Diesel VS 4 stroke outboard fuel consumption

Your outboard is in the wrong place. You can't have an outboard mounted on the extreme stern and expect it not to porpoise as your boat pitches.

I'm not a catamaran guy, which makes this easier, but my outboards will be installed well forward of the rudders as close to amidships as I can go (really about 1/3 of the way toward the bow from the stern).

I have to carefully evaluate the pitch on the outboards since my boat will be kind of fast and easily driven.

 

Joakim,
The technical editor of Passagemaker magazine recomends 75% load 75% of the time. Almost everyone underloads by that definition and it seems most get away with it. As far as I know engine loading at less than WOT can only be determined by fuel burn. If your engine burns 10gph at WOT and you usually burn 6gph your'e at 60% load ect ect. One should get the WOT burn rate from the manufacturer of course. I tried hard to get a small enough engine for my boat so I could run at 75% but w the engine I chose I am probably loaded at 60 to 65%. I run at 2300rpm on a 3000rpm engine (700 down from max). Your'e a tad lower percentage than I but we're probably doing fine in this regard but workin em a little extra at times is probably good. I run mine 2500 at times for 15 to 30 min.There are lots of pleasure boats out there running around at 30 - 40% load. That's probably NOT good.
I tried to add some spice and things to ponder with our discussion but I think clearly the diesel on our sailboat will be the fuel miser but there is so very much more to consider that clearly is more important.

Easy Rider

 

For a displacement boat power ~rpm^3, thus (2300/3000)^3 = 45% of the max load, not far from mine. I do run my engine occasionally at full power, but mostly about 2500 rpm.

I think the recommendation of 75-80% is for rpm, not power. How do car diesels survive 500 000+ km (10 000+ hours), with typical power of 10-30 hv for 100-150 hv engines?

 

Excellent point Joakim.

Lots of car diesels are rated around 100 Hp with 10 - 15 Hp used for most driving with the RPM in the 1/2 - 2/3's range.

5 - 20% of driving would be close to WOT for accelerations.

That's a lot of light load running isn't it...

-Tom

 

My understanding is that the bane of diesels is the short trips where the engine doesn't get up to full operating temperature. An experienced Mercedes/VW mechanic in Bellingham mentioned that the VW TDI's are more resistant than the typical diesel to this kind of abuse for some reason.

On these larger marine diesels, if we've been running light loads for too long we'll typically look for opportunities to run them up to full load for a stretch to blow all the crap out of the stacks. Other than the fire hazard associated with not doing this, light loads don't seem to be much of a concern to engineering.

 

It's not "crap in the stacks" its stuck rings from varnish, tar and carbon buildup in the ring lands valve guides and other places. The stuck rings will lead to hard starting, loss of compression and for some other reason "glazing" of the cylinder walls. Diesels only run a fully rich mixture at full throttle. As you dethrottle a diesel from WOT you go from about 15-1 mixture to (when you reach idle) 60-1. With a gas engine idling you can fry eggs on the exhaust manifold ect but not on a diesel. The whole engine is stone cold by comparison. This is the reason diesels take so ling to warm up. A diesel engine reaches it's continuous operating temperature long after it's coolant, and even after it's lube oil temp rises to it's stable level. This is all highly related to the engine loading question. You can run a diesel at idle at the dockside for 5hrs and it still won't be ready for 50% load. I worked in a mine in Alaska and it took 24hrs to warm up a 1440hp diesel to full load operating temp. All these trawler guys running at 35% load are running cold engines. They never warm up. Gas engines are fine w that as the combustion temperature is always the same ..idle to WOT.
This engine loading question gets kicked around a lot as there are lots of engines that aren't loaded to the standard. Fishermen troll all day at 20-30% load and trawler yachts run down to 30% load and neither seem to suffer greatly for it. I run my own diesel boat at about 60% load almost all the time as I do (more or less) believe a diesel should be worked fairly hard.

Easy Rider

 

"crap" of course is a technical term for black sooty chunks that end up all over the decks.

 

Joakim,
It could be based on rpm if the propeller was a linear load but it isn't even close. It's a bit like the resistance of a full displacement hull. Even in a car where the air drag is 100% of the resistance with a car that has a top speed of 100mph and 100hp needs only 25hp to go 50mph. A 42' Krogen goes 9 knots with 85hp and makes 4.5 knots w 5hp. A Neville 39 makes 9 knots w 105hp
and 4 knots on about 5hp. The propeller load ramps up much like the FD hull
resistancs. When you combine the two the only time there's much load at all is at or near WOT unless your boat is over propped. Engine load on a diesel powered boat is directly porportional to fuel consumption. Remember ....
It's a HEAT engine.

Easy

 

When I worked for Ford programming engine controllers, load referred to how close the engine was to max torque at a given rpm. Fuel burn was somewhat proportional to HP - something quite different. In other words, one could run at 100% load at 10% of max output. Or 10% load at max rpm (but probably neither of those with a fixed pitch prop).

 

For a displacement boat near "hull speed" with "normal" propeller the equation power ~rpm^3 is close enough. On bollard pull the equation is exact and forcing through the hull speed with excessive power is almost like bollard pull. Most displacement boats have much excessive power over the needed to reach hull speed. They will show power ~rpm^3 and they should be driven with very low load, at least from a fuel consumption (and wave making) point of view.

Yes the equation fails and depends very much on the boat on such a big scale as you show. Then we are far from hull speed and the speed of the boat changes much more with rpm. Around hull speed rpm changes primarily slippage not speed.

 

Where do these fairy tales about diesels come from?
What you find nowadays in boats up to 35 ft and sailing boats up to 45 ft are all derivatives of car engines. Both in cars and boats they are lightly loaded so burn a very lean mixture and have minimal deposits of any kind.

Since the invention of the thermostat, these engines need just a few minutes to reach a decent operating temperature. The twin VW diesels in my boat reach a stable coolant temperature within 1 mile, running at 1600-1800 rpm which is less than half of the maximum, so barely 20% load. Only at idle they need incredibly long to warm up.

My Kia Sorento has a somewhat larger common rail diesel which needs approx 1.5 miles in summer. In winter the situation is a bit different: the engine doesn't produce enough calories to heat the cabin, so the factory installed a diesel fired heater that cuts in automatically at 5 C. Warming up is almost as quick as in summer but that is no fair comparison.

Most small car diesel can drive around the globe 15 or 20 times without overhaul, both the ones in delivery vans, driven with the pedal floored and those that have never reached 2500 rpm.

I once built an automatic generator for a guy who lives in an even more remote spot than mine, using a scrapyard VW diesel that ran at a fixed 1500 rpm. It had survived over 5000 operating hours before it died from gross neglect. When he called me for help I found the engine without oil filler cap: black oil everywhere except in the oil pan.

If fact the only engines I ever owned that spat out embarrassing amounts of soot at every start-up were two Mercruiser 3.0 ltr gas engines. They never reached a decent temp. because they were raw water cooled and had thermostats opening already when the water was only lukewarm.

 

If you want to know if your engine has warmed up check the oil temp

PS the thermostats in raw water cooled sterndrives outboards are all 143F.
If you heat the salt water up too much the salt comes out of solution and deposits a very tough insulating coating on all the surfaces

 

"The stuck rings will lead to hard starting, loss of compression and for some other reason "glazing" of the cylinder walls"

The force of the gasses in the firing stroke seal the rings from BEHIND the ring , not just from pressing down on the rings.

With light load the minor pressure created burnishes the cylinder walls ,smoothing the oil retaining scratches , so the unit has a shorter service life.

The blow by into the crank case requires more and more frequent oil changes.

Props are hell on a diesel, as a small rpm reduction takes so much of the load away.

With the std sized engines used in sail boats , Either a CPP prop is needed , or for poorer folks a "cruising prop".

A cruising prop loads the engine heavily at the desired RPM , not at the max hp rpm as the engine marinizers would like.

IF a 30ft lwl boat needs say 16 hp -20hp for normal cruise (3/4 to 1 gph fuel burn) and the installed engine is 50hp at 2500rpm , its really inefficient to run slowly.

So the boat will be "overpropped" so it cruises at say 1500rpm , and when the throttle is advanced the engine will only go to 1800 rpm.

This is FINE , as long as the operator realizes 1500 is the MAX cont , and 1800 rpm can never be used.

The boat will NOT have a reserve for climbing big waves, but big waves usually have big winds , a sailors delight.

Use the Gen set power table for the engine you want to use and it is a great guide to where (RPM) the power you need is located.

FF

 

What would be this shorter service life? At least around here the diesels in sailboats last 20-30 years and 90% of them are killed by rust. Most of the 70's and 80's diesels were raw water cooled, thus I'm expecting a longer life for modern engines.

Oil is changed once a year.

A car has a very similar power demand curve as a propeller.

Why would you install a 50 hp engine, if you are going to overpropp it to be able to deliver only 20 hp? 1500 rpm at cruising speed (probably 7.5 kn for 30 ft LWL + 20 hp) means that idle speed will likely be over 4 kn. Not very nice!

Much better to install the 20-30 hp engine in the first place.

 

Check the BSFC curves for any engine and you will see that fuel economy is better not running the engine at max output. Plus it will last longer. So buying a moderately larger engine than you need and then overproping makes some sense.