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Old 15-02-2009, 12:29   #16
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Originally Posted by Celestialsailor View Post
Actually, you do not have to have a heavier boom because you are spreading the load over a much larger area at 3 points.
Sorry but if you have a loose footed main, you are wrong.

Simple principle of mechanics

if you are connected to the end of the boom, you are pulling directly in line with the clew - hence bendiness of the boom is totally irrelevant.

If you are pulling even from 3 seperate points - unless the 3 points are hardwired to a single pivot, then the pull will act to bend the boom as it tries to pull the end of the boom down, with a pivot force at the other end of the boom. Thus a boom with a loose footed main needs to be significantly stronger if you are connecting the main sheets to the middle of the boom.
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Old 15-02-2009, 12:42   #17
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Originally Posted by Talbot View Post
Sorry but if you have a loose footed main, you are wrong.

Simple principle of mechanics

if you are connected to the end of the boom, you are pulling directly in line with the clew - hence bendiness of the boom is totally irrelevant.

If you are pulling even from 3 seperate points - unless the 3 points are hardwired to a single pivot, then the pull will act to bend the boom as it tries to pull the end of the boom down, with a pivot force at the other end of the boom. Thus a boom with a loose footed main needs to be significantly stronger if you are connecting the main sheets to the middle of the boom.
Thanks Talbot...I stand corrected...If it is loose footed it would be best to end sheet.
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Old 15-02-2009, 12:48   #18
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I agree with Talbot

The whole reason end-boom sheeting has become so popular is that it reduces the amount of weight needed for the boom. Switching a lighter boom to mid-boom sheeting could lead to failure.

Most booms will be over-engineered to a certain degree, but I wouldn't want to be testing the limits in this way. One crash jibe could easily lead to a snapped boom.

I agree that no one likes a cockpit that's bisected by a traveler. But you've got to ask the question of why Bristol designed the traveler that way in the first place. Hint: it wasn't just to give you a place to hit your shins.
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Old 15-02-2009, 13:00   #19
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I moved the main sheet from the rear of the cokpit to the front of the cockpit because without doing so there was no way to install a bimini. It moved the main sheet from the end, to one third from the end. It works perfectly. It does take a little more effort to sheet it in, but I got shade now. I did talk to an engineer and send him pics of the proposed movement first though.
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Old 15-02-2009, 16:00   #20
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Originally Posted by READY2GO View Post
I moved the main sheet from the rear of the cokpit to the front of the cockpit because without doing so there was no way to install a bimini. It moved the main sheet from the end, to one third from the end. It works perfectly. It does take a little more effort to sheet it in, but I got shade now. I did talk to an engineer and send him pics of the proposed movement first though.
Mine was just the opposite. Plus, I went with a loose footed sail. It all depends on the design of the cockpit in relation to the rigging. Some booms end up in the middle of the cockpits some are far forward like on a ketch and then others are far aft like on cat boats (not to be mistaken with a catamaran).

If I were to move mine forward it would only be 1/4 back from the gooseneck, which would create such a load that it would probably tear out the cabin top let alone the stress on the boom fittings. On my last boom I had already ripped out the vang attachment on an accidental jibe. Now I use a boom brake. 16' of boom and 380 sq. ft. of mainsail can really create havoc when jibed.
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Old 15-02-2009, 17:39   #21
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The orginal main sheet was at such an angle you had to duck every time you tacked. Plus no way to put a bimini. as you can see from the pics I didn't have to move it very far forword. It's a little inconvenient to get to the companionway on a port tack, but since I singlehand 90% of the time, its not a problem. (I moved the main sheet to were the rope is hanging down in the second pic.
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Old 15-02-2009, 17:43   #22
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Wow it accually worked. That was a major acomplishment for me to post pics. Old dogs learn new tricks very slowly.
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Old 16-02-2009, 05:44   #23
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Depends on the geomtry, sometimes you can and sometimes you can't. You have an older boat and may be better served by moving the traveler behind the canvas.



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Hi All,
I am thinking about moving the mainsail traveler from the cockpit to above the companion way hatch, primarily to increase a little space. It doesn't seem to be too big a project to do this, but I wonder about changing the dynamics of the boat. Anybody have any insight?
Hank
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Old 16-02-2009, 07:41   #24
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A lot of your plan's success will depend on whether your mainsail is relatively large or not. I think a lot of mid-boom sheeting started with the IOR boats and very high aspect, short-boomed mainsails. Old CCA boats such as mine with a long cockpit and long-boomed, low aspect mainsails should not convert as 2/3 of the boom is behind the coachroof, and moving the mainsheet forward would have too little mechanical advantage and too much strain on the boom.
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Old 16-02-2009, 07:43   #25
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Experimental science

Here's a little experiment you could do to get some insight about your boom strength:

Top the boom up to its normal operating position.

Remove the sail from the boom.

Make a temporary attachment point(s) where you propose to put the new mainsheet attachment. A simple lashing around the boom will do the job.

Attach your current mainsheet to the lashing and pull on it using whatever tackle and/or winch you normally would use going to windward in a stiff wind.

See how much the boom bends! Maybe even measure the deflection. If you are not comfy with what you see, have a chat with a rigger or spar builder.

Incidentally, in my experience, the foot slugs on most non-loosefooted mains don't take much strain, and the difference in boom loading isn't all that significant.

Hang in there, mate, I think that you will be able to come to a reasonable decision despite all our advice!!

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Old 16-02-2009, 13:51   #26
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...Incidentally, in my experience, the foot slugs on most non-loosefooted mains don't take much strain, and the difference in boom loading isn't all that significant...
Yes, I concur with that too - there is little difference between a loose footed mainsail and one with slugs/slides insofar as the boom loading is concerned.

Once the outhaul is tensioned most of the sail load is at the tack and clew and that is easily seen by feeling the tension in the sailcloth at the slides (or in the tapes attaching them). One may even find the slides are loose in their tracks if the sail has little elasticity along the foot.

I know some will disagree with that but I point out that their disagreement is entirely irrelevant because in the end if one has slab reefing the moment you put the first reef in the loads are definitely all at the tack and clew.
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Old 17-02-2009, 01:04   #27
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I know some will disagree with that but I point out that their disagreement is entirely irrelevant because in the end if one has slab reefing the moment you put the first reef in the loads are definitely all at the tack and clew.
1. Slab reefing effectively reduces the length of the boom and thus reduces the bending moment.

2. using the kicker to help flatten a foot-roped sail would tend to suggest that the sail is providing an upward force on the middle of the boom

3. I had reefing points on my main in addition to the slab reefing in order to improve the sail shape
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Old 17-02-2009, 02:29   #28
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1. Slab reefing effectively reduces the length of the boom and thus reduces the bending moment.
You missed the point. The unreefed sail with slides, I said, applies little distributed load on the boom from the slides. If one disagrees with that then one has to explain why when the first reef is put in the reefed foot of the sail which is only attached to the boom at the tack and clew does not try to pull the loose sail of the reef (the slab) under it up. It doesn't do so because the reefed foot is tensioned by the outhaul (meaning the outhauling part of the reefing line) between the tack and clew. Even though that tension is not great and the reefed foot of the sail more elastic than the unreefed foot (when reefed the foot being just sailcloth) it is still sufficient to resist the vertical forces in the sail cloth along it.

Try it with a bit of cloth - pull an edge tight and then see how that resists force applied to the cloth perpendicular to tight edge (might need three hands ). The same happens at the boom with slides (ie little force is transferred through the tight edge to the slides) and is why a slab reefed sail doesn't try to pull the reefed slab up from off the boom.

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2. using the kicker to help flatten a foot-roped sail would tend to suggest that the sail is providing an upward force on the middle of the boom
If the vang flattens the sail it is bending something, either the mast (via the leech/roach) or the boom . It is uncommon that one would want to bend the boom to flatten the sail but even if one did you would be mainly increasing the clew load.

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3. I had reefing points on my main in addition to the slab reefing in order to improve the sail shape
Reefing points are provided to tidy the bunched up sail on the boom. Your sail may have been different but if so it was unusual.
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Old 17-02-2009, 05:20   #29
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You missed the point.
Nope, but I am not going to continue to argue. I could easily demonstrate with a boat and boom, but I have a slight difficulty with that approach at the moment as I am between boats.

It is your boom, you are welcome to break it if you like
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Old 17-02-2009, 07:50   #30
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Originally Posted by Talbot
2. using the kicker to help flatten a foot-roped sail would tend to suggest that the sail is providing an upward force on the middle of the boom

Well, that doesn't make too much sense to this observer. The vang/kicker or whatever you call it pulls down on the boom, The boom pulls down on the tack attachment point, which in turn pulls down on the leach of the sail, reducing twist. Works like this on my loose-footed sail. Doesn't change when you have slab reefing, just moves the tack farther forward to where the reef outhaul cringle is located.

And by the way, when you bend the boom (by whatever means) it doesn't flatten the sail, it bags it a bit. Why? BEcause when it bends,the ends have to move closer together, which in turn effectively "eases the outhaul".

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