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Author Topic:   21 Walkaround: Fuel Tank Capacity
gqwhaler posted 07-18-2005 12:49 AM ET (US)   Profile for gqwhaler   Send Email to gqwhaler  
I have a 21 Walkaround, 1992. The fuel tank has a label that says [its capacity is] 92 gallons. But I found out the hard way it only holds 75 gallons. Can anyone explain this?
gimcrack225 posted 07-18-2005 07:35 AM ET (US)     Profile for gimcrack225    
Interesting. How did you find out that it only holds 75 gallons? I have never had any reason to doubt the label on my tank. At the same time I have never had it bone dry and measured its volume with the gas pump. I can't remember the largest quantity of fuel I ever put in at one time but I think it was also about 75 gallons. Could there be fuel remaining in the tank when the gauge indicates empty? Or maybe the angle is the tank in the water might make some fuel come out the vent before the tank is 100% full?

I can't imagine a manufacturer mislabeling a fuel tank by 18%.

Backlash posted 07-18-2005 08:30 AM ET (US)     Profile for Backlash  Send Email to Backlash     
Billy--I trailer my 21 and try to keep as little gas in the tank as possible. Each fall, prior to winterizing, I syphon all the gas out of the tank and fill with 5-gallons of treated gas. I then fog the engine and store the boat. This year, prior to the first trip out, I started the engine (on muffs) to burn off the fogging oil and had 4.7-gallons remaining as indicated on my Standard FF41 fuel flow gauge. Prior to launching, I put in 75-gallons and the tank was not full, but had 79.7-gallons.

Many, many years ago on a Bahama trip, I was literally running on fumes when I pulled into a marina and it took 89-gallons to fill the tank. That being said, I have no reason to question the 92-gallon capacity of the tank.

I normally do not fill the tank completely and do not know what the actual useable capacity of the tank is, but can assume (from the Bahama experience) that it is at least 89-gallons. I do know that my Yamaha digital fuel gauge is very inaccurate and will show empty when maybe 30-gallons are remaining in the tank. That is the main reason I installed the Standard fuel flow gauge.

Like Karl, I am curious why you think the tank only holds 75-gallons of fuel.


gqwhaler posted 07-18-2005 09:10 AM ET (US)     Profile for gqwhaler  Send Email to gqwhaler     
Here is the story. I filled the tank the night before a trip to the albacore grounds. I knew I was pushing the limits of my distance and fuel consumption. I have a fuel flow meter and it has always been very accurate for me on other trips. I normally get 2.2-MPG. On the way home my fuel flow was reading 75.5 gallons and it started to flutter for about five minutes before the engine stopped. We were empty. We had put on 170 miles according to my GPS, total miles including drift. I called vessel assist and they delivered 15 gallons of fuel, just enough to get me home, 30 miles to the dock. When I got to the dock the fuel flow now reads 89.5 gallons used. I feel the fuel flow is still very accurate, considering when running out of fuel it was fluttering and pegging the needle hence reading slightly more fuel consumption that actual. That said, I trailered to the gas station and she only took 75.2 gallons. This leads me to believe that is all she holds, at least when she is on the trailer. I plan to float her and try to add the remaining 15 gallons.
JOHN W MAYO posted 07-18-2005 05:33 PM ET (US)     Profile for JOHN W MAYO    
I use a fuel flow meter also, an excellent tool. A 28 gallon Tempo-brand tank I filled new, took only around 23, a surprise to me. You expect your tank to hold what is stated.

If you set the fuel flow meter with the number of gallon you have on board, it works well.

gqwhaler posted 07-26-2005 02:32 PM ET (US)     Profile for gqwhaler  Send Email to gqwhaler     
Update on the 92 gallon fuel tank in a 21 walkaround--To conclude this question, here is what I have found: With the fuel tank empty of all useable fuel, my 21 will hold 75.5 gallons while on the trailer on flat ground. After launching it, I was able to add another 11 gallons. a total of 86.5 gallons. I feel it is very probable that there maybe 5.5 unuseable gallons sloshing in the bottom that cannot be reached by the fuel pick up.

I am now looking at ways to add another 25-40 gallons for the long runs we make to the tuna grounds. I understand some 21 Walkaround models have a holding tank of 40 gallons. Where is the factory location for this? This may be a place I can have a tank installed or maybe a fuel bladder.

gimcrack225 posted 07-27-2005 11:03 AM ET (US)     Profile for gimcrack225    
Fuel capacity on 21' Walkaround. Interesting calculations. Possibly the only way to fully fill the tank is when its perfectly level, a situation that may never happen.

My holding tank is below the instrument panel. The capacity is nowhere near 40 gallons maybe 10 gallons. There is also room under the portside cockpit wing that is used for an optional 10 gallon fresh water tank. To get to either of these places would require a lot of cutting and dealing with the main wiring harness that runs through this area. Forty gallons takes up a lot of room - Visualize 6 1/2 of those red metal 6 gallon fuel tanks that are used for small outboards.

I don't know where you could safely add extra gasoline tanks. Maybe in the fish wells under the main deck or in the center storage compartment under the bunks in the cuddy. Its a complex problem since you have to deal with filling hoses, vent hoses and switchable supply lines and potential contamination and effects of seawater on the fuel and or the tanks if you went under the main deck.

I'd be afraid to carry gasoline below the deck level in anything other than a carefully engineered situation. You probably would not be able to get insurance for a vessel with anything other that factory installed fuel tanks.
Gasoline fumes = the biggest danger on a power boat. Turn the key blow up the boat - I've seen it happen.

Maybe a 25 gallon tank mounted on the cockpit floor or a couple of 12 gallon tanks mounted on the bow in the walkways.

Good luck. You've convinced me that it would be a good idea to install a fuel flow gauge.


jimh posted 07-28-2005 09:28 AM ET (US)     Profile for jimh  Send Email to jimh     
The label on the tank is probably indicative of its ultimate capacity if fully filled. It is typical that this must be de-rated about ten percent to account for fuel which cannot be withdrawn in actual use. Thus the useable capacity would be nominally

92 - 9.2 = 82.8 gallons

If the orientation of the boat during filling prevents you form completely filling the tank, this further reduces the capacity.

Typically during filling you can bring the tank level practically to the top of the filler hose if you are diligent. However, I would not recommend that as a normal procedure because the rubber hose of the filler will not benefit from having gasoline sitting in it for long periods of time. It can soften the hose.

In your recent fill-up you added 86.5 gallons. This seems to confirm the fuel tank capacity is as stated on the label, allowing for some slop fuel in the tank which cannot be withdrawn by the fuel pick up.

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