rpm and fuel consumption

New Trawler owner soon.** Curious about crusing rpm and fuel consumption on a Ford Lehman 120 HP, 2715E .* Engine has 5700 hrs.Looking at Trawler Forum and that doesnt seem over used.* Survey coming soon and it appears I should look for oil leaks and exhaust manifol discharge pipe ware. Any other ideas and help?

36ft Universal, single, aft cabin

Plumber,* We run our 120 lehman between 1500 and 1800 rpm. Average about 2gph in the middle of that range.

Our boat is only 32ft and yours at 36ft should probably be a bit faster due to the longer waterline. We generally do only between 6.5 and 7.5 knots.

As you have stated the exhaust elbow should be looked at if it has more than 2000 hours.

Another thing to check is whether the raw water pump is the original design (Jabsco) or the much improved Johnson pump.

Once you know what you are looking for the Jabco is easy to spot because the four mounting bolts are located on a separate pump drive which is about four inches behind the front housing of the engine.. While the Johnson pump is connected directly to the front housing without any separate pump drive.

The pump drive is the problem, some of them break.* With 5700 hours this problem has probably been addressed.

The Lehman is very simple and reliable,* take* care of it and it should last and last.

Most of them are old already and just keep chugging along.

Good Luck. JohnP

I have a 40 Albin, single FL 120, run at 1700-1800, get about 6.5 to 7 knots, use less than 2 gph at that speed.

I did the Johnson upgrade when I bought the boat 4 seasons ago. Easy job.

Best of luck.

Maybe you need a Flowscan or whatever to measure consumption (top-right gauge).

Charles,

That depends a lot on the boat. For example my Willard would be running at less than 5.5 knots at that point. The Willard 40 makes 7 knots with almost half the power it takes to make that speed w a W30. At one time I thought hull speed was achieved when the bow started to rise but then discovered it happens considerably later (at a greater speed). You're right though Charles that that is a very economical speed to run.

It's hyperbolic.

My 34' marine trader with a single 120 runs about 8mph at 1650 rpm and burns 2.4 gph

one quick note I upgraded the oil filter hoses from the stock 5/16 hoes to 3/8'' hoses and picked up 10lbs of oil pressure

I have a 37 ft Puget Trawler (Taiwan built) with the FL 120. I run at 7 knots at 1600 rpm and get 2gph. When I got the boat I did the Johnson pump conversion, and changed the exhaust elbow. The exhaust elbow goes bad from the inside out. Oil leaks at the back of the engine may be from the fuel pump gasket. Change the oil and transmission coolers if they haven't been changed in the last 2 years. If you change the coolers, get the copper-nickel ones, a little more expensive, but a much longer life.

I run twin FL120's in our IG 36'.

I am right in assuming that with both engines running, at say 1700rpm, they would use slightly less fuel each than say a single engine would powering a 36' would. if this is the case then what fuel consumption could I expect to get with this twin set up?

Shrimp,

You should burn more fuel because of the much higher heat loss from two engines running. However I think you are implying that w 2 engines driving the boat at the same speed they would be doing less work each and burn less fuel. I'm sure that's true but the heat loss will prevent it from being a wash. I think Marin Faure runs his GB 36 w twin Lehmans at 3gph on each engine and I think he runs at 1800 rpm. Unless you're way up at the top end the less the load the less the efficiency. Keep in mind Shrimp that almost everybody calculates their fuel burn from engine hours (from the hour meter) and how many gallons it takes to fill up the tanks. All the idling and slow running gets mixed up in the numbers and now that fuel cost is high most do shorter trips and the less than cruise throttle engine time is a higher percentage of the total so most skippers burn considerably more than they claim. The difference could be as high as 25%. Those skippers that do very long trips like Larry H should have more accurate numbers. In answer to your question I'd say 4.5 to 5.5 gph. That may be at about 8 knots so slowing down to 7 should save at least 1.5 gph.

I think your looking for actual experience so you will have a benchmark to start with on the new boat. I run a 1980 Lehman 120 in my 40' Puget Trawler. @1750 rpm I burn 2.0 GPH at 1850 I burn 3.1 GPH. These are from the FlowScan unit which has been calibrated with a couple thousand gallons of diesel thru it. At wide open throttle I can get almost 8 GPH to flow thru. At 1500 it drops to 1.3. Depending on your cruising style and how fast you want to get somewhere, you can run your engine as you see fit.

Oil coolers, the cupro/nickel variety should be considered suspect after a certain amount of time. In my cruising waters I look, scrape, and decide after about 5 years of use. I've not had a problem with that schedule in 32 years now. The coolers are available from American Diesel for $120 each plus shipping.

Exhaust elbows can be looked at annually. Dig around with some sharp pointy object with the intent to dislodge any carbon or to drive it thru the side of the elbow. You can also tap on the outside of the elbow with a hammer like object and sometimes hear the difference in the strength of the metal. I know thats a subjective test, but if you tap lightly on the outside and the hammer crackes the metal, you have serious deterioration inside. You can buy gaskets for the elbow at AD also for I think right around $3. An elbow is about $140.

Your raw water pump may or may not have been changed. The "Johnson" pump replacement is about $400 at AD. A careful examination can be made periodically, or you can do what I did and just make the change, keeping the old as a good spare if needed.

Good luck with your new purchase,

Ken

"you can do what I did and just make the change, keeping the old as a good spare if needed."

I recall plugging up an oil hole (with a supplied tapered pin) to the drive gear as part of the new pump installation. That would make the old water pump not usable as it requires oil. No?

*

On a FL 120 with that number of hours it would be nice if you had a decent history of oil analysis. You might be able to see wear patterns,water in the oil and what not. As far as the number of hours on the engine I personally would say that 5000 hours would probable be considered half life. The Lehman is a pretty basic and not real high end engine. Just My opinion! I have a FL 120 in my Willard 40 and burn 1.6 gal per hour @ 1550, I simply don't see that much advantage running the engine*much faster on our full displacement boat. One thing that you should look at might be the injection pump, I have seen several of these that have had there lube oil contaminated with fuel. Just look for very thin oil. This pump should have the lube oil changed every 50 hours, so if it has recently been changed you won't see any problem.

Rob Hays

Lady Anne, Willard 40 PH

jleonard wrote:
"you can do what I did and just make the change, keeping the old as a good spare if needed."

I recall plugging up an oil hole (with a supplied tapered pin) to the drive gear as part of the new pump installation. That would make the old water pump not usable as it requires oil. No?

*

Click to expand...

*I suppose for those who did plug the hole it may be a problem long term. I understand many didn't plug the hole. (mine is plugged)*Since I plan to use mine as a spare because of some problem that a new spare impeller won't fix, I'm thinking that reduced oiling is probably a minor issue. If one were to use it as a spare (with the excess oil hole plugged)*for a long period of time, then you'll probably see some excess wear on the gears.

I'm optimistic that even a 24 hour run back to civilization would be possible.

Ken

*

*

120 lehman 120 GPH

I see we are talking 2 to 3 Gals per hour at about 7 knots. Is that for duel Lehmans or single. Would a duel get 4 to 6 GPH then?

Johnnie said:

I see we are talking 2 to 3 Gals per hour at about 7 knots. Is that for duel Lehmans or single. Would a duel get 4 to 6 GPH then?

Click to expand...

Fuel consumption is based on load (pushing the boat through the water). While a single engine versus twins is almost always more fuel efficient for the same boat (everything else being equal), twins often only use 25 to 75% more fuel compared to the single.

Ted

On our new-to-us Defever 44 with twin Lehman 120s, we averaged 3.5-4 GPH on our 8 day trip bringing it home to S. Fl from the Charleston SC area. One day running outside, the remainder in the ditch. We averaged 8+/- MPH. Only one night gen use during the trip. These incredibly well maintained engines have a little over 7000 hours. They purrrrrrrrrr like kittens.

Get an oil analysis done. Worth every penny.

Yachtman said:

On our new-to-us Defever 44 with twin Lehman 120s, we averaged 3.5-4 GPH on our 8 day trip bringing it home to S. Fl from the Charleston SC area. One day running outside, the remainder in the ditch. We averaged 8+/- MPH. Only one night gen use during the trip. These incredibly well maintained engines have a little over 7000 hours. They purrrrrrrrrr like kittens.

Click to expand...

We own a 1983 DF44 with twin Lehmans, 6,500 hours. In doing the Great Loop, we averaged 3.6 GPH over 5,500 miles. That included about 200 hours of generator time and a fair amount of driving in circles waiting for locks to open. Our boat weighs 56,000 lbs (half load). Normal RPM at cruise was 1,650 which gives us 8.5 MPH, no current, no wind.

Rob said:

On a FL 120 with that number of hours it would be nice if you had a decent history of oil analysis. You might be able to see wear patterns,water in the oil and what not. As far as the number of hours on the engine I personally would say that 5000 hours would probable be considered half life. The Lehman is a pretty basic and not real high end engine. Just My opinion! I have a FL 120 in my Willard 40 and burn 1.6 gal per hour @ 1550, I simply don't see that much advantage running the engine*much faster on our full displacement boat. One thing that you should look at might be the injection pump, I have seen several of these that have had there lube oil contaminated with fuel. Just look for very thin oil. This pump should have the lube oil changed every 50 hours, so if it has recently been changed you won't see any problem.

Rob Hays

Lady Anne, Willard 40 PH

Click to expand...

Bob Smith's demonstration FL120 had over 20,000 hours and was till going strong. Your 10,000 hour guess, assuming good maintenence, is, IMHO, far too low.

Notice in reading the various posts engines operated at lower RPM seem to have the longest life.

One reason is the lower number of piston miles from being operated slower.
The engine simply wears out slower

An other reason may be the higher load carried by the engine to make the power required at cruise.

The slower speed and higher load causes the fuel burn to be more complete ,so the fuel use is less.

Low RPM cruising is huge win for folks with the right prop and tranny gearing.

Many folks have shifted to the amount of fuel that’s gone through the engines rather than hours as the measure of use. It makes more sense as wear is more closely associated total RPMs during its life rather than hours on. Also lubrication and cooling systems are less stressed. Think for both reasons (less total fuel burn and less less stress on lubricants/cooling ) PS as usual is on to something.
You see Gardners and Luggers with ridiculous hours on them and no rebuilds. Not very efficient engines and more polluting then current electronic engines so think efficient burn maybe be a contributor to long engine life but not key.

RPM can be related to fuel consumption but the real creator to high fuel consumption is engine load. You don’t burn much gas in your car until you start going up hills. But unlike cars that always go down the other side the load is constant on boats.

You can INCREASE rpm and REDUCE fuel consumption as the great Marin Faure once documented. He was overpropped, reduced pitch (load) and burned less fuel.

But of course he may have run at the same rpm. The difference may be hard to find but fuel burn is more related to load than rpm.

Twin FL120s, each just over 5,000 hours, in a 50,000 pound, round-bottomed full displacement hull, at 1400rpm we burn a total of 1.8gph (each engine is .9gph). However, we round up to 2gph total for easier math...

This from the POs calculations (and flow meters when they worked) and our own calculations based on more than 4,000 miles in the last 9.5 years.

When is a gallon not a gallon?
1 Canadian gallon is 1.2 US gallon.
A litre or liter is the same. So, why not convert your usage to L so it makes it easy to compare.
My twin 108 Lehman burned an average of 8.99 L per hour.
Yes the 2714E is 108HP even if it looks like the 120