Adding a spars and centerboard to motor dory

[Adam Frederick]

Hello,

First time poster, total newb, ignoramus.

I want to add a (balanced lug, or lateen, or gaff, or sprit) rig to an old mahogany make and brake motor dory. I believe I can modify things enough to maintain the motor drive, while also having a centerboard (stored along the side of the hull, not in the center of the boat). Not 100% on that, but I’m hopeful.

I’m wondering about mast placement. My plan was to find plans of a similar boat out of Chapelle or something, and get spar and sail measurements and mast/centerboard placement from there.

Please tell me why I shouldn’t do that, and all the better ways this should be done.

Thanks!

 

[messabout]

Find the lateral center of area of your boat. You can do that several different ways. The simplist way is to let the boat float freely, tie a rope somewhere near the middle of the gunnel. Pull on the rope.....If the boat tends to point toward the direction of pull, then move the attachment point of the rope back a ways. . if the bow points away from you then move the rope attachment forward ..............after only a little bit of experimentation you can find the center of lateral resistance. CLR.

Now you need to find the center of effort of the sail that you are going to use. You can do that graphically, or you can believe whatever your sailmaker says. You have found the CE of the sail.

The CLR is some distance from the stem. Set the sail position so that the CE is about even with the CLR or perhaps a bit forward of the CLR..... Now you have gotten the basics. Your outboard "centerboard" is called a Lee board. You can locate it somewhere around the CLR for starters. You might need to move it fore or aft when you do your balance testing.

I expect that you might have a too small rudder because engine driven boats have the rudder at work within the prop wash. The rudder will probably be smaller than a sailboat rudder would be. For experimental purposes you might consider testing with a very large sweep oar.

Post a picture of your boat and we might be a bit more helpful.

 

[gonzo]

I would vote, but neither number is too many.