Pulley ratios.

General chat about boats
User avatar
trebor
Posts: 956
Joined: Mon Jan 02, 2012 1:53 pm
Location: West Midlands
Contact:

Pulley ratios.

Post by trebor » Thu Jan 22, 2015 11:43 pm

I have tried a search but no luck, how do you work out the ratios for pulleys ?
For instance what is the ratio of this arrangement?
http://s1149.photobucket.com/user/aquab ... ort=3&o=37
Robert
Minisprint 4230
Tinker Traveller 160
Mirror 61147 Anastasia
http://www.aquabatdinghy.co.uk

Rupert
Posts: 6224
Joined: Thu Sep 16, 2004 8:40 pm
Location: Cotswold Water Park

Re: Pulley ratios.

Post by Rupert » Thu Jan 22, 2015 11:47 pm

In that case, count the strings.

On a cascade system, each block doubles it.
Rupert

dronskiuk
Posts: 179
Joined: Thu Nov 15, 2012 3:23 pm
Location: Where Broadland meets the sea

Re: Pulley ratios.

Post by dronskiuk » Fri Jan 23, 2015 10:51 am

Or to elaborate a little....the 'strings' between the pulleys not including the rope to hand thus making this 3:1

Were you to add a 2:1 to the 'rope to hand' part then this would double the 'purchase' making it 6:1.

I once had a ridiculous kicker system on my Enterprise that utilised a 7:1 lever bar led via a 6:1 purchase to a drum winch the take off of which was then split to go to either side....brilliant in a blow but a lot of friction when mellow! I can't remember the final purchase but it almost took a calculator to work out.

Rupert
Posts: 6224
Joined: Thu Sep 16, 2004 8:40 pm
Location: Cotswold Water Park

Re: Pulley ratios.

Post by Rupert » Fri Jan 23, 2015 11:46 am

Ah, drums... I never did get the hang of ratios on them - I guess it depended on the size of the drum compared to the axle where the wire wraps. Horrible things, though I can't help feeling they could make a comeback with modern materials.

Sorry, yes, late night answer to begin with - thank you for the elaboration!
Rupert

User avatar
PeterV
Posts: 1168
Joined: Thu Sep 16, 2004 8:47 pm
Location: Locks Heath, Hampshire

Re: Pulley ratios.

Post by PeterV » Fri Jan 23, 2015 1:51 pm

You cannot just count the strings. A pulley block attached to the ceiling to lift a weight has 2 strings, but gives no mechanical advantage. A pulley block attached to a weight gives a 2:1 advantage when you lift the weight, and still has 2 strings. The difference is that some pulleys just turn the rope to a more convenient position, others give mechanical advantage. The pulleys which are attached to the moving part all give mechanical advantage, so if you count the strings at the moving pulley you will get the mechanical advantage. (Striclty speaking you'll get the velocity ratio and the mechanical advantage is this less friction losses).
This used to be O level Physics, not sure if it's now in GCSE.
PeterV
Finn K197 & GBR564
Warsash

User avatar
neil
Site Admin
Posts: 1610
Joined: Tue Aug 31, 2004 10:33 pm
Location: Plymouth

Re: Pulley ratios.

Post by neil » Fri Jan 23, 2015 1:59 pm

If you can find it, Chisnell's Dinghy Systems is a useful source for this type of stuff

http://www.amazon.co.uk/Dinghy-Systems- ... 1853103926
IC: K26
Harrier +: 2

Rupert
Posts: 6224
Joined: Thu Sep 16, 2004 8:40 pm
Location: Cotswold Water Park

Re: Pulley ratios.

Post by Rupert » Fri Jan 23, 2015 3:00 pm

Peter, in the block in the ceiling scenario you mention, the 2nd string is the same as the string coming to your hand from the last block, so the only counting string is the one going to the ceiling.
Rupert

Rupert
Posts: 6224
Joined: Thu Sep 16, 2004 8:40 pm
Location: Cotswold Water Park

Re: Pulley ratios.

Post by Rupert » Fri Jan 23, 2015 3:05 pm

I suppose you could also move the object a set amount, then measure the amount of rope used, and divide it?
Rupert

User avatar
PeterV
Posts: 1168
Joined: Thu Sep 16, 2004 8:47 pm
Location: Locks Heath, Hampshire

Re: Pulley ratios.

Post by PeterV » Fri Jan 23, 2015 4:20 pm

Rupert, you are correct on both posts, the difference in distances is the velocity ratio, a 'string' from a stationary block adds nothing other than a conveneint lead.

Anyone got an early Admiralty Manual of Seamanship?
PeterV
Finn K197 & GBR564
Warsash

dronskiuk
Posts: 179
Joined: Thu Nov 15, 2012 3:23 pm
Location: Where Broadland meets the sea

Re: Pulley ratios.

Post by dronskiuk » Fri Jan 23, 2015 4:27 pm

PeterV wrote:You cannot just count the strings. A pulley block attached to the ceiling to lift a weight has 2 strings, but gives no mechanical advantage. A pulley block attached to a weight gives a 2:1 advantage when you lift the weight, and still has 2 strings. The difference is that some pulleys just turn the rope to a more convenient position, others give mechanical advantage. The pulleys which are attached to the moving part all give mechanical advantage, so if you count the strings at the moving pulley you will get the mechanical advantage. (Striclty speaking you'll get the velocity ratio and the mechanical advantage is this less friction losses).
This used to be O level Physics, not sure if it's now in GCSE.
It's 'between the pulleyS' bit that counts...a single pulley, as you say, gives no mechanical advantage and in fact adds friction thus making it less than 1:1 :oops:

Many decades ago I had to do a Seamanship 'O' level paper and these questions were part of a lead up to what should the breaking strain/safe working load be on rope/wire used on an X:1 pulley system to lift Y tons on a cargo - a long way from the computerised/containerised modern systems....or a hydraulic kicker or backstay for that matter.

I also recall diameter of the sheaves was relevant...

I think I'll have a lie down...bad memories!!

Rupert
Posts: 6224
Joined: Thu Sep 16, 2004 8:40 pm
Location: Cotswold Water Park

Re: Pulley ratios.

Post by Rupert » Fri Jan 23, 2015 4:32 pm

You may get less mechanical advantage because of friction, but it is still a good deal easier to lift a weight by pulling down from a block in the ceiling than trying to lift it from above, say. I think that has more to do with the way we are built, though.
Rupert

User avatar
trebor
Posts: 956
Joined: Mon Jan 02, 2012 1:53 pm
Location: West Midlands
Contact:

Re: Pulley ratios.

Post by trebor » Fri Jan 23, 2015 6:23 pm

3 strings not counting the one in your hand = 3 to 1.
Thank you for replies.
I have another couple of queries, I am designing mainsheet system for Minisprint, I need to use an hoop to clear centre board when it swings over, here is a rough pen and paper design for hoop.
http://s1149.photobucket.com/user/aquab ... sort=3&o=0
I have designed it with corners so mainsheet block will slide to corner and I hope prevent boom from pulling to near centre line.

I am hoping this ratchet block will do the job, I intend to tie this block to top of hoop.
http://www.ebay.co.uk/itm/HOLT-NAUTOS-H ... 4181a36849

Will it work ?
Robert
Minisprint 4230
Tinker Traveller 160
Mirror 61147 Anastasia
http://www.aquabatdinghy.co.uk

JimC
Posts: 1706
Joined: Sun Oct 03, 2004 10:24 pm
Location: Surrey
Contact:

Re: Pulley ratios.

Post by JimC » Fri Jan 23, 2015 7:13 pm

Hoops are a PIA. Wouldn't it be easier just to use a bridle?

User avatar
trebor
Posts: 956
Joined: Mon Jan 02, 2012 1:53 pm
Location: West Midlands
Contact:

Re: Pulley ratios.

Post by trebor » Fri Jan 23, 2015 8:17 pm

Hi Jim, I have heard Bridles mentioned before, but I do not know what they look like, do you have a link to a photo of one ?
Robert
Minisprint 4230
Tinker Traveller 160
Mirror 61147 Anastasia
http://www.aquabatdinghy.co.uk

User avatar
jpa_wfsc
Posts: 1184
Joined: Sun Dec 04, 2005 9:52 pm
Location: Oxford (Work) Coteswold Water Park (Sailing)

Re: Pulley ratios.

Post by jpa_wfsc » Fri Jan 23, 2015 10:12 pm

IMHO... a single sailed boat needs lots of gooooood control with the kicker, to maintain bent mast and so manage power. So, the mainsheet needs to be only used to control the angle of attack of the sail - i.e. how far out from the CL it is.

Bridles work well for this IF the kicker is powerful and 'stiff' enough (low stretch), and if the same applied to the kicker mountings. i.e. if the boom and hull is stiff. You simply set up two lengths of STRONG stretch-free rope to hold one block up where the boom will be when sheeted in as far as you want it to be. The ratchet block (last before your hand) _has_ to be on the floor or perhaps top of the Centreboard case, it can not be flapping around in the air on the bridle. Most commonly, the bridle is at the outer end of the boom and is fixed to the corners of the transom like this: http://www.yachtsandyachting.com/forum/ ... veller.jpg

The great bit about the bridle is that when you have lots of kicker tension (its blowy) then the boom end comes down a little, and the boom can not then be sheeted right in to the centre line - which is perfect, its what you need on a single sailed boat. It is possible to refine this with the system that Laser use on the Laser and Pico (lots of pics on the internet about that) or for that matter the topper.

A hoop is really just a very stiff and very heavy alternative to a bridle and keeps all the rope in the middle of the boat. Again - the ratchet will be on the floor, or under the hoop. The hoop will ALWAYS be in your way in a single handed boat.

If the kicker is not strong, then you need a mainsheet traveler (like OK, Finn, most Moths etc. use) either across the cockpit or if there is no room, then at the transom.
j./

National 12 "Spider" 2523
Finn K468 'Captain Scarlet'

British Moth, 630, early 60's 'Pisces'

!!!! Not CVRDA !!!!
Comet Trio - something always ready to sail.

Post Reply