Diesel wear and tear

Every day I get closer and closer to severing my ties with 64 years of sailing. Well, the boat hasn't sold yet, but, getting closer.
Anyway, after all these years of boating you would think that I would have some informed opinions about engines in general even if the largest diesel I have ever owned is 35hp.
As we look at "trawlers" in general (forget about displacement or semi-displacement) there are single and twin engine. Ok, but, all the twins seem to be over powered for the size boats. We love traveling at 5-7 kts. We can run our 35hp diesel at 3/4 throttle and use maybe a little over 1/2 gallon an hour. We have been told that if we run large diesels at low rpm's we will ruin them in short order. If that is true, how many engines are ruined by doing the "loop?" There aren't many places where you can run up over 10kts.
At our age we want more creature comforts afforded by a trawler. However, we aren't exactly thrilled about feeding $20,000 worth of diesel through one just to do the loop.

kinda depends on the diesel...Lehmans are notorious for lasting despite continuous low RPM cruising.

The debate I have always heard and find comes full circle...is many diesels once broken in can run all day at lower RPMs and not suffer greatly. Some manuals will give guidance as to when and how long they should be run at higher RPM to overcome a day run at low RPMs.

Some engines have a history of doing well at low RPMs...others not so.

But to arbitrarily say running a diesel at less than 80-90% load is automatically bad is just uninformed.

Welcome to my world.

Also, totally agree with Psneeld.

So, for a 42 foot boat, you probably need about 65 hp to go 6 to 7 knots, a little more hp for high winds.

Having said that, I love my Sp 135

I get 1.5 gal/hr at 7 knots and if I cut back to 1500 rpm and aobut 6 kntos, will consume 1.1 gal/hr

It purrs like a kitten and puts me to bed every time

Put a bit of a load (50-70%) on a diesel and it's happy, don't let it overheat by paying attention to the cooling system.

High HP engines generally have a forced induction system (turbo/super charged) and they need to work harder to get under a load because they make more power to begin with.

Normally aspirated engines in the 75 to 130 HP range will power a trawler very happily running in the 1400-1800 RPM range for darn near ever.

Don't run a fast boat slow all the time and don't run a slow boat on the pins all the time.

Most boats are overpowered for the use they get.

If you want maximum efficiency out of a modern diesel, ideally you want to run it at or above the rpm that delivers peak torque. If you want to have it last a long time you need to burn enough fuel at that rpm to maintain proper operating temperature. Unfortunately most boats aren't setup to run 7 knots or less within those parameters.

With fuel at $4 a gallon and only likely to go up, repowering the vessel of your choice with either new or a remanufactured engine to obtain efficient operating paramenters makes all kinds of sence ...... or dollars.

Ted

Generally no harm in running even high output diesels at low load settings. I make my living with these engines and have never seen a failure from someone running hull speed.

Lots of talk on "them thar interwebs" about how you have to run a diesel at 75% load or whatever to last. Nonsense. Show me an engine that has failed due to running at 20%. Have not seen one.

It is true that the engines will "load up" with unburnt fuel/lube, but this can be handled by a daily hard run for a couple minutes. But even one boat that basically idled all the way from NC to Key West and back- engine had no apparent problems. Blew a good cloud on power up, but cleared right up.

Running an engine harder than necessary for hull speed- you will burn way more fuel, and if the trip is long enough, enough fuel to pay for cylinder kits. Which you will likely not need.

Ski in NC said:

Generally no harm in running even high output diesels at low load settings. I make my living with these engines and have never seen a failure from someone running hull speed.

Lots of talk on "them thar interwebs" about how you have to run a diesel at 75% load or whatever to last. Nonsense. Show me an engine that has failed due to running at 20%. Have not seen one.

It is true that the engines will "load up" with unburnt fuel/lube, but this can be handled by a daily hard run for a couple minutes. But even one boat that basically idled all the way from NC to Key West and back- engine had no apparent problems. Blew a good cloud on power up, but cleared right up.

Running an engine harder than necessary for hull speed- you will burn way more fuel, and if the trip is long enough, enough fuel to pay for cylinder kits. Which you will likely not need.

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Thank You! Just get in the boat and go.

Barpilot,
You are now (as a shopper) in the drivers seat. You have the power to find and purchase the boat that YOU think is right based on all the information you have at your disposal. Ask around and don't overlook engine manufacturers and engineers. If I had a 32 Nordic Tug and needed or wanted to burn 1.5 gph (or go 6.5 to 7 knots) I'd need to sell the boat or repower to do it if I found underloading unacceptable. You can avoid or minimize your exposure to being in that position before you buy a boat. After you buy you'll be stuck unless you find and buy a FD boat w a small engine .... 3 ton5hp per ton.

Having said that lots and lots of members have underloaded and overpropped boats for many years. Spend time in the archives as much opinion has been written over the years.

Please send a photograph of the drive end of whichever engine interests you. I will look at it and let you know if it is efficient and suited for the purpose.

RickB said:

Please send a photograph of the drive end of whichever engine interests you. I will look at it and let you know if it is efficient and suited for the purpose.

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First off Rick, what color do you prefer?

There is little choice in engines purchasing a used boat , so the best that can be done is to purchase what has a good rep.

The Ford Econo power marinizations (Leman)seem to be popular with their owners , and as they were never heavy duty industrial engines , the design for a small truck or tractor works well at modest power.

How much service life is lost by underloading is moot on most small boats.

OK if you ran it to perfection it would go say 10,000 , BUT you underload it constantly , refuse to change the oil on installed time , just run time , and save by only changinf filters with every 3rd change.

So your 10,000 engine gets a half life,,,, 5,000 hours big deal.

5,000 hours is longer than many 40 year old boat engines have ever seen.

Dont worry , be happy,

And do proper maint and the engine will probably not require replacement while you own the boat.