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  Topic Review (Newest First)
05-04-2012 11:13 PM
Re: Turbo charged diesel

I would have tried to remove the turbines from the turbo and plugged up the shaft hole instead of paying $1500 to have it worked on. I see what you're saying exactly.
05-04-2012 10:21 PM
Turbo charged diesel

Turbocharging of smaller marine diesels seems such a waste to me. Especially when considering how sailboat power plants are located and housed. Even with plenty of cool water available, heat soak will always be an issue due to radiance.
I have much experience with troubleshooting and repairing engines, and turbocharging adds time and money to the equation. AND the necessity of turbo-knowledge. Even for the experienced, the simpler engine will be fixed quicker and easier. And also less likely to fail when you really need it to keep going....
05-04-2012 08:40 PM
Re: Turbo charged diesel

The Harley argument was simply to exaggerate a point. Turbo charging may be efficient, but seems unnecessary on a sailboat. It may allow for a smaller engine, but we require such little horsepower that shouldn't be an issue at all. The added expense to buy and maintain is not worth it. I just paid $1,500 to repair the turbo that I don't need. That's more than I paid for fuel last year by a long shot.

Now I've also realized that I have a derated 150 hp engine to 100 hp, which somewhat explains why I have this battleship engine under my companionway.
05-04-2012 02:50 PM
Re: Turbo charged diesel

Originally Posted by klem View Post
The basic answer is that with a turbo, you can have a much smaller engine which is advantageous for many reasons. You are correct that turbos force more air into the engine. This then means that you can put more fuel in too. So for the same sized engine, you have more power.
This is the reason. Smaller engine, with more HP. Why is your engine bigger than your harley? It's because it's a compression ignition engine (diesel) and not a spark ignition engine (gas). The forces in the cylinder are about 4times that of an SI engine. It has to be bigger than a 100HP gas engine. It's part of how you get so much HP for so few RMP in a CI engine. Same HP with less revs means less explosions per second but more force with each explosion.

I've heard it argued that turbo engines are NOT more efficient than naturally aspirated engines. Yes, since you can put more air in the cylinder, your and put more fuel in there too, and get more HP BUT you ARE putting more fuel in there.

A well designed naturally aspirated engine has the correct amount of air/fuel and is not lacking in it's ability to derive HP from the liquid fuel. Increasing both the fuel and air allows more HP from the same size engine, but you can't change the fact that you're only going to get so much work from a gallon of diesel and the efficiency of one type vs the other is not that big.

Smaller engine with more HP can be very advantageous on a boat where space is at a premium. I personally don't like them because they're a little more fiddly and there's more to go wrong. Also if you over prop them (ie you see black smoke EVER, or you can't reach governed RPM under load) it kills turbo engines much more quickly than diesel engines.

(most of this drivel is plagerized form articles recently read at and a book I'm reading now from 1975 on diesel engine design.)

05-04-2012 01:49 PM
Re: Turbo charged diesel

I'm thinking what you want is to get the best fuel economy at hull speed. The optimal size of a prop gets smaller as speeds increase as reduced drag becomes more important than reduced slip.

Is there a way that you can see what the throttle after the governor is on the engine? That would directly equate to fuel use. You could look at that to see what prop angle gives you the lowest throttle / fuel use at hull speed.

If you set the governor to 1800 RPM you could end up increasing the prop angle making the throttle and fuel use increase while the boat and engine are still going the same speed and you don't notice any difference.
05-03-2012 05:45 PM
Re: Turbo charged diesel


Being able to reach slightly over the rated rpm is not a bad thing. If you can't reach full rpm, it means that there is too much load being placed on the engine (too much prop pitch). In the best case scenario, this means that you have a poorly matched propeller and you are not properly taking advantage of your engine's power. In the worst case scenario, you are lugging the engine which means that you are trying to get too much power out of each combustion event and you are putting large loads on components through high cylinder pressures and high exhaust temperatures. With derated engines, it is typically hard to do damage from lugging but you still can in extreme cases.

Some people intentionally lug their engines for better fuel economy but in my opinion, there are much better ways to get your fuel economy up. High rpm has both good and bad effects for fuel economy but overall it is bad. The good is that you have less time for leakage past rings and valves, the bad is that your mechanical efficiency goes down, your combustion efficiency typically goes down and your valve losses go up by a lot.

Earlier in this thread, someone touched on one of the things that I don't like about some turbodiesels for sailboats (I can see the justification in some other applications). When people try to put a really big turbo on, they have to lower the compression ratio to keep the cylinder pressures reasonable which hurts efficiency. I like moderately sized turbos that don't drive you towards lower compression ratios but do require some beefing up of components. There are 23:1 turbodiesels out there, that is the type of engine that I like for a sailboat. The other reason that compression ratios are coming down is for NOx emissions so it is not entirely due to turbos.
05-03-2012 04:23 PM
Re: Turbo charged diesel

Originally Posted by steel View Post
Here's the datasheet:

Steel, you were indirectly more helpful than you know. Thank you.

Our Volvo is specifically a TAMD31S

Since that wasn't listed in the brochure you provided, I went to google for it and found several interesting articles on the engine. It is actually a de-rated TAMD31P, which on the link you provided shows 150hp. Ours is only 100hp.

Here's the fascinating finding. While all these other TAMD31s, run in the 3800 rpm range, our max is supposed to be 3000rpm!!

This is significant because I've been playing with prop pitch thinking she wouldn't make top rpm and was creating backpressure. She now makes 3200 rpm at full throttle while underway and I'm going to leave it there.

If I steepen the angle, the rpm is going to drop to 2800 or 2900 and I think being above max is better than below. Can anyone confirm?

p.s. Our typical cruising rpm is either 1800 or 2000, which is either side of 8kts.
05-03-2012 03:33 PM
Re: Turbo charged diesel

Here's the datasheet:

Looks like they put on the turbo to reduce emissions and increase power. If you removed the turbo the engine should work just fine. It doesn't say what the boost pressure from the turbo is but it probably isn't much. Your compression is 17.5:1 which isn't as bad as it could be. Will it start without using the glow plug?

Since your engine is heavier and bigger than other engines of the same power rating, even those without turbos, it may indicate superior quality. Longer piston rods reduce loads on the cylinders to make it last longer but make the engine bigger.

In any case, I think your engine is either too big or not geared/propped correctly. 2200 RPM is a little higher than you would want to prolong engine life. If this was a commercial fishing boat that would be a concern, but since you don't use the engine much it doesn't matter. You could get a smaller engine and run it at 2200 RPM. That would be more fuel efficient. Or you could get some kind of gear box with at 2:3 ratio or larger prop to run the engine at 1450 RPM so it would use less fuel and last 10,000 hours.

* * * * * *
I've had a hard time finding out real numbers about engine fuel consumption. They say gasoline is about 10 horsepower-hours per gallon. Two stroke is worse. The specs for a small < 10HP Chinese air cooled diesel is 183g/hp @ 2700 RPM. That's 17.7 horsepower-hours per gallon. They are direct injection and I think the compression is 20:1. The smaller ones are a little worse on efficiency than the bigger models.

A turbo increases the back pressure on the exhaust side and increases the pressure on the intake side. A typical pressure ratio is 1 intake to 1.5 exhaust. Higher is worse. Let's say it's 1:1 to make it simple. It has the opposite effect of running the engine on a high altitude mountain top. A partially loaded engine at sea level should still have about the same efficiency at high altitude shouldn't it, even though the full throttle power will be less? If you went below sea level that should hold true also, but full throttle would put out more power instead of less. That's what a turbo does.

I was thinking about it and a turbo can increase efficiency somewhat. If you increase an engine's power by 50% but the internal frictional losses are constant you do end up getting better economy out of it. But that only applies at high throttle. You can use a smaller engine in an automobile because of the turbo which saves on economy even more.

But for a boat would you choose a 16:1 compression engine with 50% boost from a turbo or would you pick 24:1 compression and no turbo? both engines are the same size and weight (not counting the turbo.) I would pick the 24:1. The 24:1 won't put out as much power as the 16:1 with turbo, but it will put out more power than a 16:1 while using the same amount of fuel.
05-02-2012 02:53 PM
Re: Turbo charged diesel

Let me try to make my point more succinctly. I'm not taking issue with what a turbo charger does.

I believe I could get 100 hp out of a normally aspirated diesel engine of essentially the same size as the one I have. 100hp non-turbo diesels are made and they just aren't that big. I am only suggesting the simplicity and lower maintenance of a normally aspirated engine seems to make more sense on a sailboat that doesn't use the thing for primary propulsion.
05-02-2012 12:15 PM
Re: Turbo charged diesel

In a diesel it is more meaningful to use the term 'power setting' which as you point out, isn't necessarily proportional to governed RPM or what most people think of as throttle. To reach 100% power setting, you have to be at 100% rated RPM. Properly set up diesels will do this without smoke and do it for a very long time.

All smoke means is that you are injecting more fuel than you have oxygen to burn it, usually because you aren't able to make governed RPM. My kubota will run at 3600 RPM (100%) at 100% power setting very happily without smoke. It is rated to do so for an hour and rated for 75% continuous power setting.

Agreed. If you go to the trouble to have gynormous Harley engine built and installed in your sailboat, it will probably work and last a long time, but at that point, why not a small block Chev (or go figure - a turbo diesel)?

So you are saying there is a vast right wing conspiracy keeping the efficiency of american cars and trucks sub optimal. I guess Ford (numerous hybrids) and Chevy (Volt) didn't get the memo. Do you think European lorrys get any better gas mileage or avoid turbo diesels for their superior efficiency?

The reason that you can put a chip in your dodge and get better mileage or better power is that they aren't strictly focused on economy. They have to balance emissions, drivability, and longevity over a huge range of operating conditions with large margins.

No, actually effective compression ratio is what matters. Pick up any book on thermo dynamics, start with carnot cycle, then look up otto and diesel cycles.

I never asserted that you could add a turbo to any old engine and increase efficiency. But for a given displacement a turbo configured diesel will usually out perform a NA higher compression engine in efficiency and power to weight. You will also find that peak efficiency is typically in the top 1/3 of the power setting range.
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