Thoughts by Nigel Irens on directional stability and broaching

The Maggie B is often out in big seas and big winds. One area of concern for any sailor who goes in those conditions is “broaching,” where the boat is going down wind and down wave at high speed and twists sideways to the wave and capsizes, often with the lost of rigging and sometimes with the loss of life and the boat. The Maggie B has always felt quite stable in such conditions. I was particularly interested in the impact of the use of the centerboard in “big weather.” Standard practice is to raise the c/b when off the wind, but I have always found that the additional stability from having the c/b down seemed valuable, both for crew comfort as well as keeping the sails steadier. The danger is that in some boats, the c/b can cause the boat to “trip” and can bring on a broach.

I asked designer Nigel Irens his considerations on directional stability and broaching in designing the Maggie B, and below Nigel writes:

I think the first thing to say is that in the interests of helm-balance sailing boats tend to have the deeper parts of their hulls placed in accordance with the distribution of sail area down their length in order to balance the helm.

A schooner, for example, which has its mainsail aft will always have a deep heel to the keel and a fairly shallow forefoot, whereas a ketch will usually have a deeper forefoot but the heel of the keel will be less deep.

If you consider directional stability without any sail set then it’s easy to see that the schooner would be the most directionally stable downwind - simply because the undercurrent of a breaking sea is less likely to get a ‘grip’ on a shallow forefoot. At the same time the long sternpost (and consequently longer, more efficient rudder) will have tendency to stop the stern trying slew and ‘overtake’ the bow (which is what broaching is all about). This analogous to a dart or arrow - the flights want to ‘trail’ back and so keep the un-flighted end pointing downwind.

In Maggie B’s case there was another characteristic built in which I hoped would help limit the tendency to broach and that is that the entry of the waterlines forward is fairly fine, which means the bow has more of a chance of passing through the back of a wave without to much resistance. If the front doesn’t slow down too much this again limits the tendency for the back to try to overtake it.

You couldn’t choose to have fine waterlines forward if the weight of the boat was also too far forward - otherwise she would trim down by the bow. In our case the weight is moderately far aft but the forward sections are quite ‘U’ shaped so that for the weight they carry (i.e. the immersed volume) neither is the forefoot too deep nor the waterlines too bluff.

I was very pleased to see in Antigua last year that for a boat of her displacement the relatively fine waterlines mean that she is not stopped too easily when pitching into a sea - she seemed quite eager to get on with going up wind. That was a slight weakness in my ROXANE design - although the static waterline (design waterline) was relatively fine the waterlines higher up the freeboard were were progressively bluff and the boat does get stopped a bit as these enter the water. This happens when her natural pitch frequency is excited by the coincident ‘rate of encounter’ of seas advancing seas, which is in turn a function of wavelength, wave height, boat speed and the angle at which waves are encountered.)

Sometimes a boat will be stopped upwind by an unusual sea and so the helmsman will bear away a few degrees (and maybe crack off the sheets) to break her out of that resonant pitch frequency into which she has fallen. When the speed has picked up and the frequency broken again she can probably be nudged back on the wind a bit more. Most people who sail a lot do this instinctively - especially if they are often on the same boat and so get used to her ways. (I imagine you can keep Maggie going with your eyes shut these days !)

Coming back to your question about whether to use the centreboard downwind or not I think you are bound to try different ideas and see what happens. If she was a ketch - with the centreboard well forward I’d say leave it up as it could cause directional instability - or even exaggerate a tendency to broach, but as Maggie’s is well aft (like her sailplan) then she may be more steerable with a ‘pivot’ to steer around. It’s hard to say. One thing is sure and that is that I think a schooner will always be best in very strong following seas because of the above-mentioned ‘dart’ shape, but the most reassuring formula must be a schooner with main furled in strong winds so she is being ‘pulled’ and not ‘pushed’ from behind.(Try pushing a pencil across your chart table rather than dragging it !)

Not much actual science here I’m afraid, but I hope it helps….

— Nigel Irens, Ashburton, May 2007