Well, time to poke my head into the lion's den... I've been struck by the wide variation in balance in my Ikon, and have been struggling to find ways of dealing with what seem to me to be quite drastic changes in helm as the wind picks up or dies away. Let's start by laying out what I think we know.
In practical terms, the boat is balanced by either moving the sail plan forward to reduce weather helm/increase lee helm, or moving the sail plan back to reduce lee helm/increase weather helm. But why does the boat need "balancing" at all? Why doesn't she just, um, sail straight ahead? And if she does need balancing, why does the balance change with the wind strength?
Most texts discuss balance in terms of the difference between the centre of effort of the sails and the centre of lateral resistance of the hull. I've not found this at all useful. The centre of lateral resistance of the hull seems pretty much a fixed quantity once you've bolted the fin in place and centred the rudder. The centre of effort of the sail plan also seems pretty much a fixed quantity once you've stepped the mast, adjusted the jibstay to the required length, and sheeted in. Yet the balance of the boat is anything but a fixed quantity. When I "balance" my Ikon perfectly at a heel angle of about 30 degrees in a good breeze, she develops almost uncontrollable lee helm should the wind drop away to a breath, and develops strong weather helm should a gust push her to, say, 45 degrees of heel.
Instead, let's "mentally" strip away the rig from the hull, detach the fin and rudder, and run some experiments. These will have to be imaginary experiments, but what happens can be demonstrated on the water. So, we push this canoe body along, and it moves in a straight line. Of course. But what'll happen when we force the canoe body to heel, perhaps by placing some ballast on one side? Why, when we push her along, she turns up, away from the side that is lower in the water, and moves in a circle. The more we make the canoe body heel, the more sharply she turns. If you mentally now re-rig the hull, you can see that she naturally screws up in the direction that the wind that is making her heel would be coming from. Put the fin and rudder back on, and the same thing happens, but less strongly. The fin and rudder, after all, are meant to keep the hull running straight ahead.
Given that the hull naturally screws up to wind when heeled, when we rig the hull we need to place the sail plan ahead of the canoe body 'centre'. The way I look at it is that placing the sails in the right position allows the wind to blow the bows to leeward, counterbalancing the tendency of the hull to turn to windward. When a puff comes in, the boat heels more, and two things happen. First, the canoe body tends to turn to windward even more strongly, and second, the wind is less effective at blowing the bows off the wind because the extra heel presents less effective sail area to the wind. Put these together, and the boat tends to screw up to weather.
Gary Cameron has explained that the key to changes of balance is the degree of asymmetry of the heeled waterplane. In the 'Designing an IOM using circular arcs' page, I show a heeled waterplane for a skiff-type hull. It is reproduced here, and it is clear that this waterplane is almost perfectly symmetrical, although its axis of symmetry is angled with respect to the course being sailed.
Gary suggests that the higher the asymmetry of the heeled waterplane, the greater the weather helm with heel. Conversely, the more symmetrical the heeled waterplane, the less the change in balance with heel. If we look at the heeled waterplane of a narrow beam, narrow transom hull, also reproduced here from the 'Designing an IOM using circular arcs' page, we can see that the leeward bulge of the heeled waterplane shows much higher camber than the windward side. The heeled hull works just like an aerofoil, and besides generating lift (a righting moment) it is also generating a pitching moment, which for the hull is a yawing moment, turning the boat to weather.
By comparison, the skiff's heeled waterplane, being symmetrical, is not developing much lift, is not developing much of a yawing moment, and so is not tending to twist the hull up to weather as much.
There are other effects of the more symmetric heeled waterplane that are equally intriguing, discussed on the "Foil planforms" page.
©2011 Lester Gilbert