Effects of water ballast

Hi Guys,
I have a 50ft monohull. the boat draws 2.8m with a 4 ton keel. In addition to this it has wing tanks that each take 1000L of water. This water can be moved from one side of the boat to the other depending on what tack you're on.
My question is: Is it possible that in certain conditions the water ballast will actually reduce comfort or perhaps increase rolling? I haven't had much opportunity to play around with it but my understanding is that you can have too much righting moment. Could someone please explain this? Thanks in advance.
Jeremy

 

Welcome to the Forum Jeremy.

How high up are the wing tanks - are they under the deck edge, or low down under the floor boards?
Do these tanks contain fresh water, that can also be used for drinking, or are they exclusively salt water ballast, which you pump in and out to suit?

Re increasing rolling, if the tanks are fairly high up (say under the deck edge) and you had both pressed up full, and you were motoring along (no wind) in a cross swell, then your rotational moment of inertia would increase and you would tend to roll more (but I cannot see any reason why you would want to do this).

I think that generally, if they are just salt water ballast tanks and you empty them when going downwind, and you keep the windward tank full on anything from a broad reach to closer to the wind, then they will only help (ie not hinder) you.

Note that if the tanks are not pressed full, you can have what are called 'free surface effects' which will cause a reduction of your stability (depending on the amount of free surface area, and how much it can slosh around).
An analogy to this is if you pour a cup of water into a frying pan - you can easily carry the water in the cup, but if you walk with the frying pan of water, the water will slosh from side to side, effectively making the frying pan more unstable.

 

Thank you for your reply, that's very helpful!
The tanks are freshwater, used just for the boats showers, they are long and narrow and roughly from the water line to deck level in height and with a series of baffles along their length.
In a beam sea, lightish wind would you think that that amount of stability would cause a hasher righting moment as the boat rolls on the wave? I was wondering if it would be best in light winds or a broard reach to move some water to the leeward tank. Thanks again for your help. much appreciated.

 

I suggest you find some people who have experience with sailing a boat of this size with water ballast…….the only real way to define this is to sail in those different conditions and points of sail and experiment with configurations, maybe talk to the previous owner or the builder or designer……
Put 500l in each, the go sailing….sail,upwind, move the ballast to the high side and see what changes,…….then bear away onto a reach or broad reach….and then move some to leeward…….less stiff = more rolling but a softer motion….and a little slower.

 

Excellent idea. I do intend to do this.

 

It would be interesting to see how they are plumbed, the pipe size, and the pumps if any. Could be for extra moment while beating or reaching, could have been an attempt at passive anti-roll...would need to know all the details: who designed it, when, history, and any drawings. Lots of things have been tried and then discarded.

Antiroll tanks - Wikipedia https://en.wikipedia.org/wiki/Antiroll_tanks

 

Rolling is primarily a function of hull shape and moment of inertia. A side ballast tank should slow down the roll response for two reasons: it increases the moment of inertia; if above the boat's overall centre of mass it reduces the metacentric height.

You could imagine a situation though where the tank reduces the heel angle in a given set of conditions, placing a different part of the hull at the waterline and so changing the response to incoming waves. You would only notice this if your boat's GZ curve had pretty serious changes of slope; for example if there was a chine that was submerged at some heel angle but not at some other, slightly lower heel angle.

 

Having water ballast on a boat is all about balance. If you have too much water on one side, it can make the boat heel excessively, which could indeed affect comfort and potentially increase rolling. It's all about finding the right balance based on wind and sea conditions.