converting sloop to ketch?

[tim mcd]

are there guide lines anywhere to the change in sail area necessitated by changing a sloop to a ketch -presumably need more SA as ketch less efficient .

what about main to mizzen area ratio's? -from checking photos the mizzens seem to be getting larger and becoming not just a balancing sail.

and lead allowance for a ketch? difficult to calculate as there seems to be different perceptions as to what proportion of a ketches sail area to count as being effective

-any references (leads)?

[Stephen Ditmore]

For any set of anticipated wind conditions, sail area is generally a function of stability. Heeling moment (we can use sail area*heeling arm) will always equal righting moment at the equilibrium heel angle, so if the vertical center of gravity is the same for both rigs the sail area should increase as the vertical distance between the center of effort and the center of lateral resistance decreases, maintaining the same heeling moment.

[Stephen Ditmore]

On the other issues I suggest you read everything you can about Bruce Farr's Whitbred Race ketches and the Spanish boat that Ian Howlett modified to become a ketch in its second running of that race in back issues of
http://www.seahorse.co.uk/

and that you see what you can learn from
http://www.image-ination.com/sailcalc.html

[DavidG]

Pierre Gutelle's "Design of Sailing Yachts" has a really good section on two masted rigs. He quotes a lead for ketches of 11% to 15%.

Although two masted rigs are not theoretically as efficient as sloop rigs, they do break down individual sail sizes which may make handling better, and of course there is some redundancy.

In practice, the mizen needs to be carefully trimed in order to avoid loading the rudder with a lot of weather helm.

With regard to conversion, you obviously need to consider the structural and ergonomic implications to the boat, my experience is that it is quite hard to accomodate the required E mizzen + E main + J inside the deck length of the boat.

Historically racing yachts from the early 20th century were converted to ketch or schooner rig for cruising, however they had longer keels where lead was less critical.

[Stephen Ditmore]

If you came to understand how self steering vanes work, which I don't entirely, I wonder if there's some clever way you could make the mizzen act as a self correcting rudder? Concievably one could have the helm work the mizzen traveller as well as the rudder, but I'm wondering if there's something more clever still one could do.

On self steering see http://www.selfsteer.com/booklet.html