Auxillary Gas Tank Options/Questions

[San_Diego_SeaRay]

I have a 27' 1982 Sea Ray Sundancer I'm restoring. Currently has a 100 gallon aluminum fuel tank. I'm looking to remove this large fuel tank and replace it with a Moeller 50 Gallon below deck fuel tank. But I'd also like to have a separate back up supply of fuel just in case. Looking to buy one of those red Moeller or Atwood tanks that seem to be built for above deck applications for smaller boats. I'm looking to put the back up tank in the aft cabin (This is going to be a scuba diving boat so this is going to be more of a "storage" area rather than a "sleeping or relaxing" area).

I have a few questions about my idea:

1. If these are indeed designed only to be used above deck, I imagine the reason is because flammable fumes are released and may present a fire hazard. Is this the case?
2. If these types of tanks are not suitable for below deck use, what would you use to store appx 12 gallons of extra fuel below deck?
3. Do the permanent below deck tanks emit less fumes? If so, why?

Thanks in advance!

- JC

 

[Bondo]

Do the permanent below deck tanks emit less fumes? If so, why?

Ayuh,.... Because they're Vented to the outside,.....

 

[GA_Boater]

Unless the existing 100 gallon tank has a problem, why not only fill it to 50, 60 or 65 gallons. There's your reserve with 60 or 65 gallons.

I don't think it a good idea or really safe to store gas in a cabin.

 

[Scott Danforth]

your tanks need to be vented to the outside for safety reasons (and its a requirement). All below deck tanks are vented to the outside. you cannot use an above deck tank under the deck or it will go BOOM.

if you want to store extra 12 gallons of fuel below deck, get a bigger below deck tank.

you may want to check out the ABYC laws on fuel storage, specifically ABYC H-02, and Title 33, CFR 183.50

 

[San_Diego_SeaRay]

Unless the existing 100 gallon tank has a problem, why not only fill it to 50, 60 or 65 gallons. There's your reserve with 60 or 65 gallons.

I don't think it a good idea or really safe to store gas in a cabin.

The current tank is original and boat hasn't run in 5 years. After having rebuilt two engines and put new carbs on them, I don't want to have to figure out or take the chance of having fuel problems. Also, the 100 gallon tank takes up a lot of room, and I'd rather have the extra space to move around in the engine area. This will be a very local boat; never expecting to go more than 10 miles from the launch ramp. I might even be able to get away w. a 30 gallon tank.

you may want to check out the ABYC laws on fuel storage, specifically ABYC H-02, and Title 33, CFR 183.50

Ah. Thanks!

 

[GA_Boater]

The current tank is original and boat hasn't run in 5 years. After having rebuilt two engines and put new carbs on them, I don't want to have to figure out or take the chance of having fuel problems. Also, the 100 gallon tank takes up a lot of room, and I'd rather have the extra space to move around in the engine area. This will be a very local boat; never expecting to go more than 10 miles from the launch ramp. I might even be able to get away w. a 30 gallon tank.

Then you won't need a an aux tank with only 20 mile round trips. 50 should be OK to get you out and back with some reserve for bad weather. Any idea on fuel usage with the twins? Is the current tank in the engine room?

 

[San_Diego_SeaRay]

Then you won't need a an aux tank with only 20 mile round trips. 50 should be OK to get you out and back with some reserve for bad weather. Any idea on fuel usage with the twins? Is the current tank in the engine room?

Yes. You remove a rear deck hatch that's about 4' x 5' and the two engines are, of course, with flywheel bell housings butted almost against the inner transom plates. This leaves about 1.5 feet of room between the engine pulleys and the current gas tank, which forms effectively a sheer wall pretty much flush with the forward ridge of the engine hatch. Idea would be to remove the current tank and install a 40 or 50 gallon Moeller. This should allow for much more room to maneuver, as the "sheer wall" will become a recessed tank that is only 1/2 the size. Might even be able to install a shelf above the tank for add'l storage. Won't know until I remove the gas tank; which the mechanic at the boatyard says can be a %&#@ to do...

And I have no idea what the fuel usage will be. I'm *hoping* that it will be somewhere around 1 mile per gallon for the combined mpg. They are twin Merc 260s (SBCs). What do you think?

 

[Scott Danforth]

using .45#/HP-Hr BSFC fuel consumption, assume you are cruising at 170HP each motor. that is each motor using 76.5# of fuel per hour or 9.1 gallons per hour each or 18.2.

using the 1/3 rule of fuel, that would give you 50 minutes of run time prior to needing to turn around.

now assume you have to come back at WOT on both motors, assuming 260HP each, that is 117# of fuel per hour or 27.86 gallons.

a 50 gallon tank looks small.

to put this in perspective, I have a 75 gallon tank, I am a "local only" boater, and I have a single. I can burn thru my entire tank in 6 hours of boating. I have run out of fuel coming into a fuel dock

 

[Fleetwin]

My alarm bells are going off!!!!

A topside tank in an enclosed compartment. Ding Ding!!!! What doesn''t sound right?

Do this right or don't do it at all. Get a closed tank and vent it properly. Nuff said.

 

[San_Diego_SeaRay]

using .45#/HP-Hr BSFC fuel consumption, assume you are cruising at 170HP each motor. that is each motor using 76.5# of fuel per hour or 9.1 gallons per hour each or 18.2.

using the 1/3 rule of fuel, that would give you 50 minutes of run time prior to needing to turn around.

now assume you have to come back at WOT on both motors, assuming 260HP each, that is 117# of fuel per hour or 27.86 gallons.

a 50 gallon tank looks small.

to put this in perspective, I have a 75 gallon tank, I am a "local only" boater, and I have a single. I can burn thru my entire tank in 6 hours of boating. I have run out of fuel coming into a fuel dock

OK great. Thanks for that info. At least that gives me an idea. I'll have to think about this carefully, as after the engines are installed, you can't swap out the gas tank w/o removing the engines again. One of my major concerns about the current gas tank is that I have no idea whether or not there's bad gas leftovers in there, and I *really* don't want to introduce the variable of gas contamination into newly re-carbed engines.

 

[Scott Danforth]

there should be an inspection plate above the sender in the floor. one of the round plastic ones. pull it up, pull the sender and look.

if the fuel is over a year old, yes, its probably going bad
you can use a manual siphon pump and pump it out to dispose of properly

 

[smokeonthewater]

Forget storage in the engine room... Bad idea... IMHO you are way over thinking this...
Pump out the existing tank and clean it thoroughly... Install a new pickup with screen and use the aft cab to store your gear...

It takes less than an hour to pull an engine so if you need engine work that can't easily be done in the existing space it just gets yanked out, fixed, and put back in... Likely cheaper than paying someone to work below deck or easier than working below yourself.