Help calculate fuel tank capacity

[Dave22q]

Fuel tanks normally are vented, so thermal expansion will never rupture the tank but theoretically could push some fuel out through the vent. However, the coefficient of expansion is .00046/deg-F so a 100g tank holding 99g would still hold slightly less than 100g if temperature rises 20 degrees. Hard to envision a realistic scenario where thermal expansion is significant. Fill to top in winter, do not use for months is bad practice for many reasons. Anyway it is almost impossible to fill to the very top with most boats without chancing a nasty and potentially expensive fuel spill. I suspect most boaters like myself use a dipstick or gauge to determine the amount to add and watch the pump to cut off as planned. It's not like a car where the flow automatically cuts off at
'full'.

[SigaSiga]

Hi
The tank on our Taswell49 is more complex. It's under the aft berth. We estimate it holds 450 liters.

[Spot]

Originally Posted by Dave22q

Fill to top in winter, do not use for months is bad practice for many reasons. Anyway it is almost impossible to

Hi Dave, could you share please why filling up before decommissioning is bad?
I was told that a full tank is less prone to effects from condensation as there is less volume and surface area available for wringing moisture out of the air.

[N4585K]

Originally Posted by TrentePieds

You might like to know that a diesel like the 80 horse Yanmar will burn about 4 lbs of fuel per 10 HP per hour. A (US)gallon of diesel fuel weighs 7 lbs
(approx.). Therefore a diesel will "normally" use about half a gallon per 10 HP per hour.

In your boat I would expect that the "throttle" setting that will be appropriate for most operating conditions will be one that develops about 60HP. Therefore the consumption will be 6 x 1/2 = 3 gallons per hour. In consequence you would have fuel in the tank for about 33 hours of continual operation at 60HP output.

Very interesting to read your response TrentePieds! Somehow your estimate of 3gph seemed very high to me though and I looked up the numbers for my 75 HP Yanmar 4JH4-TE. At 2000 RPM (my normal power setting) my engine produces 45kW/60 HP and consumes 4 liters per hour...

[garyfdl]

Originally Posted by Dave22q

Fuel tanks normally are vented, so thermal expansion will never rupture the tank but theoretically could push some fuel out through the vent.

Obviously, you've never had the pleasure of meeting Mr. Murphy...

[TrentePieds]

N4585:

Thanx :-)

You are spot on, of course, which only proves that tables are but a starting point. Your own experience in your own boat is what really matters. 4 L/hr equates to "a gallon an hour". My little Beta20 going flat out uses a tad more than a litre an hour, at cruising revs a little less than a litre an hour, so we are in agreement.

Here is a link to the table whence the "half gallon per hour per ten HP" came.

https://barringtondieselclub.co.za/t...-aspirated.pdf

All that notwithstanding, my purpose was not to furnish the OP with definitive figures, but rather to set him thinking along certain useful lines. You have now helped me to do that, for which I thank you :-)

TP

[Cadence]

Originally Posted by garyfdl

Sorry implying that the coefficient of expansion of fuel isn't a factor would be incorrect. While it might not cause problems of bulging the tank, it could result in a spill, as someone already mentioned.

"A typical value of the coefficient of thermal expansion for diesel fuel is 0.00046 per degree Fahrenheit. Using this value, 1.000 gallon of diesel fuel at 20°F will expand to 1.037 gallons at 100°F."

Gary and others I stand corrected. Physics was ages ago so it appears liquids do expand. Not sure how significant it is practically 4 digits below the decimal point.

[waterman46]

Correction to my previous post. Had one dimension off by 10! Corrected volume of outside of tank is 104 gallons Inside maybe a gallon less of course. And assuming the sides are vertical.
Interesting how this thread wandered into fuel consumption and both fuel and water expansion with temperature!
When we accidentally ran out of fuel (new boat to me) and had to be towed the last mile to a fuel dock due to adverse wind direction, I was able to calibrate a dipstick for the tank. 140 gallon tank has such a deep vee that only 20 gallons are contained in the lower 1/3 of tank.

[Cadence]

Originally Posted by waterman46

Correction to my previous post. Had one dimension off by 10! Corrected volume of outside of tank is 104 gallons Inside maybe a gallon less of course. And assuming the sides are vertical.
Interesting how this thread wandered into fuel consumption and both fuel and water expansion with temperature!
When we accidentally ran out of fuel (new boat to me) and had to be towed the last mile to a fuel dock due to adverse wind direction, I was able to calibrate a dipstick for the tank. 140 gallon tank has such a deep vee that only 20 gallons are contained in the lower 1/3 of tank.

Dip sticks don't lie when done like that.

[mobern]

I've updated my sketches to include all sides with dimensions. My first side view was incorrect, please disregard. All sides appear to be vertical, so the top dimensions will equal the bottom dimensions. I have the sketches in pdf format. Hopefully that will make it easier to zoom in on the dimensions.
I want to thank everyone for their help and comments regarding my fuel tank.

[mobern]

Sorry I haven’t been able to monitor the “Fuel tank party” as closely as I would like to. I’m anchored off St. John, USVI with weak internet access. Looking through the responses, it appears that “Spot” recreated my tank correctly with his 3 dimensional sketch.

[mobern]

Actually Spot, the 15.5" to 18.5" would be from from front to back. I think your sketch shows those dimensions running from side to side.

[Spot]

Hello Mobern,
Jealous of your location, hope all is well and crew is healthy.
First time was tapered front to back, this time outside to inside.
Still in the 112 to 114 us gallon range no matter how I model or guesstimate. I agree with others that you may want to lower that figure to add a reserve, calibrate a dipstick, etc.

[N4585K]

Originally Posted by TrentePieds

N4585:

Thanx :-)

You are spot on, of course, which only proves that tables are but a starting point. Your own experience in your own boat is what really matters. 4 L/hr equates to "a gallon an hour". My little Beta20 going flat out uses a tad more than a litre an hour, at cruising revs a little less than a litre an hour, so we are in agreement.

Here is a link to the table whence the "half gallon per hour per ten HP" came.

https://barringtondieselclub.co.za/t...-aspirated.pdf

All that notwithstanding, my purpose was not to furnish the OP with definitive figures, but rather to set him thinking along certain useful lines. You have now helped me to do that, for which I thank you :-)

TP

TP,

Thanks for your kind reply. I have learned so much from everyone here on CF. It is great to see so many great replies on such varied topics!

[lordgeoff]

Originally Posted by mobern

Is there anyone on the forum that can estimate the U S gallon capacity of my fuel tank ? Dimensions attached.

Depending on your circumstances and positioning of your tank it may be a good opportunity to drain the fuel, clean your tank then filter the fuel as you refill. If you use say a 20lt container you can get an exact measurement of your fuel capacity.
Plus as a bonus you know your tank is clean and your fuel clean. Win win win