fuel tank replacement considerations

[ScubaDude]

Hi,

I'm new to the forum. I have read lots of good stuff here, thanks in advance for your input.

I run a 38' steel hulled scuba diving charterboat. It is powered by 2 Volvo 4 cyl gas engines (I/O or sterndrives). (Yes, I know that it is underpowered, but they have sufficed the previous owners of the vessel for decades and work well in our environment.) What doesn't work well is that the fuel tank is far too small. Previous owners carried jerry cans and would refuel even at sea, something I am not comfortable doing. It would also make life easier if we did not have to refuel between morning and afternoon runs when we are trying to get one group off and another on expeditiously.

Some numbers. The current tank is located in the engine compartment the tank is welded aluminium and is only 20 gallons!!! The center of the tank is about 8 feet forward from the transom, and it is near the front of the engine compartment, securely mounted with it's bottom at about the same level as the carburators of the engines. The vessel is 38' long and has a beam of 11' 3". I'd like to take her fuel capacity to somewhere in the 80 to 100 US gallon area. That would actually get us through a weekend of charters without having to refuel. I'm considering either a pair of Moeller 50 gal (Part# 032550) tanks (45" L x 12.5" W x 22" H) mounted with their long axis running P&S, one in front of the other or a pair of their 40 Gal tanks (part # 32640) 26" L x 20 " W x 18.5" H) mounted side by side i.e. port & starboard.

Link to Moeller tanks page:

http://www.moellermarine.com/afterma...rmanent_tanks/

Here are my questions for you knowledgeable folks. I'm going from 20 gallons of fuel weight or 122 lbs in this location to either 610 lbs or 488 lbs. The average diver (we carry 12 divers and 2 crew) likely weights 175 lbs and brings on board at least that amount of kit - or about 4200 lbs. So I'm adding the equivalent of 1 or 2 extra passengers worth of weight. That is a lot of additional weight, although given the overall vessel weight of slightly over 20,000 lbs, perhaps not so much. While we can control where the gear is stored, I am concerned about the degree to which this might affect either the stability of the vessel of her trim. (No I haven't done a new CG calc yet, I figured I'd ask for general thoughts from experienced people before I get into that level of detail.)

Since the tanks are gravity feed, I could lower them and add a fuel pump. I could also put them into the bilge under the cockpit and main cabin deck which would both lower them and move them closer to the center of the vessel. I could also put them in a fore aft configuration keeping the weight much more centered from a port stbd perspective. The downside of this is that I would not be able to inspect or see the tanks easily as I would be able to in the engine compartment. It also adds the cost and complexity of a fuel pump to the system. This bilge area, like the engine compartment, is already outfitted with blowers. In the bilge area, I might use some of their tanks like part number 032529. I should also note that the bilge is always bone dry.

Should I also be concerned with tanks of this size about the shifting of the fuel as the boat pitches and rolls in the water? I understand that most tanks of the sort I am considering don't have internal baffles.

Finally, are there other things I should be considering or aware of in what I'm seeking to do? Thanks for your input.

[gonzo]

To start with, if you have a gravity fed gas system, it is illegal. It should have a pickup from the top of the tank with an anti-siphon valve. If the current tank does not change the trim considerably between empty and full, it is located properly. All you have to do is to make the CG of the new tanks the same as the old ones.

[ScubaDude]

Gonzo,

Thanks for the quick reply. First regarding the fuel feed, this is likely my lack of understanding of terminology. The fuel pickup is on top of the fuel tank, but there is no fuel pump in the system (at least that I can see. or identify).

Second, regarding the CG of the new tanks, I would think that your statement is true only if the CG of the current tank coincides with the CG of the vessel. If the CG of the current tank is aft and higher than the CG of the vessel, then it seems to me that adding more weight also in that location will change (raise and move aft) the CG of the vessel. Why would this not be so? Thanks

[gonzo]

The CG and Center of Flotation are always vertical with respect to each other when the vessel is at rest. It is the normal state of equilibrium. If the CG of the tank and the vessel's minus the tank are not aligned vertically, then the load of the fuel will make the trim change. If the trim is not changing now, the tank is placed properly. As long as the CG of the new tank is aligned vertically as the CG of the old tank, then it would keep the trim unchanged too.

[CatBuilder]

Hmm... if you're running scuba charters in the USA, you should have a COI and the Coast Guard should have picked up on the legality (or not) of your current fuel system installation.

I'd add the tanks low and in the center, in the bilge, amidships, if possible.

Why not just have a welder come in and weld up a nice, new steel tank, centrally located, down low in that bone dry bilge? It's dead simple and he could make the tank fit the way you want it to, with inspection ports, baffles and the whole 9 yards for very little money.

[gonzo]

It is a gas tank. They have to be fabricated by a certified welder and then be tested and certified. Gas tanks can't just be welded in place. Plastic tanks are much cheaper and don't corrode. Plastic tanks are also already certified.

[ScubaDude]

Catbuilder,

Been inspected both before we bought the vessel and since and passed inspection so I assume that my description of the fuel system is what is lacking (or my ability to identify parts of it).

I have read a number of threads here about SS vs Al vs "plastic" tanks and as gonzo notes, the plastic seems to be far and away the least expensive option and the most readily available.

[CatBuilder]

Ok, I stand corrected. I'm used to diesel tanks, so my mistake.

[ScubaDude]

I would love to run diesels, but I checked into refitting the boat with them and then checked on my winning lottery ticket... which wasn't. Diesels are great to run, but very expensive upfront. Sigh... someday.

[michael pierzga]

Not sure about the practicalities of installing an extra gas tank on your vessel , but check out these guys if you need additional options concerning tank shape or capacity.

http://www.oceanlinkinc.com/tanks/gas-diesel.htm

[Ike]

So far good advice. Your fuel tanks are higher than the fuel inlet on the engine so you have to have an antisiphon valve in the fuel line. Thta keeps the fuel from accidentally siphoning out. You do have fuel pumps. They are on the engines. If you look at where the fuel line connects to the engine you will find the fuel pump.

It's good that you are concerned about the stability issue. If it gives you heart burn hire a naval architect to do a quick stability analysis. This may require doing an inclining which isn't often done on that small a boat but is the best way to find out how the stability is affected.

[ScubaDude]

Thanks for all the advice folks. Based on what I've heard here I'll do the accurate measurements of exactly where the new tanks can sit, take them to my friendly local marine architect and let him plug the numbers into his software and do the numbers. While I may be being overly cautious, I'd rather not make a decision on something which others are better trained and have better tools to evaluate. So thanks for all the input. I'll let you know what I end up doing when it gets done in April.