Checking in and a rigging question

[Jim Stumpf]

Hello All,

I sold my Vacationer and the NF 15 is put away for the season so I am looking for a winter project. I have a Bolger 16 ft sailing Pirouge with an under sized sprit sail on it. I am thinking I would like to experiment with a cat-ketch. Can anyone suggest a good source for information on the conversion, some rules of thumb for sail size in relation to the boat and to each other would be helpful as well as determining the CE using a ketch rig.

I will dig up a picture and post it soon.

Thanks Jim

[Andrew_Butchart]

A picture astounding like this one Jim?

[PAR]

Balance the combined CE of the proposed new rig over the CLP of the old, then shove it forward, say about 7% of the LWL, to provide some lead. This will make it very close and you can balance with your butt in a vessel like this. Many designers toss the CE directly over the CLP in small craft, but I prefer some lead to get a natural, rather then a neutral helm. I consider this a mandatory safety feature in small craft, so it can't sail away from you, if you manage to get dumped in the drink.

The CLP is easy enough to figure out. Take an accurate drawing of the hull's profile, with the appendages. Cut along the LWL and around the underwater areas. Fold this drawing along the LWL, just below it and attempt to balance this on a pin. Adjusting the folded underwater areas back and forth until you have it level and not dropping fore or aft. This is the geometric center of the underwater areas and accurate enough to balance the rig over. Unfold the drawing and transfer the pin prick to the drawing and a vertical line through the LWL and mark it CLP (Center of Lateral Plane).

The CE is a little more drawn out, but nothing more then simple geometry, which you should have acquired in 5th grade. The area of a triangle is 1/2 the base times the perpendicular height. The center is found by connecting the corners to the mid point of one of the other sides. You can do all three sides, but only need two. I usually use the two longest sides.

With the area and centers found, connect the two centers with a line (which will be sort of horizontal if the sails are similarly sized) then calculate the percentage of difference between the two and subtract that from the center point of the line you just drew, of course favoring the bigger sail. This is the CE. An example would be two sails, one 40 sq. ft. and the other 50. The 50 sq. ft. sail is 20% bigger, so the CE shifts toward that side of the line drawn between the two centers, by 20% of the length of that line.

You don't need a calculator it perform these tasks, but some drafting skill is necessary. You can do it entirely by mathematics, but that is quite painful.

http://marina.fortunecity.com/breakwater/274/1999/0515/index.htm#Sail%20Area%20Math

This is a link to Jim Michalak's news letter, which covers it fairly well. He does differ form my methods, but the results are the same. He is one of those that don't typically incorporate "lead" into small craft design, but consider it anyway, it's a better balance option.