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-   -   8V92 T in a trawler (http://www.trawlerforum.com/forums/s6/8v92-t-trawler-17126.html)

boating2go 10-16-2014 11:39 AM

8V92 T in a trawler
 
We are looking at a Sea Ranger with twin Detroit 8V92 T diesel engines, I know they are reliable engines and easy to maintain. However, how fuel effective will they be? I know people who only averaged between 10 and 15 gallons a hour consumption. Not an efficient trawler in my opinion.

FF 10-16-2014 12:27 PM

I know people who only averaged between 10 and 15 gallons a hour consumption. Not an efficient trawler in my opinion.

These folks are not running displacement trawler speeds.

Or have large boats.

10 GPH is at least 150hp , 15 Gph well over 200hp.

It takes 3 hp per ton to operate a displacement boat.

So at 15 gph the boat would need to be about 75-80 tons .

Or they are going too fast for trawler speed fuel economy.

With that size engine 7 or 8K cruising was not in mind.

If you want 1 1/2 to 3 gph cruising it will need a different boat.

DavidM 10-16-2014 01:04 PM

I suspect that the Sea Ranger you are looking at is the 75' in San Diego, right?

That boat has a listed weight of 55 tons and might easily be 60-65 tons with fuel, water and gear.

I agree with Fred's analysis with one exception: a semi displacement hull, which this seems to be, will use more fuel, at least 4 hp per ton. So it could easily need 250 hp to push it to displacement speed, which will certainly use 15 gph or better of fuel.

Displacement speed for that boat is about 11 kts. Slow down to 8-9 kts and you will burn half the fuel.

But to do better than that, get a smaller boat, not just smaller engines if you want good fuel consumption.

David

boating2go 10-18-2014 07:43 PM

After reading both of your posts, obtaining slip fee quotes, insurance quotes, and knowing maintenance will be costly I am trying to talk my wife out of the boat. She loves the boat but I do not love the costs involved! We shall see who wins.

stornoway7 10-19-2014 03:20 AM

Hi,I have twin 8v71n's we burn about 10 gallon an hour at 9 to 9.5 knots.
You can't idle along with Jimmies for too long,they like to work for a living.
You have to give them a good blast at least every hour or so.
Not so sure you going to get anything south of 14 gallons per hour with 8v92T's.
Big boat,big fuel use.How many miles per year do you want to do?
If your not going too far it might be OK.
Super reliable motors though.I love them.

caltexflanc 10-19-2014 07:09 AM

Two 8v92TI's on my 60' LOA, 75,000+# Hatteras 56MY . At seven knots, 900-1000 rpm around 7 GPH total , but typically run at 8 or 9 knots, 1300-1500 rpm 10 GPH total. Designed to be a planing boat but we never did, nowadays it would euphemistically be called "semi-displacement". Running them up once at the end of the day for 10-15 minutes to 1800-2000 kept them very clean and happy, got a very thorough survey at 3000 hours, came out great. Maybe engines in poorer condition need more frequent high RPM runs, mine certainly did not; I could skip a day or two and the smoke would clear up in a couple of minutes.

Given the size of the boat in the OP, but now knowing the hull form, 10-12 GPH at say 8 knots may be about right (all GPH do not include generator use). Want better economy, get a smaller, narrower, true full displacement boat and go even slower. At some point, it takes X amount of horsepower to move Y amount of boat at Z amount of speed.

makobuilders 10-26-2014 08:32 AM

Quote:

Originally Posted by caltexflanc (Post 276881)
Two 8v92TI's on my 60' LOA, 75,000+# Hatteras 56MY . At seven knots, 900-1000 rpm around 7 GPH total , but typically run at 8 or 9 knots, 1300-1500 rpm 10 GPH total. Designed to be a planing boat but we never did, nowadays it would euphemistically be called "semi-displacement". Running them up once at the end of the day for 10-15 minutes to 1800-2000 kept them very clean and happy, got a very thorough survey at 3000 hours, came out great. Maybe engines in poorer condition need more frequent high RPM runs, mine certainly did not; I could skip a day or two and the smoke would clear up in a couple of minutes.

I would run my 6-71NA's at 1350rpm continuously. Economical speed and the max noise I could handle. They were old and always put out a slight white-blue smoke.

Anyone know what the minimum continuous rating is?

kulas44 10-26-2014 08:52 AM

The problem with DDs at very low power is that they tend to "air cool" themselves. The blower moves lots of air, which is a good thing for a diesel engine, but it tends to over cool the cylinders and pistons. This causes incomplete fuel burn and loading up, mostly the air boxes get loaded with "crap" and it starts getting pulled into the cylinders causing even cooler temps. This over cooling has nothing to do with the engine coolant system or thermostats. Higher temp thermostats wont help, warmer intake air helps some. Correctly sized DD engines, sized for the job at hand, is the best and most economical way to use these engines.

pilothouse king 10-26-2014 10:04 AM

Go over to yachtforums.com and look at the thread about these engines under sportfish and specifically 48' Hatteras. The answers are all correct.

Billyfeet 10-26-2014 10:19 AM

That is quite a boat! Others have given a good idea about fuel consumption and the realities of the engines. I would like to offer the advice that on a vessel like this one your fuel cost will be a minor consideration in the total cast of ownership. I'm glad that the wife is pushing for it!

ancora 10-26-2014 10:52 AM

Being a B.V.I. boat there could be a problem with the vessel transfer. The ad states, "not for sale to U.S. residents in U.S. territorial waters." One of our club members purchased a new $1,600,000 boat and took a well documented possession off-shore before bringing the boat to Mexico for a year to avoid paying the sales tax. That could be an option.

makobuilders 10-26-2014 12:20 PM

Quote:

Originally Posted by kulas44 (Post 278545)
The problem with DDs at very low power is that they tend to "air cool" themselves. The blower moves lots of air, which is a good thing for a diesel engine, but it tends to over cool the cylinders and pistons. This causes incomplete fuel burn and loading up, mostly the air boxes get loaded with "crap" and it starts getting pulled into the cylinders causing even cooler temps. This over cooling has nothing to do with the engine coolant system or thermostats. Higher temp thermostats wont help, warmer intake air helps some. Correctly sized DD engines, sized for the job at hand, is the best and most economical way to use these engines.

Thank you for the enlightenment. I always figured that my engines at that rpm were running at about 35%-40% load (60hp out of 160hp rating) which was probably too low. Perhaps they'd be happier, and not suffer from the problems that you mentioned, if they were pushed to about 60% continuous instead.

DavidM 10-26-2014 12:51 PM

You probably would be happier running them at the 35% load that you now operate, but once a day or every 10 hours or so running them up to 70-80% power for ten minutes to blow out the air boxes and soot out of the exhaust.

David

makobuilders 10-26-2014 01:09 PM

Would 50% power be considered a proper minimum speed to not have to worry about under loading or the issues mentioned above?

caltexflanc 10-26-2014 03:11 PM

Quote:

Originally Posted by pilothouse king (Post 278570)
Go over to yachtforums.com and look at the thread about these engines under sportfish and specifically 48' Hatteras. The answers are all correct.

Well, one of them is, that's for sure.

Ski in NC 10-26-2014 04:11 PM

I would not be afraid of running an 8v92t at low power setting. I know many boats that have logged thousands of hours at low power with no issues. A once a day power up helps clean things up, but many do not bother and engines keep going just fine.

The fuel saved by low power running can soon pay for cylinder kits, depending on hours run and running numbers. If needed at all.

River Cruiser 10-26-2014 06:58 PM

Twin 8V71 350 hp in 47' 40,000+ lbs boat averaged 5.5 gph, 1.1 mpg going up the Illinois River last May.

Nomad Willy 10-26-2014 07:55 PM

Quote:

Originally Posted by makobuilders (Post 278614)
Would 50% power be considered a proper minimum speed to not have to worry about under loading or the issues mentioned above?

That's about exactly where I am and feel comfortable. Many feel this is a non issue. If I had an old trawler w two Lemans in it I'd probably run it at 25% w the oil cooler bypassed. But if I was re powering I'd install engines that would be loaded about 60 or 70%. That would give plenty of extra power and allow cruising at tempetratures that are supposed to be best for long life and minimum wear. Exactly what those temps are are not very evident to me but lube oil temps at about 170 to 180 degrees but little exists to firm up this issue.

The gear head at Passage Maker Magazine (no longer there I think) says a 70% load is best but few adhere to that much load. I could depending on the boat.

kulas44 10-26-2014 10:28 PM

750 hp 8v92 TIs will run forever at no load and 1000 rpm. They will smoke, stink and load up the whole time. bASICALLY OVER LUBRICATED. A 2-71 rated at 75 hp and at no or little load will do the same. A 8v92 750 hp TI ran at 350 hp runs clean. A 2-71 30 kw genset runs clean at half load. Go figure.

makobuilders 10-27-2014 01:16 AM

I've always been told by mechanics that my Jimmies weren't happy unless loaded up, but I always thought that was just a saying. Now per kulas44's explanation I see the actual technical basis for it.

A displacement boat powered to 100% at V/L=1.34 would be running at 50% at V/L=1.15, a typical continuous and economical cruise speed. In my case I ran my boat at 35% load which never varied. Whether in 20ft seas or flat calm or crossing a bar, I never raised my speed. Honestly I never had a need for "reserve power". I suppose on my upcoming trawler (hopefully Jimmy-powered) I will design it for 60-70% powerload at V/L=1.15, leaving it a bit underpowered.

Perhaps we should spin-off a new thread about engine loading and reserve power.


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