Vang purchase

[PabloPicasso]

I'm not clear how the theory works on the purchase/advantage gained by adding extra blocks or even a cascade to a kicker/vang

So if the kicker line goes around 4 sheaves (2static/2moving) is that considered 2:1 or 4:1

If I add an additional sheave via a cascade does that double the advantage to 8:1?

 

[PaulRainbow]

I'm not clear how the theory works on the purchase/advantage gained by adding extra blocks or even a cascade to a kicker/vang

So if the kicker line goes around 4 sheaves (2static/2moving) is that considered 2:1 or 4:1

4:1

If I add an additional sheave via a cascade does that double the advantage to 8:1?

If you had a 4:1 tackle and added a 2:1 cascade the result would be 2x4 = 8:1

 

[PetiteFleur]

I had the standard 3:1 system and added a cascade system it's 12:1 - very quick and easy to use with a cam cleat

 

[PabloPicasso]

4:1



If you had a 4:1 tackle and added a 2:1 cascade the result would be 2x4 = 8:1

So 6 sheaves (3 static 3 moving) = 6:1
Add an extra cascade sheave means 12:1?

Did I get that right?

 

[PaulRainbow]

So 6 sheaves (3 static 3 moving) = 6:1
Add an extra cascade sheave means 12:1?

Did I get that right?

6:1 is correct. You'd normally add another tackle for the cascade, say you added two blocks, that gives you 2:1 x 6:1 = 12:1.

Some images here might help you picture it : http://www.harken.co.uk/content.aspx?id=3897

 

[William_H]

The efficacy of a vang depends a lot on the height you have between the cabin top and the boom as this distance relates to the length of the boom. A small distance height here as on many cabin keel boats means that if the vang sits at 45 degrees to the boom you are pulling on a point not far from the gooseneck. Move the vang attach further out to the end of the boom the angle means you are pulling the boom forward rather than down. Move vang attach forward and you have less leverage to the clew of the sail. So on a dinghy with keel stepped mast and vang attached to the keel you can get a lot of power but not much on a high cabin KB.
On the other hand with a wide traveller on a KB you can get a lot of pull boom down power from the main sheet even with traveller out and boom fairly wide to centre line. So vang not so important to sail shape control.
When running down wind however a vang can be useful to keep the boom down to stop inadvertent gybe.
In this case not so much purchase of the vang system is necessary because the friction of the system willprovide the hold down as opposed to pull down.
So OP does not mention type of boat or relative dimensions but I am guessing he may not get much benefit from cascade vang system. olewill

 

[Daydream believer]

When running down wind however a vang can be useful to keep the boom down to stop inadvertent gybe.

Can you expand on that a little more please

 

[ex-Gladys]

Can you expand on that a little more please

It's a bit more complex than that... take the extreme case of no kicker (sorry we're in the UK, the yanks call it a boom vang...) down wind, there is little or no downward pressure on the boom when broad reaching and running, this mean the boom lifts by the wind pressure on the main, and the leech will curve forward, until at some point the effort through the sail starts to point to windward, and starts a roll. If you want to try it for yourself, take a Laser out in a bit of breeze, and just let the main out a bit too far, and you'll suicide roll in to windward...

So the point of the kicker is to keep the main in control by keeping the leech fairly tight, once a roll has started, the boat will be trying to steer itself away from the "lower" side of the roll, and as this develops it would be extremely difficult to be concentrating enough to be certain to avoid an accidental gybe.

The kicker also helps prevent the classic definition of a Chinese Gybe happening. In the days before kickers, as the boom lifted it would be possible for part of the main to gybe, whilst the other part stayed on the original side. https://en.wikipedia.org/wiki/Chinese_gybe

Of course, the reference earlier to travellers is fine, so long as you have a wide vessel. Most catamarans do not have a kicker because the main traveller is as wide as the boat, and therefore, you don't ease the mainsheet as you bear away, you ease the traveller

PS... the reason for lots of purchase is to make it easier to adjust the kicker, which also has a significant impact on sail shape

 

[Daydream believer]

So it does not prevent the gybe at all. It prevents a roll to windward if the leech is allowed to go ahead of the mast.
What causes the accidental gybe is the helmsman not steering a straight course

 

[PabloPicasso]

So it does not prevent the gybe at all. It prevents a roll to windward if the leech is allowed to go ahead of the mast.
What causes the accidental gybe is the helmsman not steering a straight course

Thread drift

This should be a new thread

 

[PabloPicasso]

Would A lever on the vang give a greater purchase? I hadn't thought of that and have a lever tucked away somewhere

 

[Daydream believer]

Would sing a over on the vang give a greater purchase? I hadn't thought of that and have a lever tucked away somewhere

??????

 

[PabloPicasso]

??????

A lever type kicker I meant. Bloomin predictive text is make it up ain't it.

Would a lever give greater purchase in the limited space between cabin roof and boom.
Or I could bring the kicker back to a winch

 

[Amulet]

I have a 3-1 one and 3-1 cascade giving 9-1 on my wee boat. Too much I think. I pulled a lump out of the bottom of the mast. Broke the weld on a pad eye etc.

In a cruising situation you really aren't tweaking the kicker all the time. You can have a modest purchase, head up to wind, crank the boom down with the mainsheet, make up your kicker hard, and then bear off again.

 

[William_H]

Can you expand on that a little more please

What Gladys has written is probably right however not an effect I have noticed. However with a vang (kicker) holding the boom down the leach tends to be closer to straight. This means that with the sail well out you can sail further by the lee before the boom will come across. Giving you a much wider margin of safety when running. " by the lee" is the term for sailing with the wind coming from the side the boom (or jib) is on, Obviously only by a small degree. olewill

 

[Daydream believer]

A lever type kicker I meant. Bloomin predictive text is make it up ain't it.

Would a lever give greater purchase in the limited space between cabin roof and boom.
Or I could bring the kicker back to a winch

A lever would not give the range of adjustment you need especially when you take in to account reefing. If you carry a spinnaker & wanted to release the kicker in a broach then it would not let enough line out to release enough to slacken the leech. ( In a broach one way to help get a boat back on track is to let the main out & let the kicker right off so the boom rides up & lets the wind right out of the sail)
If you take the vang back to a winch to get the required pull ( because you did not have enough falls in the system) that suggests that you will be putting quite a load on the final fall & then you would have to either leave the fall on the winch, thus tying up the winch, or put it into a stopper ( expensive). You could not put it it into a simple cam cleat as the load would be too great for the cleat & the line would probably slip .