Ouch!

I'm grateful I hired a diesel mechanic to check out my 1982 32 Island Gypsy after putting over 100 hours on her in 8 weeks.

I started noticing black hard ash under the block and my engine appeared to be rusting and burned near the exhaust manifold elbow. Sure enough, he tells me I need a new exhaust manifold. There goes a couple of thousand...

On the bright side, the broker who sold me the boat in January told me not to take her above 1800 rpms and that her cruising speed was 7 knots. Turns out my Ford Lehman 120 horse single engine is designed to be run at around 2600 RPMS and can cruise at 10-12 knots. I hate to spend money but sometimes the investment of an expert's time and knowledge is money well spent. I'm a great salesman, but no little about the engine room. We all have our specialty's. My goal now is to be a sponge and learn as much as I can.

It may be rated to put out 120 hp at 2,600 rpm but it is not designed to do that all day long.

I would not run that engine any harder than 2,200 rpm continuously and that assumes that it is propped to reach 2,600 at wot.

David

Agree

I agree with you. I was cruising at only 1800 max which apparently wasn't good for the engine either. I think I will now cruise a bit faster but would never keep her maxed out all day.
Thanks for your feedback.
Now can you please make my exhaust manifold issue go away?

Why would cruising at 1800 rpm do a Lehman 120 harm? Can`t see it myself. I`m getting no harm from running (admittedly twins) at around 1500rpm cruise.
Sorry about your manifold, maybe the broker knew and was trying to extend the time between sale and fail. Check out fredwarner1 on ebay, he does some Lehman cooling system products, or American Diesel, the Lehman successors.

On the advice of some very knowledgeable people in the UK with a long experience of servicing, maintaining, and repairing the Ford Dorset diesel, which is the base engine for the FL120, the operating range of this engine for maximum longevity is 1500 to 1800 rpm. Max rated rpm is 2500 so I don't know where you're getting this 2600 rpm figure, but according to our Lehman and Ford manuals, it's wrong.

We cruise at 1650 with our two FL120s, and have done so for the last 17 years. They have been to maximum rpm exactly once during this time for about three minutes to get data for the prop shop. Other than that, they have never been above 1800 rpm while in our care. The engines were built in 1973. According to the information we got from the previous owner, he ran them at less than 1800 rpm, too.

Not to say our engines won't explode tomorrow, but they run no differently, burn no more oil (a quart or less every 100-150 hours), smoke (blue oil) only for a few minutes at cold startup, and run as well as they did when we had the engines surveyed after our sea trial prior to buying the boat in 1998.

People who advise running these engines hard (fast) are giving bad advice despite all the "experts" opinions to the contrary. The FL120 is a weak engine in some respects (the head gasket in particular). It's a medium speed, medium load engine that proved to be worthless as a truck engine (for which it was designed) but very good for industrial and agricultural use, which is why it proved to be a good candidate for marinizing.

Run it like a newer generation engine and your mechanic will be rubbing his hands in delight because now he'll be able to afford that Porsche he's always wanted.

Good advice

Sounds like the broker who sold me the boat and used to own her, was correct when he told me ideal cruising speed is 1800. That's where I have been cruising at was afraid to rev up higher.

Guess I will settle for the 7-8 knots - heck why be in a hurry with a trawler

Agree with the above posters, there is no benefit to running that Ford above 1800. It's a tender engine above that. 1400-1800 and that thing will run for decades.

Post photos of the manifold issues. There are parts of the manifold that have no water jacket, and in those parts it will cook off the paint and rust. That alone is not an issue.

Give more detail about the "black hard ash". Photos would help.

I would not go into a manifold replacement without confirming the need.

You all make me glad I've got a low RPM engine (max. of 2400). Even at 2200 (fast-cruise, hull-speed), the 80-horsepower JD 4045 engine is only at 75% load pushing 14 tons.

Normal cruise (a knot below hull speed) is at 1800 RPM resulting in a 45% load.

Mid-way during engine warm-up, cranking 1400 RPM:





(Relative engine load equals actual fuel consumption compared to maximum fuel consumption.)

My boat's sweet spot appears to be 1750rpm, @ ~ 7kn. I agree with several others. Do not run it over 2000rpm for more than brief bursts, and it will last a long time with adequate maintenance. I have the same engine as yours.

Greetings,
Twin 120's here. The "sweet spot" is 1750 rpm according to Bill Smith the Lehman guru. We make 8 knots at that rpm. (vessel WEIGHT 25 tons)

"I think I will now cruise a bit faster but would never keep her maxed out all day."

A simple rule of thumb is to run up to full throttle , note the RPM and pull back 10% or 300RPM.

This will allow the engine not to be overloaded.

However the Ford Econopower marinizations are of a tractor engine , not an industrial rated engine.

Maxed out all day operation will require a different engine .

I would heed the advice of many and use 1800 or so as a LRC , long range cruise
.
If your actual fuel burn is over 4 GPH , you are pressing too hard.

In this case the broker was correct.

We have twin FL 120s. 1750 gets us 8 knots & 4 GPH fuel burn. 1800 is the max we push it. Our boat is 32 years old & these engines still purr like a kitten!