Dear Joan,

I meant to write to you a couple of years ago, but somehow never got round to it. But on re-reading No. 73 I find myself still wanting to dispute part of an article by Andrew Dawson.

In his section on the Mirror dinghy, Andrew Dawson makes the following statement: "do not put much gear in a buoyancy tank - it destroys its effectiveness." I believe that this statement is wrong, and will mislead many sailors (Mirror and otherwise) into wasting much valuable space in what is likely to be a somewhat crowded cruising boat.

My reasons for disagreeing with his statement are as follows:

Buoyancy and flotation are simply a matter of weight and volume; applying this to the situation of a capsized or waterlogged boat, that statement must be modified to the extent of saying that what matters is only the immersed, under-water, volume of the boat, though it is the overall weight of the boat which counts here (together with the weight of the crew if they are "on board" rather than floating independently in the water beside the boat.) To understand clearly the physics of the situation, imagine two identical boats in a capsized or waterlogged state; if you then add to each boat an identical load (same weight, same volume) they will float at exactly the same level EVEN THOUGH ONE OF THE LOADS WAS STOWED INSIDE A BUOYANCY TANK AND THE OTHER WAS STOWED ANYWHERE ELSE IN THE BOAT (but not under water). This is because the total weight of each boat is equal to that of the other, and so is the immersed volume.

It is worth looking at the exception - a load stowed not in the buoyancy tank but in a position where it will be underwater when capsized. In this case the load does actually contribute additional buoyancy (compared with the boat which has its load stowed 'out of the water'), the extra buoyancy being 62½ lb for every cubic foot of the volume of the load. Thus if the load was a sealed bag of clothes or sleeping bag, the difference would be substantial.

The only way that buoyancy can be DESTROYED is by ballasting the boat with 62½ 1b of extra weight for every cubic foot of buoyancy. It is true that a boat can be somewhat less buoyant when loaded inside the tank, than if it were to carry its load where it will be underwater when capsized. But (and here's the rub) the benefit in stowing NOT in the tanks relates mostly to buoyant things like clothes and sleeping bags, and then only if they are stowed deliberately where they will be UNDER WATER when capsized. Isn't that WHY we have stowage in tanks???

David Fraser