High Engine RPM but not going fast?

[FargenNewbee]

Hay folks I think I need some advise on Props/pitch or something else?
I have a Volvo Penta 5.0GL with a SX outdrive. Just did maintenance on outdrive - oil looked brand new no junk or water.
Now what I am seeing is high RPM's on the motor - 5K (6K Readline) and ~40MPH. (origional boat guage goes to something crazy like 130mph! Right!)

Now it also seems like the moror has more power to give but it does not seem to make the boat go any faster.
40 is fine but ~50 may be better for that high RPM.
Now I got the boat ~1 year ago and do not know anything history wise about the existing prop. In fact there is one under the seat I need to look at as well. (I'll post any prop #'s when I look at the prop later tonight)
I am assiming I need to increase the pitch of the prop to push more water - right? This may make the bottom end a bit but improve top speed- i.e. ability to pull Fat-Boys out of the water vs top speed?
Not sure what to look for or look into...
Looking for your advise and thanks in advance.

 

[mkast]

Re: High Engine RPM but not going fast?

How do you get a red line of 6k?
The prop is suppose to be the limiting factor.
As long as the coulper isn't slipping or any other problems, you should be turning a wide open throttle (WOT) rpm of no more than 5100 or 5200 on a good day.

 

[Haut Medoc]

Re: High Engine RPM but not going fast?

What you need to do is verify your tach .....
Verify your speed with a GPS.....
Post what pitch & size your present prop is.....
Length & weight of boat....
Then we might get somewhere.....
5K is over revving for a 4 stroke boat engine.....​

 

[Bondo]

Re: High Engine RPM but not going fast?

Ayuh,....

I'd sooner think that 4800rpms should be Max for that motor....

 

[Silvertip]

Re: High Engine RPM but not going fast?

Am I correct that your tach reads to 6000 and you think that's redline? Better have a peak at your owners manual. Running 6000 rpm will make a hole in the bottom of the boat as engine parts make their way past the oil pan preceded by a "big bang".

 

[QC]

Re: High Engine RPM but not going fast?

No need to scold . . .

Welcome aboard Flippin' Farfugnugen!!

What year? What horsepower? What boat? What Haut Medoc says above is a great start, when he says get speed by GPS he means with the throttle mashed all of the way and the boat trimmed out as fast as she'll go with a typical load. That will help us help you a lot. Marine engines don't really have a redline as you are used to. As mkast says the prop determines the end RPM and that RPM will tell us if she is propped right and/or if you have some issues etc. Info has to be dead nuts though. Tach and GPS . . . Good luck!! Oh and more pitch will make those "Fat-boys" feel even fatter . . .

 

[Silvertip]

Re: High Engine RPM but not going fast?

Not scolding -- but stating fact. Ever seen an engine grenade at 6000 RPM. It isn't pretty. What's even worse is having a flywheel come through the bellhousing. I had a trucker friend that lost some toes due to an episode like that and the engine was running at nowhere near 6000 RPM. The dash of his truck looked like a bomb had gone off behind it.

 

[BAYLINER185]

Re: High Engine RPM but not going fast?

Well Isnt there a rev limiter? If your not hittign it your tack may be off. My tach was off when I 1st got my brand new boat. Youll never know if its off unless you check and the engine sound may be deceiving as I thought I was over reving but in reality the engibe was only at like 3K RPM.

I would tend to think if you are hitting 6K the motor would sould like its ready to rip out and launch into orbit lol.

You may be proped OK as I said you may actually be around 4k RPM for a speed of about 40MPH. Id check the tack 1st before doing anything. You said your speedo is reading all over the place too?

Boat speedo's are ran by air preasure so I dont know why yours reads like that unless it is shot completly.

 

[Bondo]

Re: High Engine RPM but not going fast?

Well Isnt there a rev limiter?

Ayuh,....

Unless it's an EFI motor,..... Probably Not.......

 

[Don S]

Re: High Engine RPM but not going fast?

The 5.0GL is a carbed engine.
I don't see enough information in your original post to answer anything specifically.
What is the full model number and year of your engine, what size of boat is it in.
Just because a tach (or speedometer) has numbers a lot higher than you can get to doesn't mean you should be able to get them.
A 6000 rpm tach is industry standard size for an IO, of course very few engines see over 5000rpm in the WOT (Wide Open Throttle) rpm specs.
Are you using the trim correctly??? That makes a big difference.

 

[QC]

Re: High Engine RPM but not going fast?

My point, which was not well made, is that 6000 RPM is not gonna happen unless he is running a 9" pitch prop . . . so let's keep this to educating instead of terrorrizing . . .

P.S. To answer Mr. Silverstones' question, I've seen a lot worse than a SBC grenading at 6000 RPM

 

[FargenNewbee]

Re: High Engine RPM but not going fast?

Thanks everyone for your posts.
Ya the Tach says 6K but I only reved it up to 4.5 maybe 5 for a short period. Never hit any Rev limiter but did not want to go there.
Ya blown motor (explosion) = Bad
It just seemed that as the RPM rises, you get less and less oomph and it seems like it just is not getting the power to the water. Maybe it's me.

I'll try your ideas about the GPS next time I am out which should be in a week or so. I have messed around with different trim settings while zipping along to find the sweet spot. Tach and speedo "seem" to be working as designed - no wild fluxuations, but they may be off a but. I can check speedo w/GPS but what's the easy way to verify the RPM's? (I keep loosing count as it spinns around )

I just finished doing the regular scheduled maintenance - Oil/Filter, Plugs, Fuel Filter, Outdrive oil, Gimble and U-Joint lube. All seemed to be in good shape but changed anyway.

While cleaning the pig last night I tried to see any #'s on the prop but it looks like some weenee heavily over painted them as I can see where they Should be. I'll dig some more and post.

Boat Info: Four Winns Horizon Limited Sport, Boat Model: HRS-A98 --> HORIZON RS 10 STANDARD 1998 ? 18 Foot.
Weight: ??
Engine:5.0 GL V8 Chevy(I believe) Model: 502BPBYC - Carb'd
OutDrive: Volva Penta SX Cobra
She has about 350 hours (according to the engine meter) and was only driven by a little old lady once a week to church on Sundays

I'll post more when I have the GPS and Prop info.

 

[HT32BSX115]

Re: High Engine RPM but not going fast?

Howdy,

You did say in your first post that you're seeing up to 5000 rpm at WOT. (about right for this engine probably......maybe 200 high if REALLY 5000?) AND you're getting 40 mph. 40mph at 4500.... sounds in the "ballpark" to me.

Most marine tachs have a 6000 rpm full scale. (Full scale is never red-line btw...very common misconception)

It's unlikely that you'll blow the engine at 5k but it is possible that you have a replacement prop that someone put on there that's a little low (pitch)

Don't know about OMC for 1998. (is 502BPBYC a Volvo or OMC model number?)
The rev limiter in my 454 carb'ed Mercruiser IS built right into the Thunderbolt V ignition and is set to 4700. Maybe OMC didn't do (electronic ign) rev limiting in 98.

If you plug numbers into a typical prop slip calculator

Using 17p 1.5 ratio and 4500 rpm and 12% slip you get about 42mph. well within the realm of possibility.....Most of the prop slip "boys" claim 11-15% "slip" is normal....put 15% in there and you absolutely get "good" numbers.

Of course "regular" speedo's are notoriously inaccurate and the tach could be suspect. You should get yourself another (known) tach and GPS to verify what is really happening.

If you want to use that prop slip calculator, you need to know your prop pitch, your drive ratio, speed (don't do it on a moving river!!) and RPM accurately.



Cheers,


Rick

Now what I am seeing is high RPM's on the motor - 5K (6K Readline) and ~40MPH. (origional boat guage goes to something crazy like 130mph! Right!)

 

[FargenNewbee]

Re: High Engine RPM but not going fast?

Yes but I have not completely opened it up to WOT. Stopped before explosion Ya at around 4500-5000 I'm running at 40 MPH assuming the tach and the speed guage are correct - which I'll check next time out.

BTW I did look at the 2 props I have. Both are the same. 14 1/4 X 21 (3850302) In looking at a chart in the Prop section (http://www.volvo.com/NR/rdonlyres/2D..._2005_2006.pdf) I see that gor a 5.0 GL with a ratio of 1.60 to 1 at roughly 44-4800 RPM I should be hitting roughly 50ish. So I'm a tad low...but in the ballpark.

Verifying the actual speed and RPM should help nail this down better.
Also in the same chart I see there is a 14.25 X 23 prop available with a better top speed.
Anyone have thoughts on this? or other things to look at?

 

[QC]

Re: High Engine RPM but not going fast?

Yes a 23 would be faster IF you have enough horsepower to pull it . . . Check the tach, check the speed, check the ratio too. Gotta be sure on ALL three Don't change anything until you know for sure!!! One or more of the three numbers is wrong, I am going with the tach.