RPM issue under load

[Puregravy]

When under way we noticed the engine shift its pitch and found the rpm would not go above 2000 rpm. Thinking we may tangled a prawn trap line (none evident) we placed her in reverse and dove to double check when at anchor. No sign of fouling. Given we like cruising between 18-2000 rpm we have not really tried pushing the engine. Further research indicated that max rpm for our engine is 3600 with a max cruising rpm at 75% of that.

We are not able to go above 2000 rpm under load. She will when in neutral. There is no sign of fouling on the shaft. We have Volvo Penta 2020B with a MS2A-D Reverse gear and a 13"x12" Michigan 3 blade prop and a neoprene cutlass.

There was 2 instances last year of some short term clatter/grinding sounds coming from the gear box. I've changed the oil and there was no sign of metal flaking. The oil was very dirty. As the boat is new to us and there was no mention of gear box oil change I'm not sure when it was last changed. The cutlass area is cool to the touch when running.

I've searched this section of the forum but have not found any similar issues. Any suggestions or concerns we should look at before taking her in? We're just heading out cruising to Desolation Sound for 6 weeks and want to make sure there is not a bigger issue lurking in the background waiting to happen.

Thanks
Rob and Maura
Wandering Star
1979 Catalina 30

[a64pilot]

How high an RPM did it used to rev to under a load?
Will it rev higher in reverse than forward?

[Puregravy]

Originally Posted by a64pilot

How high an RPM did it used to rev to under a load?
Will it rev higher in reverse than forward?

We only have taken it to 2200 under load as that was our "sonic" tolerance and were not sure of the max rpm at the time. We got the same results in reverse. I got to wondering as well if it would be a fuel issue but have changed the filters between the time it first happened and now.

[billknny]

When you now push the throttle all the way up, and the engine tops out at 2000RPM, does it smoke at all?

[Puregravy]

Originally Posted by billknny

When you now push the throttle all the way up, and the engine tops out at 2000RPM, does it smoke at all?

No. Just stays at the same rpm. I was wondering if it was a governor issue but it revs higher when not under load.

[Puregravy]

Originally Posted by Puregravy

When under way we noticed the engine shift its pitch and found the rpm would not go above 2000 rpm. Thinking we may tangled a prawn trap line (none evident) we placed her in reverse and dove to double check when at anchor. No sign of fouling. Given we like cruising between 18-2000 rpm we have not really tried pushing the engine. Further research indicated that max rpm for our engine is 3600 with a max cruising rpm at 75% of that.

We are not able to go above 2000 rpm under load. She will when in neutral. There is no sign of fouling on the shaft. We have Volvo Penta 2020B with a MS2A-D Reverse gear and a 13"x12" Michigan 3 blade prop and a neoprene cutlass.

There was 2 instances last year of some short term clatter/grinding sounds coming from the gear box. I've changed the oil and there was no sign of metal flaking. The oil was very dirty. As the boat is new to us and there was no mention of gear box oil change I'm not sure when it was last changed. The cutlass area is cool to the touch when running.

I've searched this section of the forum but have not found any similar issues. Any suggestions or concerns we should look at before taking her in? We're just heading out cruising to Desolation Sound for 6 weeks and want to make sure there is not a bigger issue lurking in the background waiting to happen.

Thanks
Rob and Maura
Wandering Star
1979 Catalina 30

In reviewing past maintenance logs the previous owner undertook a engine inspection report and the mechanic noted engine rpm "noticeably restricted" and suspected that the 13"x12" 3 blade Michigan prop to be too big. Any opinions?

[a64pilot]

When I bought my boat it has a Yanmar 4JHE, a 3600 RPM engine, will rev easily to 3500 in neutral, the last 100 may be Governor slightly out or tach slightly out, not worth messing with.
She would make 8 kts at 2200 RPM at full throttle, was severely overpropped, but didn’t smoke at all or overheat.
Funny thing is prop size which was I think 18x14 was what Island Packet specified, but prop may have been repitched I guess.

Reason I asked about reverse is if they are overpropped they will usually turn higher RPM in reverse due to transmission gearing and that a prop is less efficient in reverse.

I’m betting your overpropped, which can cause harm to the engine, or not depending on how you use it, if you realize your overpropped and don’t push it too hard, you’ll be fine, if you don’t, it can overtemp and harm the engine. Overtemp the combustion chamber and exhaust valve, not necessarily the water.

[Puregravy]

Originally Posted by a64pilot

When I bought my boat it has a Yanmar 4JHE, a 3600 RPM engine, will rev easily to 3500 in neutral, the last 100 may be Governor slightly out or tach slightly out, not worth messing with.
She would make 8 kts at 2200 RPM at full throttle, was severely overpropped, but didn’t smoke at all or overheat.
Funny thing is prop size which was I think 18x14 was what Island Packet specified, but prop may have been repitched I guess.

Reason I asked about reverse is if they are overpropped they will usually turn higher RPM in reverse due to transmission gearing and that a prop is less efficient in reverse.

I’m betting your overpropped, which can cause harm to the engine, or not depending on how you use it, if you realize your overpropped and don’t push it too hard, you’ll be fine, if you don’t, it can overtemp and harm the engine. Overtemp the combustion chamber and exhaust valve, not necessarily the water.

Great points. Thank you for your thoughtful response. I have coming around to the overpropped perspective as well. I called the prop manufacturer with requisite info re engine size and gearbox ratio and was told I had the right size. But then in talking to the mechanic that mentioned in a 2015 engine survey that he thought the prop was over sized, there is a bit of 'black art' in proper prop sizing and that stepping back in pitch would be a good step especially for health of the engine. I will double check the reversing rpm no that I have a clearer mind and not one that is worried about the endless possibilities.

I appreciate your comments around heat build up especially those other than water temperature. I will pamper and not push until I can re-pitch.

Thanks again.

Rob Purgavie
Wandering Star

[Wotname]

Originally Posted by Puregravy

When under way we noticed the engine shift its pitch and found the rpm would not go above 2000 rpm. Thinking we may tangled a prawn trap line (none evident) we placed her in reverse and dove to double check when at anchor. No sign of fouling. Given we like cruising between 18-2000 rpm we have not really tried pushing the engine. Further research indicated that max rpm for our engine is 3600 with a max cruising rpm at 75% of that.
..................

Originally Posted by Puregravy

We only have taken it to 2200 under load as that was our "sonic" tolerance and were not sure of the max rpm at the time. ...............

It does sound like something has changed. Clearly the prop pitch hasn't changed so while you may be over-propped, this isn't cause the change in engine "pitch" and decrease in rpm that you report.

Have you checked the exhaust mixing elbow for corrosion / restriction? This should be a regular maintenance item, especially if using the engine regularly well under it's max rpm.

[Puregravy]

Originally Posted by Wotname

It does sound like something has changed. Clearly the prop pitch hasn't changed so while you may be over-propped, this isn't cause the change in engine "pitch" and decrease in rpm that you report.

Have you checked the exhaust mixing elbow for corrosion / restriction? This should be a regular maintenance item, especially if using the engine regularly well under it's max rpm.

I have not. As I am new to the world of marine diesels and this boat so I will explore this as it has not, to the best of my knowledge, been part of a regular maintenance regime. Will report back when I find out what and where mixing elbow is!

[Wotname]

Originally Posted by Puregravy

I have not. As I am new to the world of marine diesels and this boat so I will explore this as it has not, to the best of my knowledge, been part of a regular maintenance regime. Will report back when I find out what and where mixing elbow is!

Good start

Look at the exhaust manifold and see where the cooling water is injected into the exhaust system, usually very near where the flexible rubber exhaust hose is connected to the mixing point. Not always an elbow but more often is.

This area of the exhaust can clog up and restrict exhaust flow, sometimes dramatically. The engine apparently runs fine forever and then one day it doesn't want to rev.

[Puregravy]

Originally Posted by Wotname

Good start

Look at the exhaust manifold and see where the cooling water is injected into the exhaust system, usually very near where the flexible rubber exhaust hose is connected to the mixing point. Not always an elbow but more often is.

This area of the exhaust can clog up and restrict exhaust flow, sometimes dramatically. The engine apparently runs fine forever and then one day it doesn't want to rev.

Got it! See it! The outside 'nipple' where the elbow that takes the cooling water into what I believe is the heat exchanger has signs of an electrolysis type of corrosion. (could be the dis-similar stainless being I proximity to the cast of the elbow). Regardless if there are external signs chances are there will be internal realities.

Knowing that this is going to be a regular winter maintenance issue moving forward, balanced with the reality that we have just started a 6 week celebratory adventure (turning 60!), what are the impacts of putting g another 80-100 hours on the engine with a constricted mixing elbow?

A new elbow is available in two days for $550 CND shipped to our next port of call.

In appreciation,

R

[Compass790]

Originally Posted by Puregravy

In reviewing past maintenance logs the previous owner undertook a engine inspection report and the mechanic noted engine rpm "noticeably restricted" and suspected that the 13"x12" 3 blade Michigan prop to be too big. Any opinions?

It struck me reading your original post that your prop is waaay overpitched. I wuda thort 7-9" pitchbut you can go to vicprop propelllor calculator on Uncle google & feed in yr boat specs & see what it spits out.
I've been on a DIY re-pitching exercise recently so looked a bit at this stuff. Think that mechanic was on the ball. But it could be many things. I'm not discounting other suggestions but that prop sounds wrong for a start

[Stu Jackson]

I may be wrong, not an expert on props, BUT, having said that, it seems your boat was repowered somewhere in its life, 'cuz your engine as far as I know was not originally installed on C30s.

That said, though, there is a plethora of Catalina 30 websites, International Catalina 30 Association being one. I am sure that recommendations for prop size and pitch are there and have been for decades. They made over 6,000 Catalina 30s. Unfortunately, there are altogether too many C30 websites, instead of having everything in one place, like we do.

Ken Kloeber, a C30 owner who contributes (terrorizes?) our C34 forum and website, has just started a C30 tech wiki. Try this:

Tech WIKI

Good luck, happy research.

[a64pilot]

Did you say it actually changed RPM? I didn’t get that.

Either way in all honesty a spare exhaust elbow is a good spare to have, it’s not a matter of if you’ll ever need it, but when if you cruise.