Wednesday, October 08, 2008
Lester Gilbert on winch drum's and winching
I've had two kinds of winch drum turned in my efforts to fine-tune the response of my RMG-380 sail winch at close-hauled. The RMG-380 (and, for the IOM class, the more appropriate RMG-280) has outstanding power and speed. I have my mainsheet post as low down as possible, and deliberately use the power of the RMG to "sheet vang" -- tighten the leech of the main at close-hauled when I want to. Please DON'T try this with a Whirlwind, Futaba, HiTec, or ANY other winch! I also have absolute confidence in the ability of the RMG to sheet in from a broad reach to close-hauled when the wind is right at the top of "A" rig within an instant. In fact, I've broken my servo tray proving it can do this. But the resolution and repeatability of the RMG at just off close-hauled is not wonderful.
The 8-bit analogue to digital converter of the RMG controller electronics provides for 255 separate positions of the drum over its travel of around 5 revolutions. If we call this a round 250 positions, we get 50 positions per revolution. If the drum diameter is, say, 28mm, then one revolution pays out about 90mm of line, or about 1.8mm per position.
The problem is, although the theoretical resolution is1.8mm, repeatability is about twice this value, around 3.6mm, and at close-hauled 3.6mm is a very significant difference. The winch electronics cannot (at present) do better. So it is necessary to have a drum whose diameter is less at close-hauled. The first picture illustrates two "snail" drums, which have an ever-changing drum "diameter". The larger drum, shown upside-down, has a total travel of about 600mm, and the smaller drum has about 300mm. (You may need to turn up your screen brightness to see the detail clearly. These black, shiny things are very difficult to photograph well.) The smaller drum has a maximum diameter of about 28mm, down to about 12mm close-hauled. One revolution at close-hauled pays out about 40mm, so resolution is about 0.8mm per position, and repeatability is about 1.6mm.
The second picture illustrates two "step-down" drums. These drums have two diameters, a larger diameter, and a smaller diameter. The larger of the step-down drums illustrated (again, upside-down) has a total travel of about 550mm, through four turns at the 32mm larger diameter, and any remaining turns at the 12mm smaller diameter. For all of these drums, the sheeting line is tied through a small hole in the drum at the fully sheeted-out position, at the largest diameter of the drum just where the channel starts.
Your friendly lathe operator should be able to turn one of these drums for you to try. I understand that it is an exacting and time-consuming process, though, so be sure he owes you a favour first; it would be rather expensive if you were to be charged full commercial rates. Alternatively, Rob Guyatt now sells excellent examples of these drums to suit his RMG winches, in three diameters, at exceptionally good prices. What are you waiting for?
Closed-loop sheeting arrangement
Naturally, neither of these kinds of drum can be used in a "closed-loop" system, but I've not found that to be a drawback. In fact, just the opposite; if the sheeting line is tied to the drum so that it is fully sheeted out on the run with absolutely no more line around the drum, then should the line jump off the drum at any time, sheeting right out and then sheeting back in again always clears the problem. The diagrams illustrate the two main methods of sheeting from the winch drum -- closed-loop or open loop -- and an arrangement of the sheets.
Open loop sheeting arrangement
The "Drums" spreadsheet (about 16kb) makes some simple calculations relating to sheeting and drum turns. Given the main boom sheeting angle and the sheet attachment point, the line run is calculated. Then the number of turns needed for each type of drum to sheet that amount of line is calculated. For a "simple" drum, the number of turns is calculated straightforwardly. For a "snail" and a "step-down" drum, given the maximum and minimum drum diameter, you enter trial values for the number of turns until the calculated wind is about the same as the expected line run. Alternatively, the revised spreadsheet has some macro buttons you can click. (Note that the presence of the macros may trigger an anti-virus alarm in your system.)
Sheeting lines arrangement
A final comment concerns getting to use the small amounts of sheeting required by "sheet vanging". Normally, it would be rather tricky to be able to move the Tx winch stick by, say, 1/50th of its travel, never mind 1/250th, and some skippers wonder whether the fuss with these drums is worth it. I use a Futaba 3VC computer radio (which can command the Rx to 1/1024th of full-scale travel at highest resolution) and have "exponential" travel on the winch stick, such that one "click" on the stick, 1/20th of the stick travel, gives me about 1/150th of travel at the winch. Another click gives me about a further 1/80th of travel at the winch. I thus have three winch settings which I use at close-hauled, corresponding to pinching, normal, and footing, with three different amounts of twist pulled into the mainsail and just a small change in sheeting angle.
©2008 Lester Gilbert
Posted by Dave at 5:24 PM