2 inch change in pitch.
 
You must log in or register to reply here.