Bought a nitro boat that was performing at 50MPH had the H2O impeller replace and now at 4000rpm run 10mph

Question

Can some one help me pointing the cause of the problem

Answer 1

If it's going 10mph at 4,000 RPM and the lower unit isn't making all kinds of horrible noises, then you got one of three things happening:

1. spun hub.  Score or paint a line across the inner hub & rubber bushing, put the motor in the water (probably don't even have to take it off the trailer) and rev it up in gear, then look to see whether your mark still lines up.  If it don't, that's your problem;  take it to a prop shop to have a new hub pressed in.

2. cavitation.  Something is interrupting the flow of water over the prop blades.  Happens when you get hydrilla caught on the lower unit, but it could be caused by irregularity or deformity on the lower unit or on the prop itself.  Cavitation is actually boiling water;  the prop blades exert enough pressure on the water to bring it to a boil (pressurizing lowers the boiling point of water).  It's destructive as it actually degrades the material (alum. or SS), and leaves what looks kinda like scorch marks.

3. ventilation.  Air gets drawn in to the prop from the surface, or exhaust gas bleeds back into the blades.  Trim too high or broke antiventilation plate can cause it.  If your prop has a diffuser ring (OMC props) and it gets knocked off, or a tiny bit of gravel caught in it can lead to ventilation, but you probably have a merc, and a prop with a flared hub instead of the ring.  If your prop had vent holes that were plugged and a plug fell out, that'll cause it, too.

Only other thing I can think of is if you have a 2-part drive shaft, and the linkage between upper & lower is shot -- but seems like that would make a lot of noise, too.  Maybe not.

Answer 2

From what you posted here I would say that the cause of your problem is the new impeller.

Answer 3

Something is wrong with this question.  I need to better understand what's happening.  I'm pretty sure you don't have a jet boat, so the H2o impeller makes no sense.  This sounds like it's not in gear, which could happen if you didn't reinstalll the drive correctly.  But, from what you are telling us, I just don't know.  Good luck!

Answer 4

Bamma say he impeller expert China. Bamma say you make number one beginner mistake. Bamma say you put impeller on backwards. Bamma say Chinese never make such errors. Bamma say Chinese product best in world. Bamma say what better than disposable everything. Bamma say use one time. Bamma say just buy new one. Bamma say you cavitate. Bamma say boat no like. Bamma say brush twice daily. Bamma say repeat. Bamma otta know.