RPM issue under load

[Wotname]

Originally Posted by Puregravy

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

If you do have a constriction in the mixing elbow / bend / riser and if it causing your current issue, it could deteriorate rapidly.

You should be able to check though without replacing anything yet. Often it can be cleaned enough to be serviceable.

The item should be something like item 15 here https://www.marinepartseurope.com/en...0-25-4157.aspx . you might need a new gasket as in item 16.

Or it maybe more like item 1 here https://www.marinepartseurope.com/en...0-25-4166.aspx with a gasket 3.

[Puregravy]

Originally Posted by Compass790

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

Tried a couple of different prop sites including talking to and using Victoria Props website. Talked to Michigan prop, the supplier of my current prop as well. Vic prop said that although a tuning of the prop may well be in order that the kind of rpms I am dealing with suggest a mechanical issue as well, indicating that 1 degree of pitch can effect rpm by 200 rpm.

Would love to hear learn about how one goes about getting the correct pitch and prop as that is now on the books too.

R

[Compass790]

Originally Posted by Puregravy

Tried a couple of different prop sites including talking to and using Victoria Props website. Talked to Michigan prop, the supplier of my current prop as well. Vic prop said that although a tuning of the prop may well be in order that the kind of rpms I am dealing with suggest a mechanical issue as well, indicating that 1 degree of pitch can effect rpm by 200 rpm.

Would love to hear learn about how one goes about getting the correct pitch and prop as that is now on the books too.

R

What did the vicprop calculator tell you?

[Puregravy]

That I need a 14.2x8.4. Based on keeping a 13" diameter prop (not sure if I have room for the extra 1.2") and using their suggested 1" decrease in diameter for and 2 degree shift in pitch would put me at a 10.8 degree pitch. I have 12. If I use the 1.5" degree shift the suggested pitch is 9.2 degrees. Using the rule of thumb that 1 degree of pitch increase will create a loss of 200 rpm gives me a loss of between 240-560 rpm, still shy of the 75% factor for the maximum 3600 with nothing left power wise to push past 2700 rpm (if that makes sense). I am going to look at re-pitching this winter and in the interim cleaning out the injection elbow that leads to the vented loop and order a new elbow and gasket to have on hand as recommended by Calder. Make sense?

[Puregravy]

Originally Posted by Wotname

If you do have a constriction in the mixing elbow / bend / riser and if it causing your current issue, it could deteriorate rapidly.

You should be able to check though without replacing anything yet. Often it can be cleaned enough to be serviceable.

The item should be something like item 15 here https://www.marinepartseurope.com/en...0-25-4157.aspx . you might need a new gasket as in item 16.

Or it maybe more like item 1 here https://www.marinepartseurope.com/en...0-25-4166.aspx with a gasket 3.

Wow! Thanks. Yes. Mine looks like 15. Calder suggests packing an extra and given we are heading out for upwards of 7 weeks, as a anchor neighbour said, "you can always find a mechanic but not always the needed part" I may just order one to be delivered to our next port of call. Other than potentially needing a new gasket I may be able to clean out the existing mixing elbow. There looks to be some external calcification on the nipple heading to the siphon tube. I'll start there.

Thanks for this.

[Compass790]

Originally Posted by Puregravy

That I need a 14.2x8.4. Based on keeping a 13" diameter prop (not sure if I have room for the extra 1.2") and using their suggested 1" decrease in diameter for and 2 degree shift in pitch would put me at a 10.8 degree pitch. I have 12. If I use the 1.5" degree shift the suggested pitch is 9.2 degrees. Using the rule of thumb that 1 degree of pitch increase will create a loss of 200 rpm gives me a loss of between 240-560 rpm, still shy of the 75% factor for the maximum 3600 with nothing left power wise to push past 2700 rpm (if that makes sense). I am going to look at re-pitching this winter and in the interim cleaning out the injection elbow that leads to the vented loop and order a new elbow and gasket to have on hand as recommended by Calder. Make sense?

yes that sounds a lot better than yr existing setup. The guy who does your repitching , if he does yachts a lot, ( sailboats ) should be able to help.
When I had a go at ours the vicprop calculator was close in its recommendation. Not allowing yr engine to reach at least the 75% is a bit akin to climbing a steep hill in yr car in overdrive. Of course you will need higher revs to produce the same boatspeed if you reduce pitch so you will get into yr sonic pain barrier sooner. It's all a tradeoff.

[NYSail]

I would open the mixing elbow and check before you leave. Classic symptoms.

[sailcrazy]

If you have not re-propped in recent months, I'd doubt the prop is your problem. We had a similar issue on our Yanmar some years ago....turned out to be the fuel control. We took it off, had it stripped down and rebuilt, and no more issues since. I'd look at the injectors and the fuel control as likely suspects.

[alaskaflyfish]

I'm just going thru the same thing now. I just changed my prop due to thinking it was over-propped (3 blade 12x10). It was not the problem. I wanted a Martec folding prop (13x9)anyway, but rpm remains the same . My Yanmar 1gm10 8hp should turn 3600 rpm under load and I was only getting 2500rpm. I did not realize this was an issue because the boat was new to me and I was able to reach hull speed. I changed the prop only after completely redoing the fuel system, replacing the engine head with new, new mixing elbow, fuel supply pump, and injector. I did this engine work cause the engine was neglected and needed it anyway. The only thing left to do in the injection pump which coincidentally the Yanmar maintenance manual trouble shooting section states is the cause for lack of power. That being said; It does not sound like your over-propped based on my experience. Check your fuel system for air leaks or fuel starvation, mixing elbow restrictions. After that start checking your injector and pump.

[a64pilot]

Realize guys that he was only able to obtain 2200 RPM prior to this, not the rated 3600 RPM.
Plus he has a previous engine survey that noted that RPM was noticeably restricted.
Engine wasn’t making anything near rated RPM to start with

[alaskaflyfish]

I guess what I am trying to say (correct or incorrect) is that my little 8hp Yanmar is rated to have a 3 blade 12x10 Campbell Sailor or a Martec 13x9 2 blade folder and make 3600RPM. The OP's 20hp Volvo with 3 blade 12x13 sounds about right. The fact that the OP's Volvo 20hp diesel is only making 2000rpm and not blowing black or grey smoke leads me to believe it is fuel flow problem. I think the existing rpm issue was just transferred from one owner to the next unsuspecting owner, just like my situation. This is just speculation on my part (just a guy on a keyboard), I cannot diagnose this without a hands on inspection.

[Squanderbucks]

Sounds like the first place to consider after the usual stuff like filters, etc is the prop as mentioned above. Especially when it was noted that the engine was overpropped by a prior mechanic.
While it will do little damage to run the engine overpropped at low to moderate speeds you really don't want to run it for long periods of time with the boat "fully loaded" which could be against heavy seas, full boat weight or near the 2000 RPM you can make. Essentially if you run it at the 2000 RPM which might be near your expected normal cruise RPM - you will be essentially running the engine at full bore. It's just that you will be asking it to pull such a load that it can't make it's real fully rated RPM.
The engine is telling you I can't meet my specs because you are asking me to pull to hard ( in this case too hard against the water going around the prop). No different that any other overload.

[Wotname]

Thinking aloud for a moment

We know:
OP only ever got 2200 rpm (presumably at WOT and no smoke).
He has a 20 hp engine with max rpm of 3600.
LOA 30' and 13x12 3 bladed fixed pitch prop
Recently rpm dropped to 2000 while underway and wouldn't increase (presumably even at WOT).
It will rev higher in neutral.

We don't know the gearbox reduction ratio but are told that a suitable prop would be ~14x8.
As a data point, I had a 14x8.5 prop with 18 hp on a 31' LOA and it would reach max hp at 3600 rpm.

The absence of smoke at WOT suggest to me the prop is not way out of spec. Probably not ideal but not the only issue.
The absence of smoke suggests there is not enough fuel getting into (or out of) the engine at WOT.

Whatever caused the rpm reduction while underway is also probably causing the low max rpm (2200).

My approach is start with the easy and work towards the hard. Check engine operation after each stage and continue down the list until either max rpm improves or smoke appears at WOT. Once you can get say 2800 or better and smoke at WOT, try prop pitching etc.

Change all fuel filters
Check pick up in tank
Check air filter for obstruction or just replace
Clean exhaust mixing elbow/bend
Ensure good flow of fuel to injector pump or rig a temporary gravity feed small tank with clean fuel
Have injectors checked
Have injector pump checked.

Ideally you should be able to make max rpm at WOT with a clean prop and bottom with no smoke.

This is probably a long winded version of what alaskaflyfish has already posted (#24)

[Compass790]

Originally Posted by alaskaflyfish

I guess what I am trying to say (correct or incorrect) is that my little 8hp Yanmar is rated to have a 3 blade 12x10 Campbell Sailor or a Martec 13x9 2 blade folder and make 3600RPM. The OP's 20hp Volvo with 3 blade 12x13 sounds about right. The fact that the OP's Volvo 20hp diesel is only making 2000rpm and not blowing black or grey smoke leads me to believe it is fuel flow problem. I think the existing rpm issue was just transferred from one owner to the next unsuspecting owner, just like my situation. This is just speculation on my part (just a guy on a keyboard), I cannot diagnose this without a hands on inspection.

Just curious, have you checked on the vicprop calculator what your prop should be Alaskafly fish.We have a YSM8 Yanmar which is only 7hp continuous at 3200 & our boat is only 1ft shorter( probably the same @ LWL )
According to vicprop calc you are over-propped too. Can you reach that 3600 rpm?

[alaskaflyfish]

With the Martec 13x9 I can only reach 2500rpm. The prop specs I have were confirmed thru 3 sources. One being Martec who did the calculation based on my boat and transmission specs, the next was a online calculator offered by a prop manufacturer in Vancouver B.C. and finally the Prop Shop in Seattle who also did the calculations for me based on a 2 blade fixed prop. Online research on other boats with the 1gm10 Yanmar state that the Martec 13x9 was a good match. I will look again since you brought this up.


So I just checked Vicprop with the boat and engine specs and they list 13.1 x 8.8, so it looks like Im over by 0.2 on the pitch.