low speed - high rpm

[QC]

Re: low speed - high rpm

The 1400 @ idle begins to shed more light on this. Your tach is counting something, but it isn't engine RPM . . . If you put it in gear at 1400 RPM this thing would lurch like you have no idea. It would be exciting to say the least. Let's get an accurate tach and speed reading and start over . . . My .02

 

[texanaustinite]

Re: low speed - high rpm

OK... Took the boat out this morning. The only thing different from Tuesday to today was that I temporarily spliced the severed ground strap (cable). The gauges read as before I had the issue.

I was a bit off in my original post. It wasn't originally running 35mph @ 3500rpm. Looking at the gauges helped my memory a bit. It ran today, as it had been, 30mph @ 3200rpm and hit 38mph @4200rpm. It also idled @ 1000rpm (is that too high?).

I'm still going to have dcg9381 take a look at it this weekend to see if the gauges are correct.

Thank you all for your responses and I'll post back with the results.

BTW... the prop did not slip.

 

[QC]

Re: low speed - high rpm

It also idled @ 1000rpm (is that too high?)

It is if it's correct. Spec is 650 - 750 in forward gear . . .

BTW... the prop did not slip.

Never thought so, like Don S said. They won't make much speedwise if they are slipping. Like anything, there are exceptions though . . .

 

[dcg9381]

Re: low speed - high rpm

I'll put the boat on GPS and inductive tach, but can anyone comment on how much speed you should get out of a correctly prop'd 3.0L motor in a 17-18 foot boat? I was thinking low 40s.

 

[180shabah]

Re: low speed - high rpm

4600RPM X 1.97:1 gear ratio X 19" prop = 42mph (38mph w/ 10% slip)

 

[texanaustinite]

Re: low speed - high rpm

Met Darin (dcg9381) this morning on the lake. The boat's tach was just slightly off (a bit high). He was able to calibrate it and I was still able to hit 38mph @ 4200rpm. The speedometer was also a bit off. GPS read the speed at 36mph while the boat read 38mph.

I'm quite happy with the results. Thanks again Darin. I really do appreciate it.

Now I want to buy a faster prop. Not sure if I should go with a 21 or 23 pitch...

 

[QC]

Re: low speed - high rpm

OK, so you actually got 36 MPH (GPS) @ 4200 RPM, trimmed out as far as you could and WOT? If so, you want to go down in pitch, not up . . .

Oh, and how typical was your load? People, fuel, beer, stuff . . .

 

[180shabah]

Re: low speed - high rpm

I agree with QC, if 4200 was WOT, then dropping down to a 17" should let you rev to about 4600. Your top speed won't change much. if any, but you hole shot and accelleration will both improve.