Why use mainsheet systems described as 5:1, 7:1, or 9:1 rather than 4:1, 6:1 or 8:1?

These systems are quite common (see attached picture as example).

However, since it is only the blocks on the object which is moved (i.e. the boom) which contributes to increasing the mechanical advantage, these systems are actually 4:1, 6:1 and 8:1, respectively.

All systems which terminate the fixed end of the sheet line on the boom necessitates an extra length of rope and a block, without increasing the mechanical advantage. Hence, even if the sheet line must be pulled 5 inches for each inch movement of the boom (5:1), the mechanical advantage is 4:1.

Please correct if I am wrong!
I assume that the boom must be considered the moved object and the block on the traveller the fixed point (I know the theory of relativity, however in the moderate speeds obtained in sailing, I assume Newtonian mechanics and empirical measurements may be trusted).

Stein

I'm afraid you are incorrect. Each fall of line contributes to the overall mechanical advantage of the system regardless of whether it terminates on the "fixed" or the "moving" end (in fact, the two are moving relative to one another, so they are both moving). But regardless, a simple test: flip your blocks upside-down. You will find that you get exactly the same purcahse with the blocks in either orientation.

sm

Those blocks listed as 8:1 are 8:1. Do a load diagram...if you pull 8 inches and get 1 inch of movement than it is absolutely 8:1 - no way around it.

In hindsight, I put these block load arrows pointing the wrong way for a static diagram - but I think you get the point.






Just wait until you try to sort out a cascading system.

picture

You must both be right; your explantions seem highly plausible. It must be correct that advantage depends on ratio of (rope length pulled):(distance moving of boom).
My simple living-room experiment must be inaccuarate. And the general rule that only blocks on the object moved contribute to advantage, must be incorrect.

Thank you!

Stein

PS: I stick with my 10:1 setup for my Taipan 5.7, though.

Yeah - I think you are getting conturbulated thinking that the kinetic system (moving) is different than the static. All loads must be equal. To quicly determine the purchase of a (non-cascading) system, you can count the running lines between the two stationary points. 2 lines is 2:1, 3 lines is 3:1, 4....and so on.

If you have a cascading system, you separate the two systems and multiply them against each other. A lower block setup with 4 runs at 4:1 that attaches to a cascading system with 2 runs at 2:1 is a total of 8:1 purchase.

So just for practice, this one is 2 x 4 = 8, correct?

It looks like the black is a cascade doubler and the red on the left is 4:1.
The red-right is ineffective, turning only.

Assuming that the black and the red lines that just end at the boom are simply attached to the boom - yup. 8:1...the question is though....WHY all the complication?

I think is was Rolf that posted this as a Tornado setup. Why, you ask? Small diameter lines through the blocks, I'd guess. No taper needed. Maybe less total weight by using small blocks? Hidden mechanisms keeps the rookies staring? Easy to hide an illegal 10 or 12 setup? <img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" />

This drawing was done by Macca, I think.
It shows a low-friction, lightweight and very clean setup used on many top-notch Tornados.

There are no rules on how much purchase you can have on any sheet on the Tornado. Would be a silly rule in my opinion.

But this one has another line contributing to total boom movement...the black line on the extreme left. So won't is be 8:1 + 1:1 = 9:1?

Correct !

Wouter

Kind of like "Where's Waldo?"....I missed one.
The biggest, darkest line on the page!

9:1 is is!

I've been wanting to setup this exact system on my T...but I have not zero'd in on a double-block that is slim enough to fit into the Marstrom boom section. Plus, I have an outhaul system running inside already (that I quite like & use), so it would need to be removed/replaced with some other setup.

Right now I run a traditional 57mm harken carbomatic triple lower unit up to two carbo doubles (one with becket) hanging off the boom with spectra. I also put a 40mm single on the lower unit & run the tail back to the upper double's becket. Gives 9:1. Works pretty good.

The advantages are:
1. much lighter overall (blocks & 3mm vectran line)
2. Excellent easing (cascades are generally lower friction)
3. Lower cost can be achieved since blocks are small & cheap compared to 57 mm Harken Quads etc. (though I've seen several version of this running custom made blocks with carbon side plates...$$$)

Some disadvantages are:
1. can be a headache to work out
2. Can jam up inside boom if not setup right
3. Maintainance more difficult


Lots of top T guys run this setup, but it is not required to get to the top of fleet...Charlie Ogletree & Johonny Lovell run standard harken quads <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />

I have often thought this wold be a good way to go. I do not understand what turns the section where in the diagram the line goes from red to black... ??

some pics, someone can link them so they appear in the posts but i have never worked that part out <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

This pic is the turning block at the front of the boom, the thin rope turns around this block and attaches to the floating block in the next pic.

this is the floating block, we have a new systems that allows us to change from 9:1 to 12:1 in seconds, but more on that when i put the boat back together and take pics. way to complex to explain here <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

Many thanks for the photos as they will prove very useful to me. What I am having trouble understanding is how the larger sheet exits the boom at the sharp angle. is there a through block there?

Okay, have understood. : (

I make 8:1 + 1:1 = 9:2 [Same as 8:1]

My money is still on 8:1


Paul

right result; wrong maths

it's is actually a 8:1, with a 1:1 on it, so

8:1:1 is a 8:1; 9:2 is 4.5:1

Yes, definitely 9:1.

For anyone who doubts: 1 lb of tension on the sheet (black line) leads to 2lbs of tension on the red lines, which is obviously 9lbs of tension between the boom and main traveller, or 9:1.

Yes, and there are 4 red lines in the moving part of the system. (up and down from the boom to the back beam (well the wire to the back beam)

8:1

You have a 2:1 on the Black line system, the black line system is then pulling on a 4:1; 2:1:4:1 = 8:1

It is a 9:1 system guys.

No doubt about it.

Wouter

Agreed and I stand corrected - that is 9:1

I've had another look at this and think thus

Red system is a 4:1.

The black system is a 3:1 as the both blocks (2 and 3) on the boom are (in effect) moving in relation to the back beam.

As you pull the mainsheet in, the first block on the boom(2) is also moving down, as is the big block inside the boom(3), so the black system is a 3:1, making the total system a 12:1.

I cannot see how it can be a 9:1 as the internal cascade is 4:1 and so any multiplier must result in an even number unless there is just a "routing" block, which there are none as all the "big/black" blocks are moving except the back beam block.

Try this

Okay, let me explain how it works.

But first lets derive the tension in each line because we'll need that later.

Black line is held in hand and so all the black lines are under 1 unit tension; the pull of the skipper.

The red line is attached to the black line by an 1:2 purchase system so the tension in the red line is 2 units.

It is impossible for any line to have different tensions at different points. Think about this.

When a line makes a full 180 wrap around a block then it pulls 2 times the line tension downward, that is obvious. If a line makes a 90 wrap then the block is pulled down by 1 times the line tension. A termination point equal 1 times the line tension downward. All still pretty straight forward.

Now lets count the loads when looking only at the boom blocks, the others are unimportant.

Black line : about 1 partial wrap so roughly 1 unit down pull and nothing else

red line : 1 full wrap, 1 partial wrap and a termination point ; total = 4 times the line tension of 2 units = 8 units of downward pull

Add them all up and you get (by approximation) 9 units of downward pull for each single unit of pull by the skipper, ergo it is a 1:9 system.


And this is one of the few ways to do it right.

Wouter

Wouter,

You can't keep writing purchases a 1:x unless you are talking about an inverse purchase system. The standard nomenclature has the advantage shown FIRST.

8:1, 4:1, 6:1, 12:1 etc.

by writing 1:9 you are in effect saying that for every 1lb of effort applied at the load you need to apply 9lbs - usually bloody hard work!!!

Look, whatever the purchase is, its not enough for the new carbon mast on the T, so we have now a system with more purchase, by making the floating block inside the boom a 3:1 system.

We then run either 4:1 or 3:1 on the small blocks at the back, the change is achieved by dropping the last block in or out of the system depending on what you want at the time.

Here you go mate...... I must spend too much time doing this $hit <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" />

Dependents on which view point you have.

You can view it from a force points of view or from travelled distance point of view.

Force : 1 lbs effort => 9 lbs end result
distance : 9 units distances pulled => 1 unit distance travelled by boom

And there are many more, and several have to be called 9:1 and others 1:9

So I just the beach talk version that is fashion of here and that puts the 1 in front mostly because in Dutch it just flows betters. I think it does so too in English.


Wouter

Sorry Wouter gotta disagree here. In the UK, (IMO) people will always use the multiplier first.

Back to this multi purchase...

in the picture I agree that up to the last block on the boom, there is a an 8:1 system.

But I do not understand where "only" an extra 1 comes from.

The block is moving down, and there is a (at least a 8:1) moving over this block, BUT as this block is moving itself there MUST (I think) be some multiplyer on the rest of the system. I agree it cannot be 2 as there is not a 180 degree turn, but I cannot see how it can be 1 as the block on the boom is moving.

The "count the moving blocks" thing will only work on a standard system of blocks. The key issue here is that the black rope does not come back down to the beam. Instead, it feeds into a second system. If you removed that system and simply anchored it in the boom you'd have a 1:1 system.

I find these things easier to look at in terms of rope moved. What does it take to move the boom 1 unit down? Well, if you remove the final block and imagine yourself sitting on the boom, it's pretty obviously 8 units. Now sit back on the boat and use that final block. How much more rope do you have to pull to shift the boom that 1 unit? 1 unit, so you've pulled 9 units, so it's 9:1.

It's not going to be a multiplier because it's not a cascade. There's only one floating block in the system and that's the one that gives you the 2 x 4 = 8 : 1.

Paul

Although my first proposal on 3:1 and 5:1 was found incorrect, I cannot resist commenting on the "Macca-system".
The confusing point here is the 90 degree turn to put blocks inside the boom. If you straighten out the system, you find a 4:1 compund system and a 2:1 cascade. Total = 8:1 (Or 1:8 if you are the boom).
For cascades, you cannot simply count number of line segments.

Seriously:
May I propose that you publish mainsheet setups that you really like here. This would be most useful to all of us.

Macca has provided an exellent example.

Stein

Nope. If you straighten it out and anchor the "static" turning block (the one originally at the far right) to the boat not the boom (e.g. tie it to a cable from the top of the mast) then you'd be nearly right. In fact, you'd only get 7:1 because the black rope is tied off to the boom not the deck. Fix that and you do finally get to a standard 2 x 4 cascade.

Straightening out the system doesn't change it, and either way you can't get away from the fact that the cascade is anchored in the moving boom, which means you have to move an extra unit of rope.

Paul

Ok, here is the new system, you can change the purchase really easily by moving the red line from terminating on the boom to terminating at the bottom block on the bottom of the system. we call the min purchase 9:1 anf the max purchase 12:1 but I am sure you all will find 500 other ways to calc it out <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

Here is a simple geometric explanation:

With the sheet taught, lift the boom, pulling the 4:1 blocks 1" further apart. This pulls 4" of the red line into the 4:1 system, moving the floating block 4", and pulling 8" of the black line into the end of the boom. However, since the end of the boom moved 1" away, this pulls 9" of line through the main sheet cleat.

So, it's 9:1, with the "extra 1" coming from the boom movement as you sheet.

--

Here's another simple geometric explanation:

Imagine you had a conventional 8:1 rigged upside down like this:

     BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB Boom
       /O
      /4 4
     / 4 4
       4 4
        O

and then you take the line at the end and run it through the bottom set of blocks like this:

     BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB Boom
        O
       5 4
       5 4
        O______

Suddenly, it is 9:1. It's no different for the cascaded system. That's where the "extra 1" comes from.

Holy Carp! That must be an expensive system.

Price aside, can you tell me why you went with the Karver blocks instead of, say, a floating Harken 2651 40mm Ti-Lite Single, a Ronstan 20151A or 30151A cheek block forward, and a smaller diameter secondary line? That's what I've been thinking of using.

Of course, we all wonder what blocks you use in the aft end.

Are you using a boom-external outhaul with this setup?

Thanks for the pics!

Harken carbo blocks and all Ronstan blocks are [censored].... they simply explode in the sort of application.

I will take pics of the rear end of the system today and post shortly.

Karver blocks are awesome and never stuff up.

We use Harken Wire Sheaves in the small rope sections, 980kg working load means they hold together pretty good.

Yeah, it's definitely 10:1 and 13:1 system. :-P

The funny thing is: some people told me that the "8:1" cascaded systems are easier to sheet than the 8:1 conventional systems. Well, of course they are, since they are actually 9:1.

When I floated this same design some time ago on catsailor, I was told that cascades had fallen out of favor, as too unreliable, having limitted travel, and requiring lots of line. Has this changed due to the stiffer carbon mast? Have the downhauls also changed? On my old T, I kinda like my 7:1 downwind. I can't imagine what 13:1 would be like!

At Nationals last year, I did notice John and Charlie with a 8:1 + untapered salsa line + carbon mast on their boat. But as I get older, I do start dreaming about 13:1.

You could still use the 8:1 and be fine but we like the ability to trap flat on the wire and play sheet without using back and shoulders (sit up a bit to sheet) We use the 9: mode when its windy as the sheet loads are actually a little lower then compared to 8-14kts when you have heaps of sheet load.

The downhauls are tending towards 16:1 but some still have 8:1

Since I have a boom in my garage awaiting conversion (Thanks Dan!) I would like to pick your brain a bit. If you have the time, please tell me if I'm making sense here.



I've heard people curse Ronstan blocks on this forum before, though I've never seen a failure first hand. Do these blocks really not live up to their SWL ratings? Or do people just not understand that the SWL rating needs to be 2x the line load for a 180 degree turn?

I don't understand why the Harkens would have any trouble in the primary section, where the sheeting forces should be slightly less than in a conventional 8:1 setup. However, they obviously cannot carry the loads in the secondary.

Since the strongest Harken small boat ball bearing traveller car is rated at 1250lbs MWL, I've been assuming main sheet total load does not exceed that. Based on this assumption, the configuration with a cascaded 3:1 primary with 3:1 secondary sees the highest loads: the secondary line has 375lbs of tension, meaning you need a block rated at 750lbs at the front of the boom to carry the 180 degree turn. This is above the SWL of Ronstan and Harken ball bearing blocks, but not above the Karver's.
However, the floating block needs only a 375lb SWL rating, which is well within the 485lb SWL of the economical Harken 2651.

Cascades with a 4:1 secondary see maximum line loads of 300lbs in the secondary, requiring blocks with ratings of at most 600lbs SWL. This falls withing the rating of most of the Ronstan 30mm ball bearing series, so I would expect them to work with a 4:1 secondary. Harken ball bearing blocks cannot handle the loads.



Thanks! Thanks a lot!



That's good to know. Thanks. They are still ball bearing blocks though, which makes we wonder:

What about the Marstrom boom-internal system with needle bearings. Why have I never seen that system actually used (except I have seen the similar system on the Marstrom 20)? It is an option on the Tornado order form.



Aren't these plain bearings (not ball bears and not needle bearings)? Don't they turn less freely? Does this seem to be an issue?

Wow: there are lots of questions in my mind. Thanks in advance if you can answer any of them.

You make perfect sense explaining the advantage of purchase upwind.

Do you do the Boyer thing and set the traveller and play the sheet downwind to moderate hull flight (in addition to steering)? Or do you play the traveller and steer? Or do you just steer?

I ask because I do the Boyer thing, and 13:1 would make this sheeting much less effective. (And 'cause I want to know what technique you use!) Then again, since you fall back to 10:1 when the wind (and puffs) pick up, I guess it's not all that big of a difference in the rough stuff.

Thanks once again,

If you exceed the SWL of the Karver blocks, they break. Which is OK, as long as you know it. Only heard good things about them.

The Ronstan stuff does fail in an internal boom, we have used the friedrecksen wire blocks and they blow up after a few days sailing, they are rated high enough (cant remember exactly) but they just dont last. The Harken carbo blocks blow up as well even when used on the big rope section.

The Harken wire blocks are very well made and have not failed us yet, most guys use them for internal booms.

The Marstrom (playskool) blocks are good when you first get them but they require a lot of maintenance and they are also prone to breaking the side of the block surface off.... we had this at the europeans and it really screwed our day. I am now running the 51mm Harken wire sheaves and they have less friction than the Marstrom blocks and are indestructable.

attached is a pic of the rear of my boom, more pics to follow.

the end of the boom showing outhaul etc

shot shows the whole of the rear part of the boom and sheet system

Macca, while we have your attention..
How do much faster do you think/feel the Tornado is with the carbo mast? I notice that the rating has not changed under our Scandinavian handicap system, but the boat must be faster now.

I personally did blow one Ronstan block in my 10 years of sailing. It was one of the first series ratchet blocks. It had an issue that was recognized by Ronstan and they redesigned their ratchet blocks. They replaced my old block (broken) with a new one.

All the other harken blocks seem just fine to me. Same for Harken. I do have some misgivings about the way Ronstan designs their mainsheet systems but that is about it.

I'm using small Ronstan blocks in my cascading downhaul system and they have held up perfectly over the last three years of sailing. It is a 12:1 systems and blocks are loaded up passed their SWL.

Having said all this I think that my boats do tend to see lower loads then Tornado's or Andrew's (Macca) Super Taipan 5.7. Maybe that is the reason Ronstan fittings are just fine from my perspective.

Wouter

Then maybe it is the non-ideal alignment of the lines/blocks in the internal boom setup that is damaging the Ronstan and Harken blocks. I did find that the "plastic" blocks of both makes really do not like skewed loading or skewed line entries/exists. That tends to eat away at the plastic sides and eventually break the block. Is that what happened with the blocks Macca ?

Personally I'm interested to have a cascading mainsheet setup inside my boot like you, but I don't have big bucks for blocks like you have. So that is why I'm asking, I need to build it using cheap blocks.

Wouter

Macca why is that a line through the side of the boom wall and the orange block ?

Wouter

Macca,

What is that small diameter grey line in front of the mainsheet that is still attached to the bottom mainsheet block ? The one with its own 2:1 purchase. Is that linked up to your mastrotation setup or possible the downhaul ?

Wouter

It is a 13:1 system.



Well the claim is still true when both are true 8:1 systems. There is just less friction in the system. A similar effect can be had by tapering the sheet. And that is what many are using now over cascaded systems. Cascades are still often used as downhaul systems as tapering is less practical here.


Wouter

There is some torsion on the rear block system, the loads are skewed to one side of the blocks a little more than the other. but its pretty minimal and is no more than you would expect from a system that has these loads and hence these blocks.

The blue line through the orange block is there to stop the mainsheet ever popping off the sheave and to stop the block spinning in the boom. it is only held in place by a bolt passing right through the centre of the Karver block.

The small diameter line on its own 2:1 system is a sheeting gauge, it has a shock chord attached to the line and then to the front of the boom, there is a scale marked on the boom about a foot from the font, so you can see how tight the sheet is without looking to the back of the boat. Euan (my crew) wanted it and it doesn't bother me so we did it.

I think the T with the carbon mast is quicker but it is very marginal in most conditions. I think it would be impossible to give it a hard number in terms of increased performance. Certainly in big wind and waves it is much nicer to sail and is for sure quicker. The change to the carbon mast was never for performance, more for consistency in rigs. in this regard it is perfect.

Thanks for the pics. I admire your handiwork. Very nice.

That looks like an outhaul set with stopper knots? Yes, I have wondered about the value of messing with the outhaul at mark roundings with spinnaker. I see you've made up your mind!

Again, thanks for the pics.

Since you're talking F16 loads, you might want to try a cheap and elegant 9:1 setup I devised. I have a detailed sketch somewhere. I will try to find it. It's pretty easy to describe, though, and works with narrow extrusions:

* A harken sheave for 3/8 line at the aft boom end, supported by a through bolt and spacers. (Build-your-own Karver.)
* A harken 2651 airblock floating block.
* A hole near the aft bottom of the boom, to act as a becket for the end of the primary sheet.
* A Ronstan 30151 cheek block at the front inside the mast.
* A pair of Ronstan 30151 cheek blocks mounted on the outside of the boom, to act as the upper blocks of the 4:1.
* A small hole mid-boom for the secondary line to pass from the inside of the boom to the outside. The hole should be placed in-line with the tensioned line to prevent wear.
* An eye secured to the end of the through bolt to act as a becket to terminate the 4:1.

If you place the blocks intelligently, this system reaves beautifully, and there is *zero* torque on the boom arounds its length, nor any significant block side loading when sheeted tight. The trick is to put the internal cheek block, pass-through, and eye/becket all on the same side of the boom.

You can *eliminate* out-of-line loads on the Ronstan blocks by using a pair of singles at the bottom, so they align themselves with the loads, and by sizing the blocks for the bottom of the 4:1 that have diameters matching the width of the blocks at the top, so the top blocks are not side-loaded. Finally, note that the cheek block on the hole/becket side of the boom must be mounted at 45 degrees (to suport the 90 degree line wrap) and should be mounted forward of the opposite block.

Finally, Do not use a long pigtail below the 4:1 to "save weight". If you sized the secondary line properly, you won't save weight by doing so, and lengthening the secondary 4:1 will minimize block side loads and extend the life of the system.

It's the simple, cheap, culmination of all my thoughts on what I wanted to do on my boat up until last week.

I would be very interested in your diagram

Wouter

Macca,

Thank you for sharing ideas and valuable experience!

Are these systems heavy?
You need a stronger boom tube (than systems attached directly to the clew) plus the bolts going through the sheaves looks like heavy-duty.

Do use a similar system on your super-Taipan?

Stein

The internal system is ligther overall (including the boom) than a normal external system.

The boom is a normal section in weight, you could go smaller and lighter but then you would be forced to use an external system and thus be heavier overall.

Super Taipan is currently an external system until I make an all carbon internal boom <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

Here's the diagram, redrawn. There is a higher resolution version attached.



Please critique.

The major things I like about the design are:

• Costs about US$100, assuming you have a boom, a 3/8" mainsheet, and hexa-cat base already.
• No overloaded blocks, assuming 1250 lb total load.
• Balanced loads (no twist).
• Relatively simple construction.

My only [minor] concern about the sketch is the block at the front of the boom. When the mainsheet is eased to the limit, the load from the primary to this block will be imperfectly aligned. This could be fixed by replacing the cheek block with a single on a very short pigtail (but still installed touching the wall of the boom extrusion to minimize wear that the pass-thru hole).

Enjoy,

Macca,

The weight of our Taipan 5.7 external 10:1 mainsheet system is 3.3 kg (excluding the mainsheet line itself).

This includes the aluminium boom + all 10:1 system mainsheet blocks (including centre-sheeting).

How much do you gain by changing to an internal system?

By the way, do you contruct your new super-Taipan boom from off-the shelf carbon tube?

I will weigh the systems that i have and post over the weekend. the internal sheeting system on the Tornado is a little bit lighter then the external (carbon boom) system on the Taipan.

The new internal boom for the Taipan will be from std carbon section, looks like it will be an eliptical section from CST.

Another gain of the internal system is windage, it is much more streamlined.