Hello,

While repacking my shaft logs, it was noted the original 1966 Galvanized fuel tank was rusty and flaking metal. Fortunately this was found before the tank leaked all the gas into the bilge. So out the old tank came

My question is one larger tank (130Gal?) or two 50 gallon like the original set up

My thoughts are to go to one tank but for some reason CC went with two tanks.

One or two tanks?

Timmy

FXA 0061

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Put in 2. 1 would be too big and restrict your access to certain areas of the bilge. When you order the new ones, square them up using maximum height and width that will fit between the engines without exhaust manifolds on. I did this and increased capacity from 100 gals. to 176 gals. I no longer sweat when going from Cape May to the C&D Canal. Replace the mufflers while you down there. Dave

Dave, the plastic tank shown is only 27” wide. Although it is wider than the original tanks, you will still be able to access below the rear hatch and get to (struts bolts, rudders, trim tab pump, etc) . That tank shown is 142 gallons which is a 42 gallon increase. I need to stay plastic for im an avid scuba diver and the salt water dripping on dive gear invariably leaks into the bilge area. Aluminum and salt water..... The mufflers were upgraded in 2011 to centek 3”.

Do you have any pix of your new tank install?

Timmy

Tim,

Going with one tank that is mounted transversely like you are planning will create a few difficulties. Not that these are show stoppers but things that you should consider.

First there is the issue of fuel pickups, you can get the fuel pickups wherever you want to put them, however with a transverse mounted tank, if the boat lists, the fuel pickups will be affected. If you mount the fuel pickups on both sides of the tank, after you are below half full, one pickup or both depending on the roll rate etc. will be exposed at times and suck air. If you mount the fuel pickups in the middle, your safely available useful fuel capacity will be substantially reduced. depending on weather conditions etc., you could lose as much as 25% of available capacity. Would not be fun to have 1/4 tank on the gauge and be sputtering from lack of fuel.

Another issue that the rear mounted transverse tank creates is that you are moving the fuel tank center of mass rearward by approx. 12-16in. In addition you are adding more fuel capacity by about 30% which further exacerbates the rearward shift of the center of mass of the fuel tank. You are also likely going to mount the tank very close to the rear deck so you will be increasing the height of the center of buoyancy making your boat more tender.

One last consideration you should make is that there is a significant amount of redundancy with a two tank setup. With a single tank transverse setup, the tank will be more sensitive to water issues in that as the boat rolls and or lists, the water in the tank will always be going to the low side. Just a little bit of water in a traditionally mounted tank will go unnoticed but the transverse tank will allow all of the water to pool on the low side, greatly increasing the probability of picking up water. The dual tank system also allows you to have two separate fuel tanks that are isolated and will allow you a certain amount of redundancy if one of the tanks has a fuel quality issue.

There are some good reasons why naval architects favor longitudinally mounted fuel tanks.

Ronald,

The replacement tank/tanks have not been purchased yet so we can still go to one or two.

Just so we are on the same plane, are not the original tanks mounted transversely? Longitudinally runs the long way (bow to stern)?

I have thought about the fuel pickups   Might hit up an engineer to figure out if at what degree of roll and a what amount of gas in tank will the fuel pickups become uncovered by gas. But if you look in the picture with the tank of the deck. You will see a replacement black plastic gas tank mounted transversely for I had replaced one original tank. Only a 50 gallon tank with fuel pick located on outboard side of tank and boat. I ran that tank for about 10 years in some very rough ocean's (Boston harbor/outer harbor) for over 7,000 miles (GPS) and never had an issue of engine trouble for gas pickup. If I go with new tank shown in pix, the new tank will actually be a couple of inches lower than that of original tank, so tenderness shouldn't be an issue. Old and new tanks both sit on top of wood stringers

I had thought about center of gravity, with a difference of 252 lbs between totally full 142 gallon tank (852lbs) and 100 gallon tanks (600lbs) moved 12" aft?  I guess I should count the increase in weight of tank, but if I go from two tanks to one is there a weight difference? So maybe 250lbs moved 12". That is only with a totally full tank, when tank is 2/3rds full, the weight differential is gone. IMHO, shift in center of gravity is not an issue

Redundancy, is a valid point and still working thru cost benefit of having two tanks. That can go with nice to have a back up vice twice the chance something will break. How often have I broke down and switched gas tanks to get back in?

So many decisions;)

Timmy

Timmy B

A list or roll of 5.44 deg with the tank at half full will give you a dry corner on the high side.

At 1/4 full you will basically be out of gas in rolling seas as neither side will have sufficient periods of immersed fuel intake.

Wow, nice and thank you

Is there a online calculator for that?

Now i wonder why fuel pickups are at one end or another of tanks vice in the middle,

This gets back to the tank that you are looking at was specifically designed for mounting in a lengthwise orientation to the hull any would probably lay in the average keel very nice.

Most boats are in a bow high attitude while under way. This allows that tank you are looking at to be able to literally suck the tank dry. Mounting the fuel pickup any other location beside the natural low point will not allow you to get full use of the tank capacity.

If you were to use two tanks shaped the same but half the length, the fuel pickup issues would be greatly diminished.

To answer your question about the angle calculation, it is a straightforward trig calculation and yes there are right angle calculators that will do the problem for you.

Again, thank you

Time for plan E

Timmy

This is why I had my pickups put in the front of the tank when I designed them. Natural reserve, come off plane and there's still a few gallons left to run at hull speed with. 

Ronald Zick said:

This gets back to the tank that you are looking at was specifically designed for mounting in a lengthwise orientation to the hull any would probably lay in the average keel very nice.

Most boats are in a bow high attitude while under way. This allows that tank you are looking at to be able to literally suck the tank dry. Mounting the fuel pickup any other location beside the natural low point will not allow you to get full use of the tank capacity.

If you were to use two tanks shaped the same but half the length, the fuel pickup issues would be greatly diminished.

To answer your question about the angle calculation, it is a straightforward trig calculation and yes there are right angle calculators that will do the problem for you.

Ronald,

If possible, can you calculate the degree of roll required for the fuel pickup to become uncovered if the tank in pix is mounted transversely?

Thank you,

Timmy

Ronald Zick said:

Timmy B

A list or roll of 5.44 deg with the tank at half full will give you a dry corner on the high side.

At 1/4 full you will basically be out of gas in rolling seas as neither side will have sufficient periods of immersed fuel intake.

Tmmy B.

This is not exact in that the tank is a complex shape but should be close enough and will be on the conservative side so real performance should be slightly better.

If the tank is half half, then a list or roll of 23.3 deg will give you a dry corner on the high side.

If the tank is 1/4 full, then a list of 13.3 deg will give you a dry corner.

This is probably quite close to the original tank performance and if the new tank is properly baffled would help even more.

Timmy B said:

Ronald,

If possible, can you calculate the degree of roll required for the fuel pickup to become uncovered if the tank in pix is mounted transversely?

Thank you,

Timmy

Ronald Zick said:

Timmy B

A list or roll of 5.44 deg with the tank at half full will give you a dry corner on the high side.

At 1/4 full you will basically be out of gas in rolling seas as neither side will have sufficient periods of immersed fuel intake.

Ronald,

Thank you, I think I will go with tanks in last pix.

Again, thank you,

Timmy

Ronald Zick said:

Tmmy B.

This is not exact in that the tank is a complex shape but should be close enough and will be on the conservative side so real performance should be slightly better.

If the tank is half half, then a list or roll of 23.3 deg will give you a dry corner on the high side.

If the tank is 1/4 full, then a list of 13.3 deg will give you a dry corner.

This is probably quite close to the original tank performance and if the new tank is properly baffled would help even more.

Timmy B said:

Ronald,

If possible, can you calculate the degree of roll required for the fuel pickup to become uncovered if the tank in pix is mounted transversely?

Thank you,

Timmy

Ronald Zick said:

Timmy B

A list or roll of 5.44 deg with the tank at half full will give you a dry corner on the high side.

At 1/4 full you will basically be out of gas in rolling seas as neither side will have sufficient periods of immersed fuel intake.

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