Section 4 Resisting the transverse force generated by the sailing rig

Figure 4 shows that there will always be a transverse force often greater than and, for some points of sailing, much greater than the driving force. This force has to be resisted if the hull is not simply to move sideways and prevent the sail from driving the boat. There is only one way to do this and it is to generate the necessary force by the action of the water on the hull and/or its appendages.


So we want a hull with its appendages that, to reduce the sideways movement to a minimum, will generate a large sideways force and produce only a small drag in the forward direction. It is a tall order and I suppose that one might trace the history of all the attempts to meet this specification. When I look at this history I do not see any evidence for any application of science; I see a process of trial that is not yet complete. Perhaps the pleasure lies in the chase not in the kill. I will pick out one or two illustrative examples.


It is probable that Brunel’s great achievement in maritime affairs is the SS Great Britain. Many innovations that became standard practice first appeared on that ship. It was designed as steam-powered vessel with auxiliary sails and had no external keel. After a chequered history it was refitted for the run to Australia as an immigrant ship and to be a sailing ship with auxiliary engines for calm conditions. In order to be effective as a sailer it was fitted with a false wooden keel that was 17 inches deep and 20 inches wide along the whole length of the ship. This brought it into line with HMS Victory that had a keel of similar depth of about 6% of the draught.


So what is this about? Hulls of wind-driven boats move forwards and sideways at the same time and it is the sideways movement that resists the transverse force produced by the sails. The flow over a hull of a power driven boat is complicated enough but here we have a further complication of flow under the hull from leeward to windward. The two flows combine to produce a skewed flow that starts at the bow to leeward and ends at the stern to windward[1][9]. Add to this the effects of alternate regions of high and low pressure that form under the hull as a result of the flow off the forefoot and it becomes very complicated. As I see it we require an explanation of how a hull can resist a transverse force and how this force can be altered.


Text Box:  Fig 55       

In figure 55 I have drawn the cross section of the SS Great Britain as it was built and attempted to draw a flow pattern for water flowing under it from left to right. This is a big hull and the transverse speed of the water will be quite low and this will mean that the approaching water will lift up above the normal free surface level by only a small amount to produce a rise in pressure on the leeward side. This will drive the water downwards and under the hull. The flow lines converge and follow the shape of the hull until the sharp change in curvature on the right hand side where the flow breaks away and all sorts of eddies form in the down stream flow. A region of low pressure will form on the right hand side and the swirling water will disturb the free surface visibly. (I do not know the character of the downstream flow, only that it will swirl and eddy and the arrows are purposely vague.)


The lift of the water on the left will raise the mean pressure on the left hand side of the hull and the drop in level on the right will lower the mean pressure on the right. This difference in pressure will act on the hull to produce a force from left to right. A smoothly contoured hull like that of the SS Great Britain may be quite unable to produce a large transverse force without excessive leeway.

Text Box:  
Fig 56

The SS Great Britain was fitted with a keel as I show in figure 56. This keel causes the flow under the hull to break away upstream of the keel. Then the flow downstream of the keel generates a stream of eddies that deepen the wake and lower the mean pressure still more. The net effect is a greater transverse force on the hull.


This cross flow combined with stem-to-stern flow simply adds to the resistance to motion of the hull in the absence of cross flow and is an inefficient way of generating a transverse force. It is a method that is still used although, in some yachts, the full-length keel has been replaced by a shorter, deeper keel.


The most efficient method is the use of the aerodynamic keel or fin. This has taken a long time to find its way on to racing yachts considering that it is only the wing off an aeroplane together with its stabiliser. Its great advantage is that, at a leeway of just a few degrees, 3° to 5°, a large transverse force can be produced with very small drag. The keel operates totally submerged in relatively undisturbed water and is extraordinarily effective. Its only snag is that the force it produces adds to the upsetting moment of the rig but to offset this the keel or fin can carry the ballast weight in the form of a streamlined bulb.


However before we can understand the aerodynamic keel we must look at the balance of the boat and before we can look at that we must look at how a boat is steered.


[1][9] There are pictures about of tall ships reaching and photographed from the windward side. I think that one can see the up-welling of the flow under the hull.