Water ballast tank placement

I have scanned through the entire thread and still think there is a point to be made.
You are adding water ballast tanks off center to give yourself the option of increased righting at the cost of increased displacement ->when that is desirable.
You don't want to negatively affect trim, but you need to consider trim under the conditions increased righting is desirable. In righting limited conditions the thrust to push your hull at speed is applied at the equivalent height of the center of effort of the sails and drag is occurring a few feet below waterline at the center of drag. The designer of your boat comprehended this (and likely compromised for wind limited as well as righting limited condition) but did not account for your water ballast add. To keep the trim, your water ballast would be centered on a vector opposite this moment, aft of the center of buoyancy.

Your hull shape is closer to canoe stern compared to racers that typically have water ballast (and carry full beam to the transom). Moving water ballast aft limits distance off center reducing the benefit. Have you considered going external? By my calibrated eyeball your internal tank would be limited to 4 to 5ft off center. If you go outside it would be at least 7ft off center. The same righting could be achieved with 650l, and a failure would be of no consequence (not flood your cabin). This would make more sense if your use case is as you say all day on one tack. It's no good beating upwind but you may find your internal is also more trouble than it's worth in this situation.
It's not what you asked for so I will end there.

 

How on earth would you build an external ballast tank that does not compromise hull form and can remain attached in severe conditions?

Incidentally, the beam is pretty much 13 feet. Might need to recalibrate that eyeball.

 

Think of a RIB pontoon on the toe rail, starting from a point, growing to a half round for a bit (flat against the hull) then to a point again in back, all above the static waterline.
It would survive dips into the water at speed, but not be desirable if that happens frequently.

 

First, this is a blue water cruiser, not a river boat . You really underestimate what gets thrown at these boats.
Second, compromising the access above decks is a death trap. Moving around is hard enough in bad weather, climbing over some giant tank on deck is lethal.

But finally, unless the tank sits outside the existing hull line it has no significant improvement in leverage. And if it does sit outside the hull line it compromises the hull form.

Sorry, your idea is just plain wrong for a blue water boat. Stick to rivers.

 

I think you misunderstand (it hangs over the side, not on deck) but are clear you will not consider external. I am not surprised.

 

You want to hang a tank over the side of a 44 foot blue water boat?

That is just plain dumb. Of course I wouldn’t consider it. I didn’t think that’s what you were proposing because I didn’t think anyone could be that silly.

 

Since you were worried about the fore and aft trim, have you considered four tanks? It would only need having a bulkhead in the middle of the tanks.

 

As previously mentioned, it adds significantly to the construction challenges and plumbing complexity. Also, I don’t want to waste time and space building bits of the tank that I subsequently don’t use. Far better to get a single compartment tank in the right spot.

On balance, I’m going with a single tank each side, but set very slightly aft of the centre of buoyancy.

I was curious to see if anyone knew of a good reason for having the tanks aft. This question has been answered pretty well, I think.

 

Sorry, but that is a misunderstanding. Increasing weight towards the ends will cause a reduction in pitching frequency. As a result, the boat will pitch in resonance with waves with lower frequency, i.e. larger waves with more energy to force the pitching movement. Most unpleasant for a cruising vessel.