I want a powerful vang for vang sheeting. The present one is attached to the boom as far aft on the boom as the boom is off the deck. In other words equidistant up and aft. How much further aft can I go to maximize downward pull before it all turns into forward pull? Maybe two lengths aft to one up?
Vang attachment point
- Thread starter Quoddy
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Probably have a US Spars (Z-spars) mast and boom. In any case, its about that same for all. 30 degrees is optimal.
http://www.usspars.com/wp-content/uploads/2011/03/vanginstall.pdf
http://www.usspars.com/wp-content/uploads/2011/03/vanginstall.pdf
At 1 and 1, the down pull is about 71% of the total.. 100 pounds pull in the vang gives ya about 71 pounds of down on the boom.. 2 out and 1 down gives ya 45% of vang pull.. so 100 pounds of pull on the vang is about 45 pounds of down.. But that is not the whole story because as ya pull further out on the boom, you get the mechanical advantage of the boom lever.. Example.. if your boom is 8 feet long and you do the 1:1 pull at 2 feet from the gooseneck, then change it to 2:1 , you get a 50% increase from the lever but you lose 26% because of the extra angle.. ya end up gaining 24% downforce on the boom for the same input .. The cost is in added compression force in the boom.. in the examples, the compression on the boom increases about 19% .. first case about 71 pounds driving the boom into the gooseneck, second case, about 90 pounds driving the boom into the gooseneck.. (I think my Trig is close?? but wouldn't guarantee it)
(My assumption is that the vang is attached to the base of the mast)
(My assumption is that the vang is attached to the base of the mast)
I had a 1995 Hunter 26 which I think is the same as yours. The only sailing control on the mainsail it had was the mainsheet that was mounted in the cockpit floor and attached on the rear 1/2 of the boom. We added a boomvang that was mounted at the mast base and attached to the boom the same distance out as the distance from the mastbase to the gooseneck. 1 to 1.
What an improvement!
Ray
What an improvement!
Ray
In general its OK, but you DO have to be careful with adding 1:1 vang attachments without looking the physics involved. Remember that the vast majority of the mainsails load along the boom is at the clew. If you ease out on the mainsheet but keep the vang hard, the load on the boom moves forward from the mainsheet attach point to the vang. That extra distance creates a huge additional moment on the boom. Loads like that can snap booms. Moving the vang attach point aft not only lightens the loads on the vang, but also on the boom.
That's also why travelers are such a great idea. In addition to helping keep mainsail shape while lowering AOA in high wind, they keep loads off the vang.
That's also why travelers are such a great idea. In addition to helping keep mainsail shape while lowering AOA in high wind, they keep loads off the vang.
This is not exactly on topic but it's relevant to installing a new boom vang.
Using a boom vang to adjust main sail twist going upwind will put enormous loads on your vang, boom, gooseneck and all related hardware. It is very possible to permanently bend your boom. You can prevent this by installing a sleeve within the boom at the attachment point of the vang. I added a 36" long internal sleeve to my boom last season. The row of fasteners on the boom indicate the limit of the sleeve. There is no perceivable deflection now.
Using a boom vang to adjust main sail twist going upwind will put enormous loads on your vang, boom, gooseneck and all related hardware. It is very possible to permanently bend your boom. You can prevent this by installing a sleeve within the boom at the attachment point of the vang. I added a 36" long internal sleeve to my boom last season. The row of fasteners on the boom indicate the limit of the sleeve. There is no perceivable deflection now.
Attachments
I Added a Block to the Tang on the Mast Step
The line on the vang runs from the boom thru the block and back up to the boom. The boom already had an extra bail on it so I moved that one forward about four inches back from the original. Using the extra block gives it a greater mechanical advantage and the block with the cleat being further aft makes it easier to use. If I can get out to the boat before I forget to I'll take a picture or two.
The line on the vang runs from the boom thru the block and back up to the boom. The boom already had an extra bail on it so I moved that one forward about four inches back from the original. Using the extra block gives it a greater mechanical advantage and the block with the cleat being further aft makes it easier to use. If I can get out to the boat before I forget to I'll take a picture or two.
Any vang system will optimize at about 45 degrees to the boom and mast base. With 45° angles youll get 71% downward and 71% forward force on the boom .... for every 1 POUND you apply youll get .71 pounds of reaction force. Placing the vang-to-boom attachment any further back on the boom than at a 45° angle will require more force - at 30° angle to the boom you'll only get .5lb downward force for every pound pulled (on a single line, no 'block & tackle'); therefore needing a .71/.5 = 145% 'stronger' vang system to do the same job of keeping the boom 'down' in comparison to a 45°.
At smaller intercept angles to the boom the vang becomes more of a 'mast ram' which can bow the lower sections of the mast for sail flattening purposes on 'floppy'/bendable masts .... but increasingly makes the boom more vulnerable to 'buckling failure' due to its now increased axial 'compression load' .... thats why I recommend that 45°; plus, the typical mast/rigging on a boat like yours isnt designed for such 'bending loads'.
So, if you have enough distance between the mast 'base' and the boom, a 45° angle (rise:run = 1:1) will usually be optimal. The strength of the boom (flexure) and strength of the mast connection are vitally important ... especially if you apply a high mechanical advantage vang system.
On a boat of your size, I'd opt for a 'compound' vang ... 2:4:1 = mechanical advantage = 8! OR a 'rigid vang' with the equivalent mechanical advantage (8:1). You 'must' be careful not to 'overstress' the such a powerful vang system or let a 'gorilla' adjust the vang when under load or you'll definitely break 'something' - like a sail or a boom, etc. The advantage of 8:1 is finger-control (from the cockpit, etc.) to adjust the vang, even when the mainsail is fully windloaded (overpowered).
I prefer high strength Harken blocks, hexaratchets, etc. for such 'high-load' vang applications.
At smaller intercept angles to the boom the vang becomes more of a 'mast ram' which can bow the lower sections of the mast for sail flattening purposes on 'floppy'/bendable masts .... but increasingly makes the boom more vulnerable to 'buckling failure' due to its now increased axial 'compression load' .... thats why I recommend that 45°; plus, the typical mast/rigging on a boat like yours isnt designed for such 'bending loads'.
So, if you have enough distance between the mast 'base' and the boom, a 45° angle (rise:run = 1:1) will usually be optimal. The strength of the boom (flexure) and strength of the mast connection are vitally important ... especially if you apply a high mechanical advantage vang system.
On a boat of your size, I'd opt for a 'compound' vang ... 2:4:1 = mechanical advantage = 8! OR a 'rigid vang' with the equivalent mechanical advantage (8:1). You 'must' be careful not to 'overstress' the such a powerful vang system or let a 'gorilla' adjust the vang when under load or you'll definitely break 'something' - like a sail or a boom, etc. The advantage of 8:1 is finger-control (from the cockpit, etc.) to adjust the vang, even when the mainsail is fully windloaded (overpowered).
I prefer high strength Harken blocks, hexaratchets, etc. for such 'high-load' vang applications.
I agree with RichH, 45 deg is optimal from a stress on the boom/mast/vang standpoint. It gives the most leverage with the least amount of rope tension.
Why do you think you NEED a powerfull vang? Going to windward the sheet does most of the "vanging" and going down wind you really dont need a vang except to keep the sails off the spreaders.
Is this a case of vang envy? does the guy in the next slip have a bigger vang than you ;-)
Why do you think you NEED a powerfull vang? Going to windward the sheet does most of the "vanging" and going down wind you really dont need a vang except to keep the sails off the spreaders.
Is this a case of vang envy? does the guy in the next slip have a bigger vang than you ;-)
You guys are worried about rope tension, I'm worried about my boom! ;^)
US Spars, Forespar and others recommends a 30 degree angle for good reasons.
Going down wind the vang does much more than keeping the sail out of the spreaders. It's used to de-power the main by detwisting it. All the sail load is on the clew, and I'd rather have my vang attach point as far aft as possible.
Need more leverage?, cascade it as Rich points out.
And sometimes its simple geometry and available space. On the 36.7, the boom is 16 feet long and only 2.5 feet off the cabintop and the mast. Its can't be 45.
US Spars, Forespar and others recommends a 30 degree angle for good reasons.
Going down wind the vang does much more than keeping the sail out of the spreaders. It's used to de-power the main by detwisting it. All the sail load is on the clew, and I'd rather have my vang attach point as far aft as possible.
Need more leverage?, cascade it as Rich points out.
And sometimes its simple geometry and available space. On the 36.7, the boom is 16 feet long and only 2.5 feet off the cabintop and the mast. Its can't be 45.
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The H260 has sufficient space and configuration for a 45° vang system .... 45° is also the 'stock' vang angle.
The boat is also (should be) set up for loose footed mainsail. The boat is also set up for 'end of boom' sheeting (nice!!!) so 'naturally' a less robust but more 'flexible' boom can be used.
Applying the boom-vang connection at 30° intercept angle will move the attachment point more to toward the midspan of the boom .... and that will increase the vulnerability of buckling failure of the boom as it will more greatly induce midspan deflection of the boom while the boom is under maximum axial compression load (increased by the 30° attachment ..... cos30° = .86; cos45° = .71 ; 20% 'more' axial compression than with a 45° & as an applied force component applied from the vang system.
.... and will also need a 'more powerful' vang system (~150% 'stronger') to handle the 'vertical' loads ..... sin30°=.5; sin45°=.71 .71/.5 = 142% in comparison to a 45°.
http://h260.com/manual/index.html --- page 2
The boat is also (should be) set up for loose footed mainsail. The boat is also set up for 'end of boom' sheeting (nice!!!) so 'naturally' a less robust but more 'flexible' boom can be used.
Applying the boom-vang connection at 30° intercept angle will move the attachment point more to toward the midspan of the boom .... and that will increase the vulnerability of buckling failure of the boom as it will more greatly induce midspan deflection of the boom while the boom is under maximum axial compression load (increased by the 30° attachment ..... cos30° = .86; cos45° = .71 ; 20% 'more' axial compression than with a 45° & as an applied force component applied from the vang system.
.... and will also need a 'more powerful' vang system (~150% 'stronger') to handle the 'vertical' loads ..... sin30°=.5; sin45°=.71 .71/.5 = 142% in comparison to a 45°.
http://h260.com/manual/index.html --- page 2
Rich,
I think I'm follwing your math... how does it apply in this case:
With end-boom mainsheeting on a loose footed main, I agree that that is the most safe for the boom, very little defection loads are placed on the boom as they go straight fron the clew to the sheeting. Now we are running, the sheet is let out, the boom wants to rise and the vang takes the load. It seems to me that the closer the vang attach point is to the clew, the smaller the lever and the less stress on the boom. Am I missing something?
I think I'm follwing your math... how does it apply in this case:
With end-boom mainsheeting on a loose footed main, I agree that that is the most safe for the boom, very little defection loads are placed on the boom as they go straight fron the clew to the sheeting. Now we are running, the sheet is let out, the boom wants to rise and the vang takes the load. It seems to me that the closer the vang attach point is to the clew, the smaller the lever and the less stress on the boom. Am I missing something?
When the boom attachment point moves aft, it takes MORE force to support and maintain the loads in the 'vertical plane', (due to the 'increased lever action' ... all dictated by 'trigonometry').
Correspondingly, the vang (@ 30° intercept angle with the boom) is now delivering increased axial stress to the boom (along the 'thin axis' of the boom) and 'to balance' the 'reaction point' at the opposite end of the vang (the connection at the mast base) must also be strengthened .... or you will now be more vulnerable to 'pulling out' that connection (by about an increase of 20%) due to the 'trigonometry' of the vang.
The smaller the angle that the vang makes with the boom, the "higher" the forces will be (compression in the boom) and will require MORE force to counteract the boom from 'lifting'.
As an 'extreme' case, think of the vang connected at the aft end of a VERY long boom and the intercept angle approaching 'zero' .... those forces will approach 'infinity' - infinite force being generated along the axis of the boom by the vang .... and requiring infinite force in/on the vang and its attachment connections to keep the boom 'from rising' due to 'sail action'.
Ditto in moving the boom connection point very close to the gooseneck .... just the opposite but the developed forces become just as 'strong'.
45° is the 'optimum' .... if you have the 'geometry' to do so.
Correspondingly, the vang (@ 30° intercept angle with the boom) is now delivering increased axial stress to the boom (along the 'thin axis' of the boom) and 'to balance' the 'reaction point' at the opposite end of the vang (the connection at the mast base) must also be strengthened .... or you will now be more vulnerable to 'pulling out' that connection (by about an increase of 20%) due to the 'trigonometry' of the vang.
The smaller the angle that the vang makes with the boom, the "higher" the forces will be (compression in the boom) and will require MORE force to counteract the boom from 'lifting'.
As an 'extreme' case, think of the vang connected at the aft end of a VERY long boom and the intercept angle approaching 'zero' .... those forces will approach 'infinity' - infinite force being generated along the axis of the boom by the vang .... and requiring infinite force in/on the vang and its attachment connections to keep the boom 'from rising' due to 'sail action'.
Ditto in moving the boom connection point very close to the gooseneck .... just the opposite but the developed forces become just as 'strong'.
45° is the 'optimum' .... if you have the 'geometry' to do so.
Interesting.... I'll need to read that several times.. ;^)
You say 45 is optimum, if you have the geometry to do so... what does that mean, and why to all the rigid vangs I know of recommend a 30 install angle? Not trying to be argumentative at all, just trying to make sense of it.. ;^)
You say 45 is optimum, if you have the geometry to do so... what does that mean, and why to all the rigid vangs I know of recommend a 30 install angle? Not trying to be argumentative at all, just trying to make sense of it.. ;^)
Rich, What ya have to consider, though, is that the force to hold the boom down goes down as you put the vang point further aft.. Ya end up playing the trig loss against the lever arm gain.. as ya get to the end, there is an asymptote kinda thing that happens but in the kinda linear part of the trig curves there is some fun to be had. but like I said earlier.. compression increases a lot and ya may collapse the boom .. Euler is not a kind or predictable thing to play in sometimes
I find using the vang downwind is extremely helpful. It keeps the sail flat when you need it the most, specially for sailing by the lee in lighter winds going w/w. Curly mainsails going downwind is a speed killer, and can poke holes in mainsails, even with a standard rig, to say nothing about swept back spreader rigs. When going downwind, whether ddw or any significant component thereof, the mainsheet's downward component is negligible, so the vang is the perfect tool to use to flatten the main.
kloudie1 ..... re.: vertical components of induced/reactive force, I thought, would be made clear by: "... and will also need a 'more powerful' vang system (~150% 'stronger') to handle the 'vertical' loads ..... sin30°=.5; sin45°=.71 .71/.5 = 142% in comparison to a 45°." Aristarchus would agree. ;-)
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Jack --- The few the rigid vang systems Ive ever installed were at 45° and were attached to 'well reinforced' connection points.
I have seen powerful rigid vangs installed at ~30° and have sometimes noted the beginnings of 'shear failure' (projected load or 'saddle' stress on the bolting surfaces) of the aluminum boom at the drilled and tapped holes (vang 'shoe' connection) because probably due to the inducement of axial stress (causing shear) on the bolting and its mating surfaces. For less than (or greater than) 45° when you calculate, you must 'oversize' (and include increased 'safety factors') ... 45° is the 'optimum' and is in accordance with 'trigonometry'.
You have to be careful with a rigid set at or less than 30° as the sometimes the amount of total 'boom lift' (boom aft end amount of 'travel' in the vertical plane) will be restrained by that 'geometry'/kinematics as a rigid vang many times is limited in its amount of fully compressed to fully uncompressed distance/length. The 'good aspect' of a rigid vang is that the internal spring develops a LOT of counter-force to 'balance' against the induced compressional load induced along the boom. Mathematically a rigid vang is very different from a 'rope' vang as it can act in compression as well as tension !!!!!!!!
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Jack --- The few the rigid vang systems Ive ever installed were at 45° and were attached to 'well reinforced' connection points.
I have seen powerful rigid vangs installed at ~30° and have sometimes noted the beginnings of 'shear failure' (projected load or 'saddle' stress on the bolting surfaces) of the aluminum boom at the drilled and tapped holes (vang 'shoe' connection) because probably due to the inducement of axial stress (causing shear) on the bolting and its mating surfaces. For less than (or greater than) 45° when you calculate, you must 'oversize' (and include increased 'safety factors') ... 45° is the 'optimum' and is in accordance with 'trigonometry'.
You have to be careful with a rigid set at or less than 30° as the sometimes the amount of total 'boom lift' (boom aft end amount of 'travel' in the vertical plane) will be restrained by that 'geometry'/kinematics as a rigid vang many times is limited in its amount of fully compressed to fully uncompressed distance/length. The 'good aspect' of a rigid vang is that the internal spring develops a LOT of counter-force to 'balance' against the induced compressional load induced along the boom. Mathematically a rigid vang is very different from a 'rope' vang as it can act in compression as well as tension !!!!!!!!
Boom Vang Attachment Point
OK, attached is the math.. Question from Quoddy was what happens if I go from 1:1 to 2:1 on the vang attach .. Attachment shows that for the same boom hold-down force, vang tension goes down because needed downforce decreases as ya move out. Compression in the boom goes down (just a smidge) in the 2;1 case; back up in the 3:1 case. So Quoddy is better off to move his attachment point out to twice the rise from the gooseneck to base.
OK, attached is the math.. Question from Quoddy was what happens if I go from 1:1 to 2:1 on the vang attach .. Attachment shows that for the same boom hold-down force, vang tension goes down because needed downforce decreases as ya move out. Compression in the boom goes down (just a smidge) in the 2;1 case; back up in the 3:1 case. So Quoddy is better off to move his attachment point out to twice the rise from the gooseneck to base.
This more to my point if I follow right... The DOWN force is also the pressure the boom must be able to absorb before it deforms or breaks under the load from the clew, if the mainsheet is not part of the equation. Correct?
Yes indeed, Jack. How much and where the downforce is applied can change things a lot. I am assuming that the mainsheet stays wherever it is/was and that variable has not changed. I also assume that the boom is strong enough for all cases... Just answering a vang question.. End-of-boom sheeting does unload the boom a lot when beating, especially if there is a traveler .. A note: in the example, the compressive force in the boom was almost the same.. but a compressive load applied further down the boom (away from mast) is more likely to collapse the boom.. You can push with 20 pounds force on a 2" piece of coat hanger wire but ya can't put that same load on a 20" piece without collapsing it.
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