TOPIC:

I intend to fit an adjustable rig to my boat and have checked out various set ups .The most suitable system for my boat layout is purchase blocks above the gunwale connected to further blocks running between the hog at the mast step and under the foredeck. The system I have seen, which I know works well, uses a quad and triple block above the gunwale. My question for all the engineering brains out there is, if this was reduced to a triple and double block would it greatly impair the mechanics of the set up?

Hi all



If the above system is correct then you could add a "rough" tune by adding an extension off the dead end of either of the 2:1 sections to a cleat for adjustment. This arrangement would negate the need for the above deck throw off levers, it would also require less strength and possibly less “string” in the fine tune.



However a word of caution it may be difficult to accurately calibrate.



Derek 7176

Derek Gibbon2008-12-20 19:33:45

Phil. From what you describe it is 12:1 on the shrouds. I think this is more or less what you are describing:

2:1 on 2:1 on 3:1 gives 12:1
John

JohnW2008-12-10 13:50:32

Hi All,



I have a 2:1 cascade above the deck with a high load block. It then goes through the deck via a thru deck block with almost no friction. I then goes thru another high tension block bolted to the quad supports area. It then had a another high tension block on the end.



From this there is 2:1 another cascade that goes from the back of my mast foot plate and thru the end of the 1st cascade. The 2nd cascade has a single + becket block attached to it.




The third cascade involves the connection of the becket from the 2nd cascade which then goes thru a double block attached to the mast foot plate, back up thru the block on the end of the 2nd cascade and then back down thru the 2nd part of the double block connected to the mast foot plate, this is then lead back to the back of the centre board case thru another block and cleat.



I also have levers. I think this gives me an 8:1 ( I think) purchase on each shroud.



This involves 14 blocks and 2 cleats.



For the jib I use a double and triple block.



Hope this helps.



Phil

A6504"Nemesis"

Its important to understand the difference between mechanical advantage and velocity ratio.
Mechanical advantage is simply how much easier is it to pull on the rope. Velocity ratio is how much rope you have pull compared with how much the thing you are pulling moves. They are only the same when there is zero friction.
A 4 way block and a 3 way block you mention will give you a velocity ratio of 8:1 (or 7:1 depending where the take off is), the triple and double will give 6:1 (or 5:1) - count the strings to get the velocity ratio, but ignore the last one if it comes from the fixed pulley.



However the mechanical advantage or purchase is less than this due to friction in the system. This is why cascaded systems are popular - there are fewer pulleys in the system for the same velocity ratio, which means less friction and therefore the mechanical advantage is greater. And as a bonus it costs less in blocks (but the first one in the cascade has to be able to take the full load so is often a higher specification). For example you can get 8:1 velocity ratio with a 3 and a 4 way block or with just three single blocks.
The down side of cascade systems is that they can have restricted range. Every pulley in the system has to move twice as far as the previous one in the cascade. This has resulted in hybrid systems where a multi-block arrangement is put on the end of a cascade.
This diagram shows three configurations that all give a 8:1 velocity ratio. The cascade in the middle will give the best mechanical advantage using just three single blocks.


[edit: the left diagram is actually 9:1 (9 strings) as I have drawn it, but the principle is there].
I would not recommend putting a multi purchase system above the deck as there is more to get tangled up in it as well as more windage. I would recommend that if you have any of the system above the deck, that it is the first stage of a cascade.
[JW:edited to correct errors]
JohnW2008-12-09 01:06:16

Thanks Derek,I was asleep most of the time during "O" Level physics, this,I am sure will help me understand what I am trying to acheive.

This is am interesting discussion, and in order to understand the physics behind the ratios (long time ago since I did "O" level physics) I looked up some info. that seems to set it out clearly and may interest others at:



en.wikipedia.org/wiki/Mechanical_advantage





Derek

Thanks for the info Derek, it was the block ratio I was interested in so thanks for that.

Hi Gordon



I suppose there are a number of issues, firstly how much distance do you need, in terms of lengthening the shroud and how many purchases will you be putting under the deck. In your question you compare two block configurations, the first achieves an 8:1 the second a 6:1, depending on what you are considering below the deck will obviously multiply this up.



The second issue is the distance you are prepared to pull through to achieve the desired shroud distance; also does the "puller" have enough strength to achieve the necessary poundage on the shrouds.



Thirdly do you wish to include throw off levers to act as rough tune in the system.



You need to decide whether you wish wire or "rope" going through the deck, the position of the anchor points, how many “ropes” are going through how many “holes” in the deck.



Whatever you decide make sure that you reduce friction as much as possible and all direction pulls are as “clean” as possible. Once all of the above is taken account, you may still wish to "talk" to someone who has this type of set-up.







Derek Gibbon 7176

Saw a CVRDA FD at the Earls Court show that was using Holt Allen drum winches (the sort that used to be used for the kicker) for adjustable rig. A HA drum winch gives about 10:1 advantage with low friction, and loaded with 3mm Dynameea on the spindle and double braid on the drum can easily give you over 50cm travel. I've got one controlling my steel centreplate. *AND* they are compact. The FD had a 2:1 whip on the shroud leading to the drum spindle under the deck, but an Alb would surely be managable at 10:1? They are a bit rare now, but it would be easy enough to build some from scratch as all they are is a hollow stainless shaft with a drum with a core 10 x shaft diameter on one end running in a hole through a stainless U bracket, a cross hole in the far ewnd for a washer and split pin and a *nicely* radiussed hole to feed the wire or dynameea end into the centre and knot it or crimp a ferrule on to retain it.



You need to keep a little tension on the control line to take up any slack or have a smooth, springy plastic 'wiper' finger keeping it neatly on the drum.