What's the trim like under normal conditions? 2/ or 4/ donk, or, how heavy is your donk? Do people sit on the rear lounge a lot? If the battery you mention is the 2nd battery at the transom, don't think that's a good place to add another 50kg Dan.
Don't know this model, but the bladders I have seen in bigger CC's (and mine) have been in the side pocket area around midships.
However, as most std bladders are designed to lay flat, when you tilt them on the side (ie to fit in the fwd side pockets), it is impossible to use all the 50L due to the air trapped in the high part. What about the various shaped & size plastic tanks available from various manufacturers, have you checked these out?
Cheers
Brendon
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