Some sheet load info for you.

[W Wagner]

So I was geek-ing out with my EC21 the other day and came up with some info that might be useful in answering some of the questions about which winch or arm would work. What got me onto this was I was looking into arm winches and servos available for a dual arm winch setup and I needed to know what kind of sheet loads the EC12 developed. So I sat down with one of the navel architects I work with to answer the question of how much force is on the main sheet of an A rig. I picked the main sheet because it would represent the biggest load I would have to accommodate. The formula used was F=S*V squared divided by 800. F is the force on the sail, S is sail area in feet squared, V is velocity of the wind in feet per second, and 1/800 is the density of air. So here is what we came up with. With the main stationary and at 90 degrees to the wind and the wind blowing 15kts there is around a 4.6lb total force at the center of the area of the main, which translates to about a 2.31lb load on the sheet attached 14.5 inches out on the boom. For the sake of the post I calculated the approximant total force the jib is producing at 15kts and it is 2.9 lbs at the center of the area with a .84lbs of load on the sheet. Assuming that I followed my colleague’s calculations correctly, 2.3lbs and .84lbs seem like reasonable numbers to me, but I would still like to verify them in the real world somehow.

So to find the torque needed for an arm winch you need a couple of other bits of info. First we need to know how much sheet we need to move, about 16 inches, and second we need to know the arm length. A 4 inch arm traveling 180 degrees with a 1 to 2 purchase will move 16 inches of line. So take 2.3lbs multiply it by 2 (1 to 2 sheet) and then by 4 (length of the arm in inches) and you get 18.4 in-lb or 294.4 in-oz. So, to pull in just the main sail in 15kts of TWS you need an arm winch that has over 295 in-oz of torque. Once you pull the main into close hulled the numbers jump to like 4.6lbs on the sheet and 588.8 in-oz of torque because of the increasing angle of the sheet which is now pulling more down then in. The nice thing about using and arm that travels through 180 degrees is that angle of the arm and sheet decreases from 90 to 0 at close hulled (if you set it up like that) which in turn reduces the effective length of the arm and increases the torque output and the resolution of the system up to infinity when the sheet and arm are in line (0 effective arm length). Now I know that one would never use an A rig in 15kts but I was looking for a worst case and I had to start with something. The numbers for 10 kts would be 1lb on the sheet at 14.5 inches on the boom and 8 in-lb or 128 in-oz at the winch. Some assumptions were made and a very educated guess of the lift coefficient of one was used but in the real world it could be upwards of 1.5 or so, which could mean my 2.3lbs could be off by a factor of 1.5. But, as I said it sounds reasonable to me. Remember the above numbers are just for the main.

So to have this have some relevance and as a reference I went onto the RMG website and found the specs for the 280DL, which is recommended for the EC12. It has 194.4 in-oz of torque and if you use the 32mm drum (1.25 in diameter, 3.93 circumference) it will take 4 turns and 1.7 seconds (2.3 RPS under load) to pull in 16 inches of sheet. With that drum the 280DL can put a 311oz (constant) pulling load on the sheet. The equivalent torque needed in an arm winch with a 4 inch arm and a 1to 2 purchase is 2488 in-oz to match the 280DL! That number is with the arm at 90 degrees to the sheet in a worst case. The torque of the arm setup increases at either end of its travel up to infinity when the sheet and arm are in line. It is impossible, well more impractical, to do a direct comparison because the 280DL has a linear torque and resolution curve compared to the arm that goes from infinite torque and resolution at either end of the 180 travel to minimum at 90 degrees. Things get a little closer with the spiral drum on the 280DL, which significantly increases the torque and resolution as the radius of the drum decreases. So to say it another way, if you have an arm winch that has 2488 in-oz of torque and traveled through 180 degrees in 3.5 seconds you will have as close as you can get to the same performance as the 280DL.

Well, there you have it. Keep in mind that the numbers I came up with may be totally wrong compared to the real world so take it for what it’s worth.….


EC12 fixer-upper OZM0110 sail number 826

[PaulP]

Will,
Excellent information. To us engineering types, it's like someone passed around the jar of candy.

You probably can add an additional 40% for the jib.

I points out you need a pretty hefty servo if you want to sail aggressively in heavy winds. You can get by with a lesser one but would always have to bear off to decrease the load on the sheets.

Thanks for your input and numbers

Paul