Finding the CoE on a 10.5ft sciff?

[vinnya42]

This is my first real sailboat (I race RC Yachts of varying sizes), the hull is 1/4" ply sides and 3/8" ply flat bottom and will be under 50 pounds when done. It is 10.5ft long and 46" at the widest. I am sorry if I am not using the right terms?

I have a real aluminuim rudder and am going to using a daggerboard (30" exposed under the boat?), I know I need the rudder on it when testing the CoE but should I be in there too?? I think I should take the skiff out on calm water and push the side of the boat until it is balanced on my push...that will be the CoE, Right?

I am going to use a 20ft aluminuim mast (2 8ft stiff alum tubes from Home depot) with a hardwood dowell to join and the dowell at the mast step to get the 20ft. these poles are plenty light and strong and I will be using cables from the side of the mast about 10 ft up the mast to the boat sides for extra lateral support support, I am thinking I won't need a front stay in this setup.

The sail is going to be made of 6mil plastic sheeting with fully reenforce edges. I am using the leg of mutton sail design with the tall setup (15' x 9' @ 67sq')

So what I am looking for is any mistakes I have made or ideas I have that are way off...I don't know and am a very handy builder so I am using common sence that may not work with a sailboat.

1. is 30" daggerboard under the boat enough or too big?
2. Am I correct about finding the CoE and do I need to be in it when finding it?
3. is a light 20ft mast going to tip the boat because of the height (My weight is 215 pounds)?
4. is a under 50 pound skiff light enough to sail in an inland lake with light winds?

Thanks for any help I you may offer, I am excited to sail!
Vince

[capt'n ron]

vinnya42, hi, i'm new to this site also, but a an elementary grasp of sailing physics. your dagger board will be fine at that depth, according to the "rules", you need 3-4% of sail area for the fin. at 30" of depth and 4% of 67 sq.ft, you dagger should be about 12 inches wide.
the coE is not what you will be looking for with the hull in the water. what want to find is the center of resistance(coR), the coE is in the sail, the two work against each other to balance the sailing character of the boat, ie., lee or weahter helm. your weight should be in the boat so the water line will be correct, as when sailing. once the coR is found, the mast should be placed so that the coE is aprox. 10% of the waterline length behind the coR. this results in a slightly weather helm, which is desireable. should you loose controll of the tiller or fall overboard, the boat will turn into the wind and stall. the opposite condition will result in lee helm, which will result in the boat turning down wind and possibly sailing off without you. these are only general starting points and will most likely have to be adjusted as you sail the boat the first few times. the idea is to get a balance so that you don't have to use allot of tiller to keep the boat on course.

[vinnya42]

Thanks for the reply, it is good info for sure.

So I need to be in the boat went the CoR is found right...and I guess having a daggerboard trunk that has 6" of play would be good so I can adjust the daggerboard depending on my position and for fine tuning. I would of course need to make it solid when I get that position set....wait if I set CoR with me in it then when I fall out :rolleyes: the boat will not be set to go up wind right? Maybe what was ment was I need to get the water line with me in it so I can make the correct calulations.

You don't have any ideas on the 18-20ft mast and a 50 pound boat?

Thanks
Vince

[SailDesign]

Capt Ron, you got it a little backwards there. The CE should be AHEAD of the CLR (Centre of Lateral Resistance). Not behind. I know this sounds wrong, and for that reason it is the most often-made mistake in the book.
Since the sail is never exactly on centreline, and the boat is usually heeled, the vector is outboard to leeward, and directed just a bit forward(or the boat wouldn't go forward at all), thus making its intersection with the centreline somewhat further aft than the geometric centre of the sailplan (which is what you usually measure for CE).
So - make the CLR 10% of LWL behind the CE, and don't include all of the rudder unless you want to use the rudder all the time :)
Steve

[capt'n ron]

saildesign and vinnya42,
ooopppps, red face. how right you are, i stand corrected. i lost grasp of my sailing physics for a moment. my apologies to vinnya. that's what i get for being a newbie, trying to help a newbie! :D thank-you saildesign.