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A Question of Balance

POST #03


With the first two posts down by way of introduction I can start to get into the nuts and bolts of what is driving my argument. I tried to explain it in an article in Multihulls World a couple of years ago but I'm not sure I succeeded. Maybe I didn't understand it all that well myself at the time.

Here it is laid out in terms that even I can understand.

When we start with a new design we intuitively understand the hull type we want to create and where the centre of buoyancy needs to be based on our desire for good all round performance. We also know where the mast needs to be- and that is close to 45% of LOA aft of the forward point. This is where it is on the majority of cruising cats and where it has been for a long long time. Move the rig forward and you're pushing the bows down in a seaway. You're also stealing from headsail area which is easy to reef and adding mainsail area which is not.

On a race boat we would move the rig further aft, but for a cruiser we want a reasonable length available for cabin space and we don't want to put the mast on the cabin top for various reasons, so the 45% mark strikes a good compromise.

That gives us the fundamental geometry from which we can determine what happens next.

We start creating human spaces, firstly decks, for working. and the general functioning of the boat. That is; boarding, anchoring, sail controls etc. Then we create human spaces for lounging and socializing, and we create spaces for essential human functioning including sleeping, bathrooms, and cooking. In doing this the balance of the hulls and the superstructure is important and space is precious. The smaller the boat the more precious the space. There is the ever present temptation to stretch the platform. (see also post #02).

If we extend the platform aft we can't just balance that out by extending the mast deck forward. There is far more weight around the aft beam than there is at the leading edge of the cross structure and the aft beam and associated equipment are much further aft from the centre of buoyancy than the mast deck is forward of the centre of buoyancy. So while the extension forward will contribute to weight and windage (and possibly slamming) it won't effectively balance an aft beam that is too far aft.

There are only three possibilities for what can happen next.

1. The boat will float down by the stern and you are going to suffer from excessive transom immersion. This will inhibit performance in some conditions, especially in light air which is where cats are most vulnerable.

2. You can preempt the concentration of weight aft and design for it by increasing rocker or by giving the boat fatter water planes, especially aft at the transom.

3. You can attempt to remedy the situation after the fact by extending the transoms or putting ballast in the bows.

I will discus all three options in the next post.

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