Detroit Diesel 8V71TI - Long distance cruising? - SailNet Community
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post #1 of 13 Old 08-30-2009 Thread Starter
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Detroit Diesel 8V71TI - Long distance cruising?

I found a wonderful old post by WarpP:

"I had a 8V71 take off after it warmed up, because someone put three of the oil rings in upside down. By the time we got the plywood against the turbo intake, 10 seconds, it was screaming at 3000 rpm and climbing. The emergency air shut on the supercharger was too far on to the engine to grab, thank god for that piece of plywood. Very impressive, a ton of iron trying to destroy itself and there is no way of escape."

Good to know.

I have a question about 8V71TI diesels (8 cylinders, 71cc per cylinder, turbo charged).

What is the optimal cruising speed and maximum range of the following Deep V planing hull?

1. Fuel tank 4000 liters (879.8769932 UK Gallons)
2. LWL 48.031496063 feet
3. Beam 4.3 Mts
4. Draft 1.52 Mts
5. Gross Tonnage 31.63
6. Total Engine Power 656.48 kW. (880 Hp)

Assuming the useable capacity of the tank is 87% of the maximum, that would provide 765.49 UK gallons (3480 liters).

For optimal cruising speed do you take the square root of LWL
(6.93047589008143) and multiply it by 1.34 as you would for a full displacement hull? 9.28 knots.

As it is a Deep V planing hull, do you multiply the square root of LWL by 2 giving 8.31 knots?

It seems from Stapletons Power Boat Bible that V471 normally aspirated engines with 55mm injectors needed 11,355 liters to cover 3,818 nm in 460.8 hours of running time.

Adding 8 UK gallons per hour for each 5mm increase in the size of the injectors does not answer the question because they are four cylinder engines.

If anyone can figure out what the optimal cruising speed (for maximum range) would be, and what the maximum range is, that would be great
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post #2 of 13 Old 08-30-2009 Thread Starter
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Oops. Not 8.31 knots. Now you know why I need your help
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post #3 of 13 Old 08-30-2009
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It is best to install a fuel flow meter and optimise crusing speed and caculate distance that way.

I believe that is 71 cid not cc's

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post #4 of 13 Old 08-30-2009 Thread Starter
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You are quite correct. It is cubic inches.

This is all new to me.

Have not yet bought the boat. Trying to figure out whether it has any sort of decent range, and whether I could run it on one engine, using the other as a backup.

Thanks for responding.
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post #5 of 13 Old 08-30-2009
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The cruising speed you speak of (1.34 x the square root of the waterline length) is the theoretical hull speed (maximum speed before trying to climb its bow wave) of a displacement hull, its ideal cruising
speed for economy would be slower, probably bout 80% of that speed. You have a planing hull. It isn't as efficient following the same rule. Quite often a planing hull at the speeds mentioned, in other words displacement speeds, will be trying to climb its own bow wave and appear to be trying to pull half the ocean behind it. Not very efficient. What design is it? Your best bet is to contact the designer and find out from him what the given hull is designed to do. There are planing hulls and displacement hulls and everything in between, seemingly small changes making a fair difference. Without knowing what the hull design was optimized for it's hard to determine the optimal speed.
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post #6 of 13 Old 08-31-2009 Thread Starter
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Detroit V871TI

That makes perfect sense. Thank you.

I think it is going to be a trial and error job by watching consumption at various speeds unless I can find out from the manufacturer.
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post #7 of 13 Old 08-31-2009 Thread Starter
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Detroit Diesel


I most definitely shall not mess about with the engines

You are right about parts availabilty. There are engine overhaul kits on e-bay.

Frankly I love the sound of these engines. They make your hair stand on end

Now all I need is a piece of plywood

Sincere thanks,
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post #8 of 13 Old 09-01-2009
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That's some sailboat with a pair of 8V-71s!

The short answer is the faster you go, the more fuel you burn per mile. The optimum cruising speed is a slow drift.

For diesels you can figure 0.055 U.S. gallons/hp/hour. Wide open this boat will burn 880*0.055 = 48.4 US gallons/hour.

The boat is probably a semi-planing hull given its displacement/length ratio. You can estimate horsepower for a given speed in two steps (this is from Dave Gerr, naval architect):
SL = speed/length ratio = speed in kts/SQRT(waterline length)

HP = (displacement in pounds) * (SL/10.665)^3
where the ^3 means to the third power.

You can work out power use at different speeds. I get a maximum of about 17 knots for your boat, giving about 1/3rd of a mile per gallon wide open.

If you're in full displacement mode, assuming the hull is efficient in displacement mode, the rule of thumb is 2 HP per ton, or 63 HP for your boat. This should get you to hull speed of 9.3 kts. Fuel burn would be about 3.5 gal/hr giving about 2.7 mpg. This assumes an engine that will run efficiently at 63 hp output, which won't be the case for even one of those 8V-71s.

If you're looking at something that'll cost four grand to fill up, it'll be worth spending a few hundred bucks on a naval architect to walk you through the details of speed, range and fuel economy on this boat.

Good luck,

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post #9 of 13 Old 09-01-2009
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Cardiacpaul is right on the money. My experience with the 8-71's is in truck applications so not a direct comparison, but I would plan to carry a lot of lube oil to replace what leaks out of them. They were notorious for oil leaks.

SV Laurie Anne

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post #10 of 13 Old 09-04-2009
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wow, a real living 8v-71......

Well they do make good power.. if you can stand the mess of oil leaking out of the wet air boxes, lots of smoke,and noise.. I work with trucks, tractors ,heavy equipment, gen sets and even locomotive.. i have not seen one in a while.. the federal gov has made a point to hunt them down and put an end to them. Off shore, third world country's and oil field/ agg use on water pumps are some of the few places you will see them these days.

An 8v has an odd life cycle, when rebuilt... they will get poor fuel consumption " even for an 8v" and get better and better as they get more worn out... They run best right before they go boom, or run away on their own oil ...
Also be advised they will run backwards!!! and there is no stopping them if they do... The disadvantage of a two stroke diesel.. as for calculations... no way it will be up to your throttle input and how that engine breaks in but it will almost never be consistent.

I recommend you have a VERY competent tech to set the rack. also a intake flapper solenoid would be very recommended.. " it will kill the air flow and stop a run away.
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