three parts

[dannyb9]

a 2.5 part system is kinda hard to visualize...

[GreenLake]

a 2.5 part system is kinda hard to visualize...

This is in reference to the discussion on how to set up the mainsheet. The setup is where you attach the sheet to the end of the boom, run it to a block on a traveler on the transom back up to a block on the boom. Then along the boom, down at the middle to a block (and cleat) on the CB trunk.

The force at which a sheet pulls is equal in each part. So at the end, where two parts come together the force is twice that with which you pull. At the middle there's only one part that pulls down, so you simply pull the middle down with the force you are pulling on the sheet.

Now, for the next step, think of the boom as a teeter-totter (with a second half on the other side of the mast). One half you rig your mainsheet as just described. The other half, (imagined on the opposite end of a hinge where the mast would be) instead of a sail pulling up, you rig a single weight (at the end) that pulls down

Now to balance (or lift) that weight, your mid-boom part of the sheet contributes half as much, because it's half way out from the "hinge". On a real teeter-totter, if one kid is twice the weight of the other, you can place the heavy one at half the distance, so they balance.

The two parts that are connected to the end of the boom don't have that disadvantage, so they contribute fully. Hence, effectively a 2.5:1 purchase.

You may have read or heard somewhere that when you pull on the rope in a block and tackle, the ratio of how much rope you pull, compared to the distance the load moves also defines the purchase.

This is the case here. To move the end of the boom 1" down, you need to pull your mainsheet by 2.5". You shorten each of the two parts at the end of the boom by 1" to keep them taut. But the part in the middle of the boom only needs to be shortened by 1/2" - because the boom there only moves 1/2".

Can you visualize it now?

[GreenLake]

Now, if you have a Crosby rig, where the sheet goes around a triangle in the back between two blocks on the transom, your purchase becomes even less efficient.

Because now the parts in the back don't go straight down, when the end of the boom is pulled down 1" you've shortened them each by more than 1", by reason of trigonometry. This depends on the angle, which changes with the height of the boom, but at typical cangles it's a subtantial effect, meaning that you end up with 3 parts but say a 1.9 : 1 purchase at the moment the boom is half as far up as the distance between the two transom blocks.

[seandwyer]

This is all very interesting but I really am not grasping a lot of what is being said. Is the point here that the higher the ratio, the easier it is to operate the mainsheet, or is it just more effective in a mechanical way - or both? I use what is being called the Crosby. Once, a real racing sort of guy saw that on my boat and came over to change things around telling me what I was doing was ancient. He tied a loop through the transom blocks terminating in a figure 8 about a foot beneath the boom. The sheet fastened to the figure 8 then went down the boom as usual through the block on the end of the boom, then through the block mid boom to the block mounted on the center board locker. I sailed and it was fine but I noticed no difference at all - none! I'm trying t understand what a traveler on a DS1 would look like and then come to grips with what it can do - and if the sails are a little worn would a fella even notice?

[dannyb9]

what about the angle of the sheet's parts to the vertical as the boom moves out from the centerline? i mean, we arent trying to raise and lower the end of the boom. we are trying to control its rotation out from the centerline. maybe some vector analysis might be helpful. unfortunately i forgot how to do that. something to do with arrows.

[GreenLake]

I'll leave the fascinating vector analysis as an exercise to the reader.

Just want to point out that you have to contend with two or more angles, because the parts of the mainsheet aren't all in a plane, and certainly not always perpendicular to the boom. Have fun.

I don't think that you need complex diagrams with arrows to see that as long as your setup on the transom is fixed, pulling on a single line, the mainsheet, will always affect both the rotation of the boom and the sail tension at the same time.

Baysailer was using ratchet blocks to allow him to shift the traveller by hand, giving him a bit of independent control.

[GreenLake]

This is all very interesting but I really am not grasping a lot of what is being said. Is the point here that the higher the ratio, the easier it is to operate the mainsheet, or is it just more effective in a mechanical way - or both?

When someone says he has 6:1 midboom sheeting, that number sounds impressive at first, but then you realize that it just as good as 3:1 at the end of the boom, and still roughly comparable to the purchase of the actual setups (Crosby etc).

I use what is being called the Crosby. Once, a real racing sort of guy saw that on my boat and came over to change things around telling me what I was doing was ancient. He tied a loop through the transom blocks terminating in a figure 8 about a foot beneath the boom. The sheet fastened to the figure 8 then went down the boom as usual through the block on the end of the boom, then through the block mid boom to the block mounted on the center board locker. I sailed and it was fine but I noticed no difference at all - none! I'm trying t understand what a traveler on a DS1 would look like and then come to grips with what it can do - and if the sails are a little worn would a fella even notice?

That's my experience as well. When I tried both systems, I couldn't tell the difference. Like yours, my sails were worn at the time, but I suspect that the difference it makes is probably slight to the point where you need to be racing against another boat to see it. When you're sailing by yourself on the lake, small changes in pointing angle or upwind speed may not be easy to notice.