Catamaran uneven tank location problem

Discussion in 'Boat Design' started by groper, Oct 10, 2012.

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groperSenior Member

Ok the problem relates to a catamaran that has 1 large tank located forward in the port hull, and another tank of equal size located AFT in the starboard hull. They are positioned so that their combined longitudinal and lateral distribution is bang on the center of buoyancy, and they will always have equal fluid levels during normal operation.

The question relates to the types of loads this will put on the catamaran cross structure.

Will this arrangement of (longitudinal difference in each hull) tanks increase the torsional loads (or any other loads) on the catamaran cross structure compared with an arrangement whereby they have the same longitudinal location?

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I would venture that uneven longitudinal locations would indeed place an ill advised torsional load on the center structure. This is particularly so if the it is a sail cat where the windward hull may be lifted above the level of the leeward hull. Also the longitudinal distance between the tanks plays into the equation rather importantly.

There is the matter of tank capacity, more specifically the weight of the contents. A few liters of fluid may not matter enough to worry with. ( a beer cooler perhaps). But if there is signifigant fluid weight.................back to the drawing board.

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Like most things, it depends.

You firstly need to ensure your local structure can:
2) shirk the loads to surrounding structure
3) ensure that it does not effect the global structure in a negative way.

Your main raft structure, if you took their combined weight into account when calculating the stiffness required, (i.e. their weight was included in the total or max displacement you used for the structure), then it is simply a local load path issue, not a global structure issue.

PS..i would however be more concerned with the resulting motions, since you are increasing the gyradius.

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groperSenior Member

To clarify;
Originally the tanks were located in a longitudinally central position in each hull. But due to a general arrangement change, the tanks need to be shifted so that each hull is no longer symmetric, whilst still trying to maintain the original CoG. So the port tank has gone forward and the starboard has gone backward from their original locations by equal amounts. AdHoc, from what you are saying, the main bridgedeck and beam structure will not be affected by this change, only the local loads within each hull associated with the inertia of the tanks in their new locations... is this correct?

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Correct. So long as the displacement value you used in your raft/bridge/cross deck structure calculations included those tanks and their respective weight of fluids.

Then it is simply a local issue. Notwithstanding the possible local effects on main transverse structure being used to take the global bending loads; which is still a local issue.

6. tunnelsPrevious Member

wow there one born every day !

That is the dumbest thing i ever heard but in this day and age of not know much designers its not surprising ! I am working through simular problems were im working right at this moment . it should never have been done and you should point the finger directly at the builders as well !!
Not all designer have the right number of marbles in there bags ,most have a few missing and for doing such a dumb thing this one needs his bump feeling as well .
So one tack the boat will sail quite well and the other tack it could be a pig . and in the rough seas the racking and twisting and fatigure rate will be horrendous !! but if you leave the tanks empty and semi sealled up they could be good for bouyance !!!

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TANSLSenior Member

I do not know if this can be considered a "local" problem.

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8. Joined: Jun 2006
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Richard WoodsWoods Designs

Groper is building a 35ft powercat with an expected speed in the low 20's

I agree, pitching is going to be more of a problem than structural concerns

You didn't say how big your tanks are, but assuming they are fuel tanks they will be about 2/3rd full on average. So you may also have to consider free surface effects (fit baffles).

On my powercat I have decided to fit a number of smaller tanks, in part to act as baffles but also because of the ethanol fuel problem which sensibly means you don't want to use old fuel

Richard Woods of Woods Designs

www.sailingcatamarans.com

9. Joined: Jun 2006
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Richard WoodsWoods Designs

It probably is if the full tanks weigh less than 100kgs each

It would be a very poorly designed boat if it actually twisted as much as the sketch suggests. And I know that gropers boat will be essentially stiff

In any event it is easy to test whether the whole structure is stiff enough. After decking the boat support it under both sterns but under only one bow and then measure the deflection on the other bow. It should be well under 50mm

Richard Woods of Woods Designs

www.sailingcatamarans.com

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What is it you are showing

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TANSLSenior Member

I agree. All I wanted to say is that it should make some general test, before deciding that this is only a "local" problem. I have the feeling (not supported by any calculation) that the deck and wet deck structure will withstand this torque "additional".
If you do not know what I mean. I'm sorry, because I find it embarrassing to have to explain. Maybe I should improve the quality of my picture???

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Not necessary.

Just state you assumptions in what your picture is referring too. Without such, what does it show, other than 2 hulls being rotated.

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groperSenior Member

Richard, yes despite the fact im building a boat, the problem was more of a hypothetical problem, as i certainly dont intend to put a huge tank in the bow of one hull and stern of the other - i was more curious about the possibility and what potential effects need considering with very assymetric catamaran designs and layouts. Next boat i build will be a larger sailing cat so i have to start thinking about many things already...

But its interesting as nearly everyone ive asked this question, intuitively believes that it will effect the torsional loads in the cross / raft / bridge structure of a catamaran. Would be nice to hear an explicit explanation as to why this is NOT the case...

14. tunnelsPrevious Member

Who is the designer !! put a name on the screen !!!! is this the first ??is it a just a oncer ??or has it been done before ??

15. FrostyPrevious Member

What is the weight of one tank. What is the capacity and why do you say they will always be equal.

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