Fuel tank location - what makes a hull more stable? Centered or saddle tanks?

There is a thread going on this on another forum related to Downeast hulls, which are semi displacement keeled hulls, typically single inboard engine, typically round chine but can be hard chine.. if more info is desired and this comes down to something very specific to a particular hull I can provide more info.. In short there are people arguing that both center and saddle tanks are more stable.. By "stable" in this case it is referring to the boats tendency to roll, how far it rolls, how fast, how long does it continue to "wobble", not a question of how it resists listing or capsizing.

I don't know quite how to look at the question, on one hand inertia seems to play a greater roll of helping to minimize rocking with saddle tanks but on the other hand it feels like no matter what the forces are they are balanced if tanks are equally full.

So to put it simply, the question is basically, like my sketch - if you place a fuel tank of capacity 2X dead center in the hull, does it make a more stable boat compared to if you place two fuel tanks of size X as far outward as possible.. For the purposes of this question similarly, lets assume there is no difference in height, ie not a matter of center of gravity with one tank setup being lower than the other.

Thanks in advance!

Jon

 

The further the mass is from the center of rotation, the harder it is to give the object an angular acceleration with a specific force/torque. I believe that the equation is I=mrr Moment of inertia equals the mass times the radius from the center of rotation squared

A good example of this is a figure skater in a spin. Simply moving the arms out or upward, the angular velocity changes significantly. Ie there are no external inputs to
increase or decrease the speed, only the change in the moment of inertia

 

Considering fuel tanks as ballast is erroneous from the outset, as it will always be a varying weight.

 

There are two problems to keep in mind. One of them is the imbalance that occurs with the two symmetric tanks as liquid is consumed, but that is avoided by making a symmetrical consumption in both tanks. The other problem, probably the most important, is the negative effect on stability due to the free surfaces that occur in the tanks. This effect is minimized when it comes to two symmetrical tanks with half a free surface each.

 

Wow thanks guys!

So in this case they are equalizing fuel withdrawal via some means (which BTW is a diesel, involving feed & return lines, a whole system to try to equalize tanks), which I'm aware may or may not work, which is a whole different plumbing and practical topic .. The question is assuming that all works and lets assume tanks are baffled "ideally" so similarly free space is a non consideration.

 

In your drawing, the center tank has a lower center of gravity that will translate into more stability. However, the side tanks will dampen the roll because they are further from the axis of rotation and will make the boat more comfortable. The roll will be longer and slower.

 

The solution would be a central tank, with little breadth (give more length to get the necessary volume) and, if necessary, a swash bulkhead in its longitudinal plane.
The side tanks, without a consumption compensation system, which is expensive as you know, are not the solution nor do they make the boat more comfortable.

 

Trying to ask the question under the assumption center of gravity is equal, ie tanks mounted same height from waterline, as to get the concept clearly..

Would it be accurate to state as far as a stability test would go or from a perspective of the ability to resist capsizing, center tank wins. But then from the perspective of comfort, a slower roll with less snap, saddle tanks win?

 

With the center of gravity of either tank system being the same as per the assumptions that you want to make, and if I make the assumption that both tanks are equally filled at max heel,
AND the cross section of the tanks are similar then neither system will add to the instability, or if so, the amount will be negligible as compared to whatever sail loading is in play to take
the boat to capsize.

This is also making the assumption that the boat will rotate around the centerline of the boat so the saddle tanks are revolving equidistantly from the center of rotation.
Under max heel with a capsize a half a degree away, I am not sure if a sailboat
actually rotates around the geometric centerline of its vertical cross section. Certainly the center of buoyancy changes.

One thing to note that if you are using the engines when heeled, the shape of the tank and the position of the fuel draw will be important to ensure a flooded draw tube in a heeled position
There is no doubt that the single tank will be easier to plumb in the fuel, and return lines, possibly some issues ensuring the vent will not have a low spot, ie always self draining

 

I you decide to go with twin tanks and need to switch the tanks and have the return line switch at the same time, Anderson Brass, SC, make a 6 way selector brass manual valve.
ie one turn and both draw and return lines switch. I don't think that they are cheap and at one point we also purchased, not from them the same 6 way valve in stainless,
and they were extremely expensive. I would recommend that you NEVER hook up two simple 3 way valves, ie one for return and one for draw in case you forget to turn each exactly
at the same time and rotation, otherwise you have the risk of drawing from one tank and filling say an almost full tank on the other side from the return line
and basically pump the return fuel out the tank vent when the tank fills.

 

If the CG of the side tanks is the same as the CG of the center tank then the effect on static stability will be the same.

The side tanks will increase the roll moment of inertia, lower the roll frequency, and slow roll.

As TANSL suggested with partially filled tanks free surface effects may cause some difference. Those differences would depend on the details of the tank geometry and internal bulkheads.

Barry - The original question was about a Downeast type hull which are used for powerboats, not sailboats.

 

Thanks again! That all clarifies the considerations very well.

This one is not my boat, someone else needs to weigh the complexity of trying to equalize tanks against the potential for a more comfortable roll, and how center of gravity is impacted by moving tanks from the center, what actually fits, baffles, etc etc...

 

Equalizing the tank volume of fuel is not that hard. Two simple fuel gauges and a valve to isolate each tank and then just switch them as required.
The boat will never be balance perfectly around its symmetrical centers. People moving, wind loads, food load distribution, battery locations etc so if the tanks are not perfectly balanced to even within a few hundred pounds, the boat will operate just fine. Our standard hulls fully laden around 4000 pounds including 800 pounds of fuel, ie a large percentage of variable fuel weight to the the boat weight, were dealt with in this manner.

FYI There are various suppliers of marine sending units that include the return and draw line fittings along with the sending units attached to make the installation easy
If your tanks are long, you will want to create 2 vent lines in each tank, one on each end,
manifold them, ie tie them together in a T, and then run the single line up to a vent.
A common mistake is "not to ensure that vent line is the right inside diameter". I don"t have my ABYC manual with me but I believe that vent lines need to be maybe 5/8 of an inch where as the draw lines 3/8 and return lines can be I believe 1/4. I mention the sizing, though I cannot confirm it accuracy right now, only so that you are aware of
this issue. The draw and return obviously would change if you were trying to run fuel through to engines in the many hundreds of horsepower. We used these ID's running
engines upwards of 400 HP

Another issue is deciding on a 2 inch or 1 1/2 inch filler hose. It is best to use the 2 inch to make filling easier

Also note that there has been a change over the last several years about the venting requirements for fuel tanks, so the vent that I discussed above may not be relevant. somewhere about a year ago this subject was introduced and some learned people
educated probably many, certainly me, of the new changes. If you use the search engine above and type in fuel tank venting or some other wording you will probably find it.

EXTERNAL GAS FILL CAP In this forum, not sure if it is applicable to diesel as my ABYC manual is getting old

 

Saddle tanks will resist a roll starting but will also resist it stopping. This can result in a slower but larger amplitude roll.

A boat's response to roll will depend on the frequency of the excitation (typically waves). If the natural frequency of the loaded vessel is close to that of the waves then roll amplitude will be very large. It is easy in other disciplines to design objects (eg buildings, bridges) with resonant frequencies far away from the exciting frequencies (eg with tuned mass dampers) but this is hard to do for boats due to the wide range of wave frequencies.

NB damping is separate to inertia and position of the tanks will not affect it. Having fluid sloshing through perforated baffles or a narrow pipe joining two tanks would provide damping, but it might well be negligible compared to that from external geometry (esp bilge keels).