The George Blog – Balance and Stability & Comments

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Balance and Stability

I thought it would be interesting for builders of our designs to have a look into some of the numbers underlying the design of a sail boat. We will use Design No. 146 as our example.

I think we'll tackle first the balance of a sail boat. Here is a little sail plan of No. 146:

146

Note:

I = Vertical distance from bottom of forestay to top of forestay.
J = Vertical distance from bottom of forestay to fore side of mast.
P = maximum length of mainsail luff.
E = Maximum length of mainsail foot.

Let's first discuss what we mean by the "balance" of a sailboat. Essentially I think this is how the boat feels on the helm, how she acts if you let go of the helm and how she behaves as the wind strengthens.

Some basics here:

Weather helm is the tendency of the boat to luff up into the wind
Lee helm is the tendency of the boat to bear away from the wind

It is easier and more comfortable to sail a boat with a small amount of weather helm. And it is safer too – if there is a sudden gust you can in extremis let go of the helm and the she will simply come up into the wind and de-power. A rather more controlled version of this is luffing up to de-power in gusty weather, when only part of the sail is allowed to flap. While not a long-term alternative to reefing, every small-boat sailor will have done this at times.

We can define a well balanced sail boat as one which has light weather helm at small heel angles. What do I mean by "light" – about 5º of helm. What do I mean by "small angle of heel" – between 5º and 15º.

As the heel angle increases, weather helm will also increase – that is the rudder will need to be turned to a greater angle to keep the boat on course. We will talk about why this happens a little later on.

The increasing weather helm gives feedback to the helmsman, who generally luffs up a little to take advantage of the increased wind power to sail closer to the wind and also reduce the heel angle – which in its turn reduces the weather helm until the helmsman may allow the boat to bear off a little, when the whole process may repeat itself. This is in fact a normal part of sailing and most experienced sailors do this constantly without even thinking about it.

The forces acting on the sails and hull of a sail boat are quite complicated. Essentially however, when the wind blows over the sails a force is developed – the aerodynamic force – which basically trys to push the hull sideways. The hull resists being pushed sideways and this resistance is the hydrodynamic force. The resultant of the two forces is the forward motion of the hull through the water, which in itself modifies both the aerodynamic and hydrodynamic forces.

Calculating the theoretical centres of the aerodynamic and hydrodynamic forces is complex and uncertain. The dynamic centre of the sails is dependant on the shape and the cut, how they are set, the heel of the boat and many other factors. The dynamic centre of the hull is difficult to find because the underwater shape is a complex hydrofoil and also changes constantly as the boat heels, rolls and pitches in a seaway.

So, while it would be nice to be able to use the dynamic centres to balance the boat, it is generally rather impractical. Designers have, over the years, developed empirical methods using the geometric areas of the sail plan and the underwater body, which are constant and readily calculated.

To be continued ........

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