Mast location

hello...i have a set of plans for a 50' sailboat that uses a ketch rig. i would really be interested in changing it to a cutter sailplan, is there a formula available to the novice that would make this possible? before computers, how were these calculations determined? any and all input will be great appreciated...coyotee

 

The following is how it's described in Skene's Elements of Yacht Design by Francis S. Kinney:

Find the center of lateral plane (CLP) by cutting out the underwater profile of the boat out of thin card stock (excluding the rudder) and balancing it on a pin. Next find the centers of the main and foretriangle and draw a line between them. Find the center of effort (CE) of the sails on this line by using the areas of each sail to take moments about one end. In the case of a yawl or ketch repeat for the other sails but use only 1/2 the mizzen area.

Bring the CE down to the DWL. Skene's recommends that the rig should be placed so that the CE is forward of the CLP by about 15% of the DWL for sloops and about 20% for ketches.

Note the use of the word "about". The CE calculated this way really isn't and the CLP is really just the center of the area of the underwater profile. Kinney uses the expressions "rule of thumb" and "taken with a grain of salt" a few times when describing this method. The old timers relied a lot on personal experience from previous mistakes, either their's or other's.

By all means, try to find a copy of Skene's and/or work out the design of the new rig for yourself. However, the fees you'd pay a yacht designer to make the modifications is a rather small percentage of the cost of building a 50' sailboat and well worth the peace of mind.

 

Thank you james_r for your rapid fire response! i will research the information you mentioned.

 

Mast location. There is no recomendation on modify masts in an conclude sailboat design. I agree with mantain the original especifications of the boat, or search for a new one. Janos.:idea:

 

Some suggestions:
Have other boats been built to these plans? Perhaps you can find out from a previous owner, etc. how the boat balanced. There are alot of factors involved in the balance of a boat, and designers don't always get it right. If you can get first hand info, this would be a great advantage.

Many ketch rigged boats are designed to sail upwind w/ main and genoa/jib, with the mizzen set for reaching and running, when the center of pressure in the sail plan moves forward. This is really the main benefit of a ketch-the ability to set more sail downwind. So many ketch-rigged boats balance just fine w/o the mizzen. I have seen many retrofits where the mizzen was simply removed and it worked out ok. However, if you're planning on adding a sprit w/ foreward headstay/bobstay, chances are you'll run into a lee helm problem and thats...bad.

Having a designer take a look at the sail plan would be money well spent...

 

The ever helpfull Ted Brewer has some solid, basic information on this here: http://www.boatus.com/goodoldboat/Helm_balance.htm

Hope that helps !

Paul

 

polarity...thank you for the wealth of information, it will come in handy!

 

thanks mplane...i tried to get feedback from the architect on other owners of this design, with no luck! he has been very evasive concerning questions, leaving me with no place to turn to...hopefully there will be knowledgeable people here that can help.

 

One other consideration is the structural modifications. You will have to get someone experienced to calculate the new loads and stresse s created by the different rig. For example bulkheads, mast steps, chainplates, standing and running rigging. Also the interior layout may not work out with the new location of the mast.