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  #1  
Old 03-03-2008
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Mainsail sheeting arrangements

Following some recent threads here I spent part of the weekend trying to work out if I can relocate my traveller to the bridge-deck and got stopped by this question: Does anyone know how to work out the theoretical optimum mainsheet sheeting position??

If:
- a position of 100% of boom length (ie. end-boom sheeting) requires a maximum length of mainsheet for adjustment of the boom through a arc of, say, 160(?) degrees with the minimum amount of effort, and
- a position of 0% of boom length (totally impractical) requires a minimum length of mainsheet with a maximum amount of effort, then
- assuming a loose-footed main (ie concentrated force at 100% of boom length), a Standard crew (reasonably fit male?) and Standard winds (15-20kts?):

Q1 : What sheeting position (in % of boom length) gives the minimum amount of mainsheet for a minimum amount of effort without snapping the boom, and
Q2 : What would the mainsheet tackle ratio be at that position for the typical crew to adjust the main in typical winds??

This must have been worked out by hundreds of designers over the years...
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Old 03-03-2008
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Hartley-

It really depends on the boat and the design of it... some booms were designed for end-boom sheeting and aren't strong enough to take the loads imposed by mid-boom sheeting. It also depends on the size of the sails and the mainsheet gear being considered.

On your boat, I'd probably stick with end-boom sheeting, since IIRC, that's what it was designed with. Generally to reduce effort, you have to increase the mechanical advantage, and so you reduce the strength needed, but increase the length of the rope needed. Your boat could probably get away with using a four-to-one tackle, since the sail isn't that big... and hauling in the mainsheet would be pretty easy...

If you went with a three-to-one mainsheet, instead of needing to haul in 16 feet of rope to adjust the sails, you'd need to haul in only 12 feet of rope to get the same adjustment, but you'd need 33% more strength to do so.

On some of the larger boats, you'll see a dual mainsheet tackle setup to multiply the leverage and allow relatively easy fine-tuning of the mainsheet. Often the large adjustments will be made using a six-to-one block and tackle system, and there will be a four-to-one tackle attached to the six-to-one, allowing the fine trim to be made at 24-to-one.

What is the sail area of your main sail? What are the typical strength of winds you sail in with the full main sail up? How much effort are you willing to exert to trim the mainsail?
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Old 03-03-2008
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IMHO there is no such thing as an optimum, location and design are all a set of trade-offs, what tradeoffs do you want to make?

The sheet length and effort involved in your mainsheet doesn't (or at least should not) change as the sheet ing position moves forward on the boom, you just add more turns to the tackle so the sheet-end load remains about the same. The extra turns eat up the sheet length a mid boom location would otherwise free up.

As to to the technical questions of % load and number of turns, I suggest you slough thru the thread Unmanageable Mainsheet whcih really works the subject over.

Cameron, I gotta apologize for not paying attention, but I know that your boat already has a mid-cockpit traveler, why consider moving the traveler from the current location? Run out of boat chores?
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Last edited by sailingfool; 03-03-2008 at 07:28 PM.
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Old 03-03-2008
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Have you tried going to say Harken.com and gone thru the online program that will customize a set up for you? IIRC it will also tell you how much pull in pounds you will get from a given main sail, depending upon boom length, Sq footage etc. It allows you to play with it some.

For you, with where you have it, moving it forward some to behind the cabin as I have, Tenuki etc, may make it so you do not have a knee kicker if you will. a bit more forward, where a 6-1 setup would more than make up for increased pressure if you have a 4-1 currently.

Not sure where to find the program online that you speak of, but I am sure it exists to some degree, it may be the harken compuspec, or something else, but you do not see it working as you would like per say.

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Old 03-03-2008
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Quote:
Originally Posted by sailingfool View Post
Cameron, I gotta apologize for not paying attention, but I know that your boat already has a mid-cockpit traveler, why consider moving the traveler from the current location? Run out of boat chores?
Run out of chores? I wish!!

This came up in conversation with my old yachting Dad, prompted by photos posted on Bob's thread about traveller position. Looking at our cockpit layout over the weekend, a bridge-deck mounted traveller would make life a lot easier on my shins whilst tacking, but on closer inspection it seemed that our mid-boom sheeting arrangement would become almost exactly mid-boom - so that ended that idea for fear of snapping the thing.

What I was curious to find out was whether or not our 3/4-position traveller (and hence sheeting position) was "optimal" and close out the question once and for all...
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Last edited by Classic30; 03-03-2008 at 11:48 PM.
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Old 03-03-2008
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Quote:
Originally Posted by blt2ski View Post
Have you tried going to say Harken.com and gone thru the online program that will customize a set up for you? IIRC it will also tell you how much pull in pounds you will get from a given main sail, depending upon boom length, Sq footage etc. It allows you to play with it some.
Marty, no, I haven't - thanks for the tip!

Quote:
Originally Posted by blt2ski View Post
For you, with where you have it, moving it forward some to behind the cabin as I have, Tenuki etc, may make it so you do not have a knee kicker if you will. a bit more forward, where a 6-1 setup would more than make up for increased pressure if you have a 4-1 currently.
Yup. Spot on! The question is: Exactly how much further forward can I go without getting into trouble??

If I'm at the "sweet spot" now, I'd be stupid to shift it too far..
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Old 03-04-2008
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Hartley18-

Do you have photos of your current cockpit setup???
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her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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Old 03-04-2008
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Quote:
Originally Posted by sailingdog View Post
Hartley18-

Do you have photos of your current cockpit setup???
Dan,

see Cameron's post in this forum titled "Mainsheet-cleats" which is down four threads (for some reason the message will not properly include the url link to that thread)
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Last edited by sailingfool; 03-04-2008 at 09:39 AM. Reason: url link will not post correctly
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Look here for pics etc to Camarons boat traveler issues.

The link SF was trying to do may not work because of the "-" in the middle?!?!?! then again, it should not be an issue

it's sail net and there idiotic word association BS, ie the cleats is overriding the topic with cleats in it. That is why one can not link it!

Marty
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Last edited by blt2ski; 03-04-2008 at 10:10 AM.
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Old 03-04-2008
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Traditionally, before the popularity of roller reefing booms of the 1960's, the mainsheet attrached to the boom 10% to 25% forward of the end of the boom. It was typically attached with multiple bails with the forward most bail, or the heaviest loaded bail, directly over the traveller, or horse. End boom sheeting is the most efficient way of sheeting a sail in terms of requiring the least am out of line, force and boom size. It is generally a bad idea to have the mainsheet at the midpoint of the boom since this has the highest loadings on the boom and requires the most rope or else on a bigger boat a winch. That said it is often done on boats with cabin top mounted travellers and on small racing dinghies.

The reason that I say that mid-boom sheeting requires the most line has to do with the way that designers approach designing sail handling hardware. We start out by determining how much load that we think the sail handler can reasonably pull (say 30 lbs) and then we back into the amount of purchase required. So if we look at a boat with end boom sheeting that needed a 3:1 mainsheet, if we moved the mainsheet to a position 25% from the end of the mainsheet we would need to go to a 4:1 mainsheet. And if we went to the mid-point we would need a 6:1 mainsheet. Except that there are much higher frictional losses when we increase the mechanical advantage and typically the mechanical advantage is further increases so in the example above the 25% position might be increased to a 5:1 and the mid-boom position to a 8:1 (or put on a winch with a stopper). In other words not only are you dealing with a lot more line but also a lot greater frictional losses.

Jeff
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