Swapping the keel on a British Hunter Sonata

[BrianBrowne]

I have a lifting keel Hunter Sonata. The Sonata is a 23 ft keel boat built in the 1980s. Most versions were fin keel and very popular for club racing.

The lifting keel version doesn't sail to windward as well as the fin keeler. I thought about selling my boat and replacing it with the fin keel version but most of the fin keelers are tatty and tired, having been raced.

I have had discussions with a company that casts keels and they have a keel of the correct weight for the boat - but the shape is much more modern than the original Sonata keel. The proposed keel has a trailing bulb and a lower centre of gravity.

A reputable local boat builder is happy to carry out the work to replace the keel but wants to be told where to put it longitudinally.

1) Could anybody point me in the direction for simply working out the Maths for getting the location right.
2) How critical is the right longitudinal location? If it is out by a couple of inches for example?

Thanks in anticipation

Brian

[sorenfdk]

The answers to your questions can be found in Larsson & Eliasson: "Principles of Yacht Design", so off to the local library! (Or buy it - it's a really good book).

You mentioned that the new keel has a lower centre of gravity. This means increased stability, which puts a greater load on the rig. So if the CoG is much lower, you might want to check if the rig can handle it. This is not something an amateur should do (it could be a matter of life and death!), so I suggest that you find a naval architect or a yacht designer and have him or her do it.

I don't know much about the Sonata, but if it doesn't have a bulb, you will probably find out that you'll go a bit slower downwind, but upwind, the added stability will be a bonus.