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Capt Allan · 01-07-2013, 12:03

Can anyone tell me with a high degree of accuracy how much fuel I can expect to burn on a 45' Chris-Craft Commander gross tonage at 41000 Lbs and running at 1000 RPM with twin engins being Detroit 8V71N, Allison Transmissions M20 L/R. I am running 4 blade props in lieu or the standard 3 blade type, approx. 27"

I am trying to figure my fuel usage traveling the Intra-costal from Florida north. Leaving March early.

Any help will be valued.
Capt. Allan

Pauls · 01-07-2013, 12:27

Hi Alan,

No one can answer this question accurately because there is no information on how loaded your engines are at 1000 rpm. The props specifics, trans reduction ratio, the condition of the bottom and drag of the hull will all influence the fuel burn. The engines can be lightly or heavily loaded at 1000 rpm, and this will greatly inluence how fuel consumption.

A simple and accurate way to get your answer is to get a can of diesel, like 5 gallon cans. Fill it to the brim, connect your fuel system to it, and start your run at your parameters. Time the start and end of the run. Run till the fuel is mostly gone. Then reconnect your fuel system, fill the can back to full and read the number of gallons off the pump. This will give you an accurate reading, for the weather conditions under which you ran the test.

Capt Allan · 01-07-2013, 12:51

Yes, Pauls I was trying to avoid all the mess that could result from that method and had hoped someone could give me a idea as to consumption. Guess I will just have to do something similar to what you suggest. Thank you.
Capt. Allan

Capt Phil · 01-07-2013, 13:13

Is there a measured mile somewhere close to where you are? Do you have sight gauges on your tanks(s)? If not, can you dip the tank with a stick?
You might try running 5X the measured mile at 1000RPM taking a before and after measure, calculate the volumetric amount of fuel used, convert to gallons.
8V71's are pretty lean at 1000 RPM if you are running off a return or day tank using warm fuel. Using cold fuel out of the tank will burn a bit more.
Every boat/engine combination is different so even if you scoped out the dispacement, wetted surface, prop size and bite, transmission reduction, tide, current, etc., you still end up with a WAG. Cheers, Phil

captain58sailin · 01-07-2013, 14:56

In a word NO. I can't anyway.
What can tell the tale is the injectors, if you have the N65s, they are the stingiest with the fuel. The load will also tell the tale. Most 8V71s, I have run were singles on a 38' charter boat and we weren't running for economy, we could get about 12 gallon per hour consumption. I did run a 42' crew boat with twins, and at cruise we burned about 20 per hour for both. We are talking about 2k rpm though. These figures were vessel loaded with pax, and fuel etc... My 471 gets about 4 gallon per hour pushing a Skookum 53' at 7 knots with a 4 blade prop, @ about 1500 rpm. I realize this doesn't address your situation exactly, but hopefully it can give you a general idea of what you are up against. As Capt. Phil says, WAG, you might get better performance @ 1500 rpm, fuel consumption against distance traveled. Most mechanics I have spoken to regarding this matter seemed to think that 1500 rpm gave the most bang for the buck in terms of fuel consumption against speed and distance traveled. As others have already said, "Your results may vary".

djmarchand · 01-07-2013, 19:12

Post how fast you go at 1,000 rpm and I can take a guess at fuel burn.

David

ranger58sb · 02-07-2013, 03:45

You can often find the manufacturer's performance curves for a given engine, including fuel burn rate, on line somewhere... Not sure if that would hold true for the (older) Detroits, but it might be worth a search...

-Chris