Rigging boom vang

[svsonora]

My boat came without a boom vang - just a boom downhaul. I'd like to install a boom vang this season, along with an outhaul. I'll start a separate thread for the outhaul question. Boat is a Bristol 30.

I found this fiddle block in my spare parts box. I also found a single block.

Do I need to buy another set of blocks? Does the fiddle block seem big enough for my size boat?

My mast terminates without any provision to hook up an end of a boom vang. What hardware do I need to purchase to allow for this? Should I install this at the mast, or the deck? What about the boom end?

For the boom end mounting point, should I just use the spot that allows me to achieve the 45 degree angle?

Thank you. My last boat had a boom vang, but I'm having to install one on Sonora.





Thanks in advance. Sorry for the buttload of questions.

[Suijin]

I would use 5/16” 8mm line and size the hardware to that, or bigger.

Ideally the lower attachment point is on the mast as that keeps the geometry the same as the boom swings.

Your biggest concern should be the robustness of the attachment points. A hard gybe with the vang hard on exerts an eye popping amount of force. I’ve seen vang hardware reduced to twisted garbage from one. You really can’t over build a vang system.

[davil]

as a side note,when you get an idea of hardware needed keep in mind
https://www.garhauermarine.com/ as a source of parts,amazing quality,service and potential customized parts.
My boat came without a vang and still trying to design a system will work,not there yet but as Suijin mentioned,the load imposed on points of attachment is what keeps me on hold until am sure of what I will eventually end up with.
cheers
By eyeing the pics it impresses me as not robust enough,sorry.(more money!!!!)
I opened the answer as I am interested to see what others input.

[svsonora]

Thanks. I'll be awaiting more answers as well.

[BlowMeAway]

Originally Posted by svsonora

Thanks. I'll be awaiting more answers as well.

Perhaps this link will offer some ideas. I’ve contributed several images here that show my Vang hardware and lines on my H30.

https://forums.sailboatowners.com/in.../#post-1517007

[beetle]

The vang essentially needs to be at least as strong as the mainsheet, those blocks appear woefully under-sized. The boom is end-sheeted at an awkward angle, I would use gear at least matching the existing main sheet block loads.

And test that the boom section can handle vang-sheeting without bending too much.

45 degree angle is incorrect for a vang's moment arm (there is interplay between the downward force exerted on the boom by the vang and the lever arm determined by how far aft from the gooseneck attachment). 30 degree angle with respect to boom is a rough starting point - the goal of the vang is to apply maximum downward force to the aft end of the boom, not at the vang attachment point, and you're not trying to minimize load on the vang itself.

Boom bails thru- bolted to the mast and boom are strong and can allow the mast deck-level block to pivot somewhat. An articulating mast attachment is better. Spreading load across more surface area is better than point-loading.

- rob

[billknny]

Just looking at the shape of your sail, I’d suggest that an proper outhaul should be a much higher priority.

A boom vang imposes very large loads on the boom. If the boom was not designed for it, it can fail. Do NOT consider attaching the Vang to the deck unless you are sure the deck can take the very large upward forces involved.

You should be looking at other Bristol 30s to get ideas for how this is done, or consulting a professional rigger.

[BlowMeAway]

Originally Posted by beetle

The vang essentially needs to be at least as strong as the mainsheet, those blocks appear woefully under-sized. The boom is end-sheeted at an awkward angle, I would use gear at least matching the existing main sheet block loads.

And test that the boom section can handle vang-sheeting without bending too much.

45 degree angle is incorrect for a vang's moment arm (there is interplay between the downward force exerted on the boom by the vang and the lever arm determined by how far aft from the gooseneck attachment). 30 degree angle with respect to boom is a rough starting point - the goal of the vang is to apply maximum downward force to the aft end of the boom, not at the vang attachment point, and you're not trying to minimize load on the vang itself.

Boom bails thru- bolted to the mast and boom are strong and can allow the mast deck-level block to pivot somewhat. An articulating mast attachment is better. Spreading load across more surface area is better than point-loading.

- rob

Hey rob, perhaps you should evaluate the vang configuration based on this image. You can then understand the function and forces of the vang better when the Hunter is rigged like this for a mainsheet at the aft end of the boom. Totally agree with a thru-bolted bail on the mast. Don’t place a point loaded pad eye on the deck!

[beetle]

Originally Posted by BlowMeAway

Hey rob, perhaps you should evaluate the vang configuration based on this image.

I'm sorry, I don't quite follow what needs to be evaluated in the image. Can you explain what should be looked at?

The vang attachment/configuration isn't affected by where the mainsheet is attached to the boom, but you can gauge the loads involved by looking at the mainsheet setup, you can also look at the max shear load the gooseneck clevis pin can handle and don't engineer something that will exceed that shear load.

If you want to run numbers Harken has a simplified calculator that will provide expected mainsheet loads for end-boom sheeting and then you can run different trigonometry calculations to see which vang attachment location works best to apply that mainsheet load using only the vang. It's kinda fun to do, and the result is counter-intuitive.

- rob

[BlowMeAway]

Originally Posted by beetle

I'm sorry, I don't quite follow what needs to be evaluated in the image. Can you explain what should be looked at?

The vang attachment/configuration isn't affected by where the mainsheet is attached to the boom, but you can gauge the loads involved by looking at the mainsheet setup, you can also look at the max shear load the gooseneck clevis pin can handle and don't engineer something that will exceed that shear load.

If you want to run numbers Harken has a simplified calculator that will provide expected mainsheet loads for end-boom sheeting and then you can run different trigonometry calculations to see which vang attachment location works best to apply that mainsheet load using only the vang. It's kinda fun to do, and the result is counter-intuitive.

- rob

Hey Rob, I don’t get your point. The boom vang is only used if we want to shape or twist the sail on a broad reach to add some lift. What do we care if the down force on the vang is equal to the mainsheet. My take on this is that the vang is equally qualified to handle the load on the boom as the main sheet, and I can adjust the mainsheet to contribute.

Let’s get back to the hardware question posed by the the OP.

[beetle]

Agreed - as long as the vang can do the same work as the mainsheet then we're in violent agreement I use the vang for more than controlling twist off the wind, it's also good for setting twist upwind and then using the sheet/traveler to control angle of mainsail to wind while minimally changing the sail's shape (that's one way to set the boom on centerline or above without closing the leech).

Back to the original poster's question about the angle the vang should make with the boom, you should set the vang's boom attachment point further aft than a 45 degree angle, suggestion is 30 degrees as a starting point. You can make the vang attachment point work at 45 degrees (or even closer to the gooseneck if you want) at the expense of a stronger/heavier-than-necessary boom and gooseneck.

- rob

[svsonora]

Thank you. I'm starting to understand it. We are using the gooseneck as the pivot point to yank the boom down, and looking down the 'Y' axis (top down), the force needs to originate near where the clew is (mast). The closer it is to the gooseneck, the more stress it will be on the gooseneck.

My boom is pretty beefy. Its one of those old school on-boom furling booms. I don't use it to furl my boom. My boom may be beefy, but I am wondering if my gooseneck is made weaker due to the rotating mechanism.



I imagine if I go smaller than 30 degrees, the 'downward' force on the boom will be deminished and that I'd be wasting too much force into compressing the boom instead.

Is there a way for me to optimize this angle? I am only a pseudo-engineer (college race car build team, but don't know any engineering outside automobile chassis and suspension), but I can get concepts pretty quickly.

[svsonora]

Could I replace the function of the boom vang with the mainsheet traveler instead? I did read somewhere that Bristol 29/30s don't need a boom vang. I used to think it is balony but now I am just confused.

[billknny]

Originally Posted by BlowMeAway

Hey Rob, I don’t get your point. The boom vang is only used if we want to shape or twist the sail on a broad reach to add some lift. What do we care if the down force on the vang is equal to the mainsheet. My take on this is that the vang is equally qualified to handle the load on the boom as the main sheet, and I can adjust the mainsheet to contribute.

Let’s get back to the hardware question posed by the the OP.

The thing you are missing is that the vang attaches at a point 1/3 of the way back on the boom, so the amount of pull needed to hold the boom down is 3 times the load that would be generated by an end boom sheet. Add to that the fact that the vang is not pulling straight down, but forward, and you have to multiply that 3X by another 50% or so, so the vang has to pull with a force of 4 or 5 times the end-boom sheet to do the same job. It's all about leverage.

That why everybody is saying the hardware the OP showed in his original photos is inadequate. That is also why you can't just throw a vang onto any boom without careful analysis.

[davil]

My Cape Dory 30 MKll 1987 has been sailed and have extensive history of long range offshore sailing by PO without a vang.
Obviously is an optional piece of equipment,however does improve main shape,control of main/boom and safety? on downwind.
Having to choose decided to increase the safety factor and added a boom preventer, however will eventually add a vang,just overwhelmed with "projects" at this time.
Here is a link to West Marine on rigid booms,gives an idea on placement and angles,I realize this is a combination nevertheless gives an idea.
If money was not an issues more likely will use the rigid vang from Garhauer.
And as billknny mentions this is not a just do it without a serious consideration to forces at play,just adding a bale at boom level in the wrong place and fasteners could lead to a boom failure,


https://www.westmarine.com/buy/seola...02_063_003_501