If I buy a 800 bag I want 800 lbs, not 75% of a non vent bag.
A vented bag will not expand as much as an unvented. In fact, you will never be able to fill a
vented bag to its nominal capacity, short of some ideal conditions (outside the boat). The whole idea with running unvented bags is to be able to fill and drain completely, or at least to fill as much as the internal space allows (the bag will conform to the compartment's shape, at least to an extent). Also, keep in mind the key issue is not just the bag nominal capacity but to actually be able to fill it
inside the boat internal compartments.
Going
un-vented, the only disadvantage I am aware of is that you could damage the boat if an unvented bag were to be accidentally overfilled. I have mine on timers, but also end up always watching and sometimes adjusting them as they fill up, so I don't really see the danger. Building a little bit of pressure helps the bags to expand and fill the space.
The same with draining - if unvented they shrivel down and can be emptied completely, even if they are lying on an uneven surface with the drain
not in the lowest position, which somehow always seems to end up happening.
Yes, you can install one way valves, but every valve and every split/connector/branch point in the system creates new weak points and potential headaches down the road. I have experimented with various setups and read just about every post in every forum and concluded that the simpler the better.
With your boat, you have a lot more usable ballast space as compared to the new (2015+) 24' hulls which have wet storage compartments and a ton of soundproofing where you have just open cavities. I must say the new boats are
way quieter, and I could not complain, but the space inside the stern is pretty severely restricted.
(For comparison, there is actually more open space there in those back compartments on the side of the engine in a 190 model - but those boats are also louder than hell - the empty space acts like a huge acoustic box.)
Anyway, just my two cents. I would think twice about venting vs not, and if you really want the max volume usage, probably unvented is the way to go. If you do vent - the vent outlets need to be way higher than the top of the bag
when the boat is listed, which is easier said than done but crucial to avoid siphoning.
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