Ok, I’ve searched the site and looked at videos, but I still have to ask about the amount of travel of a boom. Some pics/videos look like the boom swings very little when the sheet is released and some looke like it swings out almost 90 degrees. Is there a standard for how far the boom swings out, or is it different for every boat/sail rig?
I’m working on a Razor with one of Brett’s single sail rigs and I want to figure out how far the boom should be able to swing out.
The boom should be able to go out a long wway - in a single sail rigMost single-sailed full sixe boats actually lit out firther than 90 degrees on the run, The (purely practical) reason for rstricting movement is that most full-size masts are held up by wires at the sides (shrouds) which resytict boom mobemenr.
Forgivie my lousy sketching but I hop rthe attached file makes the point.
Aim for 90 degrees or a little more of booom travel
Most people seem to sheet ahead of the mast, with the sheet attached about 2 1/2 inches ahead of the pivot. I get 90 degrees with a servo arm about 2 1/2 inches long and I don’t use a doubling scheme (my arm just pulls direct) FWIW Gary Sanderson uses a near-identical schems (Mine is a respectful copy of his) but he uses a 1 inch arm and double purchase sheet.
Double purchase is with a point tied off and two pulleys/blocks in the setup between the winch arm and the boom?? I’ve seen a couple pictures, but sometimes I get dense:headache:
Think of an “N”; the first free (the bottom of the first stroke of the “N”) end is attached to a fixed point on the transom (or forward if you’re following the fashion) the first angle is a pulley on the servo arm, the second is a fixed pulley close to the stern attachment of the “N” and the third is the sheet itself attached to the boom.
It is best to avoid the double purchase arrangement if possible, because it adds significant friction, which wastes torque, necessitating a bigger sail servo. On a Footy with a McRig, you can usually accomplish the same objective by proper selection of the sheet attachment point on the boom.
Sorry, I was probably unclear about what I meant by double purchase. Footys are generally toooooo small to accomodate pulleys - I simply meant a system where the linear “pull” of the servo arm is doubled by having the fixed end fixed, and the servo arm pulling the loop of the sheet so that it slides thru the hole in the arm (poor man’s pulley) and gives double the length of pull at the sail -
Sketch is attached of Gary’s method of doing this - his servo arm is aluminium (screwed to the servo disc) with a beautifully rounded hole for the sheet. This set up also includes a adjustor to set up the basic length for lots of different sails and mast pivot positions
andrew
Thanks for the drawing and all the help. I didn’t figure to do a double purchase, just a single purchase. My understanding, from reading, that a single purchase cuts the arm length in half and a double purchase cuts it to a quarter+…
I was going to copy Brett’s original and from those photos it showed the sheet tied directly at the front of the boom, but with a very long arm, so I was just going by that. I just didn’t want to use that long of an arm. Definitely the throw will be less if I move the tie farther down, as your drawing shows…
I have an aluminum arm and was going to use it. It’s a 1.5" arm.
Thanks for the help. I really appreciate the Online Lessons here:)
