Underloaded engines on old trawlers?

[Eric Smith]

I am usually pretty good searching forums for answers, but I have not been able to find a clear and complete basic answer to this question. What is the benefit of, and how does one operate a full (or even semi) displacement trawler efficiently, that has 2 engines each having as much horsepower as the single engine version of the same boat? For example:

  Marine Trader 38′ with power choices of single 135 or even 120 hp Lehman vs same model and year with twin 135 hp Lehmans.

I just figured that one could run them at reduced rpms to  travel at 6-7 knots and get nearly as high mpg as the single engine versions. 

But then I learned that diesel engines should not run under low (relative) loads. I cannot reconcile this with efficient use of displacement type hulls… I am not sure if there is a piece to this picture I am missing. If the Marine Trader had a planing hull, I could see the reason.

I am looking at buying a 40′ trawler, that was repowered with Cummins 6BTA5.9 engines that replaced the original twin Lehmans. Many of these   boats were originally supplied with single Lehmans.

Topic

[blindhog]

It’s not just the engines but how it’s geared an how it’s propped.  The 5.9’s may be more hp but they may be propped to run at the correct rpm.

[Steve Lewis]

Eric,

Welcome to the forum.  This forum spends a lot of time debunking a lot of the “Conventional Wisdom” that lacks wisdom.  Give this a read

Low Speed Running & “Break-In” of Marine Diesels

You will find that low speed or low power running is not an issue as long as you eventually run the engine hard enough to get things up to operating temps.  In a trawler the whole idea is to run low and slow, hull speed or less, so that long distance cruising is viable from a fuel consumption perspective.  You can also think about it a different way.  If an engine is designed to burn 100,000 gallons of fuel before needing an overhaul then it stands to reason that running at a lower RPM will increase the amount of hours that it takes to burn the same amount of fuel.  So an MT or GB running low and slow will take longer to burn through the 100,000 gallons than say my 6CTA 480CE’s running at 2200 rpm pushing my boat on plane.  I can burn that same 100,000 gallons but at a much faster rate, therefore fewer hours.  Here is a set of fun facts.  I can run my 6CTA’s @ 1140 rpm and make 10mph burning a total of 5 gph.  That would take 20K hours give or take.  I can also run at 2200rpm and make 22.5 mph and burn ~30gph.  That would take 3K hours.  Big difference.

The key to low and slow is making sure that at the end of each run you run the engines hard enough to get things up to operating temp so that any of the nasty stuff gets burned off.  So in the case of a semi or full displacement hull, push the engines just beyond hull speed and get them working and up to operating temp. ALSO it is very important to stay on top of ALL maintenance items so that the engines stay healthy for those hours.

The Tony’s Tips section of this site is a gold mine of real info.  Not the dock talk that is full of inaccurate info, half truths and just flat out wrong info.  Took me a while to unlearn what I had heard and learn how it really is.  Tony and the team have decades of real world experience that is second to none.  And they publish that knowledge on this site for free for all of to read.  I would take full advantage it.  Also the forum is full of like minded people looking for the truth and helping each other out.

2 engines are usually better than one.  Think redundancy.  If I am cruising a remote location in Northern Michigan or Canada I would like some redundancy just incase so that I know that if one shaft or propeller gets fouled or whatever, I have another to get me home.

You can also go to boattest.com and watch Captain Steve do a test on the Fleming55.  He covers single engine vs. twin engine with a comparison and twin is better.

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Eric Smith

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