John E Hardiman wrote the following in the forums of Woodenboat.com, he has given permission to quote his message for this web site.

" The true question is why Mark would want to add water ballast. His answer is he wants to add auxiliary tanks (what they really are) in order to compensate for design weight that isn't present (i.e. make the boat sit the design lines when minimally loaded) and as he and John Welsford say "settle her down a bit".

A quick glance at the hull form shows that the gain in stability is more due to the change in water plane inertia than to the water ballast. This is as it should be. The water ballast should compensate for the weight of the missing variable load, not its CG. The stability of this boat at its static waterline is in its form, not in the location of the water ballast. Note however, that as the first chine dips, that the growth of water plane inertia slows and the effect of water ballast begins to increase on stability.

Additionally, notice what would happen at moderate angles of heel. The wide, low angle dead rise begins to emerge, and a large shift in CB occurs. Again, this is as it should be, and that the location of the water ballast is relatively unimportant to this hull form .

Finally; notice the sinkage of the hull that is brought about by the addition of the weight. The hull is immersed deep enough to prevent pounding of the bottom while motoring and the waterline is close to the chine midships to prevent large wave slap/heaving forces from being generated while at rest. Again, the location of the water ballast is unimportant compared to the weight it adds.

Ballast material should be selected based upon the design goals required. An attempt to use water ballast to stabilize a "plank on edge" hull or in a fin keel would not be nearly as effective, if at all. "

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Why do I want Water Ballast?

Firstly, this idea has been given the "go ahead" by John Welsford but with a few caveats.

1. That the tanks must be completely FULL or EMPTY, water sloshing around in them is not ideal and could cause stability problems. Or extensive baffling is required to minimise water movement.
2. The boat could be a little less lively and possibly a bit slower to helm response.

Now for my reasons to modify.

1. I wanted to make the boat as light as possible on the trailer for one man to launch and retrieve.  I have made the centre board out of wood with a steel weight, it is about 40 kg lighter than the specified steel plate. The extra 50 kg under the bunks will not be used, instead there will a closed cell battery each side at this position, the batteries combined are 30 kg lighter than the fixed ballast. Most of the time the boat will carry only myself in an area where a short nasty steep chop can appear very quickly with a wind shift.
2. So for the above reasons there is close to 70 kg less fixed weight plus a crew member we have around 150 kg to replace. The intended tank locations will give close to 115 kg per tank or 230 kg in total, the additional surplus weight will pull the boat a little lower in the water and thus cause more stability and stiffness as John states above.
3. The local bay where I will do most of my sailing is very large, on long reaches with a steady wind the windward only tank can be full and the lee tank empty, this will have a similar trim result as would a crew man on the same side.
4. The boat is also a good design to just shoot out for a spot of fishing without the rig, or do a bit of river exploring, with the tanks empty I will have a lighter higher floating hull which will provide good performance motoring.

How are the tanks filled and emptied?

So far I have fitted two 38 mm PVC 1/4 turn gate valves, one for each tank. They are located in the cabin on the floor just in front of the aft cabin bulkhead. I used a 90 degree bend through the hull bottom into the gate valve and a straight fitting from the valve into the side of tank which is the front of the cabin bunks. Very simple and compact, with the boat sitting on water all that is needed is to open both valves and flood fill the tanks.

Each tank has a breather line going up to the hull side just under the sheer line.  This means that the tanks can be flood filled and gravity emptied when beached or on the trailer.

To control water levels when on the water I will be fitting a large volume 100 litre/min hand operated bilge pump in the cockpit near the helm. By using a series of 38 mm hoses and gate valves I can fill/empty both tanks or only one tank. The only draw back to this idea is that the valves need to be easy to get at, so will be visible in the cockpit. I am looking at maybe fitting new handles to the valves so that the valve bodies can be mounted inside the cockpit seats and only have the handles visible.

There are so many ways to deal with pumping water, and if you want the flexibility to be able fill or empty on the run then it needs careful planning, I did toy with the idea of using small electric pumps in each tank but then wondered what would happen if the batteries are flat? So I ended up staying with manual pumping, that way the batteries are saved for the radio and nav lights only.

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