low speed - high rpm

[texanaustinite]

Hello. First time posting.

I have a 1997 Regal 1700 SLR with a Volvo Penta 3.0 4 cylinder engine and a SX sterndrive. I've owned the boat for a month now and was able to run it up to 35mph at 3500rpm with two adults on board. The past two times I've taken it out, I've only been able to hit 25mph even at 4400rpm. I have been running a 14 1/2 x 19 prop. I have not done anything to it mechanically. The only repairs I've done to it have been new belts, new ignition and new engine kill switch. Any ideas?

 

[thrasher]

Re: low speed - high rpm

Not trimmed correctly? Slipping prop? slipping transmission? These are the only things I can think off that would allow the engine to rev that high but keep the speed down.. Slipping prop would seem the most likely.

Gary

 

[wire2]

Re: low speed - high rpm

Most likely scenario is prop hub slipping. Make a line with a permanent marker on the prop hub and on the shaft/nut at the 12 o'clock position. Run it once more, then look to see if the marks are misaligned.

2nd but lesser possibility is engine coupler slipping.

 

[dcg9381]

Re: low speed - high rpm

Check the hub on that prop - remove the prop.. the rubber hubs were subject to slipping... mark it as indicated above.

 

[texanaustinite]

Re: low speed - high rpm

Thanks for the responses. By "ignition" I mean ignition swtich (where the key goes), so that shouldn't be a problem.

I'll try marking it like and run it. I have to remember, it is 10 years old.

 

[Don S]

Re: low speed - high rpm

No way in the world are you going to get a prop slipping and get more than 5 mph (if that much) out of it. You will not get 25 mph.
Since you used to get 35 mph at 3500 rpm, you know the prop didn't change itself (unless you hit something), so diameter and pitch shouldn't need to change.
Most likely you have a fuel, ignition, or engine problem. Assuming the bottom of the boat is clean and you had it trimmed out properly.

 

[thrasher]

Re: low speed - high rpm

Sorry Don, but as the engine still revs to near maximum revs I can't see how you can say its ignition, fuel or engine. If the engine didn't rev up then I would agree but he says it still revs to 4400RPM so something must be slipping or it's not trimmed as originally stated.

It is easy enough to check for a slipping prop so let him do that first and then we can try other ideas..

Gary

 

[QC]

Re: low speed - high rpm

I wonder about the tach after the ignition change . . . but I always wonder about the tach. How are you getting the speed number?

 

[texanaustinite]

Re: low speed - high rpm

I'll find out tomorrow (Thurs) since I'll have a day off.

I marked the current prop so I'll try it out again, then I'll switch to the 17 pitch prop to see how it works. I was getting about 25 mph on the 17.

Would changing the ignition switch and kill switch mess with the tachometer or the speedometer?

 

[thrasher]

Re: low speed - high rpm

It shouldn't have any effect on the speedometer (most are operated by water pressure (no electricity involved)), but it could effect your rev counter if they where not correctly wired in (high resistance) but the chances are against it if the engine is running. I still think its a slipping prop or an incorrectly trimmed outdrive.

Gary

 

[Chris1956]

Re: low speed - high rpm

Texan, On an off topic, I am sceptical that you are getting 35MPH at 3500RPM on a 17' 4 cylinder sterndrive. That would put your top end at about 45MPH at 4500 RPM, which again I do not believe.

I suspect your speedo and/or Tach are incorrect.

 

[dcg9381]

Re: low speed - high rpm

Texan,
Where are you in TX - if you're in Austin, I may be able to help you. I have a GPS and inductive tach.

Look on the side of your drive.. see if there is a ratio stamped there or on a sticker.. Otherwise ask someone what drive ratio a 3.0L motor has.

Assuming the ratio is 1.8, you've got less than 5% prop slip at 3500rpm and 35mph. I can tell you that these values aren't correct... I could be off on the drive ratio, but if I'm close, your mph / RPM is off.

Kill switch shouldn't impact anything.
You didn't mention what you replaced on the ignition - it could impact the tach, but it's unlikely.

What RPM does the boat idle? Resolution at idle is low, but it can be an indicator.

 

[wire2]

Re: low speed - high rpm

This thread reminds me of the dumb blond joke, where the marina tech jumped into the water and found the trailer still strapped under the boat...

 

[texanaustinite]

Re: low speed - high rpm

Texan,
Where are you in TX - if you're in Austin, I may be able to help you. I have a GPS and inductive tach.

Look on the side of your drive.. see if there is a ratio stamped there or on a sticker.. Otherwise ask someone what drive ratio a 3.0L motor has.

Assuming the ratio is 1.8, you've got less than 5% prop slip at 3500rpm and 35mph. I can tell you that these values aren't correct... I could be off on the drive ratio, but if I'm close, your mph / RPM is off.

Kill switch shouldn't impact anything.
You didn't mention what you replaced on the ignition - it could impact the tach, but it's unlikely.

What RPM does the boat idle? Resolution at idle is low, but it can be an indicator.

dcg9381,

I do live in Austin, northwest near Jollyville. Yesterday it was idling at about 1400rpm. I don't recall what it ran at before. I would greatly appreciate your help.

As for the ignition, I just bought a generic one, new keys and all.

I don't know too much about engines so bear with me.

As for running at 35mph at ~3500rpm, it's what the gauges read.

 

[180shabah]

Re: low speed - high rpm

Nobody has mentioned it yet. what is the condition of the prop? Any dents or dings?

I am assuming that you trailer, so this is not likely, but is there any growth on the hull, or debris wrapped around the drive?

 

[texanaustinite]

Re: low speed - high rpm

Nobody has mentioned it yet. what is the condition of the prop? Any dents or dings?

I am assuming that you trailer, so this is not likely, but is there any growth on the hull, or debris wrapped around the drive?

The prop is in good condition. A few spots where paint has flaked off, but no dings. The hull and drive are clean.

 

[dcg9381]

Re: low speed - high rpm

dcg9381,

I do live in Austin, northwest near Jollyville. Yesterday it was idling at about 1400rpm. I don't recall what it ran at before. I would greatly appreciate your help.

As for the ignition, I just bought a generic one, new keys and all.

I'm don't know too much about engines so bear with me.

As for running at 35mph at ~3500rpm, it's what the gauges read.

I'm up in Lago Vista (west of Cedar Park) - I'm about 2 blocks from the boat ramp/marina, so if you can get it out to where I am sometime when I'm not at work, I'm happy to put a GPS and inductive tach on it.


1400 rpm is high for actual idle RPM on that motor.. does it sound like it's idling fast, or is that just what the tach indicates?

It's been a long time since I've been in a 3.0L boat, if memory serves, we're talking about low to mid 40mph for top end on that boat, correct?

Also have a look at the left hand side of the out drive.. see if a ratio is stamped on there anywhere...
I've sent you my email/phone.

 

[thrasher]

Re: low speed - high rpm

I would be expecting high 30's with a 3.0L engine in a 17' boat. 40's seems a bit optimistic to me..

Gary

 

[texanaustinite]

Re: low speed - high rpm

I was just looking at the sterndrive and one of the ground straps, on the left side, is severed. What exactly does a ground strap do?

Here's a pic of one:

 

[180shabah]

Re: low speed - high rpm

Those are bonding wires, the keep a maintain a solid conection between all the metal parts to allow the sacrificial annodes to do thier job. If you have one that is broken, it needs to be repaired.