over powering your vessel

[scott2640]

I have heard various things about using a larger than necessary diesel engine to power a vessel. specifically i'm referring to a full displacement trawler (21 ton) operating on a single screw at low RPM utilizing a less than a 100 HP engine (as the designer recommends).

setting aside the extra cost of a larger engine than needed and the lost fuel economy, is it bad for a modern diesel engine to be run under loaded?

i have heard it both ways. yes- the engine needs to be properly loaded to run efficiently and maintain its well being and avoid internal build up.

I have heard no- modern engines, particularly turbos, suffer no ill effects being run continually at low RPM.

As I shop for used engines, i find very few for sale in the 100hp range. however, there seem to be many 6bt's and comparable lehmans in the 200+ range at good prices.

what say you?

[Stumble]

It depends on the engine. But finding a ~100hp diesel isn't all that difficult. All of the major players make a motor that size.

[JstaRebel]

Originally Posted by scott2640

I have heard various things about using a larger than necessary diesel engine to power a vessel. specifically i'm referring to a full displacement trawler (21 ton) operating on a single screw at low RPM utilizing a less than a 100 HP engine (as the designer recommends).

setting aside the extra cost of a larger engine than needed and the lost fuel economy, is it bad for a modern diesel engine to be run under loaded?

i have heard it both ways. yes- the engine needs to be properly loaded to run efficiently and maintain its well being and avoid internal build up.

I have heard no- modern engines, particularly turbos, suffer no ill effects being run continually at low RPM.

As I shop for used engines, i find very few for sale in the 100hp range. however, there seem to be many 6bt's and comparable lehmans in the 200+ range at good prices.

what say you?

I'd say that a Lehman SP 135 would fit right into your boat just like it does on many other trawlers and motorsailers with full displacement hulls. I'd also say from experience that it moves an 18 ton motorsailer 7.4- 7.6 knots @ 1800 RPM. I'd also say to stay away from turbos and stick with a low RPM naturally aspirated motor.

Here is one used for sale @ $6,500.

Ford/Lehman Power Model 2725E Marine Diesel Engine

[blackswan]

Running diesels at low RPMs is not a good idea. It polishes the cylinder bores which over time will reduce efficiency and accelerate wear. This is caused because incomplete fuel combustion and loss of lubrication results in the cylinder bore being worn away (polished) which leads to fouling of exhaust ports and valves, along with increased amounts of exhaust pollution. Proper controlled loading is therefore essential. It increases diesel engine life by up to four times and improves the efficiency by ensuring that the engine has sufficient loading at all times.

[a64pilot]

Running an engine at excessively low load isn't a good idea, although many will correctly point out that truckers used to idle theirs overnight and sometimes longer, they key though is I believe that they would also run them up and put them under a load.
It does not take much at all really to get a Diesel up to normal operating temps and that is what you want to do, you don't have to run one at 75% power.
In my opinion as a long time mechanic unless you go crazy large with an engine that you will have no problem, just every now and again, run her up and let her roll a big wave for a few minutes.
I don't believe there will be much difference in fuel consumption either, yes a 100 HP motor will produce 75 HP with a little less fuel than a 300 HP motor will, but its not much different, the difference is the internal friction of the larger motor, and that is not much at all.

Most of the time the average Diesel pickup is run at 1500 RPM or less and at relatively low load, yet the last for hundreds of thousands of miles.
I think most often the average Diesel pickup is running empty, carry one person around town, not hauling heavy loads.

Biggest downside of a big motor, is initial expense, weight and cost of parts and oil, coolant etc is double, but if its doing the same work, likely its burning very similar amounts of fuel

[darylat8750]

I think a single Ford/Lehman 6 cyl. would be a very good engine for the money in a DD41. If the pockets are a little deeper one of the 75 to 100 hp Beta engines would be my preference. I run 2 Lehman 2715s in my 42 foot trawler and would be very happy with just one. When considering a Lehman get the specifics on it and call Brian at American Diesel. His father was (I believe) the original importer/ modifier of these engines and Brian knows about everything there is to know about them. One thing to be aware of is that there are two different types of fuel injectors and parts are easier to get for one than the other.

[a64pilot]

Myself I would put in a Cummings in a heart beat, and it would still likely be running great when they wheeled me off to the old folks home

[NewMoon]

Our 26-footer's Volvo Penta KAD44P (260hp, WOT 3800-3950 RPM) has been run at only 6 knots (1300-1400 RPM) more than 90% of the time for the last 12-13 years. Before that, at 16-18 knots (3200-3400 RPM) maybe 70% of the time. Presently has 6502 hours on the meter, and shows no sign of being messed up or tired out.

We wouldn't run at idle speed (600 RPM) for hours at a time - it would be too cool, and unevenly so. Better to make it work hard enough to keep internal temps up to nearly planing-cruise-power normal, maybe 175 degrees. You could do similarly with a Cummins.

[darylat8750]

Originally Posted by a64pilot

Myself I would put in a Cummings in a heart beat, and it would still likely be running great when they wheeled me off to the old folks home

I agree with you there! The 6 cylinder Cummings are very good engines and more modern than the Lehmans. But the 6 cyl. have more power than required for this application and the 4 cylinder Cummings, while plentiful in agriculture and construction equipment, do not have as good a reputation as the 5.9 through 6.8 liters.

If price were no object I would be inclined toward a 4 cylinder Lugger or a very close second would be the 4.5 liter Johne Deere for a 41 foot Diesel Duck.

[SVTwilight]

Originally Posted by a64pilot

Myself I would put in a Cummings in a heart beat, and it would still likely be running great when they wheeled me off to the old folks home

I have a Cummins 6BT 5.9l in my heavy 31' Duffy Lobster boat. It will run forever at 1500 rpm and is thrifty with fuel and very smooth. Parts are reasonable and easy to get. And yes, from what I have seen, I fully expect it to continue to run that way pretty much forever. SBMAR in Cal has photos of units with over 25k hours!!!

[Cheechako]

A Perkins 4-236 is I think 81HP. I had one in a little 32 ft trawler weighing probably half what yours does. They are great engines, as are the Lehman/Ford diesels in the 90-120 HP range. You are not going to get conclusive answers about running at lower RPM. Many disagree either way. Truckers idle their diesels for many hours on end while sleeping, eating etc to no apparent problems. I'm not convinced you need much of a load on them any more than you need to run your car at it's rated 5500-6000 rpm.

[JstaRebel]

Originally Posted by blackswan

Running diesels at low RPMs is not a good idea. It polishes the cylinder bores which over time will reduce efficiency and accelerate wear. This is caused because incomplete fuel combustion and loss of lubrication results in the cylinder bore being worn away (polished) which leads to fouling of exhaust ports and valves, along with increased amounts of exhaust pollution. Proper controlled loading is therefore essential. It increases diesel engine life by up to four times and improves the efficiency by ensuring that the engine has sufficient loading at all times.

So please explain how running a diesel engine that produces 135 HP @ 2500 RPM at 1800RPM for cruising speed will cause poor fuel combustion? BTW, I forgot to add with 50 psi oil pressure at operating temperature. My personal preference is a low RPM design because the engines just last forever and are very dependable. The dependability factor is what I prefer most..

[a64pilot]

Originally Posted by darylat8750

I agree with you there! The 6 cylinder Cummings are very good engines and more modern than the Lehmans. But the 6 cyl. have more power than required for this application and the 4 cylinder Cummings, while plentiful in agriculture and construction equipment, do not have as good a reputation as the 5.9 through 6.8 liters.

If price were no object I would be inclined toward a 4 cylinder Lugger or a very close second would be the 4.5 liter Johne Deere for a 41 foot Diesel Duck.

The other thing we are forgetting is just how smooth an inline 6 cylinder is, they are in fact smoother than a V 8 and among the smoothest of any engine design.
I believe also the 5.9 is what was put into the Dodge PU? So parts are inexpensive and available everywhere if it is.

[shakey doug]

Here are 3 great all time sluggers, economical, naturally aspirated, easy to live with.


1. FORD. 2715e 120hp run at 1600-- 1700rpm all day. Great pricing.


2. CUMMINS. 6b5-9 115hp 1700-- 1800rpm all day Mid range cost.


3. Gardner. 6lw 112hp Torque thru out range light years ahead of opposition

[valhalla360]

I think this is a classic case of knowing enough to be dangerous.

If you take a 300hp engine and run it at an output of 10hp, it likely won't reach full operating temps and you may run into issues long term.

If you bump that up to 75hp, it's likely plenty to get the engine up to full operating temps and is unlikely to cause any issues (see the comments about pickup engines for backup)

I think a lot of the worry was from the old days when cruisers would run the engine for a couple hours a day just to charge the batteries. With modern solar systems, there is no reason for 99% of cruisers to be using the engine purely for charging.