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Old 10-05-2007, 07:26 PM
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Default Diesel inboards and cruising speed

As some may know I'm thinking of buying a trawler in the next few years. One thing that's got me confused is the claims of some "fast trawler" or "express trawler" manufacturers that their boats can run long distances at speeds up to 20 knots, but also run for extended periods at displacement speeds and get fuel economy on par with full displacement trawlers.

My understanding was that diesel engines are generally designed to run in a narrow rpm range, and if you run them for long periods of time outside that range, you're asking for trouble. Is that not true? Could you run one of these boats at displacement speed 90% of the time and just use the extra juice to escape bad weather or make a distination by night fall?
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Old 10-05-2007, 07:41 PM
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Default Re: Diesel inboards and cruising speed

Generally maximum sustainable rpms are 80% of full rpms. Mileage will suck though. Semi displacement hulls will run at displacement speeds pretty efficiently, but not as efficient as a displacement hull. If you're going to spend most of your time as low speed with some horsies in reserve it shouldn't cause a problem.
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Old 10-06-2007, 10:02 AM
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Default RE: Diesel inboards and cruising speed

Absolutely.
The "new generation" electronically controlled diesels (sure wish I had 'em!) can be run indefinitely at low RPM for displacement cruising (like passagemaking) or opened up for speed.
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Old 10-06-2007, 10:28 AM
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Default Re: Diesel inboards and cruising speed

Cool, thanks for the info.!
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Old 10-06-2007, 11:00 AM
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Default Re: Diesel inboards and cruising speed

AdamT, The trawler market has changed significanlty over the past few years. There are a lolt of companies out there that tout their boats as being Trawlers while in fact are really more of a motor yacht. Many true passagemakers are will only be seen in a trawler with a full displacement hull and a full, prop protecting Keel with a Skeg under the prop.

Depending on the particular boat and what motor is in it will determine your cruising speed and what percentage of throttle is considered to be the optimum running area for that vessel.

I have done quite a bit with Passagemaking Trawlers over the years, if you would like to talk about them, let me know.
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Old 10-06-2007, 10:00 PM
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Default RE: Diesel inboards and cruising speed

Quote:
AdamT - 10/5/2007 7:26 PM


My understanding was that diesel engines are generally designed to run in a narrow rpm range, and if you run them for long periods of time outside that range, you're asking for trouble. Is that not true? ?
After 30 years of working on diesel powered boats I would have to disagree with that. Worst that will happen is you will get some carbon build up with long term low speed use..........an hour of high speed running once or stwice a day will take care of it though. As far as the new electronicly controlled diesels I would say that are much more capable of sustained low speed use.
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Old 10-06-2007, 10:55 PM
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Default RE: Diesel inboards and cruising speed

Here is my experience. I live in Mich. but boat in Canada on the Great Lakes. With fuel $5.00/gal I thought I would see how good I could do on fuel.
Twin diesels, top speed about 22, cruise nice at 16. At 16 I get about 1 mpg.
Running on one engine the boat handles very well and the auto pilot holds a straight course.
I ran 401 miles on one engine as much as possible. This was over a period of several days with many anchorages and some docking. So there was some idle and some use of both engines. I ran them at wot for 2 short periods.
In 401 miles I used 95.4 gal. This was running at 1000 to 1100 rpms which, depending on sea conditions was about 5.2 to 6 mph with a guess at 5.4 mph average. I also towed a 16.5 aluminum Sea Nymph with a 70 Evinrude and a small trollling motor. Usually about 30 gal gas so it was fairly heavy. From the little bit the previous year without towing I think I lost about 1/2 mph towing the boat. A rough estimate here.
Without towing perhaps as much as 10% better.
When I bought the boat last year the valves had not been adjusted in the 1300 hours on it. Should be every 500 hours.
The engines really smoked and when run at slow speeds then run wot, it would fog smoke so bad you couldn't see through it.
I adjusted the valves and the smoke went away. Not better but gone. Very clean with no soot on the boat.
Running at 1000/1100 rpms I ran it at wot after about 200 miles. Each time it smoked a little for about 15 seconds, then quit.
That told me it wasn't loading up.
Could be wrong but that was my take.
Even the marina operator when I took it for haulout remarked how clean they burned and I had just run 50 miles at slow speeds.
The engines were twin Hino's, 175 hp in a 3870 Bayliner Motoryacht.
Miles were recorded on a GPS and gallons was the old fashioned way, a fuel pump.
That's my story and I am sticking to it.
We were looking at trawlers before we purchased this boat. We fell in love with the style and room. I feel we have the economy of a trawler with the option of a decent cruise speed.
Hope this helps.
Doug
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Old 10-07-2007, 02:09 PM
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Default Re: Diesel inboards and cruising speed

Adam T Have you looked at the PDQ. Really nice looking Cats
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Old 10-07-2007, 06:48 PM
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Default RE: Diesel inboards and cruising speed

Do you own a 39' SR w/ Cats? If so, how do they perform and what is the fuel burn like. We had a 39 w/454 Mercs. She was also a little slow to respond to clutches when docking - needed a lot of throttle to make to respond. Loved the boat but not the performance with the Mercs - 17-18.5 knots @ 32 gph + (3200-3400 rpm). We found that at about 21 knots the boat seemed like it was on top and running nice - only couldn't afford the fuel burn at that rpm. Would consider another, but it would have to be diesel.
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