Yanmar 2qm20; Why Low Revs?

[sailorchic34]

With the old Qm series engines, if it starts fine and it runs well at 1800 RPM, it's probably pretty good.



Things I would suspect included partly blocked discharge elbow #1 on my list, injectors that needed cleaning, valve adjustment could be off a hair. After 40 years the cam could be worn just the hair as well.



it could even be a worn spot or worn spring in the governor linkage. Really the qms are happiest under 2000 RPM anyway.



if it starts easy it does not use much oil I would just enjoy it.

[SteveSadler]

All engines require power just to turn them over. The lower the rpm, the less power required to turn it over. The higher the rpm, the more fuel it burns at the same load.
Marine diesels rarely run at full throttle, as it just burns 3x the fuel for the extra 2 knots.
The biggest problem is being underloaded, resulting in a gooyee carbon buildup that won't burn off in a long hard run , which most engine manufacturers recommend to prevent carbon buildup. Go google your recommended exhaust temps to prevent carbon buildup.

[Chuck34]

Originally Posted by SteveSadler

All engines require power just to turn them over. The lower the rpm, the less power required to turn it over. The higher the rpm, the more fuel it burns at the same load.

Absolutely true....at the same load. Now when you over prop a boat, you completely change the load.

It takes energy (fuel) to run a boat and it takes the same amount of energy (fuel) to run a boat at 6 knots either running a slower 15" prop as opposed to a faster 12" prop.

There is no free lunch, otherwise all boat mfg's would be selling over propped boats and bragging how much they are helping their customers save fuel. When was the last time you saw a boat or engine mfg talk about the efficiency of over propping a boat?

[Want2cruz]

Originally Posted by Chuck34

I really have a problem believing you are saving fuel by overpropping. Think about it. Let's just use WOT figures, but the same logic applies to the full RPM range. You have an engine that burns 2 gallons per hour at WOT of 3000 RPM's with the correct prop. Now, let's overprop the boat so it now only gets 2500 RPM's. Guess what? You are still burning 2 gallons per hour. Why? Because you are running WOT, meaning the engine is burning 2 gph whether it's turning 3000 or 1000 RPM's or anything in between. The throttle is WIDE OPEN.

Now run at 70% throttle and let's say you are burning 1.2 gph. Whether you are running 2100 RPM's with the correct prop or 1750, overpropped, you are still burning 1.2 gph.

No, a engine running slower at WOT has less intake strokes per unit of time, and less fuel injections (3000 injections vs 1000 infections/min, in your example). The less full-throttle injections per min, the less fuel is injected/burned. The more revolutions per min, the more injection per min, the more more fuel burned per min.

[Chuck34]

Originally Posted by Want2cruz

No, a engine running slower at WOT has less intake strokes per unit of time, and less fuel injections (3000 injections vs 1000 infections/min, in your example). The less full-throttle injections per min, the less fuel is injected/burned. The more revolutions per min, the more injection per min, the more more fuel burned per min.

Since I'm not a diesel mechanic, I can't say you are right or wrong, and in that regard, I can't say positively that I am right

Assuming you are right, why aren't all boats over-propped to save fuel? Seems to me that would be a big time selling point.

"Mr. customer, even though the engine mfg recommends a WOT of 3000 RPM's, we intentionally over-prop the boat, bringing WOT to 2500 RPM's. We have found that will give you approx 15-20% fuel savings".

Pretty sure I've never heard a boat mfg make that boast.

[Cheechako]

Sounds like over propped. But , are the bottom and prop clean?

[Want2cruz]

Originally Posted by Chuck34

Since I'm not a diesel mechanic, I can't say you are right or wrong, and in that regard, I can't say positively that I am right

Assuming you are right, why aren't all boats over-propped to save fuel? Seems to me that would be a big time selling point.

"Mr. customer, even though the engine mfg recommends a WOT of 3000 RPM's, we intentionally over-prop the boat, bringing WOT to 2500 RPM's. We have found that will give you approx 15-20% fuel savings".

Pretty sure I've never heard a boat mfg make that boast.

Because engines run more efficiently at certain rpms (different for each engine, and even type mod (NA, vs Turbo, vs TA). Lugging an engine, )too much load, too little RPM), isn't good or efficient.

[Want2cruz]

Originally Posted by Cheechako

Sounds like over propped. But , are the bottom and prop clean?

The issue of low rpm at WOT with 0 load is serious and needs to be resolved first, IMHO.

[mowerandy]

Yeah - make sure it will free rev correctly in neutral before anything else.

Just an aside to the question, but my 31 footer was just over 20 years old when I bought her, fitted with the original issue Yanmar 12HP and prop. Frustrated the heck out of me that I could only get about 3.5 knots or so at full screaming throttle, so I swapped a second hand prop of similar pitch but about another inch of diameter. Over-propped? yeah, sure. Did it work. you bet. I can motor at 5+ knots now at sensible RPM. Been doing so for about 25 years and have had no significant repairs or maintenance - this motor has had virtually nothing done to it in around 45 years, and has been over-propped more than half its lifetime but still going strong.

Yes, the hull and particularly the prop need to be clean, but so did the old prop and sailing an overgrown boat isn't much fun anyway. The main problem, if you can call it that, is that I use so little fuel that it goes stale in the tank every few years. Yanmar 12s don't use much to begin with but this engine runs literally on the smell of an oily rag - it never ceases to amaze me just how little fuel it uses.

I keep the original prop as a spare but would never go back to it given the choice. Yes, I would rather repower with something bigger, but that prop was only 50 bucks. Any bigger or pitchier and it wouldn't have worked though.

[01kiwijohn]

The number of injections has little to do with fuel consumption at any particular RPM. It is the quantity of fuel, at each injection, that influences fuel consumption and that is governed by the fuel injection pump (unless you're dealing with unit injectors, which is a whole 'nother story).
So, if you are running at say 1500 RPM, with WOT, and the engine is not accelerating, it'll be getting too much fuel for the volume of air available. Is that clearer?

[Chuck34]

Originally Posted by mowerandy

Yeah - make sure it will free rev correctly in neutral before anything else.

Just an aside to the question, but my 31 footer was just over 20 years old when I bought her, fitted with the original issue Yanmar 12HP and prop. Frustrated the heck out of me that I could only get about 3.5 knots or so at full screaming throttle, so I swapped a second hand prop of similar pitch but about another inch of diameter. Over-propped? yeah, sure. Did it work. you bet. I can motor at 5+ knots now at sensible RPM. Been doing so for about 25 years and have had no significant repairs or maintenance - this motor has had virtually nothing done to it in around 45 years, and has been over-propped more than half its lifetime but still going strong.

Really glad that the new prop has worked out so well. Nothing more frustrating than a boat not running as good as it could be.
A couple of questions....what is your engine's suggested WOT from the mfg? What was your WOT with the old prop? What is your WOT with the new prop?

[Want2cruz]

Originally Posted by 01kiwijohn

The number of injections has little to do with fuel consumption at any particular RPM. It is the quantity of fuel, at each injection, that influences fuel consumption and that is governed by the fuel injection pump (unless you're dealing with unit injectors, which is a whole 'nother story).
So, if you are running at say 1500 RPM, with WOT, and the engine is not accelerating, it'll be getting too much fuel for the volume of air available. Is that clearer?

At WOT, each injection is at the maximum. The higher the RPMs, the more "maximum" injections per min. More fuel injected is more fuel burned.