Why don't catamarans have boom vangs?

[ThereAndBack]

Originally Posted by markiobe

I don't understand what you mean here... obviously we don't want to crash gybe but why does a traveler prevent this or a vang make it more likely...? And why would the fixing closer to the gooseneck be a problem as long as the boom/mast are built properly?

Ok... I'm going to try. He is not talking about how a traveler or vang would prevent a crash gybe, he is talking about the results of a crash gybe.

Catamaran rigging is subjected to forces much higher than the same size monohull due to the inability to heel over and relieve stress on the rig and sails. This is both in the form of constant loads and shock loads. This means all parts of a catamaran rig are going to need to be very strong compared to the same size boat that can heel over.

In regards to the boom... The strongest parts to connect to the boom are at the ends. The closer to the middle of the boom that you connect anything, the weaker that point will be. A traveler or sheet connected in the middle of a boom will be a weak point and when the loads get big, the boom will tend to give (read: bend or break) at the mid point.

For the two reasons cited above, catamarans with their higher relative loads and big mains tend to connect their booms at the ends. Most of us don't run a vang setup for several reasons already mentioned. You're not going to sheet out the main to 90 degrees. The spreaders/stays will stop that. You're going to have a a traveler much further outboard and usually much lower than the boom, which lets you pull the end of the boom in a good direction to maintain sail shape. And finally you're not going to have the same motion as a monohull, it just isn't the same.

I have a voyage cat (mentioned above). In regards to that, we have a strut between our mast and boom to hold the boom up to a minimum level, however, it does not function as a vane. And they do recommend using a topping lift to the end of the boom when hoisting a dingy using the boom derrick.


If you're commissioning/buying your first cruising catamaran, I highly suggest you go the common route of not having a vang and just see how it works out for you. A vang is one of those things that is "easy" to add later. I think you'll find you won't miss it. I raced monohulls for many years and even on my old cruising monohull I used a vang. On my cat, I have never missed it.

[markiobe]

Originally Posted by tamicatana

Actually, you have it backwards AFAIC. Vangs are heavy, especially for the loads the cats will deal out (will need hydraulic doodads).

Not true. You can achieve the leverage required with a lever arm which can be light and only require a simple set of blocks to adjust. At least for the weight of boat that we're building, maybe on a condomaran you'd need much more.

Originally Posted by tamicatana

Go sail a beach cat and you'll see how it works.

Been there lots, done that lots, know how it works.

Originally Posted by tamicatana

You ain't on a leaner any more. Come to the dark side.... we have cookies

Not on a leaner and no plans to do so. I'll take you up on the cookies though

[chris mac]

I added a vang to mine this year. For 3 reasons...
I wanted to rely less on the topping lift, allow it to be more slack since it chafed the old main sail(very roachy)
I wanted to be able to sheet the boom out farther when down wind and not have the main against the spreaders. The vang helps a bit, but it has increased our downwind speed
I wanted to add a Dutchman boom brake so I didn't need to go on deck to rig a preventer when down wind. It has worked fantastic for this
Another common reason for adding a vang is to rig up in mast, or in boom furling.

[markiobe]

Originally Posted by ThereAndBack

Ok... I'm going to try. He is not talking about how a traveler or vang would prevent a crash gybe, he is talking about the results of a crash gybe.

Catamaran rigging is subjected to forces much higher than the same size monohull due to the inability to heel over and relieve stress on the rig and sails. This is both in the form of constant loads and shock loads. This means all parts of a catamaran rig are going to need to be very strong compared to the same size boat that can heel over.

Yup.

Originally Posted by ThereAndBack

In regards to the boom... The strongest parts to connect to the boom are at the ends. The closer to the middle of the boom that you connect anything, the weaker that point will be. A traveler or sheet connected in the middle of a boom will be a weak point and when the loads get big, the boom will tend to give (read: bend or break) at the mid point.

Yup agreed. Would need to be built to cope.

Originally Posted by ThereAndBack

For the two reasons cited above, catamarans with their higher relative loads and big mains tend to connect their booms at the ends. Most of us don't run a vang setup for several reasons already mentioned. You're not going to sheet out the main to 90 degrees. The spreaders/stays will stop that. You're going to have a a traveler much further outboard and usually much lower than the boom, which lets you pull the end of the boom in a good direction to maintain sail shape.

Yup, as mentioned earlier.

Originally Posted by ThereAndBack

If you're commissioning/buying your first cruising catamaran, I highly suggest you go the common route of not having a vang and just see how it works out for you. A vang is one of those things that is "easy" to add later. I think you'll find you won't miss it. I raced monohulls for many years and even on my old cruising monohull I used a vang. On my cat, I have never missed it.

As said previously, we're looking at many things from first principles, having decades of sailing/racing experience. You may well be right and go with the status quo on this, we'll be discussing with the builder and rig supplier to get their take.

One of the basic principles driving us is simplicity, the fewer systems the better and those that are on the boat should be as simple as possible. That's the reason for the question. I think (to be confirmed by an engineer) that on our boat, (light and easily driven) a vang system modelled on the 49er one would be much simpler and lighter than a full traveler. But I might be wrong and there may be very good reasons not to do it. Happy to be educated, and so far IMO the chafe issue is the best reason posted not to do it.

[Shrew]

Originally Posted by markiobe

BTW is it traveller or traveler?

It's like Color vs Colour.

Traveller: UK and Commonwealth countries

Traveler: US

[markiobe]

Originally Posted by chris mac

I added a vang to mine this year. For 3 reasons...
I wanted to rely less on the topping lift, allow it to be more slack since it chafed the old main sail(very roachy)
I wanted to be able to sheet the boom out farther when down wind and not have the main against the spreaders. The vang helps a bit, but it has increased our downwind speed
I wanted to add a Dutchman boom brake so I didn't need to go on deck to rig a preventer when down wind. It has worked fantastic for this
Another common reason for adding a vang is to rig up in mast, or in boom furling.

Interesting! Is it an above or below boom vang system? What's a Dutchman boom brake?

[markiobe]

Originally Posted by Shrew

It's like Color vs Colour.

Traveller: UK and Commonwealth countries

Traveler: US

In which case: 'traveller' is obviously correct

[SV_FlyingTigress]

https://dutchmar.com/dutchman-boom-brake/

[markiobe]

Cool!! I love the idea of being able to use it to reverse the boat

[Cwens]

Originally Posted by Shrew

It's like Color vs Colour.

Traveller: UK and Commonwealth countries

Traveler: US

Helpful mnemonic for another one I frequently got wrong:

In England, it's "grEy"; in America, it's "grAy"

[capn_billl]

I just had a detailed discussion with a US Spars engineer on this very topic.

A couple features on MY boat (Leopard-44), YMMV.

My Boom is around 8,000MM long, or nearly 30 ft. with a square top fully battened main sail that looks like half an acre from the helm.

I have a boom topping lift, run to a clutch, and a rigid what looks like a boomvang, but is just a fixed support to keep the boom from dropping to the deck if a tourist forgets the boom topping lift.

I was told by the enginer "under no circumstances should any obstruction or resistance be applied to the Vang".

The reason is the upward force of the mainsail is to great for the area under the goosneck to handle, and it will break.

The ONLY place that had enough leverage to counteract this force is the mainsheet at the end of the boom.

For example if the wind exerts 10,000 Newtons on the mainsail, which is tied at the Clew to the end of the boom, a boomvang attached to the boom 5 ft from the mast will have 30/5=Ratio of 6, = 60,000 TIMES the angle 1/SIN(5/2), yields 100,000lbs of sheer force at the point of connection.

That's bolt shearing, mast bending deck shredding, cable snapping levels of force.

With the center of effort at the middle of the boom, and the mainsheet at the end, you get less than HALF that force, or 3,500 lbs, divided by a 4 to one block, which is why we use 1/2 inch Dacron rated at 8,000lbs, instead of smaller rope for our mainsheets.

The force required to move a 30,000lb boat through the water at 10 knots is exerted on the frame by the gooseneck, the main halyard, and the mainsheet added together.

SO for a smaller boat, a boomvang is cool, I like them myself, but for a larger CondoCat, they simply aren't practical.

One big reason for the insane mainsail width, is the mast is moved way forward so it doesn't obstruct the Salon leaving a really long boom that runs 3/4's the length of the boat.

Which does cause some sail handling issues.

My suggestion for other boats is to call the manufacturer of your mast, and boom, and talk to their engineer before any modifications.

[Cwens]

Originally Posted by markiobe

You can actually achieve centreline sheeting with a vang, it's also dependent on the height of the mainsheet blocks.

Not to get all pedantic about edge cases and the geometry involved, but you can't in theory get centerline sheeting with the mainsheet block on the centerline, unless you can exert infinite tension on the sheet. In practice, with a short enough distance between the mainsheet block and the boom, you can get the boom "pretty damn close" (there's that technical geometry mumbo-jumbo creeping in) to centerline.

[belizesailor]

Some do, its just not common. Most cruising monos dont have them either.

[chris mac]

Originally Posted by markiobe

Interesting! Is it an above or below boom vang system? What's a Dutchman boom brake?

Dutchman boom brake is a system that slows down the boom during a jybe .
The pic shows our rigid vang. The brake is behind the wind vane.
And yes I know that's a terrible place for a wind vane. It was a temp fix until we dropped the mast and rewired this summer[emoji16]

[AZUS]

Antares catamarans have a vang as part of the standard kit, for just the reasons you mention.