Dan DeLave wrote:
I had to send off the drawings to an engineer friend of mine so I did not make a mistake.
Indeed the drawings are right. 12 to 1 and 16 to 1. I sent him the following illustrations without purchase listings (which are in black): http://members.aol.com/ddelave/sailing/downhaulpurchases.jpg
This is his response:
The upper one is 1:12 and the lower one is 1:16. The upper one is what I had on my boat before I changed to 16:1 with a double cascade system.
The problem you will face with the 16:1 that is shown below is there will not be enough clearance between the floating bock and the spin lock cleat?.in other words the floating blocks will bottom out before the mainsail is fully down hauled?.there is simply not enough travel distance for the floating blocks?.thus the need to use a double cascade when going to a 16:1?.
This discussion has intrigued my engineering mind!
Some notes on this:
To figure out the purchase of a cascade, you multiply the purchases of each part. Your first system (12:1) seems
at first glance to be a 12:1, consisting of a 2:1 (green) on each side cascaded with a 6:1 (blue/red) (2:1-6:1, 2 X 6 = 12:1). However, it is really a double-ended 4:1 (green), which is split to be on both sides of the mast, cascaded with a duplicated 3:1 (blue and red) (4:1-3:1, 4 x 3 = 12:1). The duplication of the 3:1 has no purpose other than to allow the adjustment to be made from either side.
Your 16:1 is the same deal - initially can seem to be a 2:1 (green) cascaded with an 8:1 (blue/red) (2:1-8:1, 2 x 8 = 16:1), but is really a double-ended 4:1 (green) cascaded with a duplicated 4:1 (blue and red) (4:1-4:1, 4 x 4 = 16:1).
This is not a bad system - the routing problems of running lines to both sides of the mast and still getting a smooth-running system are tough. Plus, the room you need for the cascade really isn't that bad. If it was truly a 2:1-8:1, for a 12" downhaul range, you'd need 8 x 12" = 96" or 8' of travel available between the green-blocks (doable on the mast, but waaay up there). Since it's a 4:1-4:1, you'd only need 4 x 12" = 48" or 4' of travel between the green blocks - certainly doable.
To minimize the duplication of the second part of the cascade, as well as minimize the travel needed by the first part, what if we went 8:1-2:1? See below:
This is all made with easily available components. The entire 'green' section is the "8:1 Downhaul Kit with Double Blocks" from Murray's; two cheek blocks per side that are simply turning blocks; either a double block or two singles (probably work better with two singles) attached to the sail; and a ring to tie off the 2:1's (if you could find cheek blocks with beckets, it would be perfect, but good luck with that). This could probably be integrated into the boom gooseneck.
For 12" of range at the sail, you need 2 x 12" = 24"or 2' of distance between the lower downhaul component and the upper turning cheek blocks - no problem! Mount the lower downhaul component just below the boom; mount the upper turning blocks 2' + some above that on the mast. The lower two turning blocks could be one on either side of the boom; the two singles are on a captive-pin type shackle, attached to the sail.
I think this is a way better solution than mounting under the tramp or the front crossbar - doing that means the downhaul will affect mast rotation since you can't run the line directly through the center of rotation. Have we made a tradeoff vs the original?
Edit - part way in to this, it's way too long for a forum post. Gonna write this up as an article or two, maybe get it in the Hotline!