Fuel burn all over the place

[Shaunc]

My 53 has natural 871's.

I am trying to get an accurate fuel burn so that when it comes to doing some up coming long range trips, I know what range I have.
I am not going anywhere is a hurry and run about 1400rpm at about 8-9 knots.

I have had fuel burn rates at 1.1 mpg, .95mpg , .8mpg and as low as .65mpg over a year running around Tampa Bay area.

This is based on gps track and the amount of gallons at refill. I do run with full water tanks and fuel most of the time. Gauges dont work so its hard to tell the fuel load based on that.

I have a fairly new norther lights generator installed by the PO and has about 900 hrs on it. I am not sure of its fuel burn, but it cant be much and cant factor in to much, I would assume.

I have a clean bottom and dont have any record of any PO changing props etc, etc.

I just dont get why I am not having a consistent burn

[Pascal]

Assuming your gen is in the 15/18kw range plan on about 1GPh at 50/60% load. That can really affect how much fuel you use... sometimes we use more fuel for the generator than for the mains.

As to the mains at 1400 rpm and 9 kts you should burn about 9 to 10gph in water deeper than 9’. Less than that and your speed may be down a full knot for the same fuel

Beware than anything over 9kts fuel goes up exponentially. At 10kts you re burning 15to 18 GPH...

Full tank isn’t going to make a difference.

What you can do is feed and return both mains to the forward tank and leave the genset on the aft tank. Fill the tanks, run for a while and top off

Finally the mechanical gauges on top of the tanks are easy to unscrew and you can calibrate a dipstick while filling to get precise readings. Mark it every 50 gal. Just keep in mind the bottom of the tank isn’t flat

[RonNP]

Tides, current and wind play a huge factor.

When going north from Ventura to San Francisco I'm lucky to get 0.7 mpg. When coming back from San Francisco I get closer to 1.0 mpg.

[Shaunc]

We generally run around Tampa Bay in decent weather and the bay averages about 7-11 ft in most places.

The unknown is the generator, its a newer Northern Lights 12KW installed by the PO and I dont have any burn rates on that. I need to try get that figured out so I can factor that in.

I do keep it under 9 knots, as I read a thread that Pascal did on hull speed and took note of those recommendations.

Dick Lazzara docks his boat across from mine and has owned and run a 53 MY before.
His thought are that we may have under filled on a few occasions
He says the newer fuel pumps move fuel in way faster than pumps did when they we built and the vents are dot capable of handling the fuel and air going into the tank at a higher rate of flow causes the vent to burp and when it does often you can add 50-70 gallons more into the boat.

Most guys fueling boats in marinas dont go back and top off the tanks. I did my own filling this time and did go back and add more diesel after the 1st burp. So that is an option. I am also looking at installing Interactio sensors to get this right. https://shop.interactio.co/

The fuel gauges and fittings on the tank loo like they have been touched in 40 years so those will be a challenge to get those lose.

[Pascal]

You can, and should, download the genset manual from the NL website but 1 GPH should be oretty close, less on a light load.

It is hard to really fully fill the tanks on any boats these days as one drop in the water and everybody look at you as if you had commited a horrendous crime against humanity interesting comment from Dick Lazzara (one of the best American yacht designers) and he may have a point. Btw Lazzara always provided a beautiful wooden dipstick to check fuel level despite installing state of the art gauge systems.

The gauge should come out pretty easily as it is well lubbed by fuel... well worth the effort

[SEVEN]

The others covered the fuel. I’d like to help you with the gauges. Mine were stuck on 3/4 full and I thought I was getting great fuel economy. PM with your number and I’ll call and explain. Its a very cheap and easy fix.

[Boatsb]

The flow sensors look a bit small for a detroit.

[Scarlett]

I don’t know about your tanks, but each one of mine have a 1/4 inch pipe plug on the top next to the gauges. I use them for a dip stick and I remove them when fueling. It is amazing how much air blows out while fueling, which leads me to believe that Lazzara is correct and the vents are to small. If you have these plugs it really helps to remove them while fueling. John

[Boatsb]

Over the years the tank vents get clogged. Keeping them clear is important. My boat is from 1966 and if I dont clear them now and them I will have issues.

When fueling have someone feel for the air. If it's not blowing out you found the problem.

I know the plug inside is an easy work around but I doubt that's a legal and safe way to vent the tank.

[SEVEN]

Over the years the tank vents get clogged. Keeping them clear is important. My boat is from 1966 and if I dont clear them now and them I will have issues.

When fueling have someone feel for the air. If it's not blowing out you found the problem.

I know the plug inside is an easy work around but I doubt that's a legal and safe way to vent the tank.

When you found they are clogged how did you clean them?