tightening cross-bars that hold water and fuel tanks down.

[chriscd]

i am swapping an aft fuel tank for a water tank. i notice that the bolts that hold down the cross-bars that keep the
fuel and water tanks down are not snug. am wondering whether or not i should tighten them, or if they have been left a little loose to
allow for expansion of the plastic tanks. any thoughts on this?
my boat is a 2006 jeanneau so 49 performance model. 
chriscd in southern california

[Mistroma]

I seem to remember that tanks in 42DS have a central section where top and base are connected. The bars sit in a recess on top of this. I assumed that this was meant to limit ballooning and add stiffness. However, I think I also once noticed that the bar was very slightly loose when the tank was empty but tight when it was full.

It isn't fresh in my memory as I only lifted the aft tank once in 2011/12 to get access to screws when building a new battery bank box. I'd leave it until you fill the tank and retighten when tank is about 80% full. You could check again when 25%.

I think that some expansion should be expected but the design should prevent large movements.

Not certain if my fuel tank has a similar design but imagine it is likely.

[tilo]

From an engineering standpoint you don't want loose bolts holding a tank. Loose bolts always get looser - they never tighten themselves - and you don't want a full tank coming free in heavy seas. It's probably a good idea to give the tank some room to breathe, so I would put some washers under crossbar and then tighten the bolts.